The laser ranging experiment of the Lunar Reconnaissance Orbiter: Five years of operations and data analysis

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The laser ranging experiment of the Lunar Reconnaissance Orbiter: Five years of operations and data analysis Dandan Mao a,∗, Jan F. McGarry b, Erwan Mazarico b, Gregory A. Neumann b, Xiaoli Sun b, Mark H. Torrence c, Thomas W. Zagwodzki a, David D. Rowlands b, Evan D. Hoffman b, Julie E. Horvath d, James E. Golder a, Michael K. Barker a, David E. Smith e, Maria T. Zuber e a

Sigma Space Corporation, Lanham, MD 20706, USA Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Stinger Ghaffarian Technologies Inc., Greenbelt, MD 20770, USA d Honeywell Technology Solutions Inc., Columbia, MD 21046, USA e Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b c

a r t i c l e

i n f o

Article history: Received 14 March 2016 Revised 8 June 2016 Accepted 3 July 2016 Available online xxx Keywords: Moon Orbit determination Experimental techniques

a b s t r a c t We describe the results of the Laser Ranging (LR) experiment carried out from June 2009 to September 2014 in order to make one-way time-of-flight measurements of laser pulses between Earth-based laser ranging stations and the Lunar Reconnaissance Orbiter (LRO) orbiting the Moon. Over 4,0 0 0 h of successful LR data are obtained from 10 international ground stations. The 20–30 cm precision of the full-rate LR data is further improved to 5–10 cm after conversion into normal points. The main purpose of LR is to utilize the high accuracy normal point data to improve the quality of the LRO orbits, which are nominally determined by the radiometric S-band tracking data. When independently used in the LRO precision orbit determination process with the high-resolution GRAIL gravity model, LR data provide good orbit solutions, with an average difference of ∼50 m in total position, and ∼20 cm in radial direction, compared to the definitive LRO trajectory. When used in combination with the S-band tracking data, LR data help to improve the orbit accuracy in the radial direction to ∼15 cm. In order to obtain highly accurate LR range measurements for precise orbit determination results, it is critical to closely model the behavior of the clocks both at the ground stations and on the spacecraft. LR provides a unique data set to calibrate the spacecraft clock. The LRO spacecraft clock is characterized by the LR data to a timing knowledge of 0.015 ms over the entire 5 years of LR operation. We here present both the engineering setup of the LR experiments and the detailed analysis results of the LR data. © 2016 Elsevier Inc. All rights reserved.

1. Introduction The Lunar Reconnaissance Orbiter (LRO; Chin et al., 2007) was successfully launched on 18 June 2009. A few days after its launch, the spacecraft arrived in its designated polar orbit and has been orbiting the Moon ever since. The purpose of the LRO mission is to enable the future safe return of humans to the lunar surface and to identify and characterize scientifically interesting landing site locations. Seven instruments are onboard to perform a global and detailed geophysical, geological and geochemical mapping of the Moon in order to achieve the mission objectives. Some of these instruments have very high spatial resolution: < 50-cm pixel resolution of Lunar Reconnaissance Orbiter Camera (LROC;

∗

Corresponding author. E-mail address: [email protected] (D. Mao).

Robinson et al., 2010), 10-cm range resolution of the Lunar Observer Laser Altimeter (LOLA; Smith et al., 2010), 320 × 160-m field-of-view of Diviner Lunar Radiometer Experiment (DLRE; Paige et al., 2010), 30-m zoom resolution of Miniature Radio Frequency Technology Demonstration (Mini-RF; Nozette et al., 2010). Datasets from these LRO instruments must be positioned in a common high-accuracy, geodetic grid provided by LOLA. To take full advantage of the high resolution LRO data, the spacecraft orbit must be reconstructed precisely. The LRO mission hence requires a spacecraft position knowledge of 1-m vertical accuracy and 50 to 100-m for total position (Vondrak et al., 2010). After lunar orbit insertion on 23 June 2009, LRO entered the Commissioning phase on 27 June. The orbit in this phase was a quasi-frozen, ∼30 × 200 km elliptical polar orbit, whose periapsis was close to the South Pole. On 15 September 2009, LRO transitioned to its nominal 2-h near-circular mapping orbit, with an average altitude of 50 km, and a low eccentricity of 0.0 054 ± 0.0 019.

http://dx.doi.org/10.1016/j.icarus.2016.07.003 0019-1035/© 2016 Elsevier Inc. All rights reserved.

Please cite this article as: D. Mao et al., The laser ranging experiment of the Lunar Reconnaissance Orbiter: Five years of operations and data analysis, Icarus (2016), http://dx.doi.org/10.1016/j.icarus.2016.07.003

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After 2 years in the nominal orbit, the spacecraft entered a fuelconserving 30 × 180 km elliptical orbit on 11 December 2011, and has remained there with small orbital adjustments. As the most widely used tracking method for interplanetary spacecraft, radiometric tracking is the baseline tracking system for LRO. A National Aeronautics and Space Administration (NASA) station in White Sands (WS1), New Mexico, and a commercial network, the Universal Space Network (USN), provide up to 20 h per day of almost continuous S-band radio frequency link to LRO. All of the participating stations produce S-band range and Doppler radiometric data. The pre-launch LRO mission requirement for the S-band tracking accuracy is 1 mm/s for WS1 station (Chin et al., 2007), and 1.5–3 mm/s for USN stations. With such accuracy, the spacecraft orbits can only be determined to ∼10 m radially and 300 m horizontally (Smith et al., 2008). To attain the precision orbit requirement, a Laser Ranging (LR; Zuber et al., 2010) system was introduced to allow an Earth laser station to range to LRO whenever in view. LRO-LR operation was conducted regularly for 5 years, from 30 June 2009 to 30 September 2014. Two-way satellite laser ranging (SLR) is a mature technology and has been widely used to track Earth-orbiting satellites (Pearlman et al., 2002). The success of two previous experiments demonstrated laser ranging over interplanetary distances: a twoway laser link between the Mercury Laser Altimeter (MLA) aboard the MESSENGER (MErcury Surface, Space ENvironment, Geochemistry, and Ranging) spacecraft and NASA’s Goddard Geophysical and Astronomical Observatory (GGAO) at a distance of ∼0.16 AU (Smith et al., 2006), as well as a one-way optical uplink from GGAO to the Mars Orbiter Laser Altimeter (MOLA) aboard the Mars Global Surveyor (MGS) spacecraft at a distance of ∼0.54 AU (Neumann et al., 2006). These experiments and the LRO-LR experiment suggest that interplanetary laser ranging can greatly enhance science return from future planetary missions (Degnan, 2002; Turyshev et al., 2010; Dirkx et al., 2014). With LRO orbiting the Moon, the LRO-LR system provides a platform to demonstrate some deep space ranging technologies. For example, on March 26, 2012, a 200 × 152pixel 12-bit (4096) gray scale image of the Mona Lisa was successfully transmitted to LRO from the laser station at GGAO via the LR uplink, demonstrating a useful technique for simultaneous laser tracking and data communication to a spacecraft in deep space from existing SLR stations as an alternative to the conventional microwave links (Sun et al., 2013a). In 2013 and 2014, multiple tests have been conducted to transfer time between two SLR stations at GGAO via simultaneous LR operation to LRO (Sun et al., 2013b; Mao et al., 2014a). The purpose of the time transfer is to bring multiple stations to the same precise time reference. The success of these tests verified that nanosecond precision of time transfer is achievable and can be stable for over a year. In addition to providing a test channel for deep space tracking technologies, LR to LRO also presents a unique and new dataset for clock monitoring and spacecraft orbit determination. A few research groups have tested the LR dataset to analyze the mid- to long-term behavior of the LRO clock for 2011 (Buccino et al., 2016), or the short-term spacecraft clock stability for each LRO mission phase (Bauer et al., 2014 and 2016b). Research has also been conducted to examine LR data’s capability in determining the LRO orbit independently or together with the conventional radiometric tracking dataset for short time spans from a week to a year before 2013 (Löcher et al., 2015; Buccino et al., 2016; Bauer et al., 2016a; Maier and Baur, 2016). We here document all the engineering setup and modifications, as well as the data analysis results in both clock characterization and LRO orbit determination covering the entire 5 years of LR operation. In this paper, we first describe the LRO-LR system setup, shown in Fig. 1. The system includes the flight unit on LRO (described in Section 2.1) and the ground station network (described in

Laser Station LRO Fig. 1. Schematic illustration of the LRO-LR system, with the Earth-based ground station sending a laser uplink to LRO.

LOLA Channel 1 Detects LR Signal LR Receiver Telescope

Fiber Optic Bundle Fig. 2. Schematic illustration of the LR flight unit. The Earth uplink signal is detected by the LR telescope mounted on the HGA, and sent to LOLA channel 1 for processing via the fiber optic bundle.

Section 2.2). In Sections 2.2.2 and 2.2.3, two LR ground systems are discussed in detail to show the engineering modifications that enabled the one-way LRO ranging capability. After a summary of the LR data collection and processing in Sections 3 and 4, we discuss how the LR data are used to monitor the LRO clock’s mid- to longterm timing stability in Section 5.1. The improvement of ground station timing precision will then be presented in Section 5.2, followed by LRO orbital reconstruction solutions generated using LR data in comparison with conventional radiometric orbital solutions in Section 6. 2. LRO-LR system description 2.1. Flight unit LRO-LR conducts one-way, time-of-flight measurements from the Earth-based SLR station to the spacecraft. The laser signals from the Earth are received by the LR telescope (LRT; RamosIzquierdo et al., 2009), which is mounted on and co-bore-sighted with the High Gain Antenna (HGA). The HGA is pointed at the WS1 station during nominal operation for high-rate Ka-band telemetry downlink as well as S-band communication. The LRT has a 30mrad (∼1.7o ) field of view (FOV) and a 19-mm diameter aperture, which enables it to cover nearly the entire Earth when centered on WS1. The laser signals intercepted by the LRT are transmitted to the LOLA channel 1 detector via a ∼10-m long bundle of multimode fiber-optic cables (Ramos-Izquierdo et al., 2009) through the wire conduit of the HGA, shown in Fig. 2. The LOLA channel 1 detector is designed to receive both the 1064-nm lunar return signals and the 532-nm Earth signals using common timing hardware via separate range windows that occur at a frequency of 28 Hz, and are phase-locked to the spacecraft 1-Hz Mission Elapsed Time (MET) signals. Each 1 s interval is referred to as a major frame, which consists of 28 approximately 35.7 ms periods referred to as minor frames that begin at a time (designated as T0 ) relative to the LRO MET. Only signals falling into specified range windows are detected and processed, thus reducing the false detections and improving the receiver sensitivity. The channel 1 electronics timing is shown in Fig. 3. The Earth window

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Start of LOLA laser fire period (T0)

LOLA laser fires (~ 9ms after T0)

EARTH RANGE WINDOW (8ms)

LUNAR RWIN (5ms)

T0

Start of next LOLA laser fire period

Data Processing

T0 35.7 msec (28 Hz)

Fig. 3. Schematic illustration of the ∼35-ms LOLA dual range window.

Fig. 4. Global map showing the 10 stations participating in the LRO-LR operation.

starts 1 ms after T0 and lasts 8 ms. Signals within this window are recorded as arriving from the Earth. The lunar window opens after every LOLA laser emission with a slight time delay and spans less than 5-ms. The LOLA timing reference is based on a 20 MHz ultra stable oscillator (USO) located in the spacecraft, which is divided down to 5 MHz and is further subdivided into intervals of 178,571 or 178,572 ticks providing 28 minor frames beginning at T0 . The USO is a quartz oven-controlled crystal oscillator (OCXO), with a frequency stability better than 1 part in 1012 per h (Cash et al., 2008). The received Earth signal leading and trailing edge times are processed by LOLA, and the results are sent down in science packets over the Ka-band communication link. The Earth signal energy and event counts are additionally provided in the real-time housekeeping telemetry. 2.2. Ground network 2.2.1. Overview NASA’s Next Generation Satellite Laser Ranging (NGSLR) station at Greenbelt, Maryland serves as the primary LRO-LR ground station since the beginning of the LRO mission. The NGSLR (with the station ID of GO1L) station was originally designed to perform two-way ranging to the retro-reflectors on Earth-orbiting satellites. The system was modified to enable the station to track LRO as well as satellites around the Earth. A sub-network of 9 SLR stations from the International Laser Ranging Service (ILRS) (Pearlman et al., 2002), shown in Fig. 4, provides supporting global coverage of laser ranging to LRO. Four of these stations (Monument Peak in California, USA; Yarragadee in Australia; Hartebeesthoek in South Africa; Greenbelt in Maryland, USA) are Mobile Laser Ranging Systems (MOBLAS) deployed by NASA for SLR tracking, but modified to also perform the LR measurement. The modification of both the NGSLR and the MOBLAS systems will be described in more detail later in Sections 2.2.2, and 2.2.3, respectively. All the participating stations meet the tracking requirements of laser energy density at the spacecraft (1–10 fJ/cm2 ), wavelength (532.2 ± 0.15 nm), number of pulses per second (no more than 28-Hz fire rate), fire timing

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resolution (no larger than 100 ps), and the ability of the station software to use the ILRS Go/No-Go flag. All participating stations are equipped with oscillators to maintain a stable time base, such as rubidium, cesium (long-term stability about 10−13 s/s) or hydrogen-maser (H-maser) clocks (the most stable, long-term stability about 10−15 s/s). NGSLR has improved its time base to nanosecond precision and accuracy over 20 months by employing the H-maser clock of the nearby Very Long Baseline Interferometry (VLBI) site, via optical fibers since February 2013 (Sun et al., 2013b). McDonald has also improved its time base to ∼10 ns over 7 months, using the same technique with a cesium clock from April 2014. Details of the timing improvements of these two stations are described in Section 5.2. Some of the stations, such as NGSLR, Zimmerwald, Herstmonceux, and Wettzell, need to use knowledge of the spacecraft MET with respect to Coordinated Universal Time (UTC) to time their fires to ensure all their laser pulses arrive at LRO when the LOLA Earth window is open, at a frequency of 28-Hz, 14-Hz, or 7-Hz. This is referred to as synchronous ranging. Other stations, McDonald, Grasse, and all four MOBLAS stations, fire their lasers asynchronously at their normal 10-Hz frequency. Asynchronous ranging at 10-Hz results in 2 signals per second into the LOLA Earth Window most of the time, and at most 4 signals per second in the Earth window on occasion. Table 1 gives the system characteristics of each LRO-LR ground station and the time of their first successful range to LRO. While ranging to LRO, feedback to the operators is provided from the LOLA S-band downlink of the housekeeping data. Information provided by LOLA’s onboard signal processing software, like LOLA Earth Window receive event counts, is sent to the ground, and displayed in a graphical format via a website generated by the LOLA Science Operations Center (SOC) in Greenbelt and hosted at the Crustal Dynamics Data Information Center (CDDIS; Noll, 2010). Operators from all stations can access this web-based near-realtime plot. The plot update is typically delayed by tens of seconds, but can be delayed by up to minutes. It nevertheless provides invaluable feedback to the ground stations to optimize their pointing and tracking to LRO. Otherwise, the ground stations would rely solely on ephemeris predictions and station laser alignment knowledge, degrading overall performance and preventing realtime pointing and timing corrections. A sample of the real-time LOLA Earth receive count display is shown in Fig. 5. Each point plotted in the top panel represents 1 s’s worth of data arriving at LOLA. The average delay of the signal events into the LOLA Earth Window is represented by the location of the points, and the color of the points represents the signal count over that second. The color bar at the bottom of Fig. 5 gives the key to the signal counts for all three panels. The middle panel in Fig. 5 is similar to the top one but uses the maximum bin in the Earth Window histogram instead of the signal processing results. Results that might be too weak to show up in the signal processing (signals from asynchronous ranging stations) will show up in this panel. The bottom panel shows results outside of the signal-processing sub-window. This panel will capture results from another station that is ranging simultaneously (discussed in detail in Section 3) but that has less frequent or weaker laser pulses at LRO. For instance, when NGSLR and MOBLAS-7 are ranging together, NGSLR data would show up in the top and middle panel, and MOBLAS-7 s data would be plotted in the bottom panel. 2.2.2. Description of the primary ground station: NGSLR NGSLR (GO1L) is the Next Generation Satellite Laser Ranging prototype for the Space Geodesy Project (SGP; Merkowitz et al., 2014). Originally called SLR20 0 0, it was built to test out new concepts and new technologies for SLR (McGarry and Zagwodzki, 2006). The system was designed for single photon detection during

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Table 1 LRO-LR ground station characteristics. Station

Location

Synchronous?/fire rate (Hz)

Max number/s in LOLA window

Expected energy at LOLA (mJ/cm2 )

Station frequency source

NGSLR (GO1L)

Greenbelt, Maryland, US McDonald Obs., Texas, US Greenbelt, Maryland, US Great Britain

Yes—28 Hz

28

1–5

H-maser (18-Oct-2010)

30-Jun-2009

No—10 Hz

2–4

1–10

cesium

02-Jul-2009

No—10 Hz

2–4

1–3

H-maser (18-Oct-2010)

02-Jul-2009

Yes—14 Hz

14

1–3

H-Maser (13-May-2010)

13-Jul-2009

Switzerland

Yes—14 Hz

14

1–3

20-Jul-2009

Germany Hartebeesthoek, South Africa Yarragadee, Australia Monument Peak, California, US France

Yes—7 Hz No—10 Hz

7 2–4

1–10 1–3

oven-controlled crystal oscillator cesium H-maser

30-Oct-2009 05-Dec-2009

No—10 Hz

2–4

1–3

H-maser (14-May-2010)

25-Jan-2010

No—10 Hz

2–4

1–3

GPS-steered rubidium

03-Feb-2010

No—10 Hz

2–4

1–10

cesium

18-May-2010

MLRS (MDOL) MOBLAS-7 (GODL) Herstmonceux (HERL) Zimmerwald (ZIML) Wettzell (WETL) MOBLAS-6 (HARL) MOBLAS-5 (YARL) MOBLAS-4 (MONL) Grasse/MEO (GRSM)

First successful LRO ranging

Fig. 5. A sample of the real-time LOLA receive signal count website seen at the ground station during operation. The top and middle panel of this plot shows that NGSLR is successfully ranging to LRO, with ∼20 signals (indicated by the yellow colour of the points) reaching the end of the Earth window (indicated by the location of the points near 8 ms from the beginning of each Earth window) per second. The bottom panel shows that ∼5 signals (indicated by the dark blue colour of the points) per second fall outside of the LOLA Earth window.

ranging in both day and night. In 2013 NGSLR demonstrated twoway day and night ranging to satellites with altitudes of 300 km to over 22,0 0 0 km. During acceptance testing, the system demonstrated millimeter ranging stability over hours and ranging agreement with the current legacy NASA SLR network to within a few millimeters of the theoretically expected difference between single and multi-photon laser ranging systems (McGarry et al., 2013). The system design is flexible enough to allow for modifications to perform the 1-way ranging. A Northrop Grumman laser is added to the system above the existing SLR optical bench. This laser transmits 50 mJ of 532.2 nm light at 28 Hz in a 5.5 ns pulse-width. The light from the laser is directed toward an insertion mirror on

the optical bench that reflects the light out through the Coude path and the telescope. The insertion mirror is manually placed into the beam path for LRO-LR operations and then removed for SLR operations. A detailed block diagram in Fig. 6 shows the modifications to accommodate one-way LR to LRO. At the time that the LRO modifications were being made to NGSLR, the SLR tracking was performed using eye-safe energies (<0.3 W). With the introduction of the non-eye-safe energies for LRO (at 1.4 W), an aircraft avoidance radar is added to the system to detect aircraft near the beam’s path. A control chassis is also added to the system to take the detections from the radar and block the outgoing laser light to avoid illuminating aircraft.

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Fig. 6. NGSLR block diagram for LRO-LR operations, with all modifications added shown in green.

Given LRO-LR is a one-way range, the laser fire times must be captured with as much precision and accuracy as possible in order to form the best possible ranges. The method of capturing the fire time is different for LRO-LR than for SLR. The Honeywell Event Timer is used for both SLR and LRO-LR event times, but for LROLR the raw fires are captured to ensure the maximum resolution of the unit (a few picoseconds). The timing instrument for both SLR and LRO-LR was the Symmetricom XL-DC Global Positioning System (GPS)—steer rubidium. The steering XL-DC proved difficult to model and did not have the stability needed, so the 10 MHz from the co-located VLBI H-maser clock is brought into the system and used as an external input to the Event Timer. The 1 pps from the H-maser is also captured in the Event Timer data stream and used as a reference for the laser fire times. A cesium timing source (Symmetricom 4310B) is additionally used in the system as an alternate during certain periods of time when the H-maser signals are not available. In order for the pulses to arrive in the LOLA Earth window, the ground station software has to control when the laser fires. Given that the LOLA Earth window opens 1 ms after the start of each 28 Hz laser fire period, and the 28 Hz fire periods are synchronized to the onboard MET, the software determines the ground laser transmit time by the following equation in order for the signal to arrive at the middle of the open LOLA Earth window: UTC UTC Ttran + δtLOLA_Earth_window /2 − δtone−way_light_time , smit = T0

UT C where Ttransmit is the time when the laser pulse is transmitted from the ground station in UTC, T0UT C is the start time of the LOLA Earth window (∼1 ms after the 28 Hz LOLA laser fire time) in UTC (details of the conversion from MET to UTC are described later in Section 5.1), δ tLOLA_Earth_window is the width of the LOLA Earth window (8 ms), and the δtone−way_light_time is the laser light time from the station to LRO. This method works extremely well and produces data that consistently arrive at the same location in the Earth window. Because the LOLA detectors are all single stop, once an event is received (including noise events), no other event will trigger the detector until the next 28 Hz minor frame. Early in the LRO-LR efforts, to minimize the chance that noise triggers before the NGSLR signals arrive, the operators at NGSLR use a bias less than 4 ms for the laser pulses to arrive early in the range window. However, the signal from NGSLR proved to be so frequently received at LOLA (synchronized ranging at a frequency as high as 28 Hz) that once other stations start ranging to LRO at lower repe-

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tition rates than NGSLR, the NGSLR operators have to apply a time bias larger than 4 ms to make their laser signals arrive late in the Earth window in order to allow other stations to trigger the detector. Because no other station operated at 28 Hz, NGSLR signals can still be recorded in the LOLA Earth window when other stations’ pulses do not appear. The telescope pointing angles are calculated from the Consolidated Prediction Format (CPF; Ricklefs, 2002) files that are supplied by the Flight Dynamics Facility (FDF; Stengel and Hoge, 2008) at NASA’s Goddard Space Flight Center (GSFC). These files give the spacecraft position in Earth Centered Earth Fixed (ECEF) rectangular coordinates at times separated by 1-min intervals throughout the pass. The station software converts the ECEF vectors to local azimuth and elevation to point the telescope, and also produces the range to the target, which is used in the determination of the laser fire times. The accuracy of the predictions is on the order of ±2 km at lunar distances, which is accurate enough for the 55-μrad laser beam divergence (FWHM). The accuracy of LOLA MET to UTC time conversion provided by the LRO mission is sometimes not sufficient to put the NGSLR laser events into the LOLA Earth Window at first try. However, the operator is able to choose a time bias during the time conversion until events are recorded, as seen in the web based Graphical User Interface (GUI). Once the MET to UTC time bias is determined, it remains relatively fixed throughout a shift of tracking, and would normally drift slowly from day to day, allowing quick acquisition when LRO is being ranged to regularly.

2.2.3. Description of the NASA MOBLAS ground stations NASA’s current SLR network was developed in the early 1980s and consists of eight SLR stations, deployed around the world, which perform satellite laser ranging. Most of the systems in this network are Mobile Laser Ranging Systems, or MOBLAS for short. Modifications are performed to four MOBLAS systems to allow them to range to LRO: MOBLAS-4 in Monument Peak, California USA, MOBLAS-5 in Yarragadee Australia, MOBLAS-6 in Hartebeesthoek South Africa, and MOBLAS-7 in Greenbelt, Maryland USA. The modifications for LRO-LR consist mainly of designing and developing a separate computer system to time the laser pulses to the accuracy needed. A Guidetech timer card (GT658) is installed into a Dell workstation. The 10 MHz signal from the MOBLAS system’s XL-DC is used as the external source for the Guidetech timer card, and the 1 pps signal from the XL-DC is used as the data input for the first channel of the card. Prior to ranging to LRO, the MOBLAS operator would ensure that a spare output from the start diode discriminator is connected to the second channel. For MOBLAS systems that have access to a VLBI H-maser, the 10 MHz from the H-maser is used as the external source for the Guidetech cards. A block diagram with the modifications made to MOBLAS 4, 5, 6 and 7 in support of laser ranging to LRO is shown in Fig. 7. MOBLAS 5, 6 and 7 have access to and use a VLBI H-maser for their 10 MHz external reference. MOBLAS 4 uses the 10 MHz from the GPS-steered rubidium. The Start Discriminator signal is normally input to the Time Interval Counter for SLR, but for LRO-LR, this signal is manually moved to the Guidetech card prior to each LRO operational session. The Guidetech card provides a higher precision fire time measurement than what the MOBLAS systems currently have for SLR. The LRO system computer also has an input from a GPS receiver, which is part of an installed CNS clock system. The computer time is synchronized to UTC through the use of the CNS clock hardware and software. The software with the corresponding UTC computer time tags the event time of the 1 pps signal from the Guidetech card. The date and time of each fire event is then

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Due to the different station orientations to LRO, not all simultaneous passes are full-length; many of them have shorter segments of overlapping ranges. Almost all the successful simultaneous ranging passes have been performed by stations within the same continent, either all lying in North America or in Europe. Cross-continental simultaneous passes are much more difficult to achieve due to the different LRO visibility time windows for SLR stations from different continents, the limitation of the LRT FOV on LRO, and the downlink pointing of the HGA to specific radio ground stations, to which the LRT is attached. After some nearsuccessful attempts, a ∼2-min long simultaneous ranging segment was eventually obtained between NGSLR in USA and Zimmerwald in Switzerland on 15 September 2014, establishing the longest separation between ground stations (∼6300 km) for simultaneous ranging. Simultaneous ranging statistics for the entire 5-year LR operation are given in Tables 4 and 5. The global network of the LR stations has demonstrated that it can provide close to 24-h coverage for LRO-LR when LRO is on the Earth side of the Moon. In each year from 2011 through 2014 there were over 100 days with ranging at or over 16 h per day. 4. LR data processing and delivery

Fig. 7. Block diagram with modifications to the NASA MOBLAS systems for LRO-LR operations. The box with the dashed lines contains the new equipment added to the systems.

referenced to the 1 pps date and time. Like NGSLR, the MOBLAS system computer uses the LRO CPFs to point the telescope at LRO. 3. LR data collection During the 62 months of LRO-LR operation from 30 June 2009 to 30 September 2014, there were over 4173 h of successful LR tracking data recorded at LOLA from the 10 participating ground stations. The number of minutes and the percentage of total data for each station are given in Table 3. (The detailed weekly data contribution from each station is listed in the supplementary Table.) The length of a pass for LR is nominally 1 h (half of LRO’s orbit period) and depends upon the orientation of the LRO orbit with respect to the station. The HGA, on which the LRT is mounted, nominally points toward the Earth. Since the FOV of the LRT receiver is about 30 mrad, many stations on the Earth can fire to and hit LRO at the same time. For the first few months of LR operation, only a single station was typically scheduled to range to LRO at any time. It is shown that laser ranging pulses from two or more ground stations at the same time can be separated. The LOLA onboard LR signal processing was not designed to handle multiple stations ranging simultaneously, but only to time tag the first laser pulse arrival in the LOLA Earth range window, regardless of the ground stations. Nevertheless, we found that we could separate the returns of multiple ground stations on the real-time website when they range at the same time. Thus, simultaneous ranging to LRO by two or more stations was scheduled and achieved regularly during the LRO-LR operation. Simultaneous ranging provides the Science Team with additional information to determine measurement biases, and it enables time transfer between clocks located at different ground stations. Three-way simultaneous ranging can potentially provide a geometric solution of the spacecraft position after correction for biases (Hoffman et al., 2014). The exact dates of the first successful 1-way, 2-way, 3-way, and 4-way ranging to LRO are listed in Table 2.

Ground laser transmit time and other related information from all LR stations are sent to the LOLA SOC via CDDIS in the Consolidated Ranging Data (CRD) format (Ricklefs, 2006) for processing. These transmit times must be matched with corresponding LOLA receive times from the downlink science telemetry (Experiment Data Record; Smith 2016a) to form range measurements. To match each LOLA-received LR time to the corresponding ground SLR station transmit time, a predicted time of flight is calculated for each out-going laser pulse by using the ground station location, the Earth orientation, the lunar orientation and ephemeris based on the NASA Jet Propulsion Laboratory (JPL) DE421 ephemeris (Williams et al., 2008), and a spacecraft ephemeris provided by the LRO navigation team at NASA GSFC’s FDF. The predicted light time is added to the LR transmit time to form an approximate LOLA arrival time. A LOLA receive time is considered to be matched to the given transmit time if the difference between the approximate arrival time calculated from the transmit time and the LOLA received signal time satisfies the following condition: UTC UTC T0 < Tappr oximate_arrival − TLOLA_receive + δ tbias < T0 + δ tLOLA_Earth_window ,

where T0 is the start time of the Earth window in which the LOLA UTC receive signal arrives, Tappr is the approximate arrival oximate_arrive time in UTC calculated as the sum of the laser transmit time and UTC one-way light time, TLOLA is the LOLA receive time in UTC, _receive and δ tbias is a timing bias, which is chosen to achieve the maximum number of matches for all ground stations. To absorb the clock and orbit errors, the timing bias is adjusted every 3 months to ensure the maximum number of matches obtained over the entire LR operation. After accounting for the calculated light time, a low-degree polynomial (quadratic or cubic) is used to model the difference of LOLA receive time and the ground laser fire time arising mainly from orbit errors. The LOLA LR receive times are nominally the midpoint time of the start and the end time of the received wave at the receive threshold. Ideally, the midpoint time is when the pulse centroid arrives at LOLA. However, non-ideal laser output and distortion of the received pulse waveform by the LRO electronics (Riris and Cavanaugh, 2008), together with atmospheric signal fading (Andrews and Phillips, 2005) lead to an energy-dependent offset between the midpoint time and the actual wave centroid (i.e., range walk). To achieve high precision range measurements, an empirical range walk correction approximating that produced by convolution of

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7

Table 2 LR network’s first successes.

Ranging to LRO 2-way simultaneous 2-way simultaneous cross Atlantic 3-way simultaneous 4-way simultaneous

Date

Station(s)

30-Jun-2009 28-Jul-2009 15-Sept-2014 01-Nov-2010 11-Mar-2011

GO1L GO1L, GO1L, GO1L, GO1L,

GODL ZIML MDOL, MONL GODL, MDOL, MONL

Table 3 Total minutes of data (row 2) and percentage of total LR data for each station from June 2009 to September 2014. GO1L

GODL

MDOL

HERL

ZIML

WETL

HARL

YARL

MONL

GRSM

81,049 33%

19,587 8%

26,935 11%

3618 1%

3647 1%

290 < 1%

1878 1%

35,299 14%

70,793 29%

4694 2%

Table 4 Number of 2-way simultaneous ranging passes/segments between June 2009 and September 2014. Stations

Number of 2-way passes

Stations

Number of 2-way passes

GO1L, GODL GO1L, MDOL GO1L, MONL GODL, MDOL GODL, MONL MDOL, MONL ZIML, GO1L

61 139 318 58 93 151 1

HARL, HERL HARL, ZIML GRSM, ZIML GRSM, HERL ZIML, HERL HERL, WETL ZIML, WETL

1 4 49 16 5 3 1

a 6-ns wide Gaussian waveform with an exponential decay electronic response is added to the receive time in order to reduce the errors caused by the distortion of the received photon waveform at the LRO electronics, as shown in the formula below (in ns):

δtrange_walk = 6 −







62 + p × tpulse_width − 1

2

,

where tpulse_width is the pulse width of the Earth signal LOLA receives, and p is a coefficient depending on the ground station laser pulse properties. The value of p is chosen by examining the first 2 years of the LR data and finding the coefficient that provides best precision. Among all stations, NGSLR has a laser with a pulse width of 5.5 ns, while the maximum pulse width of lasers from other stations is 200 ps. Thus, two coefficient p values, 0.33 and 0.45, are used for NGSLR and other stations, respectively. Without the range walk correction, the residuals of a successfully-matched LR pass usually have a standard deviation ranging from 30 to 100 cm for most of the ground stations, and 50–150 cm for NGSLR and Herstmonceux, depending on the laser properties of the ground station. After applying the range walk correction, the standard deviation was improved to ∼30 cm for NGSLR and Herstmonceux, and ∼15 cm for all other stations. While NGSLR’s larger light time residual scatter is caused by its larger laser pulse width, the large light time residual scatter at Herstmonceux is due to its laser pulse train

Fig. 8. Newtonian light time residuals of full-rate (gray) and normal point (black) data from a 60-min-long NGSLR pass taken on 20-Sep-2013, after removing the calculated light time and a quadratic polynomial from the difference between LOLA receive times and ground station laser transmit times. Constructing normal points from the full-rate data improves the data precision from 29.39 cm to 5.45 cm for this NGSLR pass.

(Wilkinson and Rodriguez, 2013; Gibbs et al., 20 0 0), which consists of multiple smaller laser pulses following the first and main pulse. The pulse width of the waveform LOLA received from Herstmonceux is usually larger than that of the main pulse (200 ps), yielding a larger light time residual scatter than other ground stations. To further improve data precision, 5-s normal points are formed from the full-rate data according to the ILRS normal point algorithm (Sinclair, 1997). The precision of the normal point light time residuals is nominally 1–5 cm. Fig. 8 shows an example of NGSLR full-rate and normal point residuals from a typical 60-min pass. All of the LR full-rate and normal point data products have been delivered to NASA’s Planetary Data System (PDS) Radio Science archive (Smith, 2014) in the CRD format for public access.

Table 5 Number of 3-way and 4-way simultaneous ranging passes/segments between June 2009 and September 2014. Stations

Number of 3-way passes

Stations

Number of 4-way passes

GO1L, GODL, MDOL GO1L, GODL, MONL GO1L, MDOL, MONL GODL, MDOL, MONL GRSM, HERL, ZIML

48 116 257 58 2

GO1L, GODL, MDOL, MONL

6

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5. LRO-LR timing 5.1. . LRO USO timing In the LR operation, all the ground fire times are recorded in the UTC seconds of day, while the LRO timing system provides LOLA receive times in MET. A conversion from MET to UTC is needed to obtain accurate range measurements. At the beginning of the LR operation, the LOLA receive time in MET provided by the LRO timing system is converted into UTC by adding a Spacecraft Time Correction Factor (STCF) derived from two-way radio time packet measurements. This factor is updated regularly to keep the LRO MET within 3 ms of UTC (Andrews, 2006). The spacecraft clock correlation is obtained by taking the difference between the time stamps of when a command leaves the ground station WS1 and when the spacecraft receives the command. The WS1 station has an accurate clock as time standard. Using the value of its clock and the delays in the system, the ground system can determine the time, relative to the ground time, that the spacecraft command is received, since the spacecraft orbit produced by the mission navigation team is accurate enough. The total timing error in this system is estimated to be 0.5 ms, but ultimately relies on the stability of the WS1 station clock. The time correlation system produces the piecewise-linear coefficients in the SCLK file to convert MET time to UTC or dynamical time that may then be used to match ground station fire times and LOLA receive times. Although the 3-ms accuracy requirement for STCF is sufficient for LR acquisition, the LR data, once collected and paired up, are later used to better predict and characterize the mid- to long-term LRO USO behavior. From the ground testing, the frequency of the USO is known to not only have an offset with respect to UTC (referred to as the drift of the USO), but a gradual change in time, which is referred to as the oscillator aging. To keep the accuracy of the MET to UTC conversion, the drift is updated every 2–3 months to accommodate the aging and higher order effects. The USO frequency drift and aging are obtained from LR data by comparing the predicted LOLA receive time in UTC to actual receive time in MET. A Newtonian light-time correction is added to the ground laser transmit time to obtain the predicted LOLA receive time in UTC. This correction is calculated from the ground station location, the reconstructed spacecraft ephemeris provided by FDF, the exported lunar ephemerides with respect to Earth, and Earth orientation parameters. The time difference between the predicted LOLA receive time in UTC and actual receive time in MET is fitted with a cubic polynomial, describing the constant time offset between LOLA USO and the ground station clock, the USO frequency, the aging of frequency, and higher-order frequency behavior at the reference epoch. The LR data used to monitor the USO long-term behavior are from the primary ground station NGSLR, which has a stable Hmaser as its time base since January 2010, and contributes most of the LR data among all participating stations. The sub-second timing data of the spacecraft MET in the first 4 months of the LRO mission (Fig. 9) showed that the frequency of the USO, which provides the MET timing, has a drift of approximately 72.10 parts per billion on 01 July 2009 with respect to UTC, and a slow aging (∼5 × 10−12 s/s2 ) yielding a gradual increase of the USO oscillation frequency over time. Our results from NGSLR’s LR data show that the spacecraft clock has been speeding up steadily (Fig. 10). Long-term frequency stability is about ±0.68 × 10−12 s per day (with no temperature correction), and the frequency drift was about 65.67 parts per billion on 08 July 2014. The drift and aging are used to convert the LOLA receive time from MET counts to UTC in the process of pairing up the ground laser transmit times and the corresponding LOLA receive times via the following formula:

Fig. 9. LRO USO frequency drift estimated by the sub-second data of the LRO MET for the first 4 months after the launch of the spacecraft. The number of MET count per day lagging behind the UTC seconds (left axis) is plotted in the dash line, and the USO frequency deviation derived from the MET counts (right axis) is plotted in the solid line. The figure shows a 72.1 ppb frequency drift for the LRO clock oscillator on 01-Jul-2009.

Fig. 10. LRO USO long-term frequency behavior for the entire 5-year LR operation. The gaps in this plot are caused by either the LRO spacecraft sun-safe incidents (e.g. gaps around day 400, 1340), resulting in gaps of a few days long, or the NGSLR laser anomalies when no NGSLR data were available (e.g. gaps around day 780, 1180), usually lasting for 5–20 days. The figure shows that the USO frequency drift is gradually changing, from ∼71.97 ns/s on 02-Sep-2009 to ∼65.67 ns/s on 08-Jul-2014. The spacecraft sun-safe events also affect the aging of the USO frequency.



 

UTC MET TLOLA _receive = TLOLA_receive + δ trange_walk ∗ 1 + δ f 2

MET + TLOLA _receive ∗ α f +



δtoffset ,

where δ trange_walk is the ground-station-dependent range walk correction, δ f is the USO frequency drift, α f represents the frequency aging of the USO, and δ toffset is a constant time offset for the USO. In addition to the slowly changing frequency, we must also account for sudden discontinuities in the USO frequency drift and frequency aging, as shown in Fig. 10. These are the result of spacecraft safe mode events, during which the USO oven temperature undergoes large excursions. The Newtonian residuals of the predicted and actual LOLA receive time are obtained by removing the fitted cubic polynomial function (modeling the frequency drift, aging, and any higher order changes of the spacecraft clock) and the light time correction. As shown in Fig. 11, the maximum absolute value of the light time residuals is less than 0.015 ms for the entire mission, which is about 200 times better than the 3-ms mission

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Fig. 11. The difference between the actual and the estimated LOLA receive time over the entire 5-year LR operation. The discontinuous periods in the plot follow the spacecraft safe mode events, or the lack of NGSLR data due to ground station incidents (as in Fig. 10). After removing a fitted cubic polynomial function and the light time correction, the Newtonian light time residuals of LRO USO are less than 0.015 ms, with a RMS of 2.18 μs. This precision is about 200 times better than the 3-ms mission requirement.

requirement (Zuber et al., 2010) for USO time precision, and the scatter of the residuals is as small as 2.18 μs.

9

Fig. 12. Differences between NGSLR H-maser time and USNO master clock time are plotted in A in blue, a linear fit of the differences is plotted in red. The residuals of the time differences (blue in A) after removing a linear fit are shown in B in blue, a quadratic fit of these residuals is plotted in red. This quadratic shape of the residuals indicates a dominant linear frequency drift of the H-maser. In C, the residuals of the time differences (blue in A) after removing a 6th order polynomial fit are plotted in blue, showing a precision less than 10 ns over 20 months. The Gaussian smoothing results, ∼1 ns over the same time span, are plotted in green. All the time differences and the residuals are in nanoseconds. The gaps in the plot are the results of the ground station H-maser anomalies (e.g. accidental power off around day 128 of year 2014, etc.), and periods when the data were not available. Extra offsets and independent polynomial fits were applied to data after day 128 of year 2014 to obtain residuals in B and C.

5.2. LR ground station timing The stability of the clocks both on the spacecraft and on the ground is important for the quality of the one-way time-of-flight measurement. Although the ground clocks at each participating SLR station are carefully calibrated and monitored with reliable time sources, such as cesium clocks and H-masers, the time references used at the ground stations are not synchronized, and the long-term clock stability varies from station to station. In order to achieve better timing on the ground, some SLR stations applied further hardware and data-processing modifications on their timing systems. Early in the mission, NGSLR used a TrueTime GPS receiver as its reference, and had a 100 ps precision in the one-way LR measurement, as well as a slowly drifting bias within 100 ns. Since October 2010, the time base at the primary LRO-LR station, NGSLR, has been upgraded by switching from a cesium clock to the H-maser clock from the nearby VLBI site connected via optical fibers. The time information of this H-maser has been monitored against GPS time via an All-View GPS receiver since January 2013 to further improve the long-term ground time precision. The United States Naval Observatory (USNO) in Washington DC, about 32 km southwest of NGSLR, also provides its All-View GPS receiver data referenced to its master clock. By differencing the GPS receiver data from NGSLR to those from USNO, the NGSLR H-maser clock can be referenced to USNO’s master clock. Given that USNO and NGSLR are close in location, the ionosphere effects on the GPS signals are largely canceled, and NGSLR can effectively reproduce the USNO master clock. The USNO master clock time can thus be transferred to a distant SLR station as it ranges to LRO simultaneously with NGSLR. The time differences between the NGSLR H-maser and the USNO master clock from January 2013 to September 2014 are shown in Fig. 12. The gaps in the figure are due to either the lack of H-maser data, like the gap at the beginning of 2014, or Hmaser power down incidents, like the gap around day 140 of 2014. Fig. 12-A shows that the NGSLR H-maser has an obvious longterm drift. After removing a 6th order polynomial fit to the direct time differences shown in Fig. 12-A, the residuals over 20 months

are generally less than 10 ns. By adding a 6-h FWHM Gaussian smoothing function to the residuals, the long-term root-meansquare (RMS) can be reduced to about 1 ns. MDOL, the station in Fort Davis, Texas, adapted the same method used by NGSLR to reference their cesium time base to the USNO master clock from February 2014 on. Generally, the longterm stability of cesium clocks is about 10−13 s/s, while the Hmaser has a stability of 10−15 s/s. Thus the ground timing precision achieved at MDOL is ∼10-ns over the 7 months after referencing to the USNO GPS time, as shown in Fig. 13. 6. LRO precise orbit determination with LR LR normal points are used to determine LRO orbits, both independently and in combination with S-band radio tracking data. The results are compared to those generated with only radiometric data to examine LR’s capability in orbit determination and to reveal possibilities to improve the LRO orbit solutions when supplementing the radiometric data. The orbit analysis software GEODYN, developed at NASA GSFC (Pavlis et al., 2006), is used to perform the LRO precise orbit determination (POD), following our extensive experience with planetary satellite POD work, e.g. with MGS (Rowlands et al., 1999) and LRO (Mazarico et al., 2012). The force and measurement models used here are described in the LRO POD work (Mazarico et al., 2012), and the lunar orientation and ephemeris are based on the JPL DE421 ephemeris (Williams et al., 2008). The solar radiation pressure, and the constant once-perrevolution non-conservative acceleration parameters are estimated independently every 5–7 days. With the success of the Gravity Field and Interior Laboratory (GRAIL; Zuber et al., 2013) mission, high-resolution GRAIL gravity models have become the standard used in LRO POD work (Bauer et al., 2016a; Maier and Baur, 2016; Buccino et al., 2016). Previous LRO orbit determination (OD) work with radiometry data showed that GRAIL gravity models can provide significant improvements to the LRO POD, unless the model is truncated too excessively at the order and degree as low as 150 (Mazarico et al., 2013). We use the GRGM900C GRAIL model (Lemoine et al., 2013) in our POD process,

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Fig. 13. Residuals of the time differences between MDOL cesium clock time and USNO master clock time after removing a 6th order polynomial fit (in gray) with Gaussian smoothing (in black) in nanoseconds. Both results show that the ground time precision achieved at MDOL is ∼10-ns (RMS value) over 7 months after referencing to the USNO GPS time.

truncated at degree 270, which is a good compromise between the computing time and the orbit quality. For comparison purposes, two other gravity models are also examined, up to their maximum degree of 150: LLGM-2, which includes 2 years of LRO S-band tracking data, as well as LOLA altimetry crossovers; and SGM150J, determined with the SELENE 4-way Doppler tracking data (Namiki et al., 2009) to improve the knowledge of the Moon’s farside gravity field. The LRO trajectory is integrated and converged independently over time periods, referred to as “arcs”. Arcs as short as 2.5 days have been typically employed to determine the LRO orbit (Mazarico et al., 2012), similarly to previous Lunar Prospector studies (Konopliv et al., 2001; Mazarico et al., 2010). Recent LRO orbit determination efforts from various investigators also used such short arcs (Bauer et al., 2016a; Maier and Baur, 2016; Löcher et al., 2015). However, because the one-way LR measurements require the estimation of both the spacecraft clock and the range biases for ground stations, longer arcs are preferred to allow accurate recovery of clock and orbit parameters. Thus, we divide the complete LR and S-band tracking dataset, over the 5 years of LRO-LR operation, into 2-week-long arcs, primarily at the spacecraft maneuvers. This approach can avoid modeling those maneuvers, which generally degrade the orbit results. As such, these arcs contain an average of ∼160 2-h LRO orbits, with occasional exceptions for arc duration of 1 and 3 weeks. There are no orbital overlaps between any two arcs. Three cases are studied for each of these 2-week arcs: LR data only, S-band data only, and the combination of the LR and S-band data. Orbit solutions are generated with these 3 sets for each of the 3 gravity models (GRGM900C truncated at degree 270, LLGM2 with degree 150, and SGM150J with degree 150), and we compare them with the definitive orbits obtained from 2.5-day arcs with the GRGM900C GRAIL gravity model up to degree 600. These orbits are used as our baseline, as they are the definitive trajectories of the LOLA team and archived at the PDS (Smith, 2016b). The 2.5day time span for S-band data balances the defining of the orbit and the modeling errors, and yields a total position error of 10 m and 0.1 m radially (Mazarico et al., 2013). Given the actual precision of the different data types, 0.3 – 0.8 mm/s for the radiometric Doppler, 0.2–0.4 m for radiometric range, and 5 cm for the LR normal points, the data weights are set to be several times larger than the data noise level in our OD process: 1 mm/s for the radiometric Doppler, 10 m for the radiometric range, and 1 m for the LR data. The same weights have been previously applied to the radiometric data sets to obtain the definitive LRO trajectories. Here we report results from 58 2-week arcs converged for all three data cases and all three gravity field models. All these arcs have robust temporal coverage from both LR and radiometry data. The LR normal points used for POD consist both the LOLA laser signal receive time tags in MET and the imperfect ground station

Fig. 14. LRO USO drift estimated from direct comparison between LOLA receive time and NGSLR transmit time over 5 years (in blue), and from the OD process using all LR data for 2-week arcs and GRAIL gravity model GRGM900C (in red). Due to the paucity of LR data in some of the 2-week arcs, the orbit solutions are not well converged enough to provide meaningful USO parameters, causing the gaps in the GEODYN results in red. The results from the two estimation methods agree well with each other.

laser transmit time stamps in UTC. The LR range is calculated as:



2



MET MET UTC R = TLOLA _receive ∗ δ f + TLOLA_receive ∗ α f + δ toffset − Ttransmit ∗ c,

where R is the one-way range observable used in GEODYN POD, and c is the speed of light. The LRO clock parameters, δ f , α f , and δ toffset , are estimated in GEODYN to obtain accurate range measurements from the LR normal points, and are determined for every 2-week arc with all available LR data from multiple SLR stations. The GEODYN-estimated spacecraft clock frequency drift using GRGM900C is shown in Fig. 14. As described previously in Section 5.1, another method to obtain the USO frequency drift and aging is by direct comparison of the LOLA receive MET times with the laser transmit times in UTC without using GEODYN. Given that data from the main LR station, NGSLR, are best calibrated with the most temporal coverage, only these data are used to characterize the USO mid- to long-term behavior over every 2-3 month time span. During this time period, no clock interruption at either NGSLR or LRO occurred. After removing the calculated light time from the difference between the transmit time in UTC and receive time in MET, a quadratic function is fitted to the residuals to obtain the USO frequency drift and aging. The results of the drift are shown in both Figs. 10 and 14. The USO frequency drift values estimated by both methods generally agree well, as shown in Fig. 14. Rarely, the GEODYN estimated USO aging has a trend opposite to the overall trend. This is caused by an inconsistency of the ground station clock behaviors, which are highly correlated

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Table 6 The mean RMS for the measurement residuals of the LR and radiometric data. Data type

Gravity model

Mean RMS of S band range data (m)

Mean RMS of S band Doppler data (cm/s)

Mean RMS of LR data (m)

S band data only

GRGM900C LLGM-2 SGM150J GRGM900C LLGM-2 SGM150J GRGM900C LLGM-2 SGM150J

19.527 18.643 21.337 n/a n/a n/a 35.132 34.794 35.679

0.630 0.700 0.668 n/a n/a n/a 0.697 0.861 0.810

n/a n/a n/a 0.251 0.334 0.319 2.757 6.412 2.964

LR data only

S-band and LR data

Fig. 15. Observation residuals of LR data in a 2-week arc from 29-Aug-2014 to 10Sep-2014. Different colors indicate data from different LR ground stations as listed in the figure. The inserted plot shows a ∼2-h period in the arc around day 248. There are three passes in this time period, two from NGSLR (in black) and one from Greenbelt (in red). The residuals from all three passes are less than 2 m.

with the estimated LRO USO parameters. The USO frequency parameters estimated by GEODYN with the less-accurate gravity models, LLGM-2 and SGM150J, also agree well with the results of the high-resolution GRGM900C model. The standard deviations of the USO frequency differences between GRGM900C are ∼5.99 × 10−12 s/s with LLGM-2, and 4.32 × 10−12 s/s with SGM150J. Considering each LR ground station’s clock is free running, and not characterized against any common reference, the different ground clock behaviors (Bauer et al., 2014 and 2016b; Mao et al., 2014b) are first accommodated by adjusting the LR range biases station-by-station in our OD process. To remove the remaining systematic trends in the post-orbit-fit observation residuals, the LR range biases are later estimated as frequently as pass-by-pass. For the case of combining the LR and radiometric data, the number of LR observations used in each 2-week arc to determine the LRO orbit is less than 22% (with a standard deviation of 5%, occasionally as small as 1%) of the radiometric measurements used. The post-fit LR observation residuals with respect to the orbit solution are generally as good as the radiometric residuals and sometimes slightly better when the two types of data were used independently in GEODYN to determine the LRO orbits. An example of the LR residuals in a converged 2-week arc is shown in Fig. 15. The typical RMS of the LR residuals ranges from 1 m to 10 m in a well-converged 2-week arc (see the histogram of LR observation residuals in this example arc in supplementary Figure). The post-orbit-fit observation residuals are usually used to indicate the difference between the observations and the orbital model. The mean RMS values of LR and radiometric measurement residuals are listed in Table 6 for all data cases and gravity models. Our results show that the high-resolution GRGM900C model yields best LR residuals with or without radiometry data. In the LR-

only data case, the average LR residual RMS is as small as ∼25 cm with GRGM900C model, while Bauer et al. (2016a) reported 7 m LR residual RMS for 7-day-long arcs and 0.91 m for 2.5-day-long arcs using the same gravity model with a lower degree cutoff at 180. Although LR data result in residuals smaller than 0.4 m when used alone, combining them with the radiometric data did not yield any better constraints to either data type. The RMS of the radiometry data and LR data both largely increased regardless of the gravity model applied. Table 6 also suggests that S-band range data are not very sensitive to the gravity models. On the other hand, LR residuals benefit more from the SGM150J far side gravitational information than the LLGM-2 model, whether used independently or with radiometric data. Similar to the LR data, Doppler data are also better constrained by the SGM150J model than the LLGM-2 model, behaving oppositely to the S-band range data. For the obit comparison purpose, the position of LRO is expressed as a function of time in the along-track, cross-track, and radial reference frame for all our orbit solutions. The orbit differences among various solutions are calculated as the spacecraft position difference in all three directions, as well as the total position difference. Orbit differences between 2-week solutions using all three data types with the GRAIL GRGM900C model and the definitive LRO orbits from the LOLA team are plotted as histograms shown in Figs. 16–18, respectively. Table 7 shows the mean and the RMS values of the orbit difference from the definitive orbit in along-track, cross-track, radial directions and the total position difference orbits determined with only S-band data, with only LR data, and with S-band and LR data combined, each determined with the three gravity models. The LRO definitive orbits used as the benchmark here have an accuracy of ∼10 m in total position and 0.5 m radially (Mazarico et al., 2013). Confirmed by the 50cm high-resolution narrow angle camera (NAC) images from the LROC team, these orbits represent the “truth” well (Wagner et al., 2016; Mazarico et al., 2012). Table 7 shows that the GRGM900C model yields the best orbits in each data case, with the mean total position differences all less than 55 m. For the other two less accurate gravity fields, the mean value of the total position difference is larger than 65 m for all data cases. The results from Sband data only with the GRGM900C model nearly reproduce the definitive orbit solutions with an average total difference of less than 27 m. When adding the LR data to the S-band data with the GRGM900C model, the mean value of the total position orbit difference and its RMS value increase slightly from the S-band only solutions, by ∼6 m for both. This change is within the orbit accuracy, thus does not necessarily indicate that combining the two types of measurements degrades the orbit solution. On the other hand, the average orbit difference in the radial direction is significantly improved from ∼0.6 m (S-band only) to ∼0.14 m by combining the LR and S-band data with GRGM900C model. This improvement shows that the combination of radiometric and LR data sets can provide stronger constraints on the radial direction and help to enhance

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Fig. 16. Histogram of orbit difference between 2-week solutions with S-band data only using GRGM900C gravity model truncated at degree and order 270 and LRO definitive orbit in the along-track, cross-track, radial directions and total position. The y-axis is normalized to be the percentage of data in each bin. The results from S-band data only with the GRGM900C model nearly reproduce the definitive orbit solutions with an average total difference of less than 27 m.

Fig. 17. Histogram of orbit difference between 2-week solutions with LR data only using GRGM900C gravity model truncated at degree and order 270 and LRO definitive orbit in the along-track, cross-track, radial directions and total position. The y-axis is normalized to be the percentage of data in each bin. The histograms show a mean total orbit difference of ∼50 m and a ∼20 cm mean radial difference, demonstrating that the LR data alone are capable of providing good orbits, which are comparable to the LRO definitive orbits.

Table 7 Mean values and RMS of the orbit difference between solutions for 2-week arcs and definitive LRO orbits from the LOLA team. Data type

Gravity model

Mean along-track (m)

Mean cross-track (m)

Mean radial (m)

Mean total (m)

RMS along-track (m)

RMS cross-track RMS radial (m) (m)

S-band data only

GRGM900C

3.876

−0.437

0.653

26.609

34.372

30.705

10.251

46.213

LLGM-2 SGM150J GRGM900C LLGM-2 SGM150J GRGM900C

16.384 8.413 −6.731 12.965 −0.947 −3.070

0.205 0.233 1.826 5.235 4.608 0.534

0.361 0.294 0.200 1.824 1.044 −0.143

69.244 77.954 53.065 101.051 113.460 33.589

81.984 84.563 78.325 119.985 130.808 39.954

66.568 77.417 64.256 110.771 113.884 34.522

13.256 14.757 21.669 33.715 31.041 6.322

105.235 114.075 100.989 162.376 172.281 52.534

LLGM-2 SGM150J

13.594 6.506

1.506 1.321

0.141 −0.411

66.416 75.502

78.813 80.340

66.051 79.035

10.056 11.574

101.940 112.446

LR data only

S-band and LR data

RMS total (m)

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Fig. 18. Histogram of orbit difference between 2-week solutions with both LR and S-band data using GRGM900C gravity model truncated at degree and order 270 and LRO definitive orbit in the along-track, cross-track, radial directions and total position. The y-axis is normalized to be the percentage of data in each bin. The histograms show that combining the LR and S-band data reduces the scatter of the mean orbit difference in radial direction from ∼10 m (S-band only) to ∼6 m.

the orbit accuracy of the radial component. Furthermore, combining the LR and S-band data with GRGM900C model greatly reduces the RMS of the mean orbit difference in radial direction from ∼10 m (S-band only) to ∼6 m. Similar improvement of the radial orbit difference scatter is also reported by Löcher et al. (2015). The average value of the orbit differences represents the accuracy of the orbit solution, and the RMS of the orbit differences is an indicator of the orbit precision. The improvement on both the average radial orbit difference and the RMS of this difference is beneficial to construct a highly accurate global lunar geodetic grid, on which the high-resolution LRO data are geolocated for scientific research. For the two less accurate gravity models, LLGM-2 and SGM150J, adding LR data to the radio data results in improvement in the RMS value of both the radial difference and the total position difference in comparison with the radio data only solutions. It suggests that more high-accuracy measurements can better constrain the orbit solutions in the presence of the less rigorous force modeling that is provided by these gravity models. On the other hand, because the LR-only solutions have much weaker temporal coverage than the S-band solutions, the LR-only orbit differences are generally larger than those in the S-band only in along- and crosstrack position. However, the mean difference and its RMS scatter in the radial direction are much smaller for LR-only data case than S-only case with GRAIL gravity model. The mean total orbit difference of ∼50 m and ∼20 cm mean radial difference with GRGM900C model demonstrate that the LR data alone are capable of providing good orbits, which are comparable to the LRO definitive orbits. A good average orbit, as the one determined by LR data independently, can be used for operational or navigational purposes. The usage of the LR data in POD is greatly complicated by the high correlation between the LRO clock parameters and the LR range biases in the 2-week arcs. This suggests that 2-week time span may not be ideal to minimize the modeling errors. Instead of using long arcs (e.g. 2-week long) and adjusting clock parameters for each arc, possibilities of using a set of shorter arcs (e.g. 2.5-day long) with the same fixed long-term clock parameters are explored to take advantage of the known stability of the USO. We construct 2.5-day arcs for 7 months from February 2013 to September 2013, when no interruptions occurred to either the LRO spacecraft clock or the NGSLR ground clock. For each arc, all the spacecraft clock parameters are fixed, and only the range biases are adjusted. In the

Fig. 19. NGSLR pass-by-pass bias residuals after removing the fixed USO drift and aging for 2.5 day arcs from February to September, 2013. Adding temperature and linear β -angle corrections improved the LR range biases modestly, reducing the RMS of the bias residuals from ∼1760 m to ∼910 m. However, such large bias residuals suggest that it is not enough to solely depend on an improved USO model to resolve the high correlations between the spacecraft clock parameters and the LR biases in the OD process.

test, only LR data from NGSLR are used because NGSLR provides most of the LR data, and its clock is carefully calibrated against the USNO master clock to provide the best timing accuracy. On the other hand, the behavior of the spacecraft USO is more difficult to model. To maintain the long-term stability of the USO, the temperature of the oscillator is oven-controlled. However, events like the regular high-power Ka-band downlink between the spacecraft and WS1 a few hours per day, and the change of the β -angle (the angle between the LRO-sun vector and the LRO orbit plane) can all cause the fluctuation of the USO temperature, leading to the change of the spacecraft clock behavior. First in the test, to follow the USO as close as possible, a temperature correction and a linear β -angle correction are applied to obtain the LRO clock parameters over the 7-month time span. This set of the spacecraft clock parameters are then fixed for all the 2.5 day arcs, and the LR range biases are allowed to adjust to a converged orbit solution. The final LR range biases are plotted in Fig. 19. The figure shows that adding the corrections modestly improved the LR range

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biases. The residuals of these range biases yielded a RMS of ∼910 m over the whole span after temperature and β -angle corrections, while the RMS of the biases with no corrections was as large as ∼1760 m. The ∼1 km RMS of the LR range biases obtained after corrections to the USO parameters is still very large, and precludes attempts at multi-month-long LRO arcs which might have benefited from the long-term stability of the USO. This suggests that a better clock model alone may contribute very little to the orbit solutions even with the combination of the LR and S-band data. Our results agree with a recently study by Löcher et al. (2015), who used 2.5-day arcs with both data types and a software system different from GEODYN. This discrepancy hints at some inconsistencies between the LR and radiometric data are yet to be fully understood, let alone resolved. As such, currently, LR arcs are limited to relatively short time spans, and would benefit most from a refined force model with more frequent parameter adjustments to improve on the POD process presented here. 7. Summary The laser ranging experiment has operated over 5 years to track LRO via a one-way laser uplink, a novel measurement type for planetary science and exploration. The engineering setup of the operation provides opportunities to test new space technologies and mission concepts (e.g. Degnan, 2002; Turyshev et al., 2010; Dirkx et al., 2014), such as the successful laser-communication tests conducted at the primary SLR station NGSLR to LOLA (e.g. Sun et al., 2013a), and time transfer tests between two ground laser stations via simultaneous ranging to LRO (Sun et al., 2013b; Mao et al., 2014a). With 10 participating international stations regularly ranging to LRO, LR has established the ability to obtain near 24-h data coverage globally to a lunar spacecraft, and paves the way for future applications in deep space. There are over 100 days of 16 h or more data coverage in the last 3 years of operation. A total of 4173 h of successful LR data are obtained including all the regular simultaneous tracking data from 2 to 4 stations. The precision of the full-rate data is about 15–30 cm, and the normal points have a precision of 5–10 cm. Thanks to lessons learned as the experiment progressed, NGSLR improved its timing accuracy in the last 2 years of the LR operation down to the ns level, by referencing its timing source to the USNO master clock. Another ground station, MDOL, followed the procedure in 2014, and improved its timing accuracy to 10 ns over the last 7 months. In order to obtain accurate range measurements from the LR data, it is necessary to characterize the behavior of the LRO spacecraft clock. The USO characteristics are used to convert the time records LOLA provides for its received LR signals from MET to UTC. In this paper, we presented two methods to use LR data to monitor the spacecraft USO behavior. One is to directly compare the ground laser transmit time in UTC and the corresponding LOLA receive time in MET, subtracting a calculated light time, for a time period as long as 3 months, when there are no interruptions to either the spacecraft clock or the ground laser station clock operations. The spacecraft clock is monitored to better than 0.015 ms for the entire LR 5-year operations. The USO frequency drift and aging are also estimated with NASA’s OD software, GEODYN, through analysis of 2-week long arcs using the high-resolution gravity model GRGM900C truncated at degree and order 270. The results from these two methods agree well. The GEODYN results suggest that even when lower resolution gravity fields, LLGM2 and SGM150J, are used in the OD process, LR data can provide good estimates of the spacecraft clock parameters which are as accurate as with the GRGM900C GRAIL gravity model. Thus LR data can be used to determine spacecraft clock stability even in the early stages of an orbital mission when there is nascent knowledge of the gravity model.

We also present LRO orbit reconstructions determined by LR data alone, radiometric S-band data alone, and the combination of LR and S-band tracking data for 2-week arcs. The high-resolution GRAIL gravity models such as GRGM900C provide the best orbit solutions for all three data types compared with non-GRAIL gravity models. With GRGM900C model, our results show that the average orbit differences over 5 years are ∼50 m in total position and ∼20 cm in radial direction between solutions obtained with LR data only and the LRO definitive orbits archived on the PDS. This demonstrates that LR data can be used alone and provide good orbit reconstruction. In addition, combining LR and S-band data yield great improvement from S-band only results in the radial direction for both the orbit difference and its scatter. Nevertheless, complexities remain when combining LR and S-band data to achieve better orbit solutions than the current definitive trajectory. The complexities are mostly from the high correlation between the LR range biases and the LRO clock parameters. Shorter integration arcs, a better force model, and more frequent adjustment of solar radiation pressure and spacecraft acceleration parameters may be needed to further improve the quality of orbit solutions using LR data. Acknowledgments We thank Kopal Jha from Sigma Space Corporation for preparing the LR full-rate and normal point data products, now archived at the PDS LRO Radio Science Archive. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.icarus.2016.07.003. References Andrews, L.C., Phillips, R.L., 2005. Laser Beam Propagation through Random Media, second ed. SPIE Press Chap. 12. Andrews, S., September 8, 2006. Lunar Reconnaissance Orbiter Project Timing Specification. NASA/Goddard Space Flight Center, Greenbelt, MD LRO NGIN 431-SPEC-0 0 0212. Bauer, S., Dirkx, D., Hussmann, H., et al., 2014. Implementation of one-way laser ranging data into LRO orbit determination. In: Proceedings of the 19th International Workshop on Laser Ranging. Annapolis, Maryland. Bauer, S., Hussmann, H., Oberst, J., et al., 2016a. Demonstration of orbit determination for the Lunar Reconnaissance Orbiter using one-way laser ranging data. Planet. Space Sci. doi:10.1016/j.pss.2016.06.005, in press. Bauer, S., Hussmann, H., Oberst, J., et al., 2016b. Analysis of one-way laser ranging data to LRO: Performance and calibration of spacecraft and ground station clocks. Icarus, submitted for publication. Buccino, D.R., Seubert, J.A., Asmar, S.W., et al., 2016. Optical ranging measurement with a lunar orbiter: Limitations and potential. J. Spacecraft Rockets 53 (3), 457– 463. doi:10.2514/1.A33415. Cash, P., Emmons, D., Welgemoed, J., 2008. Ultra-stable oscillators for space applications. In: Proceedings of the 40th Annual Precise Time and Time Interval (PTTI) Meeting. Reston, Virginia. Chin, G., Brylow, S., Foote, M., et al., 2007. Lunar Reconnaissance Orbiter overview: The instrument suite and mission. Space Sci. Rev. 129, 391–419. doi:10.1007/ s11214- 007- 9153- y. Degnan, J., 2002. Asynchronous laser transponders for precise interplanetary ranging and time transfer. J. Geodyn. 34 (3–4), 551–594. Dirkx, D., Vermeersen, L.L.A., Noomen, R., et al., 2014. Phobos laser ranging: Numerical geodesy experiments for Martian system science. Planet. Space Sci. 99, 84–102. doi:10.1016/j.pss.2014.03.022. Gibbs, P., Benham, D., Sherwood, R., et al., 20 0 0. An overview of quality control at the Herstmonceux SLR station. In: Proceedings of the 12th International Workshop on Laser Ranging. Matera, Italy. Hoffman, E., Sun, X., Skillman, D.R., et al., 2014. A method of time transfer between remote stations via LRO. Geophys. Res. Abstr. 16 EGU2014-9673-2. Konopliv, A.S., Asmar, S.W., Carranza, E., et al., 2001. Recent gravity models as a result of the Lunar Prospector mission. Icarus 150, 1–18. doi:10.10 06/icar.20 0 0. 6573. Lemoine, F.G., Goossens, S., Sabaka, T.J., et al., 2013. High-degree gravity models from GRAIL primary mission data. J. Geophys. Res.: Planets 118 (8), 1676–1698. doi:10.1002/jgre.20118. Löcher, A., Hofmann, F., Gläser, P., et al., 2015. Towards improved lunar reference frames: LRO orbit determination. In: International Association of Geodesy Symposia, pp. 1–6. doi:10.1007/1345_2015_146.

Please cite this article as: D. Mao et al., The laser ranging experiment of the Lunar Reconnaissance Orbiter: Five years of operations and data analysis, Icarus (2016), http://dx.doi.org/10.1016/j.icarus.2016.07.003

JID: YICAR

ARTICLE IN PRESS D. Mao et al. / Icarus 000 (2016) 1–15

Maier, A., Baur, O., 2016. Orbit determination and gravity field recovery from Doppler tracking data to the Lunar Reconnaissance Orbiter. Planet. Space Sci. 122, 94–100. doi:10.1016/j.pss.2016.01.014. Mao, D., Sun, X., Skillman, D.R., et al., 2014a. Time-transfer experiments between satellite laser ranging ground stations via one-way laser ranging to the Lunar Reconnaissance Orbiter, 3060. In: Proceedings of the 19th International Workshop on Laser Ranging. Annapolis, Maryland. Mao, D., McGarry, J.F., Torrence, M.H., et al., 2014b. Summary of ground station performance in 5 years of laser ranging operation to Lunar Reconnaissance Orbiter, 3158. In: Proceedings of the 19th International Workshop on Laser Ranging. Annapolis, Maryland. Mazarico, E., Lemoine, F.G., Han, S.-C., et al., 2010. GLGM-3, a degree-150 lunar gravity model from the historical tracking data of NASA Moon Orbiters. J. Geophys. Res.: Planets 115 (E5). doi:10.1029/20 09JE0 03472. Mazarico, E., Rowlands, D.D., Neumann, G.A., et al., 2012. Orbit determination of the Lunar Reconnaissance Orbiter. J. Geodyn. 86 (3), 193–207. doi:10.1007/ s00190-011-0509-4. Mazarico, E., Goossens, S.J., Lemoine, F.G., et al., 2013. Improved orbit determination of lunar orbiters with Lunar gravity models obtained by the GRAIL mission. In: 44th Lunar and Planetary Science Conferences. The Woodlands, TX. McGarry, J., Zagwodzki, T.W., October 2006. SLR2000, the path toward completion. In: Proceedings of the 15th International Workshop on Laser Ranging. Canberra, Australia. McGarry, J., Merkowitz, S.M., Donovan, H.L., et al., 2013. The collocation of NGSLR with MOBLAS-7 and the future of NASA satellite laser ranging. In: Proceedings of the 18th International Workshop on Laser Ranging. Fujiyoshida, Japan. Merkowitz, S.M., Esper, J., Hilliard, L., et al., October 2014. NASA’s next generation space geodesy network. In: Proceedings of the 19th International Workshop on Laser Ranging. Annapolis, Maryland. Namiki, N., Iwata, T., Matsumoto, K., et al., 2009. Farside gravity field of the Moon from four-way Doppler measurements of SELENE (Kaguya). Science 323, 900– 905. doi:10.1126/science.1168029. Neumann, G.A., Cavanaugh, J., Coyle, B., et al., 2006. Laser ranging at interplanetary distances. In: Proceedings of the 15th International Workshop on Laser Ranging. Canberra, Australia. Noll, C., 2010. The crustal dynamics data information system: A resource to support scientific analysis using space geodesy. Adv. Space Res. 45 (12), 1421–1440. doi:10.1016/j.asr.2010.01.018, ISSN 0273-1177. Nozette, S., Spudis, P., Bussey, B., et al., 2010. The Lunar Reconnaissance Orbiter miniature radio frequency (Mini-RF) technology demonstration. Space Sci. Rev. 150, 125–160. doi:10.10 07/s11214-0 09-9607-5. Paige, D.A., Foote, M.C., Greenhagen, N.T., et al., 2010. The Lunar Reconnaissance Orbiter diviner lunar radiometer experiment. Space Sci. Rev. 150, 285–302. doi:10.10 07/s11214-0 09-9529-2. Pavlis, D.E., Poulose, S.G., McCarthy, J.J., GEODYN Operations Manuals. Contractor Report, SGT Inc., Greenbelt, MD, 2006. Pearlman, M.R., Degnan, J.J., Bosworth, J.M., 2002. The International Laser Ranging service. Adv. in Space Res. 30, 135–143. doi:10.1016/S0273-1177(02)00277-6. Ramos-Izquierdo, L., Scott III, V.S., Connelly, J., et al., 2009. Optical system design and integration of the Lunar Orbiter Laser Altimeter. Appl. Opt. 48, 3035–3049. Ricklefs, R., 2002. Consolidated laser ranging prediction format. In: The 13th International Workshop on Laser Ranging. Washington, D.C.. Ricklefs, R., 2006. Consolidated laser prediction and data formats: Supporting new technology. In: Proceedings of the 15th International Workshop on Laser Ranging. Canberra, Australia.

[m5G;August 8, 2016;20:2] 15

Riris, H., Cavanaugh, J., 2008. Calibration Document for the Lunar Observer Laser Altimeter (LOLA) Instrument. NASA/Goddard Space Flight Center, GreenbeltMD. Robinson, M.S., Brylow, S.M., Tschimmel, M., et al., 2010. Lunar Reconnaissance Orbiter Camera (LROC) instrument overview. Space Sci. Rev. 150, 81–124. doi:10. 1007/s11214-010-9634-2. Rowlands, D.D., Pavlis, D.E., Lemoine, F.G., et al., 1999. The use of laser altimetry in the orbit and attitude determination of Mars Global Surveyor. Geophys. Res. Lett. 26, 1191–1194. doi:10.1029/1999GL900223. Sinclair, A.T., 1997. Re-Statement of Herstmonceux Normal Point Recommendation http://ilrs.gsfc.nasa.gov/data_and_products/data/npt/npt_algorithm.html. Smith, D.E., Zuber, M.T., Sun, X., et al., 2006. Two-way laser link over interplanetary distance. Science 311 (5757), 53. doi:10.1126/science.1120091. Smith, D.E., Zuber, M.T., Lemoine, F.G., et al., 2008. Orbit determination of LRO at the Moon. In: Proceedings of the Seventh International Workshop on Laser Ranging. Poznan, Poland. Smith, D.E., Zuber, M.T., Jackson, G.B., et al., 2010. The Lunar Orbiter Laser Altimeter (LOLA) investigation on the Lunar Reconnaissance Orbiter (LRO) mission. Space Sci. Rev. 150, 209–241. doi:10.10 07/s11214-0 09-9512-y. Smith, D.E., 2014. Radio Science Subsystem on the Lunar Reconnaissance Orbiter Normal Point Data, data set LRO-L-RSS-1-TRACKING-V1.0, NASA Planetary Data System . http://pds-geosciences.wustl.edu/missions/lro/rss.htm. Smith, D.E., 2016. Lunar Orbiter Laser Altimeter on the Lunar Reconnaissance Orbiter Experimental Data Record, data set LRO-L-LOLA-2-EDR-V1.0, NASA Planetary Data System. http://pds-geosciences.wustl.edu/missions/lro/lola.htm. Smith, D.E., 2016b. Radio Science Subsystem on the Lunar Reconnaissance Orbiter Spice Kernel Files, data set LRO-L-RSS-1-TRACKING-V1.0, NASA Planetary Data System http://pds-geosciences.wustl.edu/missions/lro/lola.htm. Stengel, T., Hoge, S., 2008. Evolution and reengineering of NASA’s Flight Dynamics Facility (FDF). In: SpaceOPS Conference http://dx.doi.org/10.2514/6.2008-3276. Sun, X., Skillman, D.R., Hoffman, E.D., et al., 2013a. Free space laser communication experiments from Earth to the Lunar Reconnaissance Orbiter in lunar orbit. Opt. Express 21 (2), 1865–1871. doi:10.1364/OE.21.001865. Sun, X., Skillman, D.R., McGarry, J.F., et al., 2013b. Time transfer between satellite laser ranging stations via simultaneous laser ranging to the Lunar Reconnaissance Orbiter, 13-Po54. In: Proceedings of the 18th International Workshop on Laser Ranging. Japan. Turyshev, S.G., Farr, W., Folkner, W.M., et al., 2010. Advancing tests of relativistic gravity via laser ranging to Phobos. Exp. Astron. 28 (2), 209–249. doi:10.1007/ s10686-010-9199-9. Vondrak, R., Keller, J., Chin, G., et al., 2010. Lunar Reconnaissance Orbiter (LRO): Observations for lunar exploration and science. Space Sci. Rev. 150, 7–22. doi:10. 1007/s11214-010-9631-5. Wagner, R.V., Nelson, D.M., Plescia, J.B., et al., 2016. Coordinates of anthropogenic features on the Moon. Icarus, in press, doi:10.1016/j.icarus.2016.05.011. Wilkinson, M., Rodriguez, J., 2013. SLR energy density estimations and measurements for the Herstmonceux station, 13-0414. In: Proceedings of the 18th International Workshop on Laser Ranging. Japan. Williams, J.G., Boggs, D.H., Folkner, W.M., DE421 Lunar Orbit, Physical Liberations, and the Surface Coordinates. JPL Memorandum IOM 335-JW, DN. WR-200803140 01, 20 08. Zuber, M.T., Smith, D.E., Zellar, R.S., et al., 2010. The Lunar Reconnaissance Orbiter laser ranging investigation. Space Sci. Rev. 150, 63–80. doi:10.1007/ s11214- 009- 9511- z. Zuber, M.T., Smith, D.E., Watkins, M.M., et al., 2013. Gravity field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission. Science 339 (6120), 668–671. doi:10.1126/science.1231507.

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