Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite

Available online at www.sciencedirect.com

ScienceDirect Advances in Space Research xxx (2019) xxx–xxx www.elsevier.com/locate/asr

Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite Bing-Xuan Li a, Tzu-Wei Fang a,b,⇑, Bo Han a, Benjamin Lim a, Amal Chandran a, Wee-Seng Lim a, Yung-Fu Tsai c a

Satellite Research Centre (SaRC), School of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Singapore 639798, Singapore b CIRES/University of Colorado at Boulder, Boulder, CO 80027, USA c National Space Organization (NSPO), HsinChu 30078, Taiwan Received 23 February 2019; received in revised form 9 August 2019; accepted 11 August 2019

Abstract VELOX-CI was launched on December 16, 2015 into the near-equatorial orbit. It is the second micro-satellite of Satellite Research Center (SaRC) in Nanyang Technological University in Singapore. VELOX-CI is designed to explore the potential of using the commercial-off-the-shelf (COTS) GPS receivers for RO mission for the first time. Three GPS antennas are located at the zenith, forward-velocity, and after-velocity directions. In total, VELOX-CI collected 240 radio occultation (RO) missions (570 h) from 2015 to 2018 with mission durations ranging from 0.5 to 16.7 h. The lowest penetration altitude from the COTS receivers reaches 6 km. In this paper, the RO performance of the VELOX-CI is evaluated and validated with both ground-based and space-based measurements. Tropospheric and ionospheric profiles obtained from VELOX-CI are also compared with assimilative atmospheric models and empirical ionospheric models, respectively. Results show that with proper sampling frequencies (5–20 Hz), the refractivity error estimated by VELOX-CI RO is below 5% at altitudes below 25 km compared to reanalysis model estimations and radiosonde measurements. The observed ionospheric peak density and height show reasonable ranges for both daytime and nighttime. This study demonstrates the capability of COTS receiver in observing atmospheric and ionospheric parameters. In the future, utilizing COTS receivers with lower-cost low Earth orbit (LEO) satellite missions can largely increase the data volume of RO and enhance our capability in monitoring the Earth’s atmosphere. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: LEO satellite; Radio occultation; Atmosphere physics

1. Introduction Global Positioning Systems (GPS) radio occultation (RO) is a robust atmospheric remote sensing technique that can derive vertical profiles of the Earth atmosphere. GPS receivers on the low Earth orbit (LEO) satellites are common platforms for hosting RO experiments. Through the

E-mail addresses: [email protected] (B.-X. Li), [email protected] (T.-W. Fang)

relative movement between the GPS and LEO satellites, the signals emitting from the GPS satellites pierce through the Earth’s atmosphere and get refracted. The obstructed GPS signal then gets picked up by the receivers on the LEO satellites. The atmosphere induces bending and attenuation to the GPS signal, which can be translated into useful atmospheric information such as the refractivity, temperature, pressure and humidity. These measurements can also be assimilated into current weather models to improve weather and climate predictions.

https://doi.org/10.1016/j.asr.2019.08.017 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

2

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

The GPS/MET experiment launched in April 1995 was the first GPS RO mission that was developed for monitoring the Earth’s atmosphere. The mission consists of a 2 kg GPS TurboRogue receiver piggybacked on the MicroLab I satellite with a circular orbit of 730 km altitude and 60° inclination (Hajj et al. 1996; Kursinski et al. 1996). This mission successfully demonstrated the RO concept. Since then, GPS RO missions began to flourish. Most missions, like Scientific Application Satellite-C (SAC-C), TerrSARX, Gravity Recovery and Climate Experiment (GRACE), and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC-1), adopt the BlackJack series receivers developed by NASA’s Jet Propulsion Laboratory (JPL) (Yunck et al., 1999; Wickert et al., 2001; Fong et al., 2007; Beyerle et al., 2011). The BlackJack receiver is specially designed which allows for open-loop (OL) tracking when required. The OL tracking method was proposed in (Schreiner et al., 2010) to deal with severe refractivity gradients that are commonly occurred in the lower troposphere. So far, most RO missions are conducted by LEO satellites carrying similar space-borne receivers. Recently a number of new satellites are launched or scheduled to be launched for RO missions (e.g. the constellations of COSMIC-2, Spire Lemur-2 and PlanetiQ). Dataassimilation results using currently available data have shown great improvement over the past years, but the number of RO data and its spatial distribution are still insufficient to capture the entire ionospheric variability (Yue et al., 2012). Nevertheless, the commercial off-the-shelf (COTS) receivers have advantages of low cost and short development cycle. These receivers are also commonly used to perform precise orbit determination and relative positioning in other missions. The CanX-2 mission carries a COTS receiver and successfully demonstrated the ionosphere data collection capability (Swab et al., 2012). However, verification of their result was hindered from the lack of validation data. Furthermore, the receiver’s performance on the troposphere data collection capability was not shown. In this paper, the results and performance of a pioneering RO mission using COTS receiver onboard the VELOX-CI low-earth orbiting satellite are presented and discussed. VELOX-CI is the first RO mission developed by the Satellite Research Center (SaRC) in Nanyang Technological University in Singapore. It was launched into the near equatorial orbit with an altitude of 550 km. Consequently, this orbit would allow the satellite to achieve more observations in the equatorial region. VELOX-CI carries three GPS receivers with their positions on the payload illustrated in Fig. 1. The antenna’s field of view (FOV) is 120°. Receiver B is mounted in zenith direction for precise orbit determination purpose, while receiver A and C are mounted at forward-velocity and after-velocity directions to receive more signals from the GPS satellites. All the antenna are passive, which means low energy consumption can be achieved. When all the receivers and microcontrollers are switched on, the total power of the

GPS payload subsystem is less than 7.8 W. The RO payload is able to support a logging rate up to 100 Hz. However, in the actual experiment, receiver A and C logs at 1–20 Hz for RO data collection, while receiver B logs at 0.1–1 Hz for precise orbit determination. At 20 Hz logging rate, the VELOX-CI’s bus can support up to 5 data logging channels simultaneously. Currently, the entire recorded raw data from VELOX-CI is made publicly accessible (Li, 2018). The majority of the collected data use 1 Hz sampling frequency (logging rate). A few missions with high sampling rate were also carried out. Due to the performance constraint of the COTS receiver in the troposphere, lose-locks are commonly observed in the GPS L2 frequency. As a result, even after applying necessary postprocessing algorithms to reconstruct the L2 signal, only 23 troposphere RO profiles are able to be retrieved. Due to the high dynamics of the LEO orbit, it is important to have prior knowledge of the channel and Doppler frequencies for the COTS receiver in order to get the initial position fixed. The channel allocation and Doppler shift for each channel are pre-determined from the orbit and attitude information and provided to the receivers through the ground commands. With prior knowledge of the Doppler shift, the COTS receivers in VELOX-CI are able to achieve the time to first fix (TTFF) of 40–80 s in most cases. In this study we present RO results from VELOX-CI measurements for troposphere refractivity and ionosphere analyses. The RO results are compared against groundbased measurements, assimilative weather models as well as other LEO based measurements. The troposphere refractivity profiles are further compared with radiosonde data from the integrated global radiosonde archive (IGRA) (Durre et al., 2006), the ERA5 product from the European Centre for Medium-Range Weather Forecasts (ECMWF), and COSMIC-1 observations. The ionosphere profiles are compared with digital ionogram database (DIDBASE), COSMIC-1 observations and empirical ionospheric model. In Sections 2 and 3, data post-processing procedures and analysis of troposphere data using GNSS payload are introduced. Section 4 presents the plasmasphere and ionosphere data analysis and comparisons against other simultaneously collocated data sources. In the last section, the overall performance and challenges of the VELOX-CI RO mission are summarized. Suggestions are also made for the future development of similar missions. 2. Troposphere refractivity retrieving technique Fig. 2 illustrates the geometry of a radio occultation event. The excess phase is defined as the difference between actual signal traveling distance and straight-line distance of the LEO and GPS satellite. From the excess phase, bending angle (a) of the signal can be calculated. There are two ways to describe the height of the RO event, namely, impact parameter height (a) and straight line tangent height (SLTH). As illustrated in Fig. 2, impact parameter

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

3

Fig. 1. VELOX-CI and the GPS payload subsystem and allocation of GPS receiver.

Fig. 2. Radio occultation geometry.

height describes the tangent height of the ray path more accurately. On the other hand, SLTH is typically used to denote the height of the occultation during the data postprocessing as it is easier to calculate and is always lower than the impact parameter height. Typically, VELOX-CI would completely lose lock after the SLTH reaches
2 km below the Earth’s surface, which corresponds to an impact parameter height of 8 km above the Earth’s surface. The following equation is used to calculate the excess phase for GPS PRN i from L1 phase measurement /si;1 and geometric distanceqi : /si;1
qi ¼ T si;1 þ k1 N si;1 þ I si;1 þ cðdti
dts Þ þ msi;1 þ esi;1

ð1Þ

The excess phase measurement shown in Eq. (1) includes noises such as the clock difference dti
dts and ionosphere delay I si . The ROPP software (Culverwell et al., 2015) used in our study is able to remove the ionospheric delay I si with measurements from both L1 and L2 signals. The clock difference term can be removed with the single difference approach (Schreiner et al. 2010). This approach assumes that the GPS satellite clocks are all synchronized and the receiver clock is relatively stable. The excess phase of a high elevation GPS satellite at the same time is subtracted from

Eq. (1), which cancels the clock difference terms, leaving the atmosphere terms T si;1 unaffected. The multi-path msi;1 and thermal noise term esi;1 are neglected, and the ambiguity can be estimated with the assumption made on the boundary condition that T si;1 ¼ 0 at 100 km. Due to the lower power of L2 signal and the rapid signal change caused by ionosphere, the receiver carried by the VELOX-CI often suffers from loss of lock at the SLTH range of 50–100 km. These data typically would not be able to pass quality checks for obtaining tropospheric information due to their low amplitude and discontinuity. However, since the VELOX-CI L2 data commonly suffers from discontinuity at this region, a post-processing technique was specially designed to recover the data. Processing with discontinuous L2 data involves many assumptions, which are described below in further detail. The first assumption is that the distortions due to nonionosphere effects are identical for L1 and L2 signals in the region that the discontinuities are observed. Therefore, the difference in L1 and L2 measurements are solely caused by ionosphere effect. The occultation phase measurement after single difference is expressed as: /si;1 ðtÞ ¼ qi ðtÞ þ T si ðtÞ þ k1 N si;1
I si;1 ðtÞ þ msi;1 ðtÞ þ esi;1 ðtÞ ð2Þ /si;2 ðtÞ ¼ qi ðtÞ þ T si ðtÞ þ k2 N si;2
I si;2 ðtÞ þ msi;2 ðtÞ þ esi;2 ðtÞ ð3Þ The occultation range measurement after single difference is expressed as: lsi;1 ¼ qi ðtÞ þ T si ðtÞ þ I si;1 ðtÞ þ msi;1 ðtÞ þ esi;1 ðtÞ

ð4Þ

lsi;2 ¼ qi ðtÞ þ T si ðtÞ þ I si;2 ðtÞ þ msi;2 ðtÞ þ esi;2 ðtÞ

ð5Þ

The second assumption is that the time sequence of ionosphere-free phase measurement can be fitted by a high order polynomialf ðtÞ. This is derived from Eqs. (2)–(5)

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

4

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

/si;3 ðtÞ ¼ /si;1 ðtÞ þ I si;1 ðtÞ  /si;1 ðtÞ þ

  f 2L2 lsi;1 ðtÞ
lsi;2 ðtÞ f 2L1
f 2L2

¼ f ðtÞ

ð6Þ

With all the assumptions satisfied, L2 phase can be subsequently estimated from Eq. (6) as: /si;2 ðtÞ ¼

s 2 ðf 2L1
f 2L2 Þf ðtÞ ð1
f L2 Þ/i;1 ðtÞ þ f 2L2 f 2L2

ð7Þ

With all the steps listed above we are able to reconstruct continuous L2 phase measurement from L1 phase measurement and pseudorange measurementslsi;1 andlsi;2 . Fig. 3 shows an example of the data before and after the processing with interpolation. The data used in Fig. 3 was taken at 11:48:36 universal time (UT) on 20th June 2016. Fig. 3 presents the differences between the phase and range measurements in L1 and L2 against time during one descending RO event in yellow and blue solid lines, respectively. The interpolated phase difference between L1 and L2 is plotted with orange solid line, where the SLTH is indicated by the black dashed line in the same time frame. As shown in the figure, phase difference after processing follows the trend of the range difference with opposite trend as expected from the ionospheric effect. The discontinuity happens when SLTH is greater than 50 km. This interpolation will introduce error to occultation results especially when SLTH is in the region above 50 km. Radio occultation performance is also sensitive to the accuracy of LEO position and velocity determination (Han et al. 2014). The onboard position solution generated from the receiver has inherited large errors induced by the GPS ephemeris error, receiver and satellite clock errors, ionosphere and troposphere delays in low elevation measurements, etc. The precise orbit of VELOX-CI is generated from the reduced dynamics algorithm using the Bernese software (Dach et al., 2015) to achieve the accuracy required by RO data processing. The final product

Fig. 3. Example of L2 interpolation with data collected from the VELOXCI on 20th June 2016 11:48:36 UT.

from Center for Orbit Determination in Europe sever (Dach et al., 2018) are used in the data processing. Due to lack of absolute ranging measurement, the orbit overlap method is used to validate the precise orbit determination (POD) result generated by Bernese software. The onboard data taken at 14:00:00 17th–06:30:00 18th June 2016 was analyzed and shown as an example. The overlapping time is 18:00:00–23:59:59 on 17th of June. Fig. 4 shows the difference in the position and velocity of the second arc with respective to the orbit solutions of first arc in the overlapping time. As depicted in Fig. 4, the position difference is less than 9 cm while the velocity difference is less than 0.05 mm/s throughout the overlapping period. The RMS error for the position is 3.28, 2.91 and 0.92 cm in radial, alongtrack and cross track directions. The RMS error for velocity is 0.01, 0.03 and 0.02 mm/s in these directions. We have also compared the Bernese POD result with the result achieved from the GHOST software (Wermuth et al., 2010) by German Aerospace Center DLR using the same set of data. The position difference of our result was found to be less than 20 cm and the velocity difference is less than 0.2 mm/s. These results demonstrates that VELOX-CI POD performances satisfy the general requirement for RO applications (Schreiner et al. 2011). Details of the POD processing has been discussed in (Li et al., 2017). 3. Troposphere refractivity data and analysis RO experiments from VELOX-CI provide the atmospheric refractivity and dry temperature. In this study, several examples are selected from a total of 21 profiles to demonstrate the RO performance in troposphere. All the measurements are collocated and compared with the closest radiosonde measurements. The details of these examples with different sampling rates are summarized in Table 1. Geographic locations of the RO measurements, refractivity and refractivity errors, as well as the dry temperature profiles of these examples are also illustrated in Figs. 5–8. ECMWF and the NASA modern-era retrospective analysis for research and applications (MERRA) both provide gridded worldwide simulation and reanalysis results for the tropospheric temperature, pressure and humidity at predefined vertical layers from the ground to around 80 km. These results are commonly used as reference in weather forecast and climate studies. Therefore, they are utilized for comparisons when the occultation height exceeds the radiosondes limit (i.e. no available data from radiosondes). Fig. 5a shows the geolocation of a radio occultation event occurred at 12 UT on June 20, 2017 in South America. The sampling rate for this event is 20 Hz. The locations of tangent points (the point that is closest to the Earth’s surface in a signal path) are plotted for VELOX-CI and COSMIC-1. The colors along the occultation path represents the height of tangent points with lowest altitudes shown in dark blue. Since the RO events of VELOX-CI

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

5

Fig. 4. Result of the orbit overlapping test. Positional difference is shown in red curve and velocity difference is shown in blue curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 VELOX-CI RO experiment Summary. Radiosonde Measurement

VELOX-CI Measurement

Other Data Sources

Time*

Location

Delay

Distance

Sampling rate

Delay

Grid samples

Source

2017–6-20 12:00:00 (Fig. 5) 2017–6-20 12:00:00 2017–6-16 12:00:00 2016–8-14 00:00:00

27.45°S 59.05°W


12 min

260.58 km

20 Hz

2.43°S 54.72°W 4.70°N 74.15°W 0.05°N 51.07°E


15 min

454.62 km

20 Hz

128 min

385.42 km

5 Hz


33 min

487.42 km

1 Hz

0 min 0 min 71 min 0 min 0 min 180 min 180 min 0 min 0 min

36 (23°–26°S, 55°–60°W) 35 (23°–26°S, 55°–60°W) 9 (5°–6°S, 51°–54°W) 5 (5°–6°S, 51°–54°W) 36 (2°–5°N, 76°–80°W) 30 (2°–5°N, 76°–80°W) 3 (0°–1°N, 54°–56°W) 3 (0°–1°N, 54°–56°W)

ECMWF MERRA COSMIC-1 ECMWF MERRA ECMWF MERRA ECMWF MERRA

*

Time is in Universal Time (UT).

Fig. 5. Comparing VELOX-CI RO 20 Hz data with radiosonde, COSMIC-1, ECMWF and MERRA for the case at 12UT in June 20, 2017.

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

6

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

Fig. 6. Comparing VELOX-CI RO 20 Hz data and radiosonde ECMWF and MERRA for the second case at 12UT in June 20, 2017.

Fig. 7. Comparing VELOX-CI RO 5 Hz data and radiosonde ECMWF and MERRA for the case at 12UT in June 16th, 2017.

and COSMIC-1 are in close to the radiosonde station (i.e. approximately 250 km apart), these three datasets are used to provide comparisons. Fig. 5b, c and d show the comparisons of dry temperature, temperature difference, relative refractivity of VELOX-CI measurements (in red) and other observations, respectively. The inverted atmosphere profiles denoted as ‘AtmPrf’ was provided in database from the COSMIC-I. Unfortunately, the inverted profile for this example failed to collocate against the VELOX-CI measurements and is not available. Therefore, COSMIC-1 results shown here are generated with ROPP software from the ‘AtmPhs’ product from the NCAR COSMIC program office (CDAAC). Results from ECMWF and MERRA within the longitude-latitude range specified in Table 1

were averaged at each altitude level to obtain their mean temperature, temperature difference and the refractivity error profiles. Fig. 5b shows that the dry temperature obtained by the VELOX-CI agrees well with other datasets below 25 km. Above 25 km, the temperature difference between VELOX-CI and COSMIC-1 becomes pronounced. However, the difference in refractivity between 25 and 30 km (black line in Fig. 5d) is small. In Fig. 5d, the refractivity obtained from the VELOX-CI is closer to the measurement taken by radiosonde station than those from COSMIC-1. The differences in refractivity from VELOX-CI and COSMIC-1 measurements are less than 8% at the 7–35 km altitude range. Studies has shown that the COSMIC-1 data at this range has significant impact

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

7

Fig. 8. Comparing VELOX-CI RO 1 Hz data and radiosonde ECMWF and MERRA for the case at 00UT in August 14th, 2016.

on the 24 h weather forecast model (Florian et al., 2013). Good comparisons between results from the VELOX-CI and COSMIC-1 demonstrate that VELOX-CI measurements at this altitude range can be useful for weather prediction. Results also exemplify that the difference between VELOX-CI’s and MERRA’s data is less than 3% in the bottom atmosphere region (8–35 km). Larger difference between ECMWF and MERRA may come from differences in data sources used in their simulations and dataassimilation techniques. In the upper atmosphere region (>35 km), both temperature and refractivity errors become greater (see Table 2). Fig. 6 shows another set of 20 Hz data occurred at the same time as the previous case (i.e. Fig. 5) but located in South America. COSMIC-1 observation is not available for this case. The radiosonde location is located slightly further way (i.e. approximately 454.62 km) from occultation event. Similarly to the previous event, the dry temperature from VELOX-CI yielded good agreement with other data below 15 km which subsequently exhibited more significant differences above 15 km. As observed earlier in

Fig. 5, here the atmospheric refractivity and temperature measured by VELOX-CI is biased compared to other simulated data in the region above 35 km. The large distance between the radiosonde station and the RO location may be one of the reasons for the difference. The two 20 Hz profiles all have the discontinuities problem which have been adjusted using the techniques mentioned in Part 2. Thus, the errors may also be attributed to inaccurate interpolation process of L2 signals. Fig. 7 presents the results of 5 Hz VELOX-CI data in Colombia on June 16th, 2017. Note that in this case the occurrence time of RO measurement is 128 min after the record of radiosonde data. Results of the dry temperature and refractivity obtained from VELOX-CI and radiosonde show good agreement except a bias observed in the refractivity error near 30 km. By reducing the sampling frequency to 5 Hz, the data storage and requirements of downlink time become much more achievable while details of atmospheric height variations can still be captured. Fig. 8 shows an example of 1 Hz VELOX-CI data and comparisons. This data was taken at 00 UT in August

Table 2 VELOX-CI RO refractivity error summary comparing with ECMWF and MERRA. RO event

<15 km

15–25 km

25–35 km

mean

var

mean

var

mean

var

2017-6-20 11:48:00

ECMWF MERRA


5.75%
0.59%

0.29% 0.34%


3.23%
1.21%

1.75% 0.88%


1.12%
0.96%

1.23% 1.30%

2017-6-20 11:45:00

ECMWF MERRA


6.93% 0.82%

0.91% 0.06%


1.86% 1.98%

12.22% 1.18%

6.83% 6.44%

3.21% 3.73%

2017-6-16 14:08:00

ECMWF MERRA


8.54%
0.80%

2.75% 0.56%


6.70%
2.36%

5.09% 0.47%


5.91%
5.48%

8.81% 2.89%

2016-8-14 11:27:00

ECMWF MERRA


8.65%
0.86%

1.25% 0.32%


8.17%
5.19%

16.52% 38.52%


8.96%
8.19%

8.33% 13.89%

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

8

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

14th, 2016. The distance between the RO event and the radiosonde station is about 487.42 km. Continuous L2 data are collected for this profile, therefore no interpolation is required. In this comparison, a large refractivity difference is observed. With the low-frequency sampling rate, details of the dry temperature profiles shown in Figs. 5–7 are not able to be captured in the observation. However, the temperature reversal at 18 km and 22 km is still reflected from Fig. 8b. The difference in the curve shape might be due to under-sampling. From the four examples above, the CI measurement seems to agree better with radiosonde measurement and MERRA results at an altitude below 25 km. The other 17 profiles of 1 Hz RO missions are also analyzed and compared with ECMWF. The overall 1 Hz RO performance is shown in Fig. 9. Fig. 9a shows that the standard deviation of absolute refractivity error is about 3 N and the mean error up to 5 N under 25 km altitude. Fig. 9b shows the fractional error is actually smaller in the lower altitude. Fig. 9c shows the data volume at each altitude level. These results shows that 1 Hz logging rate is too low for RO purpose. The overall 1 Hz RO has larger refractivity error as compared with the case shown in Fig. 8. This is due to the interpolation errors introduced. 4. Plasmasphere and ionosphere measurements analysis Depending on the elevation angle of the observation, VELOX-CI can provide topside ionosphere/plasmasphere measurements as well as ionospheric profiles up to the orbit height. When the elevation angle is positive, the piercing point of the observation is calculated based on a single layer model assumption. Ionosphere delay along the line of sight is assigned to the location and topside ionosphere can be mapped with continuous observations. When the elevation angle is negative, a vertical ionosphere profile can be obtained from the multi-frequency measurements through the Abel inversion. Calibration and first order dif-

ferencing applied on the phase measurements are similar to data processing used in the COSMIC-1 satellites (Yue et al. 2011).

4.1. Topside ionosphere and plasmasphere results When the GPS signals come from altitudes above VELOX-CI, signals between VELOX-CI and GPS satellites can be used to calculate total electron content (TEC) for the integrated plasma density in topside ionosphere and plasmasphere. The slanted TEC (STEC) is calculated when the elevation angle of GPS signal is greater than 30°and signal amplitude greater than 40 dB (Hz). This is to improve TEC data quality by removing noisy data due to multipath caused by antenna arrangement. The slanted TEC is given as: STEC ¼

lsi;1
lsi;2 þ DCBGPS þ DCBreceiver 40:3ð1=f 2L1
1=f 2L2 Þ

ð8Þ

Differential code bias for GPS satellite (DCBGPS ) as well as the receiver (DCBreceiver Þ shown in Eq. (8) are calibrated. The DCB of GPS satellite are available from Center for Orbit Determination in Europe sever (CODE 1992) while the DCB for receiver is calculated by assuming the DCB is a constant when the receiver operates for more than 30 min, and night-time minimum STEC is equal to zero (Kao et al., 2013; Lee et al., 2013). For the purpose of receiver DCB calibration, long duration missions which last for more than 5 orbits are performed occasionally. The DCB for these missions are calculated and shown in Fig. 10. The estimated DCB shows consistency within each PRN for missions executed at various times in 2017. The white space in the figure indicates unavailability of data. The ionosphere pierce point (IPP) location for each sampling point is calculated by assuming that the ionosphere is a single layer (i.e., thin shell) concentrated at the altitude of

Fig. 9. Overall 1 Hz RO performance compared with ECMWF.

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

Fig. 10. Estimated DCB for 32 GPS satellites PRN01-PRN32. Calculated based on the data collected from the 23 missions planned from January to October in 2017.

600 km (i.e., just above orbit height). TEC dataset from a long duration VELOX-CI mission on 15th March 2017 was analyzed and depicted in 2D map in Fig. 11. Fig. 11 shows the topside STEC value collected during the mission in 15th Mar 2017 and illustrates the typical sampling paths of TEC data over the near-equatorial region. This particular mission lasted for 5 orbits, during this time the local STEC changes due to earth rotation and solar radiation, thus causing STEC over the same region to be different in each track. Extents of TEC derived from the VELOX-CI are further presented in Fig. 12a, where maps of topside-ionospheric TEC generated using the VELOX-CI are shown. Due to the nature of the near-equatorial orbit (i.e. 15° orbit inclination) and availability of line of sight acquisitions to GPS satellites, increased data coverage is typically obtained at the equatorial (i.e. 8° S to 8° N) parts of south-east Asia and the American continent. Localized pockets of high TEC may be observed. Fig. 12b also shows the regional TEC map over south-east Asia and also the ground station with S/X-band 6.1-meter antenna dish used for satellite downlink at Nanyang Technological University, Singapore. Unlike ground-based GPS receivers, satellite-based TEC data can provide valuable data over the ocean (Fig. 12b). Therefore, when utilized as a complementary TEC data source, the high temporal sampling rate (i.e. approximately 100 mins revisit) and short spatial sampling

9

intervals (i.e. approximately 6 km) of the VELOX-CI is useful to improve the understanding of regional space weather dynamics as well as ground-ionosphere coupling relationships (Lim and Leong, 2019). The time series plot of STEC obtained by VELOX-CI for several 5-orbits and 10-orbits missions (i.e. total of 23 days) conducted during 2017 is presented in Fig. 13 (note that data is not collected on consecutive days due to downlink restrictions and mission operation requirements). Hourly STEC variations were daily-averaged to yield diurnal variations. For the corresponding period, Fig. 14 exhibits the variations of geomagnetic Disturbance storm time (Dst) and solar flux F10.7 indices. Since the geomagnetic observatories (as shown in Fig. 12) providing Dst measurements reside within the equatorial latitudes, Dst index serves a good proxy for assessing effects of space weather forcing on the VELOX-CI TEC. Fig. 14 further classifies the Dst values into weak, moderate and strong storms (Loewe and Pro¨lss, 1997). The event of interest during this period is the strong geomagnetic storm occurring on 8 Sep 2017. Approaching a minimum of solar cycle 24, space weather conditions were highly active between 6 and 10 Sep 2017, which culminated with a series of X-flares outbursts on the 6th and 7th (Blagoveshchensky and Sergeeva, 2019). Subsequently, shock waves from solar Coronal Mass Ejection (CME) was reported to reach Earth on two occasions, 22:38 UT on 7 Sep 2017 (i.e. 06:38 LT the following day, which is 8 Sep 2017) as well as 11:00 UT (i.e. 19 LT) on 8 Sep 2017. The shock waves resulted in two Dst minimums corresponding to the CME times. On 8 Sep 2017, Fig. 14 clearly shows the daily Dst value reaching a significant low of
88 nT while the F10.7 index registered a value of 118.5 sfu. As shown in Fig. 13, VELOX-CI topside TEC was able to capture the effects from the flare event on this day, where STEC experienced a marked increase to 19.0 TECu. This corresponds to approximately 84.4% increase from the average values of the previous measurements in the earlier part of 2017. In fact, an X8.2 class intense solar flare occurred just two days (i.e. 10 Sep 2017) after the Dst minimum on 8 Sep 2017. Unfortunately, the VELOX-CI was not scheduled for operation during that time and could not provide any insights into this event.

4.2. Ionosphere electron density profiles and analysis

Fig. 11. Topside ionosphere and plasmasphere measured on Mar 15th, 2017 (between 21:52 and 06:10 UT next day) when daily averaged F10.7 index is 69.1 and Kp is 1.3; colorbar indicates topside STEC values in TECu (1016 el/m2).

When the elevation angle of the GPS signal is negative, electron density profile may be calculated purely with the phase measurement from VELOX-CI. The constant ambiguity part is removed during the Abel inversion by assuming TEC at the orbit level is equal to zero. Although this assumption is not completely accurate, it most affects points close to the satellite height. In this section, comparisons of VELOX-CI electron density profiles with ionosonde measurement, COSMIC-1 observations, as well as

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

10

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

Fig. 12. Typical extents of topside ionospheric TEC map generated from the GPS receivers on-board VELOX-CI. (a) daily-averaged near-equatorial TEC, showing geomagnetic Dst observatories. (b) south-east Asia regional TEC map showing data density over a 2-hour period as well as downlink ground station located in NTU Singapore (1.34° N, 103.7° E); data sampling interval is illustrated (approximately 6 km).

Fig. 13. Topside ionospheric STEC from VELOX-CI from 23 missions in year 2017; TEC increase associated with flare event is highlighted in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

results from International Reference Ionosphere Model (IRI-2016) are presented. Fig. 15 shows a direct comparison of plasma frequency between the VELOX-CI measurement (blue line) and

ionosonde observation (black solid line). IRI simulated electron density along RO path (orange line) and at ionosonde location (yellow line) were converted into plasma frequency and shown in the plot. Ionosonde measurement

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

11

Fig. 14. Space weather indices for geomagnetic Dst (top) and solar flux F10.7 (bottom); occurrence of geomagnetic storm found on 8 Sep 2017.

can only provide information of bottomside ionosphere. Topside profile (black dotted line) was estimated by fitting the ionosonde measurement using the Chapman layer assumption. The comparisons of two IRI profiles show that despite variations in the horizontal distance between the occultation tangent point and the ionosonde station, similar results are observed from the IRI. The minimum distance from the occultation path to the ionosonde station is 106 km, and the time difference between VELOX-CI and ionosonde measurements is less than 5 min. Ionospheric density profile obtained by VELOX-CI was also converted into plasma frequency here for the comparison. It is observed that the height of peak electron density (hmF2)

in VELOX-CI measurement and from the IRI are lower than the ionosonde data. The underestimation of hmF2 in VELOX-CI may be partially related to the assumption made at the top layer in the Abel transformation. The overall shape of the profile and the maximum ionospheric density agree well with the radiosonde measurement. Fig. 16 shows the comparison of electron density profiles from VELOX-CI and COSMIC-1 at 3:19–3:40 UT on 2 Sep 2017. Time difference between the two occultation events is 13 min. The two occultation events happens to be descending in roughly the same direction which gives a very similar geometry. The electron density profile of COSMIC-1 was obtained from the ionPrf product. The measurement taken by VELOX-CI shows good agreement with COSMIC-1

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

12

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

Fig. 15. Comparison of VELOX-CI plasma frequency (blue line), ionosonde observation (black solid line), and IRI simulations (orange and yellow line) on 14th August 2016. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 16. Comparing VELOX-CI electron density profile with COSMIC-1 at 3:19–3:40 UT on 2nd Sep 2017. IRI results at the two locations are also shown.

result between 200 and 500 km. The electron density estimated by IRI model along the occultation path of the two events are also depicted. The IRI model simulated electron density of the two occultation path are almost the same with two times over estimation of peak electron densities (NmF2) compared to satellite measurements. Fig. 17 shows a case when two occultation events from COSMIC-1 and VELOX-CI are very close but their tangent heights are moving toward lower altitude in opposite directions. The collocation event happened at 3:12–3:30 UT on 21 Jan 2017. Although the time difference between the two events is only 7 min, electron density profiles shown in these two cases are completely different. The differences indicate that the horizontal gradient of TEC on these occultation paths are possibly very pronounced, rendering the assumption of Abel inversion invalid. This is one of the limitation of the GNSS occultation based electron

density profile measurement. Interestingly, the difference seen in two observations were also reproduced by IRI. Similar to the previous case, the IRI simulations overestimate the NmF2 and hmF2 compared to LEO base measurement. To evaluate how well the general ionospheric morphology in the daytime and night-time captured by the VELOX-CI, an examination of more than 2-year worth (Dec 2015 - Jan 2018) of VELOX-CI data was conducted. In Figs. 18 and 19, locations of tangent points for VELOXCI occultation events are plotted in geographic coordinate. IRI results at exact same local times and locations are also calculated. NmF2 and hmF2 for these events are shown in color. Absolute Values and percentage differences (VELOX-CI - IRI) between 10 and 14 LT and 22–2 LT are shown in Figs. 18 and 19, respectively. Overall, NmF2 and hmF2 calculated from VELOX-CI measurements shows reasonable ranges for the daytime and nighttime

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

13

Fig. 17. Comparing VELOX-CI electron density profile with COSMIC-1 at 3:12–3:30 UT on 21st Jan 2017. IRI results at the two locations are also shown.

Fig. 18. Comparison of VELOX-CI NmF2 and hmF2 with IRI at daytime 10:00–14:00 local time.

ionosphere under moderate to lower solar activity and quiet geomagnetic conditions. The results also show that compare to IRI, the peak ionospheric density NmF2 from VELOX-CI tends to be underestimated in the daytime but agrees well in the nighttime. For hmF2, VELOX-CI slightly overestimates the values at low-latitudes but underestimates them at mid-latitudes in the daytime. Larger fluctuations can be seen in the NmF2 and hmF2 differences especially at night time. These observations demonstrate

that VELOX-CI ionospheric measurements are reliable and can be useful for ionospheric weather assimilation with increased data volume. 5. Discussion and conclusion In this paper, the data processing procedure and the results of VELOX-CI radio occultation experiments were presented. The radio occultation experiments performance

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

14

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx

Fig. 19. Comparison of VELOX-CI electron density NmF2 and hmF2 with IRI at night 22:00–02:00 local time.

is limited by the satellite and receiver hardware in terms of both sampling rate and signal strength. However, meaningful results were derived from the available data collected. A new interpolation method is proposed to recover L2 phase measurement from the continuous L1 measurements. A series of collocation events were evaluated for VELOXCI’s radio occultation performance. The refractivity error was found to be around 5% in the well collocated events as compared with the radiosonde and the COSMIC-1 measurements in the bottom atmosphere region. Simulation of ECMWF and MERRA shows that in the upper atmosphere (>35 km), the refractivity error of VELOX-CI measurement can be greater than 10%. For the ionosphere observations, STEC is measured and mapped into a 2D layer above the satellite orbit, daily variations of the STEC shows correlation with solar activities. The intense solar flare leading to an intense geomagnetic storm on 8 Sep 2017 was also capture by the VELOX-CI TEC measurements. The vertical electron density profile generated with VELOX-CI data is verified in the collocation events with both ionosonde and COSMIC-1 data. The NmF2 and hmF2 of the collected profiles are extracted and compared with the IRI simulation. These profiles helps to enrich the sparse ionosphere observations that are available for public access. In the future development of similar missions with COTS receivers, the receiver loss-of-lock problem in L2 need to be further addressed. This may be overcome by

employing an active antenna instead of the passive antenna used in VELOX-CI. In conclusion, the COTS receiver on VELOX-CI is able to carry out radio occultation experiment for troposphere and ionosphere studies at a lower cost and shorter development time. Acknowledgement The research work and VELOX-CI mission are supported by Singapore Economic Development Board (EDB). The VELOX-CI data can be obtained through accessing https://doi.org/10.21979/N9/XODIPT, which is hosted by DR-NTU project in Nanyang Technological University in Singapore. The authors would also like to thank all the individuals who have involved in developing the payload and helped with data collecting throughout the VELOX-CI project References Beyerle, G., Grunwaldt, L., Heise, S., Ko¨hler, W., Ko¨nig, R., et al., 2011. First results from the GPS atmosphere sounding experiment TOR aboard the TerraSAR-X satellite. Atmos. Chem. Phys. 11, 6687–6699. https://doi.org/10.5194/acpd-10-28821-2010. Blagoveshchensky, D.V., Sergeeva, M.A., 2019. Impact of geomagnetic storm of September 7–8, 2017 on ionosphere and HF propagation: a multi-instrument study. Adv. Space Res. 63 (1), 239–256. Culverwell, I.D., Lewis, H.W., Offiler, D., Marquardt, C., Burrows, C.P., 2015. The radio occultation processing package, ROPP. Atmos. Meas. Tech. 8 (1), 1887–1899. https://doi.org/10.5194/amt-8-1887-2015.

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017

B.-X. Li et al. / Advances in Space Research xxx (2019) xxx–xxx CDAAC, COSMIC Data Analysis and Archive Center, 2001. Available from: https://cdaac-www.cosmic.ucar.edu/cdaac/products.html. DIDBASE, Digital Ionogram Database Reinisch, E. B. W., Lowell GIRO Data Center, 1987–2018. Dach, R., Lutz, S., Walser, P., Fride, P., 2015. Bernese GNSS Software Version 5.2. User manual, Astronomical Institute, University of Bern. Bern Open Publishing, http://doi.org/10.7892/boris.72297. Dach, R., Schaer, S., Arnold, D., Prange, L., Sidorov, D., et al., 2018. CODE final product series for the IGS. Astronomical Institute, University of Bern. Durre, I., Vose, R.S., Wuertz, D.B., 2006. Overview of the integrated global radiosonde archive. J. Clim. 19, 53–68. https://doi.org/10.1175/ jcli3594.1. Florian, H., Healy, S.B., Bauer, P., English, S., Yen, N., 2013. Optimising the number of GNSS Radio Occultation measurements for Numerical Weather Prediction. In: Presentation at 93rd American Meteorological Society Annual Meeting, Abstract 8.1. Fong, C.J., Yen, N., Yang, S.K., Chen, S.S., Chi, S., 2007. GPS Radio Occultation and Mission Results from FORMOSAT-3/COSMIC Spacecraft Constellation. In: 3rd International Conference on Recent Advances in Space Technologies. http://doi.org/10.1109/rast.2007. 4284093. Hajj, G.A., Kursinski, E.R., Bertiger, W.I., et al., 1996. Initial Results of GPS-LEO Occultation Measurements of Earth’s Atmosphere Obtained with the GPS-MET Experiment, GPS Trends in Precise Terrestrial, Airborne, and Spaceborne Applications, International Association of Geodesy Symposia, Springer Berlin Heidelberg, vol. 115, pp. 144–153. http://doi.org/10.1007/978-3-642-80133-4_23. Han, B., Erry, G., Low, K.S., et al., 2014. Simulation study of the effect of orbital errors on open-loop tracking in GPS radio occultation. In: 2014 IEEE/ION Position Location and Navigation Symposium (PLANS). https://doi.org/10.1109/plans.2014.6851360. Kao, S., Tu, Y., Chen, W., Weng, D.J., Ji, S.Y., 2013. Factors affecting the estimation of GPS receiver instrumental biases. Surv. Rev. 45 (328), 59–67. https://doi.org/10.1179/1752270612Y.0000000022. Kursinski, E.R., Hajj, G.A., Bertiger, W.I., et al., 1996. Initial results of radio occultation observations of earth’s atmosphere using the global positioning system. Science 271 (5252), 1107–1110. https://doi.org/ 10.1126/science.271.5252.1107. Lee, H.B., Jee, G., Kim, Y.H., Shim, J.S., 2013. Characteristics of global plasmaspheric TEC in comparison with the ionosphere simultaneously observed by Jason-1 satellite. J. Geophys. Res. Space Phys. 118 (2), 935–946. https://doi.org/10.1002/jgra.50130. Li, B.X., Han, B., Lim, W.S., Tsai, Y.F., Low, K.S., 2017. Performance Assessment of the Radio Occultation Experiment for Commercial

15

of-the-shelf Receivers on VELOX-CI Microsatellite. In: Proceedings of the 30th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+), pp.1669–1677. Li, B.X., 2018. VELOX-CI Raw Data, DR-NTU (Data). http://doi.org/ 10.21979/N9/0WWIGJ. Lim, B.J.M., Leong, E.C., 2019. On the detection of anomalous seismoionospheric behavior in the presence of space weather stimuli for large earthquakes. Adv. Space Res. 63, 1961–1978. https://doi.org/10.1016/ j.asr.2018.10.020. Loewe, C.A., Pro¨lss, G.W., 1997. Classification and mean behavior of magnetic storms. J. Geophys. Res. Space Phys. 102 (A7), 14209–14213. https://doi.org/10.1029/96JA04020. Schreiner, W., Rocken, C., Sokolovskiy, S., Hunt, D., 2010. Quality assessment of COSMIC/FORMOSAT-3 GPS radio occultation data derived from single- and double-difference atmospheric excess phase processing. GPS Solut. 14 (1), 13–22. https://doi.org/10.1007/s10291009-0132-5. Schreiner, W., Sokolovskiy, S., Hunt, D., Rocken, C., Kuo, Y.H., 2011. Analysis of GPS radio occultation data from the FORMOSAT-3/ COSMIC and Metop/GRAS missions at CDAAC. Atmos. Meas. Tech. 4 (10), 2255–2272. https://doi.org/10.5194/amt-4-2255-2011. Swab, M., Kyle, O.K., Susan, S., 2012. Single-frequency Ionospheric Profiles from the CanX-2 Nano-Satellite. In: Proceedings of the 25th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2012), pp. 2007–2021. Wermuth, M., Montenbruck, O, Wermuth, M., 2010. GPS High Precision Orbit Determination Software Tools (GHOST). In: Presentation at 4th International Conference on Astrodynamics Tools and Techniques, Madrid, Spain, May 2010. Wickert, J., Reigber, C., Beyerle, G., et al., 2001. Atmosphere sounding by GPS radio occultation: first results from CHAMP. Geophys. Res. Lett. 28 (17), 3263–3266. https://doi.org/10.1029/2001GL013117. Yue, X., Schreiner, W.S., Kuo, Y.H., et al., 2012. Global 3-D ionospheric electron density reanalysis based on multisource data assimilation. J. Geophys. Res.: Space Phys. 117 (A09325). https://doi.org/10.1029/ 2012JA017968. Yue, X., Schreiner, W.S., Rocken, C., et al., 2011. Evaluation of the orbit altitude electron density estimation and its effect on the Abel inversion from radio occultation measurements. Radio Sci. 46 (1), 1–10. https:// doi.org/10.1029/2010RS004514. Yunck, T., Liu, C.H., Ware, R., 1999. A History of GPS Sounding. Terrestrial, Atmospheric and Oceanic Sciences (TAO). pp. 1–20. http://doi.org/10.3319/tao.2000.11.1.1(cosmic).

Please cite this article as: B.-X. Li, T. W. Fang, B. Han et al., Performance assessment in the commercial off-the-shelf receiver radio occultation mission on VELOX-CI satellite, Advances in Space Research, https://doi.org/10.1016/j.asr.2019.08.017