A DORIS determination of the absolute velocities of the Sorsdal and Mellor glaciers in Antarctica

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Advances in Space Research 45 (2010) 1523–1534 www.elsevier.com/locate/asr

A DORIS determination of the absolute velocities of the Sorsdal and Mellor glaciers in Antarctica R. Govind a,*, J.J. Valette b, F.G. Lemoine c a

Geospatial and Earth Monitoring Division, Geoscience Australia, P.O. Box 378, Canberra, ACT 2601, Australia b Collecte Localisation Satellites, 8-10 Rue Herme`s 31520 Ramonville St-Agne, France c Planetary Geodynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Received 23 July 2009; received in revised form 9 February 2010; accepted 9 February 2010

Abstract The Lambert–Amery System is the largest glacier–ice shelf system in East Antarctica, draining a significant portion of the ice sheet. Variation in ice sheet discharge from Antarctica or Greenland has an impact on the rate of change in global mean sea level; which is a manifestation of climate change. In conjunction with a measure of ice thickness change, ice sheet discharge can be monitored by determining the absolute velocities of these glaciers. In order to demonstrate the capability of the DORIS system to determine glacier velocities, Geoscience Australia undertook a Pilot Project under the auspices of the International DORIS Service. A DORIS beacon was deployed on the Sorsdal (November 2001 – January 2002 and November 2003 – January 2004) and Mellor (December 2002 – January 2003) glaciers. The DORIS data, transmitted from the autonomously operating ground beacon for each satellite pass, were stored in the receiver on-board the satellite and later downlinked to the DORIS control centres for processing. This paper describes the campaigns that were conducted at the Sorsdal and Mellor glaciers, the data processing standards for modelling the Doppler measurements, precise orbit determination of the satellites using the data from the globally distributed DORIS network, tracking station position and reference frame modelling, the point positioning mode employed for determining the position and velocities of the transmitting beacon antennas located on the glaciers and provides the velocity estimates that have been determined from the analysis of these tracking data. For the Sorsdal 2001/2002 campaign, using SPOT-4 data only, the measured effective horizontal ice motion was estimated to be 30 ± 0.4 cm/day (azimuth of N246°E ± 0.1°). The inferred velocities for the Sorsdal 2003/2004 campaign, using SPOT-4 and SPOT-5 data, was 5.7 ± 0.8 cm/day (azimuth of N264°E ± 7.5°) for the first eight days and 11.4 ± 1.4 cm/day (azimuth of N241°E ± 1.5°) for the subsequent 21 days. There was a noted decrease in the inferred velocities between the beginning and the end of the observing period. A sub-division of the latter 21 day observing period into three segments showed a decrease in 2D velocity from 18.3 ± 0.7 to 11.2 ± 0.7 cm/day and then to 7.4 ± 0.9 cm/day for the first, second and third segments, respectively. In comparison, a GPS derived velocity over the time-span of the 2001/2002 Sorsdal campaign gave a mean ice flow rate of 31 cm/day. The GPS velocity was derived from two daily position estimates 65 days apart. The DORIS determination from 26 days of continuous SPOT-4 and SPOT-5 data compared well with the GPS derived velocity. For the 2002/2003 Mellor glacier campaign, using SPOT-4 and SPOT-5 data, the estimated average ice velocity was 104 ± 25 cm/day (azimuth of N33°E ± 0.1°); which compared well with an InSAR derived velocity of between 110 and 137 cm/day. The point positioning technique as implemented in this study was further validated and assessed by replicating the computational process to determine the position and velocity of the permanent International DORIS Service site at Terre Ade´lie, Antarctica. Through these experiments, it has been successfully demonstrated that the DORIS system is capable of determining the velocities of glaciers with an accuracy of a few cm/day over a period of several weeks; operating in remote regions in an autonomous mode. With an increasing number of DORIS-equipped satellites and multiple daily passes, it has the potential to measure glacial velocities at a high temporal resolution (sub-daily). Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.

*

Corresponding author. Tel.: +61 2 62499033; fax: +61 2 62499929. E-mail addresses: [email protected] (R. Govind), [email protected] (J.J. Valette), [email protected] (F.G. Lemoine).

0273-1177/$36.00 Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2010.02.011

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Keywords: DORIS; Geodesy; Antarctica; Mellor; Sorsdal; Glacier velocity

1. Introduction The mass balance of an ice sheet, which is the differential rate between ice accumulation and loss, contributes to the rate of change of global mean sea level (Meier, 1993). Hence, over the last two decades, research has been focused on the mass-balance of the Antarctic ice sheet; 90% of which is discharged by fast-flow features such as outlet glaciers and ice streams into the oceans (Drewry et al., 1982; Morgan, 1982). Rignot and Thomas (2002) outline three methods to measure the mass balance of an ice sheet; the mass budget method, measurements of elevation changes over time using satellite and airborne radar altimeters and a measure of the temporal variation of the Earth’s gravity field derived from satellite tracking data such as GRACE. The mass budget method compares loss with input; that is, melt and ice discharge with respect to snow accumulation. The measure of ice discharge inland of the grounding line requires a measure of the ice velocity. The rationale for this pilot project was to establish the capabilities of the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system (Dorrer et al., 1991; Tavernier et al., 2006) to monitor the velocity of glaciers at a high temporal resolution. Glacier velocity is an important parameter for understanding the dynamics of ice sheets, the computation of its mass balance, its rate of discharge into the world’s oceans, and hence its impact on the rate of change of global mean sea level. In addition, there are several other potential applications for DORISdetermined absolute positions and velocities on ice sheets, such as, SAR Interferometry and altimetry; particularly if the beacon is placed at the ground track intersections of altimetry satellites. As part of a Pilot Experiment of the International DORIS Service (IDS) (Tavernier et al., 2005, 2006), DORIS observations, of short duration (less than 2 months), were undertaken on the Sorsdal glacier during the austral summers of 2001–2002 and 2003–2004, and on the Mellor glacier during the austral summer of 2002–2003. Figs. 1–3 show the relative locations of the glaciers. During the period of the 3 experiments on the Sorsdal and Mellor glaciers, the number of DORIS-equipped satellites increased from 3 (SPOT-2, TOPEX/Poseidon and SPOT-4) to 6 (with the addition of Jason-1, SPOT-5 and Envisat). The Low Earth Orbit SPOT and Envisat satellites (98° inclination) provide excellent polar visibility. Accurate daily station positions for the global tracking network are achieved by undertaking high precision satellite orbit determination, using data from the globally distributed DORIS network and implementing a state-of-the-art computation standard for measurement, satellite orbit and station position modelling (Soudarin et al., 1999; Willis et al., 2005). Utilising these high accuracy orbits as determined for the respective satellites, a point posi-

tioning mode was employed to estimate the daily position and velocity of the DORIS antenna located on the Sorsdal and Mellor glaciers. In this paper, Section 2 describes the 3 DORIS campaigns in East Antarctica, Section 3 outlines the data processing and computation standards. The results for the three campaigns, which includes an assessment and validation of the point positioning technique and a comparison with available GPS and InSAR results, are presented in Section 4. Section 5 discusses further enhancement and future application of DORIS for monitoring ice sheets. 2. The DORIS campaigns in East Antarctica 2.1. Site installation and remote control Figs. 1 and 2 show details of the location of the DORIS beacon at the Mellor and Sorsdal glaciers in Antarctica, in comparison to the location of other geodetic stations on the continent. Table 1 lists the permanent GPS and DORIS geodetic Antarctic sites at the epoch of the campaigns, which includes eight GPS sites of the International GNSS Service (IGS, Dow et al., 2005) and three DORIS beacons (Fagard, 2006; Amalvict et al., 2007). Although there are other permanently operating GPS stations in Antarctica, these do not operate under the auspices of the IGS and hence are not shown (Bouin and Vigny, 2000). As is standard with all DORIS installations, the beacon is programmed with the necessary parameters (identification number, transmission sequence and the satellites to be observed) as supplied for the tracking configuration (Coutin-Faye, 2000). The beacon also needs to be aligned to International Atomic Time (TAI). Identical to any station of the permanent network, the beacon transmits a signal at two radio frequencies (401.25 and 2036.25 MHz) for the duration of the pass of the DORIS-equipped LEO satellites. Doppler data are available at SSALTO1 (Jayles et al., 2006), the operational processing center, within a few hours of the satellite pass being observed. Any observed anomaly could immediately be transmitted to the operation manager for the field experiment at the Davis Antarctic base. If necessary, parameters in the beacon program could be changed. 2.2. Sorsdal glacier site (SORB) The Sorsdal glacier is 31 km. from the Davis base. The DORIS antenna was placed on a zone that is affected by complex fracture patterns and anomalously high flow rates (Patrick et al., 2003), at an elevation of approximately 1 Aviso Newsletter N7 http://www-aviso.cnes.fr:8090/HTML/information/ publication/news/news7/coutin-faye_uk.html.

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Fig. 1. General map of Antarctica and of the Amery Ice Shelf, also indicated the location of GPS and DORIS antennas at the time of observations (see Table 1) and the Mellor and Sorsdal glaciers.

During the second Sorsdal campaign only two satellites were tracked providing 8 days of continuous observations until a strong katabatic wind dislodged the antenna. After a few weeks interruption the observations resumed for a further 30-day period (Table 2). 2.3. Mellor glacier site (MLLR)

Fig. 2. Location of DORIS antenna (star) on the Mellor Glacier, lightly above the confluence with the Lambert glacier, hash lines indicates the grounding line (from ERS, B. Legresy personal com.).

440 m. During the first Sorsdal campaign, from December 2001 to January 2002, SPOT-2, SPOT-4, TOPEX/Poseidon and Jason-1 satellites were available. As per the data acquisition plan, the DORIS instrument onboard the three satellites, SPOT-2, SPOT-4 and TOPEX/Poseidon and the Sorsdal beacon were programmed accordingly. In comparison with the expected number of passes, a substantial part of the data was lost. It was subsequently learned that only part of the DORIS signals were being received due to the low voltage; resulting in the beacon’s batteries being completely discharged. The power supply of the deployed beacon could not accommodate both the large number of available passes and the 15 min time period allocated to warm the ultra stable internal oscillator. In spite of this technical problem, 26 days of observations were obtained over the 2 months of the first IDS Antarctic campaign. The summary of the processed DORIS data is given in Table 2.

Three outlet glaciers carry most of the flow from the interior to the Amery Ice Shelf (AIS). The Mellor, Lambert and the Fisher glaciers merge in a zone which is heavily crevassed at the west Mawson escarpment (Fig. 2). The DORIS antenna was installed in the Mellor glacier drainage basin near the confluence with the Lambert glacier; at the center of the stream. It is located on a grounding zone 673 km from the Davis Australian base at an elevation of about 500 m, (Fig. 1) and approximately 30 km off the floating ice limit of the AIS, as redefined by Fricker et al. (2002). Fig. 2 clearly shows the associated ice flow lines. As shown in Table 2, 8 days of SPOT-4 and SPOT5 data was observed from 12 to 19 January 2003. 3. Data processing Initially, the data from the permanent network (Fagard, 2006; Noll and Soudarin, 2006) were used to determine the high precision orbits of the DORIS satellites (SPOT-2, SPOT-4 and SPOT-5) over the period of the Antarctic campaigns. The orbit determination analysis used the NASA GEODYN orbit determination and geodetic parameter estimation program (Pavlis et al., 2006). The GGM01S GRACE-derived earth gravity field model (Tapley et al., 2004) was applied to degree and order 90. The GOT 99 ocean tide model (Ray, 1999), was used and earth tide modelling followed the conventions as applied in EGM96 (Lemoine et al., 1998), including k2 = 0.30, k3 = 0.093, and special modelling for the Free Core Nutation (FCN). The non-conservative forces (atmospheric

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Fig. 3. Davis base, Sorsdal glacier and location of DORIS antenna.

Table 1 DORIS and GPS site identification at the time of observations. Station

Location

Latitude (°S)

Longitude (°E)

GPS stations OHI2 VESL SYOG MAW1 DAVR DAV1 CAS1 MCM4

O’Higgins Sanae IV E. Ongle Island Mawson Davis Davis Casey McMurdo

63.321 71.674 69.007 67.605 68.577 68.577 66.283 77.838

302.099 357.158 39.584 62.871 77.972 77.972 110.520 166.669

DORIS stations ADEA ADEB ROTA SYPB SORB SOSB MLLR

Terre Ade´lie Terre Ade´lie Rothera Syowa Sorsdal 2001 Sorsdal 2003 Mellor 2002

66.665 66.665 67.571 69.005 68.637 68.637 73.394

140.002 140.002 68.127 39.579 78.718 78.718 66.699

Type

In operation Dec-01

Dec-02

Dec-03

P P P P P P P P

N N Y Y N Y Y Y

Y Y Y Y Y Y Y Y

Y Y Y Y Y Y Y Y

P P P P C C C

Y N Y Y Y N N

N Y Y Y N N Y

N Y Y Y N Y N

P, Permanent station; C, Campaign (temporary) station.

drag, solar radiation pressure, and planetary radiation pressure) were computed using satellite-specific macromodels and an attitude model (e.g. Milani et al., 1987; Marshall and Luthcke, 1994). The satellite specific macromodels for SPOT-4 and SPOT-5 were provided by the CNES through the IDS data centers. The SPOT-2 macromodel was devel-

oped at the University of Colorado (Gitton and Kneib, 1990) and was applied during the development of the JGM-1, JGM-2 and EGM96 gravity models (Nerem et al., 1994; Lemoine et al., 1998). The accelerations due to drag were computed using the MSIS86 atmospheric density model (Hedin, 1988). The ITRF2000 coordinates for

R. Govind et al. / Advances in Space Research 45 (2010) 1523–1534 Table 2 DORIS data acquisition during the three Antarctic campaigns.

tions were separately generated using GEODYN (Pavlis et al., 2006). These were subsequently combined as normal equations with the NASA SOLVE program, a companion program to GEODYN (Ullman, 1992).

Campaign

Period of usable data

Days with computed observations

Total DORIS dataset (no of satellite passes/ no of observations

Sorsdal 2001–2002

18 Dec-2001 to 13 Jan 2002

20

SPOT-4: 63/1205

4. Results

Mellor 2002–2003

12 Jan 2003 to 19 Jan 2003

8

SPOT-4: 64/1860

4.1. Sorsdal 2001/2002

SPOT-5: 23/290 Sorsdal 2003–2004

1 Dec 2003 to 8 Dec 2003

8

SPOT-4: 24/653 SPOT-5: 29/1191

2 Jan 2004 to 23 Jan 2004

22

SPOT-4: 122/2101 SPOT-5: 147/5130

Comments: 1. Sorsdal 2001/2002: Power supply deficiency because too many satellites were tracked: Clock offset. 2. Mellor 2002/2003: Nominal operations after full beacon initialization. 3. Sorsdal 2003/2004: Interruption due to wind storm (Dec 8). Data collection resumes on January 02, 2004.

the global DORIS network were used (Altamimi et al., 2002). Site deformation due to ocean tide loading was applied to determine the instantaneous position of the station antenna using the GOT 99 ocean tide model (Ray, 1999). The IERSCO4 values for the Earth Orientation Parameters were adopted (Gambis, 2004). The effect due to tropospheric refraction (wet and dry) on the Doppler measurements was computed using Goad and Goodman (1974) mapping function, derived from Hopfield (1971). The estimated arc parameters for the orbit determination included the state vector for each 7-day arc, a drag coefficient at 8-hourly intervals for SPOT-2 and SPOT-4, and at 6-hourly intervals for SPOT-5, daily empirical once-per-rev acceleration terms along-track and crosstrack, and a pass-by-pass Doppler measurement bias and tropospheric scale factor in the zenith direction. The satellite specific macromodels for solar radiation pressure, stated above, were directly used and no estimate of a scale factor was undertaken. The typical RMS of fit for the weekly orbit determination solution was 0.5 mm/s for the DORIS Doppler data. The a priori data weight for the observations was 2 mm/s, or roughly 4 times the typical DORIS data fits for all satellites. The estimated SPOT satellite orbits, from the above step, were held fixed while the daily positions of the campaign beacons were estimated. As with the data from the global network, pass-by-pass Doppler and troposphere biases were estimated for the beacons. For the Mellor campaign, the larger signal (1 m/day) and the availability of data from both SPOT-4 and SPOT-5, made it possible to also estimate daily velocities in addition to the daily positions. The daily position estimates (and velocity for the Mellor beacon) were iterated to convergence using GEODYN. For the Mellor beacon, in addition to the daily data-level combination, SPOT-4 and SPOT-5 normal equa-

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The data from the Sorsdal 2001/2002 are the weakest of the three DORIS campaigns since only one satellite (SPOT-4) produced usable data. Although the data spanned 26 days, solutions were estimable on only 20 days due to the availability of data or anomalies in the processing. The estimated daily positions have uncertainties of 11 and 13 cm (for North and Height) and 24 cm (for East) (see Table 3). The larger uncertainty in the eastern direction is a consequence of the polar orbit geometry of SPOT-4 and the fact that daily cross-track positioning is more difficult if the beacon is tracked only once or twice per day. The beacon velocities were computed from a weighted least squares fit, taking into account the daily position uncertainties, and were determined to be 11.4 ± 0.2 cm/day North and 27.9 ± 0.4 cm/day East, that is, a net velocity of 30 ± 0.4 cm/day with an azimuth N246°E ± 1° (see Fig. 4) for the 26-day observation period. However, the least squares fits to subsections of the daily beacon position estimates, as seen in Table 3, show that the velocities have decreased from 30 ± 2 (azimuth N245°E ± 2.5°) to 18 ± 3.5 cm/day (azimuth N250°E ± 1°) between the beginning and at the end of the 26-day observation period. 4.2. Sorsdal 2003/2004 During the second Sorsdal occupation, the DORIS beacon was tracked by two satellites (SPOT-4 and SPOT-5), and therefore the daily position determination was improved. Daily position uncertainties of 3–4 cm for each component were achieved (a factor of 3–6 improvement over the first experiment). Due to the interruption of data collection by a severe storm, two solutions were determined; one spanning 8 days from December 1, 2003 and the second solution spanning 21 days from January 2, 2004 (see Table 4). The total horizontal velocities are 5.7 ± 0.8 cm/day (azimuth N264°E ± 7.5°) for the December 2003 DORIS data, and 11.4 ± 1.4 cm/day (azimuth N241°E ± 1.5°) for the January 2004 DORIS data, a factor 3–5 less than the total horizontal velocity of 30 ± 0.4 cm/day (azimuth N246°E ± 1°) found during the first Sorsdal occupation (see Table 4 and Fig. 5). The difference in the recovered glacial velocity when compared to the first experiment is quite surprising, and one possibility is that indeed the Sorsdal glacier was flowing slower during the second campaign in 2003/2004 compared to 2001/2002. However, the initial beacon positions differed by 1088 m (in ITRF2000 coordinates), and it is possible that the local topography at the two beacon locations alone was respon-

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Table 3 Sorsdal 2001/2002 Campaign Results (N : North, E : East, Height: H, Az: Azimuth). Estimated Cartesian positions (ITRF2000)

Start of campaign (2001-12-18) X: 455904.796 ± 0.098 m Y: 2285266.698 ± 0.052 m Z: 5917765.123 ± 0.043 m

Cumulative displacement

2001-12-18 to 2002-01-13 (26 days) N: 3.011 ± 0.128 m E: 6.773 ± 0.223 m Ht: 0.353 ± 0.087 m

Average daily position uncertainties

N: 11.4 cm; E: 24.3 cm; Ht: 12.5 cm

Average total observations per day

SPOT-2: 64 obs.

Inferred horizontal velocity from weighted least squares fit – divided into three segments over the observing period

2001-12-18 to 2002-01-13 (all 26 days) N: 11.4 ± 0.2 cm/day E: 27.9 ± 0.4 cm/day Ht: 1.1 ± 0.2 cm/day 2D Vel: 30 ± 0.4 cm/day, Az: N246°E ± 1° 2001-12-18 to 2001-12-27 (subset-1) N: 12.6 ± 0.8 cm/day E: 27.7 ± 1.9 cm/day Ht: 0.1 ± 1.3 cm/day 2D Vel: 30 ± 2 cm/day, Az: N246°E ± 2.5° 2001-12-29 to 2002-01-04 (subset-2) N: 10.3 ± 1.6 cm/day E : 24.5 ± 2.7 cm/day Ht: 1.3 ± 2.7 cm/day 2D Vel: 26 ± 3 cm/day, Az: N237°E ± 4.5° 2002-01-05 to 2002-01-13 (subset-3) N: 5.7 ± 1.8 cm/day E : 17.2 ± 3.0 cm/day Ht: 3.1 ± 3.0 cm/day 2D Vel: 18 ± 3.5 cm/day, Az: N250°E ± 1°

sible without the need to invoke other explanations. From Fig. 5, we also discern that the beacon motion was non-linear during January 2004. The non-linearity of the beacon motion in the horizontal component could describe a meandering motion of the glacier track. Since this paper aims to demonstrate the capability of DORIS to monitor the kinematics of glaciers at a high temporal resolution, any further discussion on the glaciological aspects of the geodetic results are beyond the scope of this paper. The residuals from a weighted least squares fit are 8.4 cm (north), 13.8 cm (east) and 10.1 cm (height). When the January 2004 Sorsdal data were divided into three segments (Jan. 2–8, 2004; Jan. 9–15, 2004; and Jan. 16–23, 2004), there was a noticeable velocity decrease of the beacon. As shown in Table 4, the horizontal velocities decrease from 18.3 ± 0.7 (azimuth N245°E) to 11.2 ± 0.7 (azimuth

Fig. 4. DORIS antenna position changes from daily solutions – Sorsdal glacier, 2001-12-18 to 2002-01-13.

N264°E ± 1°) to 7.4 ± 0.9 (azimuth N239°E ± 12°) cm/day for each respective segment. It can therefore be concluded that there is about a factor of 2.5 in the velocity differences of the Sorsdal beacon between the beginning and the end of the experiment in January 2004. As shown in Fig. 5, the analysis also showed a change in the height of the beacon of over one meter over 21 days in 2004. The recovery of the absolute height change with daily vertical position uncertainties of only 3–4 cm (using only two DORIS satellites) is highly encouraging, but leaves us with a puzzle. The possible explanations would include flow around local topography, local snowmelt over 21 days, or a simple tilt of the antenna over the course of the experiment. Since the local support team did not report any significant tilt in the antenna, we would tend to exclude this possibility, however we cannot completely rule out any tilt (or sinking) of the DORIS beacon. 4.3. Mellor glacier (2003) From the combination of the SPOT-4 and SPOT-5 data, seven solutions of the daily position of the DORIS antenna were determined; as shown of in Fig. 6. The effective mean horizontal velocity of the Mellor glacier is determined as 104 ± 26 cm/day (azimuth N34°E ± 0.13°). There is also a notable decrease in the daily estimated horizontal velocity between the beginning and end of the observation period; and a daily change in the direction of its flow. This may imply a meandering motion, although the time series is too short to draw any firm conclusions (see Table 5 and Fig. 6). 4.4. Validation and assessment of the DORIS solutions In order to establish a measure of confidence, reliability and validity of the results of the (single day) point positioning technique employed in this study, the computation technique was replicated for the permanent IDS station

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Table 4 Sorsdal 2003/2004 Campaign Results (N : North, E : East, Height: H, Az: Azimuth). Estimated Cartesian positions (ITRF2000) Start of campaign (2003-12-01) X: 456161.462 ± 0.038 m Y: 2286250.211 ± 0.019 m Z: -5917376.563 ± 0.024 m Cumulative displacement (2003-12-01 to 2003-12-08, 8 days) N: 0.050 ± 0.043 m E: 0.484 ± 0.080 m Ht: 0.009 ± 0.049 m Avg. daily position uncertainties: (Dec. 2003 and Jan. 2004)

After system restart (2004-01-02) X: 456163.677 ± 0.043 m Y: 2286249.153 ± 0.033 m Z: -5917376.779 ± 0.035 m (2004-01-02 to 2004-01-23, 21 days) N: 1.091 ± 0.073 m E: 2.005 ± 0.106 m Ht: 1.000 ± 0.079 m N: 3.0 cm; E: 4.4 cm; Ht: 3.4 cm

Average total observations per day

SPOT-4: 104 obs.; SPOT-5: 201 obs.

Inferred horizontal velocity from weighted least squares fits – divided into four segments over the observing period: one segment in Dec. 2003, and three segments in Jan. 2004

2003-12-01 to 2003-12-08 (8 days) N: 1.2 ± 0.4 cm/day E: 5.6 ± 0.7 cm/day Ht: 0.4 ± 0.5 cm/day 2D Vel: 5.7 ± 0.8 cm/day, Az: N264°E ± 7.5° 2004-01-02 to 2004-01-23 (21 days) N: 5.3 ± 0.1 cm/day E: 10.1 ± 1.4 cm/day Ht: 4.8 ± 0.1 cm/day 2D Vel: 11.4 ± 1.4 cm/day, Az: N241°E ± 1.5° 2004-01-02 to 2004-01-08 (7 days) N: 8.2 ± 0.4 cm/day E: 16.4 ± 0.6 cm/day Ht: 8.8 ± 0.5 cm/day 2D Vel: 18.3 ± 0.7 cm/day, Az: N245°E ± 1.3° 2004-01-09 to 2004-01-15 (7 days) N: 6.0 ± 0.4 cm/day E: 9.5 ± 0.6 cm/day Ht: 6.5 ± 0.5 cm/day 2D Vel: 11.2 ± 0.7 cm/day, Az: N240°E ± 0.2° 2004-01-16 to 2004-01-23 (8 days) N: 2.5 ± 0.5 cm/day E: 7.0 ± 0.8 cm/day Ht: 1.3 ± 0.6 cm/day 2D Vel: 7.4 ± 0.9 cm/day, Az: N239°E ± 12°

Terre Ade´lie (ADEB) in Antarctica (Table 1); using the identical SPOT-4 and SPOT-5 only data, as for the Mellor Glacier campaign for the week 2003-01-12 to 2003-01-18. The SPOT-4 and SPOT-5 data used for determining the position and velocity of ADEB are shown in Table 7. In total four different solution types, as shown in Table 8, were undertaken to assess the results obtained from the

Fig. 5. DORIS antenna position changes from daily solutions for Sorsdal glacier 2003–2004: Top: Solutions for December 2003: Bottom: Solutions for January 2004.

point positioning technique which were compared to the daily ITRF2005 (Altamimi et al., 2007) coordinates and velocities. Solution A, which was a seven-day arc solution using all data from the global network and estimating the position of only one station (ADEB) formed the benchmark solution for this one week’s data. Solution B, was identical to Solution A, but in daily arcs. Solution C was the point positioning mode; the satellite orbits were determined using the identical data as in solution B with the exception of ADEB. ADEB coordinates were subsequently estimated by keeping the satellite orbits fixed. In Solution D, the daily velocities for the ADEB station were also estimated in the point positioning mode.

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R. Govind et al. / Advances in Space Research 45 (2010) 1523–1534 Table 7 Validation and assessment of computation technique – ADEB DORIS data used. Date

030112 030113 030114 030115 030116 030117 030118

Fig. 6. DORIS antenna position changes from daily solutions – Mellor glacier, 2003.

Table 5 Mellor 2003 Campaign Results (N : North, E : East, Height: H, Az: Azimuth). Estimated Cartesian positions (ITRF2000) – Start of campaign (2003-01-13)

X: 723317.381 ± 0.124 m Y: 1679413.438 ± 0.077 m Z: 6090438.092 ± 0.089 m

Cumulative displacement: (2003-01-13 to 2003-01-19, 7 days)

N: 6.613 ± 0.190 m E: 4.412 ± 0.108 m Ht: 0.266 ± 0.066 m

Average daily position uncertainties

N: 14.3 cm; E: 8.1 cm; Ht: 4.7 cm

Average total observations per day

SPOT-4: 265 obs. SPOT-5: 41 obs.

Mean 2D velocity and standard deviation about the mean (from 7 daily estimates, m/day) Average daily velocity uncertainties (from the daily GEODYN/SOLVE solutions)

104.2 ± 25.7 cm/day Az N34°E ± 0.13° N: 24.5 cm/day; E: 7.8 cm/ day; Ht; 7.2 cm/day

Table 8 and Figs. 7 and 8 show the deviations from the ITRF2005 coordinates and velocity for each respective solution type. From Table 8, for the point positioning solution C, the mean deviations of the daily estimated coordinates from the ITRF2005 values for ADEB are 14, 7 and 6 mm with an RMS of 15, 22 and 15 mm for the north, east and height components, respectively. From Fig. 7, it is noted that the largest daily deviation for the point positioning solution is approximately 50 mm for the east component on 2003-01-12 and approximately 50 mm for the north component on 2003-01-17. The largTable 6 DORIS and GPS derived velocities of the Sorsdal glacier (Dec. 2001, Jan. 2002) (GPS results from Patrick et al., 2003).

Days between observations. Velocity (cm/day) Azimuth

DORIS

GPS

21 (continuous) 30 ± 0.4 N246°E

65 (2  1 day epochs) 31 N244°E

Number of observations per day per satellite at Terre Adelie, Antarctica SPOT-4

SPOT-5

254 254 254 258 319 255 242

435 440 420 447 408 390 405

est daily deviation from the ITRF2005 in the height component is 30 mm on 030118. In comparison to the average daily position uncertainties obtained for the Mellor campaign as shown in Table 5, the mean deviations of the ADEB coordinates from the ITRF2005 values are better by one magnitude in all components. These comparisons are similar for the Sorsdal 2001/2002 campaign (Table 3). However, comparisons for the Sorsdal 2003/ 2004 campaign (Table 4) show differences in the daily average position uncertainties of between a factor of 2 (in the north component) and a factor of 5 for the east and height components. The mean of the daily deviation of the directly estimated velocities are 1, 4 and 1 mm per year from the ITRF2005 values; having a RMS 12, 8 and 6 mm per year for the north, east and height component, respectively. The largest daily deviations from the ITRF2005 are 20 and 20 mm per year for the north component on 2003-01-14, 2003-01-16 and 2003-01-18. The inclusion of the mean and RMS of these position and velocity deviations to the uncertainty of the estimated daily Sorsdal and Mellor glacier positions and velocities have no significant effects on the results. The results for the ADEB position and velocities have established that the DORIS point positioning technique is valid and reliable for determining glacier velocities. 4.5. Comparison to GPS solutions Between 1995 and 1998, the University of Tasmania conducted five seasons of GPS campaigns (Manson et al., 2000) in order to determine ice surface velocities over the Lambert glacier. More than seventy sites were observed all around the drainage basin. The average velocities of the ice flowing towards the Antarctic sea are approximately 40 m/yr (11 cm/day) in the outlet glacier with a maximum of 65 m/yr (17 cm/day). The DORIS absolute velocity for the Mellor glacier observed during the week of 13–19 January 2003 is much higher at 104 ± 26 cm/day. However, the DORIS site is located in the core of the Mellor glacier basin where the ice flow is amplified compared to the AIS periphery where the GPS stations were located. During the first Sorsdal occupation, the DORIS antenna was surveyed using GPS both at the time of deployment

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Table 8 Validation and assessment of computation technique – results. Terre Ade´lie Antarctica (ADEB) (030112–030118) solution type

Estimated position deviation from ITRF (mm) North Mean

East RMS

Height RMS

RMS

ADEB position estimated in one 7-day arc within the fixed global network

B

ADEB position estimated for seven 1-day arcs within the fixed global network

14.0

14.9

4.6

23.2

5.2

13.5

C

ADEB position estimated for seven 1-day arcs in point positioning modea

13.8

15.1

6.7

22.3

5.5

14.6

a

ADEB position and velocity estimated for seven 1-day arcs in point positioning mode

28.4

Mean

A

D

0.2

Mean

5.5

17.1 40.3 5.6 19.7 8.1 13.1 Estimated velocity deviation from ITRF (mm/year) 1.1 12.2 4.1 7.6 1.0 5.6

a Point positioning mode = (1) Orbit determination with global DORIS network excluding ADEB; (2) ADEB only data and fixed orbits for position and velocity estimates.

and when the equipment was retrieved (Patrick et al., 2003). The comparison results are shown in Table 6. There is a good agreement between the DORIS and GPS ice-flow rate. The GPS velocity estimate with two single session occupations 65 days apart was 31 cm/day with an orientation of N244°E. This is in good agreement with the effective velocity of 30 cm/day with an orientation of N246°E for the DORIS solution; having 26 days of continuous data from the SPOT-2 satellite only.

The interferometric analysis of pairs of coherent SAR images produces a dense set of velocities of the ice sheet. During the 1997 and 2000 RADARSAT Antarctic Missions (AMM-1 and -2), velocity maps for the whole AIS were established with a few m/yr accuracy at least in the grounding zone (Joughin, 2002; Young and Hyland, 2002). Both determined the same flow regime of the ice in the Mellor and Lambert glaciers. The velocity increases from the interior at 100 m/yr (27 cm/day) and reaches a maximum at the confluence to around 800 m/yr (219 cm/day). Along the center-line of the flow band and upstream to the grounding zone, the velocity increase follows the surface elevation derived from satellite radar altimetry measurements (Fricker et al., 2000).

The DORIS antenna is located at the maximum centerline of the flow band but at the border of the Mellor basin drainage where the InSAR velocity is between 110 and 137 cm/day. King et al. (2007) established that a geographically correlated bias of between ±8 and 11 cm/day existed between the InSAR velocities of Joughin (2002) and Young and Hyland (2002) in comparison to the Terrestrial/GPS determined velocities in the AIS. Although this study was confined to the northern AIS, the identified error sources contributing to this bias, in particular the measured slant ranges, can be considered to be common to the entire RADARSAT solution. That is, a horizontal positional error of 20 m could also infer that this bias is probably present in the InSAR derived velocities of the Mellor glacier. The DORIS-determined velocity of 114 cm/day can then be considered to be in good agreement with the InSAR results. Also, any changes in the motion of the Mellor glacier between the RADARSAT Antarctic missions AMM-1, AMM-2 and the 2003 DORIS campaign are unaccounted. Whereas the DORIS antenna position is determined at least daily (with a potential for sub-daily estimates), the temporal resolution of the InSAR solutions are restricted by the 24-day repeat cycle of RADARSAT, that is only one possible solution every 24 days for ice velocities between 20 and

Fig. 7. Terre Ade´lie – Estimated daily coordinate deviation from ITRF2005 values.

Fig. 8. Terre Ade´lie – Estimated daily velocity deviation from ITRF2005 values.

4.6. Comparison to InSAR

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30 cm/day. Although the areal extent of the InSAR solutions are significantly larger; 911  687 km for the Joughin (2002) solution, the use of the “speckle tracking” method for the higher velocity of Mellor glacier results in a spatial density of the ice velocity of 1 km. At this time and spatial resolution, the actual daily meandering or non-linear path of the ice flow is then not visible. 5. Discussion 5.1. Lessons learned from station implementation Through these three occupations at the two glaciers in Antarctica, it has been demonstrated that the DORIS system is capable of monitoring glacier displacement over a period of several weeks. The potential exists to determine glacier velocities at a high temporal resolution from multiple daily passes of the growing number of DORISequipped satellites. The main limitations that emerged from these experiments were due to power and beacon stability. Complimentary power systems using both solar panels and wind generators could provide long term, autonomous operation of the beacon programmed to track multiple satellites. Secure installation and stability of the antenna over a long period in the harsh Antarctic environment also need to be addressed; especially reducing the risk of the antenna being blown over by katabatic winds. Over the last several years glaciologists have improved techniques to prevent equipment failure due to severe weather conditions and any future DORIS site installation could benefit from these experiences. 5.2. DORIS versus GPS The main difference between the two systems is the direction of the radio-signal links. With GPS the signal comes from the GPS constellation and consequently the data are managed at the receiver level. Data retrieval requires regular site revisits or an autonomous data relay system. In contrast, the DORIS uplink system provides an autonomous and centralised system that collects and processes the data several times a day (Valette et al., 2000). In principle, it requires less logistics and is appropriate for long term campaigns in remote locations. The DORIS-determined positions and velocities can be delivered to the user within a day. However, DORIS presents some limitations. The power consumption of a DORIS beacon is about twice or more that of a GPS receiver. A disadvantage of DORIS when compared to GPS, for this current study, is the inability to undertake densification of observing stations due to the narrow band frequency signal and the risk of inter-beacon interference. Also currently, its does not have the ability to undertake multi-satellite/multi-station simultaneous observations as is the strength and general practice for GPS. However, beginning with Jason-2, the installation of the new generation DGXX receiver, which can track up to seven beacons simul-

taneously, on all subsequent DORIS-equipped satellites overcome the current limitations of station spacing (interference) and multi-satellite/multi-station tracking (Mercier et al., 2010). 5.3. DORIS contribution to radar techniques DORIS absolute positioning and velocity may also be used to calibrate space radar techniques such as SAR interferometry and altimetry. The radar interferograms provide relative surface velocity and DORIS is potentially of interest for providing the absolute reference velocity. Differencing interferograms also gives access to the ice surface topography that needs absolute calibration. Moreover, the radar signal differently penetrates the covering snow depending on its composition and its characteristics (Legresy and Remy, 1997). DORIS is able to furnish an altimetric reference for inland Antarctica, should a beacon be deployed there. Absolute ice flow may also be obtained. Similarly, a DORIS beacon could also be emplaced at the intersection of ground tracks for CRYOSAT-2 and contribute absolute calibration for altimeter satellites (albeit at one discrete point) at the cm level. 6. Conclusion In order to demonstrate the capability of the DORIS system to continuously monitor the velocity of ice sheets, three observation campaigns were undertaken during the austral summers between 2001 and 2004; at the Sorsdal Glacier, which flows into the Prydz Bay and Mellor Glacier, which flows into the Amery Ice Shelf, in East Antarctica. These pilot experiments were led by Geoscience Australia and performed under the auspices of the IDS. The velocities of these glaciers were derived from the daily estimates of the antenna positions determined from observations to the SPOT-2 (Sorsdal, 2001) and SPOT-4 and SPOT-5 (Mellor and Sorsdal, 2004) satellites. For the Sorsdal 2001 campaign, the effective horizontal glacier velocity over the 26-day observing period was 30 ± 0.4 cm/day (azimuth N246°E ± 1°). However, subsets of the observing period (10, 7 and 9 days duration) show a decrease in the velocity from 30 ± 2 (azimuth N245°E ± 2.5°) to 18 ± 3.5 cm/day (azimuth N250°E ± 1°). Similarly, for the Sorsdal 2003/04 campaign, the estimated effective horizontal velocity was 5.7 ± 0.8 cm/day (azimuth N264°E ± 7.5°) for the first 8 days (December 2003) of the observations. For the subsequent 21 days, the effective overall horizontal velocity increased to 11.4 ± 1.4 cm/day (azimuth N241°E ± 1.5°). However, 7-, 7- and 8-day segments of January 2004 show a decrease in velocity from 18.3 ± 0.7 (azimuth N245°E ± 1.3°) to 7.4 ± 0.9 cm/day (azimuth N239°E ± 12°). The azimuth changes of the glacier motion for the respective segments show a meandering motion of the glacier. This may be due to the surrounding topography, changes in the friction between ice and the underlying terrain, accumulation of ice

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mass in the direction of motion of the glacier as it slows down. The velocity of the Mellor glacier was determined to be 104 ± 26 cm/day (azimuth N34°E ± 0.13°). To optimize data acquisition, a wind generator could be a solution for future campaigns replacing the solar panels and providing possible multi-year continuous observations. The advantage of DORIS compared to GPS is the much simplified data acquisition logistics and the ability to provide continuous daily solutions for position and velocity. However, due to the current limitations imposed by the station spacing requirements of the DORIS system, GPS remains the choice for densification of geodetic networks. DORIS however, may still contribute for high resolution monitoring of glacier motion and for radar instrument calibration in an absolute height reference frame. In light of these successful campaigns at the Sorsdal and the Mellor glaciers, we may envisage the deployment of DORIS beacons on floating ice shelves to aid in the improvement of tide models around Antarctica, as a complement to similar field campaigns undertaken with GPS (King and Aoki, 2003; Zhang and Andersen, 2006). Any further comparison or calibration of the three techniques (DORIS, GPS and InSAR), is limited by the availability of simultaneously observed data in Antarctica. Although only single comparisons were possible between DORIS-GPS (Sorsdal) and DORIS-InSAR (Mellor), a mutual validation of the results of these techniques has been achieved. It is suggested that a repeat experiment of a long-term occupation, designed to track as many DORIS-equipped satellites as possible in order to achieve the highest signal-to-noise ratio, could obtain position estimates approaching 1 cm. Further improvements in troposphere modelling are possible using a more recent mapping function such as Niell (1996) or GMF (Boehm et al., 2006). The orbit determination analysis for the DORIS satellites and hence the daily positioning of the campaign beacons could also benefit from the inclusion of other satellite tracking data (Satellite Laser Ranging or GPS) for those satellites tracked by multiple geodetic techniques.2 Acknowledgements We acknowledge the International DORIS Service (IDS) for providing the data from the DORIS global network (Tavernier et al., 2006), and the Centre National d’Etudes Spatiales (CNES) and the International DORIS Service (IDS) for providing the Version 2.0 beacon for this experiment to Geoscience Australia. The Service d’Installation et de Maintenance des Balises (SIMB), of the Institut Ge´ographique National (IGN) provided the training for the equipment operation. Gary Johnston, Ben Patrick and Adrian Corvino undertook the installation of the DORIS beacon in Antarctica. The Davis base team are 2

The Envisat and (future) CRYOSAT-2 satellites are also tracked by Satellite laser ranging (SLR) in addition to DORIS ; The Jason-1 and Jason-2 satellites are tracked additionally by SLR and GPS.

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