Impact of tracking loop settings of the Swarm GPS receiver on gravity field recovery

Abstract The Swarm mission consists of three identical satellites equipped with GPS receivers and orbiting in near-polar low Earth orbits. Thus, they can be used to determine the Earth’s gravity field by means of high-low satellite-to-satellite tracking (hl-SST). However, first results by several groups have revealed systematic errors both in precise science orbits and resulting gravity field solutions which are caused by ionospheric disturbances affecting the quality of Swarm GPS observations. Looking at gravity field solutions, the errors lead to systematic artefacts located in two bands north and south of the geomagnetic equator. In order to reduce these artefacts, erroneous GPS observations can be identified and rejected before orbit and gravity field processing, but this may also lead to slight degradations of orbit and low degree gravity field coefficient quality. Since the problems were believed to be receiver-specific, the GPS tracking loop bandwidths onboard Swarm have been widened several times starting in May 2015. The influence of these tracking loop updates on Swarm orbits and, particularly, gravity field solutions is investigated in this work. The main findings are that the first updates increasing the bandwidth from 0.25  Hz to 0.5 Hz help to significantly improve the quality of Swarm gravity fields and that the improvements are even larger than those achieved by GPS data rejection. It is also shown that these improvements are indeed due to an improved quality of GPS observations around the geomagnetic equator, and not due to missing observations in these regions. As the ionospheric activity is rather low in the most recent months, the effect of the tracking loop updates in summer 2016 cannot be properly assessed yet. Nevertheless, the quality of Swarm gravity field solutions has already improved after the first updates which is especially beneficial in view of filling the upcoming gap between the GRACE and GRACE Follow-on missions with hl-SST gravity products.

[1]  H. Bock,et al.  GPS-only gravity field recovery with GOCE, CHAMP, and GRACE , 2011 .

[2]  W. Bosch,et al.  EOT11A - Empirical Ocean Tide Model from Multi-Mission Satellite Altimetry , 2008 .

[3]  Pieter Visser,et al.  Precise science orbits for the Swarm satellite constellation , 2015 .

[4]  A. Bezděk,et al.  Time-variable gravity fields derived from GPS tracking of Swarm , 2016 .

[5]  Frank Flechtner,et al.  Simulating high‐frequency atmosphere‐ocean mass variability for dealiasing of satellite gravity observations: AOD1B RL05 , 2013 .

[6]  H. Lühr,et al.  Swarm An Earth Observation Mission investigating Geospace , 2008 .

[7]  Keith M. Groves,et al.  Specification and forecasting of scintillations in communication/navigation links: current status and future plans , 2002 .

[8]  H. Bock,et al.  GOCE: assessment of GPS-only gravity field determination , 2014, Journal of Geodesy.

[9]  L. Mervart,et al.  The celestial mechanics approach: theoretical foundations , 2010 .

[10]  A. Jäggi,et al.  Gravity field models derived from Swarm GPS data , 2016, Earth, Planets and Space.

[11]  Grzegorz Michalak,et al.  GFZ GRACE Level-2 Processing Standards Document for Level-2 Product Release 0005 : revised edition, January 2013 , 2013 .

[12]  H. Bock,et al.  GOCE: precise orbit determination for the entire mission , 2014, Journal of Geodesy.

[13]  Torsten Mayer-Gürr,et al.  Precise orbit determination based on raw GPS measurements , 2016, Journal of Geodesy.

[14]  Chao Xiong,et al.  The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities , 2016 .

[15]  Franz Zangerl,et al.  SWARM observations of equatorial electron densities and topside GPS track losses , 2015 .

[16]  H. Bock,et al.  Swarm kinematic orbits and gravity fields from 18 months of GPS data , 2016 .

[17]  Oliver Montenbruck,et al.  Spaceborne GNSS-Receiving System Performance Prediction and Validatio , 2014 .

[18]  O. Montenbruck,et al.  Impact of Swarm GPS receiver updates on POD performance , 2016, Earth, Planets and Space.

[19]  Mohammed Mainul Hoque,et al.  Estimate of higher order ionospheric errors in GNSS positioning , 2008 .

[20]  Frank Flechtner,et al.  What Can be Expected from the GRACE-FO Laser Ranging Interferometer for Earth Science Applications? , 2016, Surveys in Geophysics.