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source: https://doi.org/10.7892/boris.72572 | downloaded: 18.8.2021
Astronomical Institute University of Bern
1Astronomical Institute, University of Bern, Switzerland 2German Research Centre for Geosciences, Potsdam, Germany
GRACE Science Team Meeting 2015 Sep 21-23, 2015
Austin, TX
Orbit and Gravity Field Solutions from Swarm GPS Observations
C. Dahle1,2, A. Jäggi1, D. Arnold1, U. Meyer1
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Slide 2 Astronomical Institute University of Bern
Contents
Mission Overview
Swarm Kinematic Orbit Determination SLR Validation Ionosphere disturbances
Swarm Gravity Field Recovery Processing method Presence & mitigation of systematic errors Comparison with GRACE hl-SST Differences in Swarm and GRACE GPS tracking Preview on time-variable Swarm solutions
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Slide 3 Astronomical Institute University of Bern
The Swarm Mission
Orbit: Low Earth Orbiter; near-polar Swarm-A/C: h=460-300 km,
i=87.3°, 1.4° side-by-side sep. Swarm-B: 530 km altitude, i=88° 90° difference in orbital plane
between lower pair and higher satellite after 3 years
© ESA
Payload: Magnetometers, electric field instrument, GPS receiver,
accelerometer, star-trackers, laser retro-reflector Secondary mission objective:
Measuring the Earth’s gravity field (GPS hl-SST observing system)
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Slide 4 Astronomical Institute University of Bern
SLR Validation of Kinematic Swarm Orbits
Mean = 2.7 mm RMS = 32.5 mm
Mean = 1.0 mm RMS = 27.4 mm
Mean = 0.6 mm RMS = 31.1 mm
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Slide 5 Astronomical Institute University of Bern
Ionosphere Disturbances
Random errors: Overall RMS is rather high
Systematic errors: Along geomagnetic equator May be reduced by additional
data screening (dL4/dt criterion)
Dominated by polar areas
RMS of Swarm-A, 320-365, 2014, descending arcs
mm/s RMS of dL3/dt
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Slide 6 Astronomical Institute University of Bern
Processing Method
Celestial Mechanics Approach Orbit, gravity field and stochastic parameters are estimated
simultaneously Kinematic orbit positions are used as pseudo-observations
(weighted with epoch-wise covariance information)
Models: Earth gravity: EGM2008 (up to d/o 90x90) Ocean tides: FES2004
Estimated Parameters: Initial states at beginning of each 24-hour arc Constant empirical accelerations over 24 hours 15-minute piecewise constant empirical accelerations
(constrained) Gravity field coefficients up to d/o 90x90
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Slide 7 Astronomical Institute University of Bern
Bi-Monthly Gravity Field Solutions up to d/o 90
Mar/Apr 2014
Jun/Jul 2014
Nov/Dec 2014
Differences wrt GOCO05S 400 km Gauss smoothing adopted
Original GPS Data Screened GPS Data
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Slide 8 Astronomical Institute University of Bern
Bi-Monthly Gravity Field Solutions Mar/Apr 2014 Jun/Jul 2014 Nov/Dec 2014
Impact of screening the raw RINEX GPS data files (dL4/dt criterion): Difference degree amplitudes are significantly improved, especially for
periods with strong ionosphere conditions (spring, fall).
Very low degrees (n < 15) tend to be weakened due to the very “crude” data screening.
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Slide 9 Astronomical Institute University of Bern
Static Gravity Field Solutions
(Differences wrt GOCO05S, 400 km Gauss smoothing adopted) Systematic signatures along the geomagnetic equator may be efficiently reduced for static Swarm gravity field recovery when screening the raw RINEX GPS data files with the dL4/dt criterion.
Original GPS Data (13 months)
Screened GPS Data (13 months)
Screened GPS Data (18 months)
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Slide 10 Astronomical Institute University of Bern
Static Gravity Field solutions
(Differences wrt GOCO05S) Systematic signatures along the geomagnetic equator cause the artificial “bumps” and may be reduced for static Swarm gravity field recovery when screening the raw RINEX GPS data files with the dL4/dt criterion.
8 months of screened data 13 months 18 months
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Slide 11 Astronomical Institute University of Bern
Comparison with GRACE hl-SST Solutions
Processed data: Dec 2013 – Nov 2014 Swarm-A/C (screened) GRACE-A/B (GPS-only)
(original L1B data)
Worse performance for higher degrees can be explained to some extent by higher orbital altitude, but probably is mainly caused by the higher noise of the Swarm GPS data.
Worse performance for short wavelengths
Results: Similar performance for
long wavelengths
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Slide 12 Astronomical Institute University of Bern
Comparison with GRACE hl-SST Solutions
(Differences wrt GOCO05S, 400 km Gauss smoothing adopted)
Original GPS Data (Swarm)
Original GPS Data (GRACE)
Systematic signatures along the geomagnetic equator are almost not visible in GRACE solutions when using official L1B GPS data.
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Slide 13 Astronomical Institute University of Bern
Number of available Kinematic Positions
Swarm-A, doy 060-090, 2014 (ascending arcs)
For the selected period descending arcs are predominantly affected by
ionosphere disturbances.
Swarm-A, doy 060-090, 2014 (descending arcs)
Missing kinematic positions are found over the geomagnetic poles,
but not along the geomagnetic equator.
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Slide 14 Astronomical Institute University of Bern
Number of available Kinematic Positions
GRACE-B, doy 060-090, 2014 (ascending arcs)
GRACE-B, doy 060-090, 2014 (descending arcs)
For the selected period ascending arcs are predominantly affected by
ionosphere disturbances (nodes happen to be separated by ~180°). Missing kinematic positions are found along the geomagnetic equator
=> problematic signatures cannot be present in the gravity field.
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Slide 15 Astronomical Institute University of Bern
Number of missing Observations in RINEX files
GRACE-B, doy 060-090, 2014 (all arcs)
Significant amounts of data are missing in GRACE L1B RINEX files
=> problematic signatures cannot propagate into gravity field.
Swarm-A, doy 060-090, 2014 (all arcs)
Swarm RINEX files are more complete (gaps only over the poles)
=> problematic signatures do propagate into the gravity field.
L1A
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Slide 16 Astronomical Institute University of Bern
“True” signal: GFZ-RL05a
(DDK5-filtered) “Comparison” signal: GFZ-RL05a
(500km Gauss)
Swarm signal: 90x90 solutions
(Gauss-filtered)
Time-Variable Gravity (Amazon)
Result: Best agreement
for Swarm-C Some outliers to
be investigated
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Slide 17 Astronomical Institute University of Bern
“Comparison” signal: GFZ-RL05a
(500km Gauss)
Swarm signal: 90x90 solutions
(Gauss-filtered)
Result: Very noisy series,
trends are hardly recognized. Not very promising.
Time-Variable Gravity (Greenland)
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Slide 18 Astronomical Institute University of Bern
“True” signal: GFZ-RL05a
(truncated at 10) Swarm signal: 90x90 solutions
(truncated at 10) Nicer, but: It is a point-wise
evaluation of the truncated field, expressed in geoid heights instead of EWH.
Time-Variable Gravity (Greenland)
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Slide 19 Astronomical Institute University of Bern
Summary (1/2)
SLR RMS of 3cm for Swarm kinematic orbits. Ionosphere disturbances affect orbit and gravity
field solutions. GPS data screening for large ionosphere changes
helps to reduce the geomagnetic signatures, but also weakens low degrees.
Very low degree coefficients are of similar quality as from GRACE GPS hl-SST.
Different behavior of GRACE solutions is related to missing GPS data along the geomagnetic equator.
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Slide 20 Astronomical Institute University of Bern
Summary (2/2)
Preview on time-variable signals is encouraging for largest annual signals, but trend estimates seem to remain a challenging task.
For more information, see upcoming paper: A. Jäggi et al, “Swarm kinematic orbits and gravity fields from 18 months of GPS data”, Adv. Space Res. (accepted after minor revision)