1. Introduction to geodesy & accelerometry with Swarm
Christian Siemes and the
ACC/GPSR Quality Working Group
Eelco Doornbos
Jose van den Ijssel
Joao de Teixeira da Encarnação
Aleš Bezděk
Radek Peřestý & VZLU team
Heike Bock
Sean Bruinsma
Third Swarm Science Meeting Copenhagen, Denmark
20/06/2014

2. Introduction to geodesy & accelerometry with Swarm
Consider S/C as proof-mass
Orbit is shaped by gravitational
and non-gravitational forces

3. Introduction to geodesy & accelerometry with Swarm
Consider S/C as proof-mass
Orbit is shaped by gravitational
and non-gravitational forces
Earth’s gravity field
g00 ~ 101 m/s2
g20 ~ 10-2 m/s2
grest ~ 10-4 m/s2
All other ~ 10-6 m/s2 and smaller
Non-gravitational acceleration ~ 10-7 m/s2

4. Introduction to geodesy & accelerometry with Swarm
Consider S/C as proof-mass
Orbit is shaped by gravitational
and non-gravitational forces
GPSR: Determine orbits
 presentation by Jose vd IJssel
Accelerometer: Measure
non-gravitational acceleration

5. Introduction to geodesy & accelerometry with Swarm
Consider S/C as proof-mass
Orbit is shaped by gravitational
and non-gravitational forces
GPSR: Determine orbits
 presentation by Jose vd IJssel
Accelerometer: Measure
non-gravitational acceleration
Determine the gravity field
Study thermospheric density and wind  presentation by Eelco Doornbos

6. Introduction to geodesy & accelerometry with Swarm
Swarm A, B, and C
High-low satellite-to-satellite
tracking using GPS
Swarm A and C
Inter-satellite baseline
processing using GPS

7. GPSR Dual-frequency GPS receiver that can track up to 8 GPS satellites
L1b data has been released to all users
Higher noise level near geomagnetic poles and at South Atlantic Anomaly
(still within specification, most likely due to ionospheric scintillations)
SLR validation confirms high quality of L2 orbits for Swarm A, B and C
Reduced dynamic orbit shows 0–3 mm mean and RMS = 2.0 cm
Kinematic orbits fit the reduced-dynamic PSO solutions to ~10 cm
Lucky coincidence
Swarm A and C track simultaneously since 2 March 2014
(~0.2 µs, last checked on 31 May, chances: 1:9)
Swarm C is flying in constellation with Swarm A since mid April
Running investigations following QWG recommendations
Enlarging mask that limits the antenna field-of-view (FoV) to 80 degrees
Increase data rate from 0.1 Hz to 1 Hz (RINEX files)
Synchronization GPS measurement epochs across satellites (simultaneous tracking)
Credits:
Jose vd IJssel
Credits:
Heike Bock

8. GPSR First gravity field results based on data of two months Credits:
Heike Bock, Adrian Jäggi, Ulrich Meyer (Astronomical Institute, University of Bern)
Christoph Dahle (GFZ, German Research Centre for Geosciences, Potsdam)

9. Accelerometer Measurement principle
Accelerometer rigidly connected to satellite
Cubic proof-mass free-floating in cavity (no connection to cavity)
Position of proof-mass controlled by electrostatic forces applied through 6 electrodes located on the inner walls of the cavity
Control voltages applied to electrodes are representative for forces acting on satellite

10. Accelerometer Specifications of the instrument
Measurement band – 100 mHz
Linear acceleration range ±1 x 10-4 m/s2
Linear acceleration resolution x 10-9 m/s2
Angular acceleration range ±9.6 x rad/s2
Angular acceleration resolution x 10-7 rad/s2
Overall random error (1 sigma) x 10-9 m/s2
Accuracy of linear acceleration better than 0.2%
No dedicated temperature control (conscious decision in mission design)
Raw accelerometer data is sensitive to on-board temperature variations
On-ground characterization of temperature dependency
Full testing of performance not possible on-ground
Instrument is designed for micro-g environment
Control voltages do not have the capability to lift proof-mass in 1-g environment

11. Accelerometer status
All accelerometer instruments are working

12. Accelerometer status
All accelerometer instruments are working, however:
Unexpectedly large temperature dependency
Instrument bias discontinuities (steps) and other disturbances (spikes, pulses)
Swarm C: Along-track acceleration
Swarm B: ACC temperature
versus along-track acceleration

13. Accelerometer status
First promising results for temperature correction model (work in progress)
The heat transfer model relates temperature measured at sensor locations to temperature at sensitive points
Credits:
Radek Peresty & VZLU team

14. Accelerometer status Swarm B Next talk: Comparison of Swarm A and C
First promising results for temperature correction model (work in progress)
Swarm B
Next talk:
Comparison of
Swarm A and C
Credits:
Eelco Doornbos

15. Accelerometer status
Example for steps & other
Swarm C

16. Accelerometer status Example for steps & other
Swarm C: Nadir acceleration

17. Accelerometer status
Example for steps & other
AOCS thruster firings

18. Accelerometer status
Example for steps & other ?

19. Summary GPSR L1b data released to general users High quality L2 orbits
Consistent performance of Swarm A, B, and C
Accelerometer
Swarm C provides significantly better accelerometer data than Swarm A and B
Large temperature dependency observed on Swarm A and B
New temperature correction gives promising first results, in particular for along-track axis (ongoing investigations)
Accelerometer data contains unexpectedly large number of steps and other disturbances that are not understood
Physical cause not clear
Treatment of steps under investigation
Accelerometer data not yet ready for general user community (release date unclear)
Industry, science and ESA are working hard to solve the problems!