Atmospheric fluctuations measured by

the accelerometers onboard GRACE and

their influence on gravity field solutions

Astronomical Institute University of Bern

IAG

Scientific

Assembly

2009

Buenos Aires

Argentina

08.31. – 09.04.

Ulrich Meyer, A. Jäggi, G. Beutlerulrich.meyer@aiub.unibe.ch

August 31, 2009

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

2

Session 2.2

Overview

• Acceleration-differences GRACE B – A

• Thruster pulses� Modeling: raw pulse => L1A => L1B� Estimation of scale factors� Cleaning of accelerations� Effect of pulses on Orbit

• Interpretation of cleaned accelerations for atmosphere sciences

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

3

Session 2.2

Accelerometers onboard GRACE A and B

28 s

GRACE B

GRACE A

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

4

Session 2.2

Accelerations GRACE B-A

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

5

Session 2.2

Thrusters on GRACE

++ --

+

+-

-

1

1

3

3

4

22

4

5

5

6

6

6 pairs of thrusters per satellite

•Accelerometer should be in center of mass

•Thruster pairs should only cause angular accelerations

S

W

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

6

Session 2.2

Thruster Modeling 1Raw => L1A L1A => L1B

•On board: Butterworth Filter

•Preprocessing L1A to L1B:

• Low-Pass Filter (CRN)

• Downsampling to 1 Hz

•Integrated Pulses L1A = L1B

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

7

Session 2.2

Thruster Modeling 2

•Choose isolated pulses from THR1B-Files

•Model raw, L1A and L1B pulses

•Estimate scale, bias and drift per pulse

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

8

Session 2.2

Detail

•Skip badly modeled pulses (RMS of fit > sigma)

•Estimate mean scale factor per thruster pair, axis and satellite

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

9

Session 2.2

Estimated Scale Factors R

GRACE BGRACE A2007

Thr6

Thr5

Thr4

Thr3

Thr2

Thr1

R sigmafactorsigmafactor

±0.005-0.043±0.0060.027

±0.003-0.026±0.0030.026

±0.005-0.043±0.0030.034

±0.0150.032±0.0340.024

±0.006-0.038±0.0060.040

±0.012-0.065±0.0310.034

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

10

Session 2.2

Estimated Scale Factors S

GRACE BGRACE A2007

Thr6

Thr5

Thr4

Thr3

Thr2

Thr1

S sigmafactorsigmafactor

±0.002-0.002±0.0030.017

±0.0010.005±0.0010.008

±0.001-0.009±0.0010.006

±0.0030.000±0.003-0.002

±0.0020.001±0.0020.012

±0.0020.001±0.003-0.006

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

11

Session 2.2

Thruster Modeling 3

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

12

Session 2.2

Cleaned dACC GRACE B-A

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

13

Session 2.2

Cleaned Accelerations GRACE B

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

14

Session 2.2

Residuals of Orbit Adjustment

Integrated and high-pass filtered L1B-pulses

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

15

Session 2.2

Detail

Thruster pulses (mismodeling) cannot explain systematics in residuals!

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

16

Session 2.2

RMS of Orbit Fit

Thruster pulses are real accelerations and have to be considered!

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

17

Session 2.2

Orbital Plane: u and beta

beta

u

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

18

Session 2.2

dACC B-A at same point, 28s apart

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

19

Session 2.2

dACC B-A at same time, 220km apart

Aug. 31, 2009

IAG

Scientific

Assembly

Ulrich Meyer

20

Session 2.2

Conclusions

• Acceleration-differences between GRACE satellites are full of information about surface forces (atmosphere)

• Thruster pulses complicate interpretation of acceleration-differences• can be modeled and subtracted for atmosphere

sciences

• should not be removed for orbit computation

• are not responsible for systematics in residuals