Astronomical Institute University of Bern

Astronomical Institute, University of Bern, Switzerland

G1.3

EGU General Assembly 2014

27 April -2 May 2014

Vienna, Austria

GOCE – Validation of last days’ orbits with kinematic PPP

H. Bock, A. Jäggi, U. Meyer

source: https://doi.org/10.7892/boris.53908 | downloaded: 9.6.2020

Slide 2 Astronomical Institute University of Bern

Background and Motivation

Copyright: Bill Chater

•The first ESA Earth Explorer core mission GOCE ended officially on 21 October 2013, because the satellite ran out of fuel.

•Three weeks later, on 11 November 2013, the satellite re-entered the Earth’s atmosphere near the Falkland Islands in the South Atlantic.

•GPS-based orbit determination was possible until few hours before re-entry.

Slide 3 Astronomical Institute University of Bern

Background and Motivation

GOCE orbit height derived from GPS

21 October 2013

10 November 2013

Last available GPS measurements: 10 November, 17:15:20 UTC

Slide 4 Astronomical Institute University of Bern

Background and Motivation In the frame of the European GOCE Gravity Consortium (EGG-C) AIUB

was responsible for the generation of the GOCE Precise Science Orbit (PSO) product.

The PSO product consists of a reduced-dynamic and a kinematic orbit (comparable to a kinematic PPP of a ground station).

Internal validation: Orbit overlap analysis and differences between reduced-dynamic and kinematic orbits for consistency checks.

External validation: Satellite Laser Ranging (SLR) measurements.

Reduced-dynamic orbits were generated with the same orbit parameterization for the entire mission.

Two main questions for this study:

Can the kinematic orbits/PPP be used for validation, because SLR measurements are no longer available (only three passes)?

Is the orbit parameterization of the reduced-dynamic orbit still reasonable for the last three weeks of GOCE?

Slide 5 Astronomical Institute University of Bern

Differences red.-dyn. kinematic orbits

−0.1

0

0.1

m

−0.1

0

0.1

m

0 6 12 18 24

−0.1

0

0.1

Hours

mradial

along-track

out-of-plane

29 December 2009

Differences between reduced-dynamic and kinematic orbit

show consistency between the two orbit types and

reveal data problems and gaps in the kinematic orbit

Slide 6 Astronomical Institute University of Bern

Differences red.-dyn. kinematic orbits

−0.5

0

0.5m

−0.5

0

0.5

m

0 6 12 18 24−0.5

0

0.5

Hours

mradial

along-track

out-of-plane

28 December 2012 and 29 December 2009

End of 2012 the data quality is worse than end of 2009

Kinematic orbit shows more “outliers” and systematic effects

It should, however, be possible to validate the reduced-dynamic orbit modeling

Slide 7 Astronomical Institute University of Bern

GOCE internal orbit validation

Jul Jan Jul Jan Jul Jan Jul Jan Jul0369

121518

RM

S (

cm)

Date in 2009−2013

radial +9cm along−track +6cm out−of−plane +3cm 3−D

RMS of differences between red.-dyn. and kinematic orbits for official mission

Bock et al. (2014)

285 290 295 300 305 310 3150

20

40

60

80

100

RM

S (

cm)

Day in 2013

radial along−track out−of−plane 3−D

21 October 2013

Larger RMS values for the last three weeks probably reveal that the parameterization of the reduced-dynamic orbit is not ideal at all

Slide 8 Astronomical Institute University of Bern

Differences red.-dyn. kinematic orbits

−0.5

0

0.5m

−0.5

0

0.5

m

0 6 12 18 24−0.5

0

0.5

Hours

mradial

along-track

out-of-plane

Original solution; 31 October 2013

Large once-per-revolution signal in radial and along-track componentshows that the orbit parameterization is not ideal for the reduced-dynamic orbit

Slide 9 Astronomical Institute University of Bern

Reduced-dynamic orbit determination

30 h processing batches (not for the last 10 days), 10 s sampling, undifferenced processing, ionosphere-free linear combination, CODE Final GNSS orbits and clocks (5 s) and Earth Rotation Parameters

Orbit models and parameterization: EIGEN5S 120x120, FES2004 50x50 (fixed by GOCE Standards) Six initial orbital elements Three constant accelerations in radial, along-track, out-of-plane 6-min piece-wise constant accelerations in radial, along-track,

out-of-plane (2*10-8 m/s2) Test solutions with weaker constraints:

2.5 x 2*10-8 m/s2

5 x 10 x 25 x 50 x

−2

0

2

μm/s

2

−15

−10

−5

0

μm/s

20 6 12 18 24

−2

0

2

μm/s

2

Hours of day 294/2013

Slide 10 Astronomical Institute University of Bern

Solutions with weaker constraints

Test solutions with weaker constraints show better consistency with kinematic orbits.

Differences between 5x and 50x weaker constraints are marginal.

Except the very last days, these solutions are acceptable.

295 300 305 310 3150

20

40

60

80

1003D

RM

S (

cm)

Day in 2013

orig 2.5x 5x 10x 25x 50x

3D RMS of differences between red.-dyn. and kinematic orbits

Slide 11 Astronomical Institute University of Bern

Differences red.-dyn. kinematic orbits

−0.5

0

0.5m

−0.5

0

0.5

m

0 6 12 18 24−0.5

0

0.5

Hours

mradial

along-track

out-of-plane Large once-per-revolution signal is

very much reduced

Original solution and 10x weaker constraints; 31 October 2013

Slide 12 Astronomical Institute University of Bern

Differences red.-dyn. kinematic orbits

−1

0

1m

−1

0

1

m

0 6 12 18 24

−1

0

1

Hours

mradial

along-track

out-of-plane

Orbit modeling and data screening for the very last hours need further investigations

The GPS data quality at this stage of the mission (150 – 130 km altitude) is still surprisingly good!!!

10x weaker constraints; 10 November 2013

Slide 13 Astronomical Institute University of Bern

Improved background modeling

No or only small improvements with respect to the old solutions can be noticed with the better gravity field model.

294 296 298 300 302 3040

10

20

30

3D R

MS

(cm

)

Day in 2013

orig 2.5x 5x 10x 25x 50x

In order to improve the background models the gravity field model EIGEN5S 120x120 is replaced by GOCO03S 200x200 for the first 11 days of the decay phase.

Test solutions with original and weaker constraints are repeated.Old solutions

Slide 14 Astronomical Institute University of Bern

Summary Can the kinematic orbits be used for validation? => Yes, except that

the very last hours/days are difficult because of data screening problems leading to larger data gaps.

Is the orbit parameterization of the reduced-dynamic orbit still reasonable for the last three weeks of GOCE? => No, the constraints are too tight; 10x weaker constraints are reasonable.

First updates in the background modeling of the reduced-dynamic orbit determination did not improve the results of the reduced-dynamic orbits.

Further work:

Detailed check of background modeling and sampling of piece-wise constant accelerations.

Improve data screening procedure for the very last days/hours.

Comparison with and possibly use of accelerometer data in the reduced-dynamic orbit determination (as long as they are available).