esoc navigation support office egu vienna, 18-04-2008, 1/18 ground support network for the metop...

ESOC Navigation Support Office

EGU Vienna, 18-04-2008, 1/18

Ground support network for the Metop GRAS atmospheric sounding mission

R.Zandbergen, A.Ballereau, M.Lorenzo, J.M.Dow, C.Marquardt, F.Wollenweber

18 April 2008


EGU Vienna, 18-04-2008, 2/18

• GNSS

• Receiver for

• Atmospheric

• Sounding

• Ground

• Support

• Network

… on board Metop


EGU Vienna, 18-04-2008, 3/18

Atmospheric Sounding

• GPS signal travels through atmosphere

• Through relative S/C motions, occultation profiles are generated

• Fore and aft antennas on Metop allow monitoring of both setting and rising occultations

• Two occultations can be observed by each antenna

• Max. per day per satellite ~500 occ.


EGU Vienna, 18-04-2008, 4/18


EGU Vienna, 18-04-2008, 5/18

Generation of sounding profiles

• EUMETSAT will operate three Metop S/C (polar LEO)

• … and generate atmospheric sounding products in NRT

• … for delivery to European Met Offices within 3 hours

• The generation of profiles requires precise orbits and clock solutions for both Metop and the occulting GPS satellite

• Metop POD to be performed in-house, using GRAS GPS receiver (zenith antenna)

• The GPS products are considered ‘Support Data’, to be provided externally


EGU Vienna, 18-04-2008, 6/18

• GPS orbits (for Metop POD and occultation processing)

• GPS clock solutions

– Low-rate for Metop POD

– High-rate for occultation processing

• EOP data (for Metop POD)

• Auxiliary data (TZD, Meteo, Nav msg…)

Support data for occultation processing


EGU Vienna, 18-04-2008, 7/18

• GPS orbits (for Metop POD and occultation processing)

• GPS clock solutions

– Low-rate for Metop POD

– High-rate for occultation processing

• EOP data (for Metop POD)

• Auxiliary data (TZD, Meteo, Nav msg…)

• High-rate, selected, ground receiver data

• High-rate ground clock solutions

Support data for occultation processing

} why?


EGU Vienna, 18-04-2008, 8/18

Supporting differenced processing methods


EGU Vienna, 18-04-2008, 9/18

Ground (Sounding) Support Data

• Method 1: undifferenced

• Method 2: eliminate GRAS clock

• Method 3: take out the GPS clock (replace by ground clock)

• Method 4 not yet implemented

• For method 3, accurate ground clock solutions and ground tracking data of the occulting satellite are required

• EUMETSAT deliver a table of predicted occultations to ESOC and ESOC return the necessary ground data (SSD)


EGU Vienna, 18-04-2008, 10/18

Support data key requirements

• The most fundamental requirements were studied in 2001:

– Timeliness: 60 min. for orbits and clocks

– Clock accuracy: 1 ns at 2-sigma for each satellite

– Clocks to be interpolated at 50 Hz

– GPS satellite velocity accuracy requirement!

– Guaranteed high availability (99% & limited interruptions)

– Can be operated for 15 years, 2 satellites in parallel

– Extensible to other missions with similar requirements


EGU Vienna, 18-04-2008, 11/18

High-level design

• A dedicated and fully redundant network of approximately 20 sites (i.e. in total 40)

– Resulted in contracts with existing network operators: GFZ, NRCan and Fugro Seastar

• A processing centre in ESOC

• GPS orbits computed every 3 hours, delivering predictions

• Clocks computed every 15 minutes, not predicted

• To support the 50 Hz GRAS data rate, compute and deliver 1 Hz GPS clocks (not interpolated)


EGU Vienna, 18-04-2008, 12/18

The GSN Network (prime sites)


EGU Vienna, 18-04-2008, 13/18

Key issue: timing of the clock products

• Using predicted orbits, batch 1-Hz clock determination every 15 minutes for 23 minute arcs (15 + 8 minutes overlap)


EGU Vienna, 18-04-2008, 14/18

Current status

• Successful validation of GRAS GSN in February 2006

• Launch of the first satellite (Metop-2) October 2006

• Activation of GRAS GSN operations one month prior to launch

• First year of operations completed September 2007

• Accuracy and availability requirements met by a good margin (first year availability 99.71%)

• Support to COSMIC mission has started


EGU Vienna, 18-04-2008, 15/18

Achieving the accuracy requirements

• Requirements verification by comparing to IGS finals

• Accuracy requirements are met almost continually

• Accuracy requirements from EUMETSAT equivalent to 95-percentile

• In IGS terms (‘typical RMS’), orbit prediction accuracy based on the relatively small network is around 10 cm.

• Clock accuracy is corresponding to this (around 0.3 ns, removing only a single common clock bias)


EGU Vienna, 18-04-2008, 16/18

NRT orbit and clock accuracy (Nov. 2007)


EGU Vienna, 18-04-2008, 17/18

Preliminary GRAS results

• Operational level-2 data will be provided by GRAS SAF

• Validation of these results in progress


EGU Vienna, 18-04-2008, 18/18

Acknowledgments are due to:

• Station network providers:

• Météo France TahitiFor the excellent quality of their support and data products

• The EUMETSAT team:F. Wollenweber, C.Marquardt, Y. Andres

• The ESOC team:ESA: A. Ballereau, R.Zandbergen, J.M.Dow

GMV SA: M. Lorenzo, P. Alfaro, J. Tegedor, I. Romero, F. Lopes Pereira, C. Garcia, J.Fernandez

esoc navigation support office egu vienna, 18-04-2008, 1/18 ground support network for the metop...

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