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
ESOC Navigation Support Office
EGU Vienna, 18-04-2008, 2/18
• GNSS
• Receiver for
• Atmospheric
• Sounding
• Ground
• Support
• Network
… on board Metop
ESOC Navigation Support Office
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.
ESOC Navigation Support Office
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
ESOC Navigation Support Office
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
ESOC Navigation Support Office
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?
ESOC Navigation Support Office
EGU Vienna, 18-04-2008, 8/18
Supporting differenced processing methods
ESOC Navigation Support Office
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)
ESOC Navigation Support Office
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
ESOC Navigation Support Office
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)
ESOC Navigation Support Office
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)
ESOC Navigation Support Office
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
ESOC Navigation Support Office
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)
ESOC Navigation Support Office
EGU Vienna, 18-04-2008, 16/18
NRT orbit and clock accuracy (Nov. 2007)
ESOC Navigation Support Office
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
ESOC Navigation Support Office
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
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