1. GRAS Level 1 Processing and Products Juha-Pekka Luntama and Julian Wilson EUMETSAT Am Kavalleriesand 31, D-64295 Darmstadt, Germany

2. Itinerary 1) Introduction 2) GRAS measurement system 3) GRAS level 1 data processing 4) GRAS level 1 products 5) Conclusions

3. Introduction • EUMETSAT Polar System (EPS) is the European component of the joint European/US polar satellite system (IJPS) • EPS mission consists of three Metop satellites with a payload of eight meteorological instruments • The first Metop satellite is launched in second half of 2005 • The total duration of the EPS mission is 14 years => NRT products for NWP applications and continuity of radio occultation data for climate monitoring • GRAS receiver has been developed especially for radio occultation measurements within the EPS framework by ESA and EUMETSAT

4. Metop spacecraft GRAS GOME AVHRR HIRS IASI AMSU-A1 ASCAT AMSU-A2 MHS

5. NRT Challenge • EPS GRAS is the first RO mission that has been from the beginning designed for operational applications • NRT requirements for GRAS product dissemination are 2 h 15 min for level 1b and 3 h for level 2 from the observation • Analysis of the NRT feasibility has revealed two main risks: • Timely availability of the auxiliary data • NRT Precise Orbit Determination of the LEO satellite • Mitigation of these risks is incorporated in the GRAS Measurement System design • There has been no reason to compromise the occultation data processing due to the NRT requirements

6. GRAS NRT Product requirements

7. GRAS measurement system • GRAS GSN provides GPS POD products and ground based measurements • Level 1b products are processed and disseminated by the EPS CGS • Level 2 products are processed and disseminated by the GRAS Meteorology SAF • Archived products include raw data, level 1b, level 2 and all GSN products • Off-line user access to archived products via EUMETSAT UMARF

8. GRAS receiver • 12 bi-frequency channels • Codeless-mode capability • Oven stabilised USO (Allan deviation 10-12) • Directional high gain occultation antennas for minimising local multipath • Onboard DSP => autonomous operations

9. GRAS GSN Service • GRAS GSN is a service to provide GPS state vectors, clock offset estimates, and clock differencing data for GRAS data processing • GSN will contain a network of 25 fiducial stations • GSN coverage for the GPS constellation is > 200 % to ensure service availability via redundancy • The GSN Service is designed to support the GRAS NRT data processing requirements

10. GRAS GSN products • GPS NRT state vectors (15 min sampling) • GPS NRT clock offset estimates (1 s sampling) • Fiducial station clock offset estimates (1 s sampling) • Troposphere Zenith Delay (TZD) • Sounding Support Data (SSD) for clock correction: • L1 and L2 carrier phase • L1 and L2 pseudorange • L1 and L2 SNR • GSN Configuration and Status database • EOP

11. GRAS level 1 processing

12. GRAS level 1a processing

13. GRAS level 1 NRT data products

14. GRAS level 1 NRT data products (cont.)

15. Level 1 product accuracy • Combined bending angle error at 30 km for azimuth 30: • GRAS dependent errors: 0.420 rad • GRAS independent errors: 0.662 rad • RSS 0.784 rad • When azimuth angle is > 45° the GRAS dependent errors increase to 0.651 rad => Total RSS for 90 % of the measurements < 0.935 rad. • Analysis by the instrument manufacturer indicates that statistically about 8 % of the observations are outside the accuracy specification => 40 occultations out of the daily 500 have errors larger than 1 rad

16. GRAS level 1 processing summary • GRAS does not directly produce phase, amplitude, SNR, or pseudorange measurements => they have to be derived from the raw data using a measurement re-assembly function • GRAS level 1 processing will include GO, WO, and raw sampling mode data processing chains • Partially overlapping bending angle profiles from all processing chains are disseminated to the users • A software tool for combining and smoothing the bending angle profiles for NWP is planned by the GRAS SAF • Metop NRT POD problem has been solved by using a SRIF algorithm together with the NAPEOS S/W and by solving the Metop orbit in short (< 10 min) arcs

17. Conclusions • The objective of the EPS GRAS mission is to provide NWP users operationally radio occultation data products • The EPS mission starts in 2005 and provides data continuously at least for 14 years • The stringent NRT timeliness requirements have made the development of the GRAS measurement system and the data processing chain challenging • GRAS data processing and dissemination are performed by the EPS CGS (level 1b ) and by the GRAS Meteorology SAF (level 2 ) • All archived GRAS products (level 0, level 1b, and level 2) shall be made available via the EUMETSAT UMARF facility • Analysis and study results indicate that the NRT data processing with the required accuracy is feasible for about 92 % of the observations