GRAS Level 1 Processing and
Products
Juha-Pekka Luntama and Julian Wilson
EUMETSAT
Am Kavalleriesand 31, D-64295 Darmstadt, Germany
GRAS SAF User Workshop
11 - 13 June 2003
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Itinerary
1) Introduction
2) GRAS measurement system
3) GRAS level 1 data processing
4) GRAS level 1 products
5) Conclusions
GRAS SAF User Workshop
11 - 13 June 2003
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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
GRAS SAF User Workshop
11 - 13 June 2003
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Metop spacecraft
GRAS
GOME
AVHRR
HIRS
IASI
AMSU-A1
ASCAT
MHS
GRAS SAF User Workshop
11 - 13 June 2003
AMSU-A2
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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
GRAS SAF User Workshop
11 - 13 June 2003
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GRAS NRT Product requirements
Level 1b products
Horizontal sampling
Sfc – 1 hPa
0.4 – 2 km
0.3 – 3 km
1 – 3 km
1 – 3 km
15 – 35 km
-
1 – 3 km
35 – 50 km
-
1 – 3 km
0.25 - 1 g/kg
0.5 – 3 K
0.05 - 0.2 g/kg
0.5 – 3 K
15 – 35 km
-
0.5 – 3 K
35 – 50 km
-
0.5 – 5 K
3h
3h
0 – 5 km
RMS
1)
accuracy 5 – 15 km
GRAS SAF User Workshop
11 - 13 June 2003
Total bending angle Specific humidity
Temperature
The average distance between individual soundings over a period
of 12 h is less than 1000 km
Sfc – 100 hPa
Vertical range
0 – 5 km
Vertical
sampling 5 – 15 km
Timeliness
Level 2 products
Sfc – 80 km
2 - 5 Hz
1 rad or 0.4 %
2 h 15 min
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GRAS measurement system
GRAS SAF User Workshop
11 - 13 June 2003
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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
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GRAS receiver
•
ISAC
DISC
RFCU-Z
GZA
IF
LO
AGGA
IF-Filter
A/D
DSP
BUS
IF
LO
LNA
•
AGGA
RFCU-V
GVA
Test
DSP
DISC
IF
•
RBI
AGGA
RF/IF
RFCU-AV
IF-Filter
A/D
Spacecraft
Interface
IF-Filter
A/D
FMU
•
LO
GAVA
Antennas
RFCUs
Freqency
Generator
Reference
Oscillator
GEU
GRAS SAF User Workshop
11 - 13 June 2003
Power
Supply
•
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
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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
GRAS SAF User Workshop
11 - 13 June 2003
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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
GRAS SAF User Workshop
11 - 13 June 2003
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GRAS level 1 processing
GRAS SAF User Workshop
11 - 13 June 2003
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GRAS level 1a processing
GRAS
raw data
(level 0)
GRAS Instrument
characterisation
database
GRAS
GSN
Service
Metop attitude, true
latitude, and
maneuvre information
GRAS level 0
processing
Level 1a processing
Source packet pre-structuring
USO frequency
correction
Occultation table
generation
Level 1a quality check
Occultation table
generation
Measurement reassembly
Level 1a pre-processing
Measurement identification
Instrument corrections
Raw sampling mode level 1a
processing
Incidence angle derivation
Code phase
correction
Carrier phase
correction
Amplitude
correction
Level 1a product quality check
Metop NRT POD
Level 1a product formatting
to level 1b processing
GRAS SAF User Workshop
11 - 13 June 2003
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GRAS level 1 NRT data products
Level 1 measurements data products
Height Range
Residual phase, amplitude, residual Doppler
L1
< 5 - 80 km
L2
< 12 - 80 km
Bending angle, impact parameter (GO)
< 5 - 80 km
< 12 - 80 km
Bending angle, impact parameter (WO)
< 5 - 25 km
< 12 - 25 km
Neutral bending angle (GO)
< 5 - 80 km
Neutral bending angle (WO)
< 5 - 25 km
TEC
< 5 - 80 km
Raw sampling mode data
Measurement quality flags
0 – 5 km
Telemetry based quality info
Selected instrument telemetry
NA
Estimated and true geolocation of the profile
NA
Level 1b data processing parameters
NA
GRAS SAF User Workshop
11 - 13 June 2003
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GRAS level 1 NRT data products
(cont.)
Supporting level 1 products
Notes
Observation error characteristics
Covariance matrices for bending angles for
NWP
GPS and Metop state vectors
Full arcs, non-interpolated
GPS, GRAS, and fiducial station clock
offsets
All satellites and stations, non-interpolated
Troposphere Zenith Delay (TZD)
For each fiducial station
EOP
EOP used in the level 1 processing
GRAS SAF User Workshop
11 - 13 June 2003
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Level 1 product accuracy
• Combined bending angle error at 30 km for azimuth 30:
– GRAS dependent errors:
– GRAS independent errors:
– RSS
0.420 rad
0.662 rad
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
GRAS SAF User Workshop
11 - 13 June 2003
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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 reassembly 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
GRAS SAF User Workshop
11 - 13 June 2003
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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
GRAS SAF User Workshop
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