gras saf user workshop 11 - 13 june 2003 1 gras level 1 processing and products juha-pekka luntama...

GRAS SAF User Workshop11 - 13 June 2003

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

IASI

HIRS

AVHRR

AMSU-A1

AMSU-A2MHS

GOMEGRAS

ASCAT

Metop spacecraft


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 requirementsLevel 1b products Level 2 products

Total bending angle Specific humidity Temperature

Horizontal sampling The average distance between individual soundings over a periodof 12 h is less than 1000 km

Vertical range Sfc – 80 km Sfc – 100 hPa Sfc – 1 hPa

0 – 5 km 0.4 – 2 km 0.3 – 3 km

5 – 15 km 1 – 3 km 1 – 3 km

15 – 35 km - 1 – 3 km

Verticalsampling

35 – 50 km

2 - 5 Hz

- 1 – 3 km

0 – 5 km 0.25 - 1 g/kg 0.5 – 3 K

5 – 15 km 0.05 - 0.2 g/kg 0.5 – 3 K

15 – 35 km - 0.5 – 3 K

RMSaccuracy1)

35 – 50 km

1 rad or 0.4 %

- 0.5 – 5 K

Timeliness 2 h 15 min 3 h 3 h


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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

M etop

EPS Polar s ite

EU M ETSATEPS C G S

U SER S

G R ASM eteorology SAF

hosted by D M ILevel 2 products

G R AS G SN ServiceLevel 1b products

G PS systemO ccultation &

navigation

Tracking


8

RFCU-V

RFCU-Z

RFCU-AV

ISAC

IF-FilterA/D

IF-FilterA/D

IF-FilterA/D

DISC

DISC

AGGA

FreqencyGenerator

ReferenceOscillator

Pow erSupply

SpacecraftInterface

RBI

DSPIF

LNA RF/IF

IF

IF

LO

LO

LO

Antennas RFCUs

GAVA

GVA

GZA

GEU

AGGA

AGGA

Test

FMU

DSPBUS

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


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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


11

GRAS level 1 processing


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GRAS level 1a processing

Level 1a processing

Instrum ent corrections

O cculta tion tab legeneration

Level 1a pre-processing

G R A Sraw data(leve l 0 )

G R A SG S N

S ervice

Metop NRT POD

Source packet pre-structuring

M easurem ent reassem bly

Incidence angle derivation

Level 1a product quality check

Occultation tablegeneration

M easurem ent identification

Raw sam pling m ode level 1aprocessing

GRAS level 0processing

Level 1a quality check

USO frequencycorrection

GRAS Instrum entcharacterisation

database

Code phasecorrection

Carrier phasecorrection

Am plitudecorrection

Level 1a product form atting

M etop attitude, truelatitude, and

m aneuvre inform ation

to level 1b processing


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GRAS level 1 NRT data products

Height RangeLevel 1 measurements data products

L1 L2

Residual phase, amplitude, residual Doppler < 5 - 80 km < 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 0 – 5 km

Measurement quality flags Telemetry based quality info

Selected instrument telemetry NA

Estimated and true geolocation of the profile NA

Level 1b data processing parameters NA


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GRAS level 1 NRT data products (cont.)

Supporting level 1 products Notes

Observation error characteristics Covariance matrices for bending angles forNWP

GPS and Metop state vectors Full arcs, non-interpolated

GPS, GRAS, and fiducial station clockoffsets

All satellites and stations, non-interpolated

Troposphere Zenith Delay (TZD) For each fiducial station

EOP EOP used in the level 1 processing


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


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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 11 - 13 june 2003 1 gras level 1 processing and products juha-pekka luntama...

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