Revised ERS-1 & ERS-2 Tandem Scenario
This document describes the impact of the ERS-2 AMI anomaly on the
Tandem Mission which has been presented at previous EOPAG, EOSTAG
and DOSTAG meetings and which was approved by the ESA Council on 23
March 1995. The nominal Tandem Mission Scenario was described in
The ERS-2 AMI anomaly has delayed the completion of the SAR
Commissioning and has so far prevented initiation of the Wind
Scatterometer Commissioning. A reliable operation of the AMI has
been achieved in SAR mode, with the transmitted power reduced by
about 2 dB to avoid switch-off by the receiver overload protection.
The investigation of the ERS-2 AMI anomaly has proceeded and a
work-around to establish full operation of the Wind Scatterometer
is under evaluation.
A more detailed status of the investigation will be presented
to DOSTAG on 10 November 1995.
- Status of AMI Commissioning
- SAR and Wave mode
Beside the verification of the proper functionning of the
instrument, the main objectives of the AMI Commissioning in Image
and Wave mode were to compute the in-flight Elevation Antenna
Pattern correction from data acquired over the rain forest, and to
derive the calibration constants from data acquired over ground
transponders and corner reflectors. The determination of these
constants required a stable on-board configuration of the AMI,
which was reached on 12 July 1995 when the setting of the on-board
gain was finalized. The commissioning activities outlined above
have proceeded thereafter, and successfully completed, as reported
at the last DOSTAG meeting.
The work-around solution for the wind scatterometer, currently
under evaluation, will eventually introduce an additional
attenuation in the transmission path also in Image and Wave mode,
and the AMI on-board gain setting in these modes shall be adjusted
accordingly to compensate for this attenuation. In this way, the
same RF power output level in Image and Wave mode as during the
calibration activities shall be achieved. This shall be confirmed
during transponder overflights, and a new computation of the
calibration constants might be required only if the accuracy of the
compensation is not sufficient. In any event, the computed
in-flight Elevation Antenna Pattern correction will not
- Wind Mode
The commissioning of the ERS-2 AMI in Wind Mode cannot start until
a stable working configuration has been identified and tested. From
there on, the calibration and validation objectives of this
commissioning, i.e. mainly the estimation of the antenna pattern
correction and calibration coefficients, and the validation of the
wind field estimates, can only be achieved after a suitable number
of passes over the tropical rain forest and over the scatterometer
transponders. Namely, completion of this commissioning requires
three 35-day cycles.
The schedule of the Wind Mode commissioning activities cannot
be established today. It will be defined after final conclusions of
the investigation of the AMI anomaly; this schedule will also
ensure that the on-going Tandem activities are not disturbed.
- Mission scenario during Tandem
The SAR Mission Scenario during Tandem is identical to the original
Tandem scenario, except that the roles of ERS-1 and ERS-2 are
exchanged. This was specifically requested by Delegations at the
second DOSTAG meeting, in order to contribute to extend the
lifetime of the ERS-2 SAR.
The ERS-1 SAR continues to provide the nominal service to the
users. Acquisitions are planned according to user requests and the
background mission, as defined in the HLOP. This background mission
is extended to fulfil the Tandem objectives related to
interferometry. FD processing and dissemination are limited to user
requests. All acquired data are catalogued and available for
The ERS-2 SAR acquisition plan is dedicated to the Tandem Mission.
It reproduces the ERS-1 SAR acquisition plan, with a 24-hour delay.
In addition, the ERS-2 SAR acquisition plan may include
acquisitions during ascending passes, when not in conflict with the
tests of the Wind scatterometer. All acquired data are catalogued
and available for off-line production.
- Low bit rate
The low bit rate Mission Scenario during Tandem is identical to the
original Tandem scenario, except that the roles of ERS-1 and ERS-2
are exchanged. This results from the unavailability of the ERS-2
ERS-1 continues to provide the nominal low bit rate FD service to
the users. Global acquisition, fast-delivery processing and
dissemination continues as during Phase G. All off-line products
are generated. ATSR near-real time service from Tromsoe continues,
until the ERS-2 ATSR processing chain is available.
ERS-2 performs the global acquisition of Radar Altimeter, GOME,
ATSR, MWR and PRARE data. Fast-delivery processing and
dissemination of Radar Altimeter data is executed at Kiruna only,
for 10 orbits/day. Off-line products are available as follows:
ATSR near-real time service from Tromsoe starts when the ERS-2 ATSR
processing chain is available.
- Radar Altimeter: OPR only, as specified in the approved Tandem
- GOME, ATSR: nominal products
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