Minimize Revised ERS-1 & ERS-2 Tandem Scenario
  1. Introduction

  2. 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 ESA/PB-EO(95)20.
    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.
  3. Status of AMI Commissioning
    1. SAR and Wave mode

    2. 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 change.
    3. Wind Mode

    4. 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.
  4. Mission scenario during Tandem
    1. SAR

    2. 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.
      1. ERS-1

      2. 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 off-line production.
      3. ERS-2

      4. 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.
    3. Low bit rate

    4. 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 wind scatterometer.
      1. ERS-1

      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.
      3. ERS-2

      4. 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:
        • Radar Altimeter: OPR only, as specified in the approved Tandem scenario
        • GOME, ATSR: nominal products
        ATSR near-real time service from Tromsoe starts when the ERS-2 ATSR processing chain is available.