# ERS-1 Orbit Information

To carry out its mission ERS-1 must orbit so that its instruments can scan along predetermined paths designed to give optimum coverage for a set number of orbits. To achieve this, ERS-1 is a three-axis-stabilised, Earth-pointing satellite in yaw steering mode (YSM). The elliptical orbit is sun-synchronous, near polar, with a mean altitude of 785 km, an inclination of 98.5deg. and a local solar time at the descending node of 10.30 a.m.

ERS-1 has a range of altitude sensors. The long-term reference in pitch and roll is obtained from an infra-red Earth sensor. The yaw reference is obtained once each orbit from a narrow field Sun sensor, which is aligned to view the Sun when the satellite is at a particular point in its orbit. The short term attitude and rate reference is obtained from three of six gyroscopes. Finally, there are two wide-field Sun-acquisition sensors for use in safe mode, when the satellite is Sun, rather than Earth, pointing. The primary means of attitude control is provided by a set of reaction wheels, which are nominally at rest. They can be spun in either direction, exchanging angular momentum with the satellite in the process. It is also possible, if there are permanent torques on the satellite due, for instance, to radiation pressure on the solar array, to bias one or more wheels to be nominally not at rest, but rotating at a defined speed. Monopropellant-type thrusters aligned about the spacecraft's three primary axes are used in different combinations to maintain and modify the satellite's orbit and to adjust its attitude during non-nominal operations.

The ERS-1 mission will also include a number of adjustment manoeuvres to synchronise the orbital period with various requirements for ground coverage. The orbital parameters for the three planned repeat cycles of 3, 35 and 168 days are as follows:

                         3-day           35-day         168-day
    semi major axis      7153.138        7159.496       7147.191
    inclination          98.516 deg.     98.543 deg.    98.491 deg.
    mean altitude        785 km          782 km         770 Km
    orbits per cycle     43              501            2411

The transition from one orbital configuration to another, to adjust the repeat cycle, requires up to two weeks to stabilise the orbits to within 1 km of the nominal ground track. With the transition made at certain times in the orbital cycle stabilisation of orbit to within 5 km can occur after 24 hours. LBR and SAR operations are re-started 48 hours after the start of the orbit change manoeuvre. However, because of the need to update various parameters both on-board the satellite and on the ground the quality of fast delivery LBR products are not guaranteed for two weeks after any manoeuvre.

For the roll tilt mode (RTM) mode campaign in early April 1992 the satellite body is rotated by 9.5deg. allowing operation of the AMI SAR imaging mode at 35deg. incidence angle. The attitude control system performances of the roll tilt mode are not significantly different to the nominal YSM with the same angular rates and harmonic and random errors and only slightly different static errors, i.e. about 0.05deg. maximum difference. During the roll tilt sequences the satellite is in a Fine Pointing Mode1 (FPM), which features no yaw steering and Earth centroid pointing, rather than geocentric pointing.

## ERS-1 Overview

  Satellite Concept
  ERS-1 Orbit
  Payload Summary
  Operational Capabilities
  ERS-1 Specification Table

ESA Earth Observation