ERS-1 Mission Phase D

Mission objectives during the Second Ice Phase (Phase D)

1. The first priority is to provide to the user community the capability to overflight the same SAR swath with a very short delay ( 3 days). This is mainly interesting for applications requiring monitoring of natural phenomena (eg. Ice applications...).
2. Another interest is to increase significantly the number of opportunities for interferometric applications. For this specific applications, the requirements for the orbits maintenance (Deadband and inclination) are modified for this second Ice phase:
   • An Out-of-Plane manoeuvre shall be performed in the middle of the phase. As a consequence, the inclination will drift approximately the same way between the 2 half phases.
   • Within each semi-phase, the deadband shall be maintained such a way that only one In-Plane manoeuvre is necessary, in the middle of each semi-phase.

Orbit maintenance requirements during the 2nd Ice Phase (Phase D)

For interferometric SAR, the baseline requirements change depending on the specific application. For differential interferometry, the objective is to observe surface movements disregarding the topography. With baseline equal to zero (<5m), the topography information in the interferogram will disappear. Such short baseline are therefore wanted in applications such as glacier movement studies.

For normal interferometry on the contrary, it is the topography that is sought in the process. Signal processing constraints such as noise and the correlation criterion lead to optimal results with baseline ranging from 150 to 300 m, but results can be obtained with baseline up to 600 m.

In order to increase significantly the number of Interferometric orbit pairs, the Ice Phase shall be divided in sub-phases, for which the orbit drifts shall restart from the same points.

Ground track maintenance at the Poles, Out-Of-Plane manoeuvres

The Ice Phase is divided in 2 sub-phases, separated by an Out-Of-Plane manoeuvre (OCM). The objective is to repeat as much as possible the inclination drift. Each sub-phase duration should be around 7 weeks.

Ground track maintenance at the Equator, In-Plane manoeuvres

During each 7 weeks sub-plane, the ground track will be maintained at the Equator in such a way that only one In-Plane manouvre is performed in the middle of each sub-phase. The ground track dead band at the Equator shall then be computed by ESOC in order to fulfil the manoeuvre spacing requirement, and should be in any case lower than +-1 Km. A third In-Plane manoeuvre is necessary at the same time as the Out-Of-Plane Manoeuvre.

Phase D parameters

• 3 days repeat cycle
• 43 orbits per cycle
• 1547 orbits in phase

Orbit number characteristics

• First absolute orbit number is: 12754 on 23-Dec-93, 17:39:06.043 UTC
  
• First internal track: 27
• End of the phase: orbit 14300 on 10-apr-94, 15:58:38.136 UTC.

Orbit parameters

• orbit eccentricity: 0.001165
• orbit inclination: 98.5162 deg
• asc. node Long.: 72.7302 deg
• semi-major axis: 7153138.10 m
• mean nodal period: 6027.906977 s
• mean anomaly: 270.1334 deg
• argument of perigee: 90 deg

Transformations from relative to absolute orbit number

ABS_ORB_NUM = REL_ORB_NUM + CYCLE_NUM*43 + 8341

CYCLE_NUM = INT( ( ABS_ORB_NUM - 8342)/43 )

REL_ORB_NUM= ABS_ORB_NUM - 8341 - CYCLE_NUM *43

Where:

• the "Absolute Orbit Number" is counted from the beginning of the mission;
• the "Relative Orbit Number" (track), in a given cycle, is counted starting from the first orbit East of Greenwich;
• the "Cycle Number" is increased whenever the track 1 is rivisited (note that cycles can be uncomplete).

This page is maintained by Remko Scharroo, remko.scharroo@lr.tudelft.nl