4 Final Report - Emits - ESA

4 Final
Report
Table 4.5-2: Characteristics of Maspalomas and Redu S-Band stations
MAS-1 RED-1
EIRP 72.1dBW 72.5dBW
G/T 29.2dB/K 29.6dB/K
4.5.6 Attitude and Orbit Control
4.5.6.1 Introduction
The AOCS plays a significant role within the functional process chain to fulfil the demanding pointing
requirements of a very high performance mission, both in terms of absolute and relative pointing /
pointing knowledge. This overall process chain consists of the payload, the platform (including the
AOCS) and the on-ground post-processing (INR).
Based on system pointing requirements and an associated pointing budget for the whole process
chain, preliminary requirements for the AOCS have been derived. Out of the various pointing
requirements, the following ones are driving the AOCS concept and will be checked in this chapter:
• the absolute pointing error (APE),
• the absolute measurement error (AME),
• the pointing drift error (PDE) over 100 msec.
A feature special to the Geo-Oculus mission is that the whole spacecraft is turned in order to move to
the next image which shall be acquired in a step and stare mode. Based on the findings related to the
mission scenario trade-offs, a medium agility is requested from the AOCS concept in order to support
a reasonable number of mission products within the dedicated revisit cycles. The allocated budget
assigned to this medium agility is 70 sec for the total manoeuvre time between 2 images. The agility
itself is driven by the AOCS actuator selection and the overall platform design (moments of inertia,
flexible modes). Especially the flexible modes come into play when high torque actuators are used.
This is due to a high initial deflection and the related long tranquilisation time in order to reach again
the required pointing budgets. Sun avoidance manoeuvres in regular intervals also represent an agility
aspect but are not driving the design because the slew times can be reasonably long. In order to
reduce the negative impact of flexible modes on the manoeuvre time, an active damping strategy for
the solar array modes could be assessed. This is currently kept in mind as a back-up solution but, so
far, only the performance of actuators without such a damping technique has been evaluated for this
study.
For the assessments performed in this chapter it is assumed that the AOCS architecture to support the
various operational modes (transfer, acquisition, nominal operation, orbit maintenance, safe mode)
can be established on the basis of existing E/O or telecomm platforms (e.g. Eurostar) in order to
benefit from long-time heritage, risk mitigation and cost minimisation. The focus for this study is to
select a suitable set of sensors and actuators which fit with the dedicated pointing and agility
requirements of the Geo-Oculus mission.
4.5.6.2 Pointing budgets for AOCS
The requirements in Table 4.5-3 represent requirements assigned for the AOCS performance (thermoelastic
distortions are covered by a separate budget):
Page 4-58 Doc. No: GOC-ASG-RP-002
Issue: 2
Astrium GmbH Date: 13.05.2009