4 Final Report - Emits - ESA

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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
4 Final Report Table 4.5-3: AOCS 100% pointing requirements Pointing Index Preliminary Values (100%) Remark APE ±100 μrad Derived to minimise overlap of neighbouring images PDE 0.5 μrad over 100 ms 1.0 μrad over 100 ms For VNIR7 channel (panchro) For other channels AME ±100 μrad Currently the same value assumed as for APE 4.5.6.3 AOCS sensors and actuators Given the high performance requirements for Geo-Oculus, only high performance sensors are considered . The main sensor will be a star tracker (STR) which will be operated together with an inertial measurement unit (IMU) in a gyro-stellar estimator set-up (GSE). In the GSE the STR data is combined with the IMU data to benefit from the advantages of both sensors. The STR provides noisy but stable attitude information and the IMU provides low-noise data which drifts over time. The IMU cancels the noise from the STR and the STR cancels the drift in the IMU to a large extent. Several options for STR are available on the European market: • Sodern Hydra • Jena Optronik Astro APS • Galileo AA Several options also exist for the IMU selection: • EADS Astrium Astrix 120 HR • EADS Astrium Astrix 200 GEO • Northrop Grumman Scalable SIRU The baseline STR is the Astro APS and the baseline IMU is the Astrix 200 GEO, as these are the baseline sensors for a reference mission which is similar in many respects. The performance of the three listed STR are similar and the baseline can easily be changed if necessary. For the IMUs there is a clear performance difference between the Astrix 120 on one hand and Astrix 200 and SIRU on the other hand. The performance level of the Astrix 200 and SIRU is necessary to meet the relative pointing requirements. The SIRU is produced in the US and is subject to ITAR restrictions. It can therefore not be selected as baseline. The attitude control and manoeuvre actuator selection has gone through several iterations, and the current choice stands between using Magnetic Bearing Wheels (MBW) and an Electric Propulsion System (EPS). • Rockwell Collins MBW • EPS system The Rockwell Collins (RCD) MBW is the only MBW option available in the European market. It is currently not flight proven but RCD indicates that they will have flight proven models available in 2013. Thales in Ulm, Germany, is currently developing a HEMPT based EPS system called HEMPT 3050. This system is much to powerful for fine attitude control and a theoretical, scaled down microHEMPT thruster has been developed for comparison. Doc. No: GOC-ASG-RP-002 Page 4-59 Issue: 2 Date: 13.05.2009 Astrium GmbH
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Geo-Oculus: A Mission for Real-Time
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4 Final Report - Emits - ESA

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