4 Final Report - Emits - ESA

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3 Final Report better than 0.5 SSD at SSP (threshold) / 0.25 SSD at SSP (goal), with a confidence level of 99.73% over each image. This requirement shall be fulfilled for any images, where land areas are included (presence of GCP). For images which contain only water areas (no GCP), the requirement can be relaxed. For these cases, additional AOCS requirements have to be established, assuring a certain knowledge drift stability between two marine images (see pointing requirements table below). The absolute geolocation knowledge requirement assures also the image-to-image registration (knowledge) performance, which is twice (worst case) the absolute geolocation knowledge (1 SSD at SSP (threshold) and 0.5 SSD at SSP (goal) for a sample of two consecutive images). The inter-channel co-registration requirement (knowledge accuracy) is 0.3 px (99.73%) between each two channels (referring to pixel size of channel with worse resolution). Pointing Requirements Following pointing requirements have been derived and established for Geo-Oculus: • Pointing coverage – European area shall be covered nominally, with the potential to cover the whole Earth disc; • APE – to acquire a coverage without gaps for the marine applications; • RPE over integration time – to assure high resolution for the panchro channel; • PDE over integration time – to limit image post-integration effort; • PDE for mosaic imaging – to have products without gaps; • PDE knowledge (reference to images with landmarks) – for marine images without coastline. Timing Requirements The timing requirements can be found in detail in RD [3]. Following requirements have been defined: • Acquisition Delay; • Timeliness; • Product Acquisition Time; • Temporal Co-registration. Sensing and Instrument Requirements A set of sensing and instrument requirements have been established for Geo-Oculus, documented within RD [3]. They are also discussed within the Instrument section of this document. Most of these requirements are purely instrument related, therefore they are not discussed in detail within this system chapter. What should be mentioned, is that several system pointing requirements can be derived from these instrument requirements, which has then be traded on system level (see next chapter). Following requirements have been established: • Radiometric Requirements; • Spectral Accuracy; • Modulation Transfer Function (MTF); • Polarisation. Page 3-16 Doc. No: GOC-ASG-RP-002 Issue: 2 Astrium GmbH Date: 13.05.2009
3 Final Report Ground Segment Requirements The ground segment requirements can be found in detail in RD [3]. It is based on a centralised Flight Operations Segment (FOS). Requirements have been specified for: • Functionalities of FOS; • S-Band TM/TC Ground Station and X-Band Ground Station for PDT; • Centralised Payload Data Ground Segment (PDGS); • Standardised User Portal; • Centralised processing facilities; • High-speed communication connections to the PDT receiving stations; • Interfaces to meteorological service providers for the provision of nowcasting and very short range forecasting information of cloud coverage. 3.2 Major System Trade-Offs In this chapter the major system trade-offs performed after the MTR, leading to the proposed Geo- Oculus baseline, are summarised. They are discussed in detail within the dedicated chapters / documents. • Field of View vs. resolution The combination of instrument FoV and resolution is limited by the detector technology (number of pixels). Additionally, both the maximum FoV size and the resolution are limited by the telescope size. Due to the coverage requirements for the marine applications, the choice is to go for a maximum possible FoV size (300km x 300 km with the proposed telescope concept), with medium resolution. For disaster monitoring, the best resolution possible with the proposed telescope concept has been chosen. This leads to a smaller FoV size, which requires mosaic imaging for disaster monitoring. • Magnetic Bearing Wheels vs. Electric Propulsion for manoeuvres MBWs allow high torques and therefore short manoeuvre times. The drawback are the microvibrations, which are much lower than with ball bearing wheels, but still impact the image quality. EPS would create no microvibrations, but increase the manoeuvre time due to the relative small torque, which leads then to a small number of missions. Furthermore, the propellant demand is significant, especially for EPS with high thrust. As a consequence, it has been decided to go for the MBW solution. • Manoeuvre time vs. image post-integration effort The pointing instability impacts the image quality. With a good pointing stability, no postintegration (including image motion compensation) is needed, as long as the MTF requirements are met. The major contributors to pointing instability are microvibrations (which are not time varying at a short timescale) and solar array oscillations. The solar array oscillations are a direct function of the waiting time after a manoeuvre. The trade-off is therefore between a long manoeuvre time, needing no post-integration and shorter manoeuvre times, requiring post-integration. Doc. No: GOC-ASG-RP-002 Page 3-17 Issue: 2 Date: 13.05.2009 Astrium GmbH
Page 1: Geo-Oculus: A Mission for Real-Time
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