4 Final Report - Emits - ESA

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3 Final Report Manoeuvre Times and Image Acquisition Times The manoeuvre times can be separated into two contributors: • Actual manoeuvre time; • S/A tranquillisation time. The actual manoeuvre time depends on the size of manoeuvre, chosen technology (wheel size, etc.) and manoeuvre strategy. The S/A tranquillisation time depends mostly on the pointing drift required for the image acquisition (the more challenging the requirement, the longer the tranquillisation time). An allocation of 70 seconds per manoeuvre has been considered. 3.4 Cloud Coverage Analysis Two of the key features of Geo-Oculus, the possibility for real-time commanding and the capability for short revisit cycles have been found to give an essential asset in order to maximise the mission performance - the optimisation of mission planning for cloud cover. The intention of this analysis has been to identify the potential of Geo-Oculus that can be gained, to validate the system requirements, to identify a possible optimisation strategy and to assess the performance compared to reference missions. Due to its geostationary orbit, Geo-Oculus has the capability to access every spot within its footprint at the time the spot becomes cloud free. Considering the applied FOV and the possible agility of the system, this capability confined. In result only a certain image acquisition frequency is achieved; hence a selection of the images is required. This leads to the point that the system will have to apply a permanently updated optimisation of the mission planning, to gain maximum possible ground coverage. This optimisation should take into account the current cloud cover situation, possibly supplied by MTG and Metop, the changing illumination conditions throughout the entire day, nowcasting and short range forecasting information on the expected cloud cover situation and the constraints placed by the on-demand missions. In the analysis described in here, a simplified optimisation strategy and mission planning have been used, considered to represent a realistic approach. This strategy accounts for the illumination conditions and maximises the achieved ground coverage. The entire cloud coverage analysis is based on cloud mask data from MSG with a revisit time of 15 min. The time span, considered in this analysis range from 01/2004 to 05/2007. In a preliminary low level analysis representative days for a detailed evaluation of the cloud coverage are filtered out of the complete dataset. To gain representative results from the analysis, representative days are indicated by analysing every day concerning: • Cloud amount • Cloud coverage changes • Time of sufficient illumination conditions By comparing the values of each day with the mean value of the whole data set, several days for detailed analyses have been indicated. Page 3-22 Doc. No: GOC-ASG-RP-002 Issue: 2 Astrium GmbH Date: 13.05.2009
3 Final Report Referring to these days the detailed cloud coverage analysis is conducted. It comprises four stages: • Analysis of evolution of geometrical conditions through the day (Illumination situation) • Analysis of cloud coverage amount and dynamics • Evaluation of the performance of Geo-Oculus for different system set ups • Comparison of the performance provided by Geo-Oculus with other planned EO Systems The analysis of geometrical conditions regards especially the system requirements on the solar zenith angle and the view zenith angle. For Geo-Oculus the view zenith angle of every region is constant all times. By contrast, the solar zenith angle, hence the illumination condition changes through the day and is depended to the season. To consider this in the analysis, the illumination conditions are calculated for each cloud mask file by computing VZA and SZA for each pixel. These information are one necessary input for the simplified mission planning, applied in the performance evaluation of Geo- Oculus. The second necessary input information are evaluated in the analysis of cloud coverage amount and dynamics. Herein the cloud mask data is evaluated concerning cloud amount and cloud coverage changes through one day. • Cloud amount is defined as how long a pixel was clouded in the time between 06.00 UTC and 18.00 UTC. It is provided in percent. Analysing the cloud amount allows to point out areas, where observation is possible, in general. • Cloud cover changes is defined as number of changes of a pixel from clouded to unclouded or vice-a-versa within the considered time-frame (06.00 UTC to 18.00 UTC) in the 15 min time interval of the MSG data. Since 49 cloud mask files are available in this time-frame, a maximum of 48 cloud cover changes can occur. With the evaluation of the cloud coverage changes, the dynamics of the cloud situation are indicated. With the help of this, it is possible to point out areas where (nearly) cloud free products can be generated, by acquiring the same area several times, as it is possible with Geo-Oculus. Some results of this analysis are to be seen in figure 3.4-1. Doc. No: GOC-ASG-RP-002 Page 3-23 Issue: 2 Date: 13.05.2009 Astrium GmbH
Page 1: Geo-Oculus: A Mission for Real-Time
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