4 Final Report - Emits - ESA

4 Final
Report
This is based on a worst case data rate scenario with the following assumptions:
• Post-integration with (simple) image motion compensation is done on-board (necessary for the
UV-blue channels of the marine applications with up to 30 images to be summed up).
• Due to the required image summation, 18 bits per pixel are assumed to provide an adequate
quantisation. For the sake of a simple and reliable algorithm, the 18 bits per pixel are assumed
for all channels even if no image summation is required.
• For the panchromatic channel VNIR7 required for the disaster monitoring mission, it is
assumed that 4 images are down-linked which may then be subjected to motion compensation
and post-integration with a more accurate and sophisticated algorithm.
• 2 x 2 pixel binning of the UV-blue and red-NIR channels of the marine applications has not
been considered for the data rate.
With these partly conservative assumptions, an evolution of the data rate throughout the next study
phases may be compensated such that no change of the selected data transmission technology
becomes necessary. For details about the choice of the location and technology of post-integration
and image summation, see Ref. [RD 8].
The instrument data are routed via cross-coupled high rate serial interface to the PDH. In the PDH the
data are buffered and formed to a continuous data stream with formatting (CADU generation), RS
encoding and scrambling. The buffer size has to be determined in the coming study phase since it
strongly depends on the ratio of average to peak instrument data. The PDH has a fully redundant
structure with the input modules, the buffer and the TMFE output modules interfacing with the cold
redundant transmission chains. The TMFE outputs provide full cross-coupling to the modulators.
The PDT is based on cold redundant transmit chains with each consisting of modulator and SSPA,
followed by a non-redundant chain selection switch and an output filter. In order to reduce power
consumption, a high gain satellite transmit antenna of 0.8m has been selected together with a ground
station receive antenna of 13m diameter. The satellite transmit antenna has a beamwidth of approx. 3
degree (3dB double sided beamwidth). The peak gain of the antenna is considered in the link budget
requiring antenna pointing via a 2 axes pointing mechanism in case of satellite re-orientation.
Payload data handling, modulator, amplifier and antenna are based on existing components or require
minor modifications to be suitable for Geo-Oculus. The only exception is the antenna pointing
mechanism due to the large amount of operational cycles. It is expected that upgrading of existing
designs or delta qualification will be sufficient.
Carrier frequency selection (ITU constraints)
The choice of carrier frequency is dependent on a number of technical and regulatory constraints,
including the ease of frequency coordination and location of ground station. For the less than 300 MHz
required bandwidth proposed, use of X-Band has been chosen as the baseline (maximum bandwidth
available in the 8 GHz downlink band is 375 MHz). Use of the currently unused EES spectrum at 26
GHz (Ka Band) could also be feasible, if a ground station is capable of overcoming propagation
issues.
Antenna design baseline
Due to the high downlink data rate (250 Mbit/s), a High Gain Antenna has been selected as a solution
Page 4-54 Doc. No: GOC-ASG-RP-002
Issue: 2
Astrium GmbH Date: 13.05.2009