4 Final Report - Emits - ESA

4 Final
Report
for the PDHT system. The need for a body-mounted instrument affects the contact to the ground
station. Due to the necessity to transmit while the satellite is repointing, a steering mechanism for the
HGA will be required. This will probably involve a boom mounting scheme on the spacecraft. The
impact on the spacecraft in terms of pointing disturbance has not yet been assessed. Control of the
HGA motion will be within the PLIU.
Other options such as an electrically steerable (phased array) antenna or a Low Gain Antenna (LGA)
do not provide sufficient gain for the high data rate and have now been removed.
Modulation and coding scheme
As OQPSK has been selected as modulation scheme, standard RS encoding 255/223 is proposed.
This results with an instrument output rate of 250 Mbps in an occupied bandwidth of < 300 MHz in Xband.
The Reed-Solomon code to be used is in agreement with ECSS standard ECSS-E-50-01A
(Telemetry Sync and coding), Chapter 6. A pseudorandomiser could be used to provide sufficient
channel symbol transitions and hence improve received symbol lock.
Link Budget
Considering a bit error rate of 10 -9 and a transmit power of 8W, a satisfactory link margin of 3.9dB is
achieved with a ground station of 13m diameter.
Ground station interface
Selection of ground station locations would be dependent on the geostationary longitude of the
spacecraft, as described in the System requirements document. A location in central Europe such as
Italy or Germany would be capable of viewing a spacecraft regardless of whether it is situated at a
longitude more than 45 degrees East or West. This would provide some flexibility in choosing a
suitable position for the spacecraft. In all cases the spacecraft should be continually more than 10
degrees above the horizon as seen from the ground station in order to provide sufficient link margin.
Additional considerations to the ground station would include the interface to the user segment,
including archiving availability, offsite data links, processing centre capability and user access and
security. These factors are especially important where mobile terminals could be deployed as part of a
network.
The block diagram of the PDT is shown in Figure 4.5-6.
Nominal
Chain
Data
Clock
Redundant
Chain Data
Clock
OQPSK-
Modulator
OQPSK-
Modulator
Figure 4.5-6: PDT Block Diagram
SSPA
X-Bd
SSPA
X-Bd
Doc. No: GOC-ASG-RP-002 Page 4-55
Issue: 2
Date: 13.05.2009 Astrium GmbH
HGA