4 Final Report - Emits - ESA

4 Final
Report
launch, remaining on until completion of LEOP.
RF signals from the ground stations at Usingen in Germany, or Maspalomas in Gran Canaria, will be
received from both receive antennas, superposed in the combiner and routed to both receivers. The
first receiver achieving a subcarrier lock will be selected by the SCU telecommand decoder as the
‘active’ receiver.
The nominal transmitter will send the generated RF signal to both nadir and zenith antennas for
transmission to ground. In the case of a failure, the nominal transmitter will be deactivated and the
redundant transmitter will take over operation.
Telemetry, Tracking and Command
The uplink frequency will be within the 2025 – 2110 MHz band whereas the downlink frequency will be
within the 2200 – 2290 MHz band. Coherence will be implemented, applying the turnaround ratio of
221/240, and can be enabled or disabled using appropriate commands.
During TT&C operation, the uplink Telecommand data and the downlink Housekeeping Telemetry
data are modulated on to subcarriers with typical bitrates of 2000 bps and 8192 bps, respectively. The
subcarriers are phase modulated onto the respective uplink and downlink carriers, together with the
ranging signal.
The helix antennas provide hemispherical coverage with a worst case antenna gain of about -3 dBi.
The Link Budget has been developed to achieve required minimum link margins, e.g. >3dB nominal
TM recovery margin, while fully respecting the maximum Power Flux Density (PFD) requirements.
Link Performances
A sample link budget for S-Band TT&C and Ranging link is shown in Table 4.5-1 below. Basic 223/255
Reed-Solomon encoding has been assumed for the telemetry downlink. The chosen ground station is
Maspalomas on Gran Canaria, which has a 15m diameter dish and a reception G/T of 29.2 dB/K.
Table 4.5-1: Assumptions for S-Band TTC Links
Programme: Geo-Oculus Orbit: GEO
Ground Station: Maspalomas-1 (S-Band)
Height: 35800.00 km
S/C Antenna: AEOLUS LGA (S-Band)
Elevation: 10 degrees
Type: Ranging
Amplifier RF Output: 5.0 W Ranging Possible: TRUE
Downlink Data Rate: 8,192 s/s Uplink Data Rate: 2,000 s/s
Information Rate: 7,142 bps
Coding Scheme: 223/255 Reed-Solomon Coding
Modulation Scheme: PCM(NRZ-L)/PSK/PM
Subcarrier Type: Sine Wave
The analysis yields very healthy recovery margins for Telecommand (26.45 dB nom.) and
Telemetry (18.65 dB nom.). Power Flux density limits are not violated.
It should be noted that two S-band stations are required for the envisaged spread spectrum ranging
method - for an optimal performance, an additional third station should be implemented.
Consequently, it makes sense to consider not only Maspalomas but also a second station, such as an
S-Band station in Redu, for instance. Table 4.5-2 shows the differences in EIRP and G/T for these two
stations. Redu will have a marginally better link than Maspalomas.
Doc. No: GOC-ASG-RP-002 Page 4-57
Issue: 2
Date: 13.05.2009 Astrium GmbH