GSC Sentinel-2 PDGS OCD - Emits - ESA

ESA UNCLASSIFIED – For Official Use
GSC Sentinel-2 PDGS OCD
Issue 1 Revision 2 (draft) - 25.07.2010
GMES-GSEG-EOPG-TN-09-0008
page 132 of 350
whenever no data is fed by the MMFU, the satellite XBS will automatically transmit idle
frames with an adequate noise pattern to make the modulated signal compatible with CCITT
regulations.
4.5.4.2.3 Ancillary-Data Packet-Store Management
As introduced in section 4.3.8.5, the asynchronous handling of the Ancillary and MSI data on
board, each one routed to different packet stores separately operated for downlink, imposes a
specific scenario to ensure that the ancillary data coinciding in time with the MSI data
downlinked at a specific CGS or LGS is appended to the downlink.
The ancillary data acquired along one orbit can be downlinked in a very short time at any
given station pass unlike the corresponding MSI data which will necessarily be scattered
throughout several station passes. It is hence required that the ancillary packet store is
managed as a time-rolling buffer systematically ensuring via its complete playback the
coverage in time of the MSI data timeline received at every CGS or LGS.
To this aim, and coupled with the ancillary data acquisition scenario described in section
4.5.4.1.2, the ancillary-data circular buffer will be operated as follows for data recovery onground:
○ It will be minimally sized such as to hold at any given time the data down to a couple of
orbits in the past corresponding to the maximum uncertainty inherent to the MSI data
downlink strategy;
○ It will be managed as a time-rolling buffer (cf. 4.5.4.1.2) and downlinked entirely by
playback to every CGS or LGS following the MSI downlink. This will ensure that the
acquired ancillary-data covers entirely the MSI timeline recovered at the station.
Playback commanding of the Ancillary memory will be driven by the PDGS through the
mission-planning loop with the FOS.
Adequate sizing of the packet store can be performed in-flight via a specific telecommand.
The PDGS will manage this parameter as part of its mission planning activities such that it
can be re-adapted at will following changes on the overall data downlink scenario.
4.5.4.2.4 On-Ground Data Reception
At every CGS, the PDGS will acquire the mission data downlinked from the spacecrafts as
driven by the CGS schedules derived from the downlink plan (cf. section 4.5.4.2.1.4).
According to the downlink agenda, the reception activities will be activated shortly before the
planned AOS event and include the following real-time operations:
o The antenna selected for acquisition will be moved to the AOS position and signal
search will be activated enabling tracking on the RF carrier signal with the associated
antenna movement until the planned LOS;
o Bit synchronisation, locking on the Sentinel-2 signal and demodulation from AOS to
LOS will be autonomously performed by the demodulation hardware. For Ka-Band
acquisitions via EDRS, a dedicated EDRS compatible hardware will perform the
demodulation;
o Front-end processing activities will be performed in parallel according to the front-end
processing schedule including frame synchronisation, decoding (descrambling and
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