GSC Sentinel-2 PDGS OCD - Emits - ESA

GSC Sentinel-2 PDGS OCD
Issue 1 Revision 2 (draft) - 25.07.2010
GMES-GSEG-EOPG-TN-09-0008
page 90 of 350
Over oceans or open seas, the WGS84 geoid altitude is considered. Presently WGS 84 uses
the 1996 Earth Gravitational Model (EGM96) geoid, revised in 2004. Over land and inland
waters, the altitude information is given with respect to the geoid altitude reference EGM96.
4.3.9.3 Data Compression versus Data Quality Trade-Off Constraints
An average acquisition of 6640 km along the Sentinel-2 orbit will amount to about 60GBytes
of on-board compressed raw instrument data. Therefore the daily amount of compressed data
received on-ground will result to about 887GB per satellite. Besides issues related to data
processing and storage, these large volumes raise a non-negligible constraint for the efficient
dissemination of final products to end users. Indeed, it was computed that a Sentinel-2
square image of 290km x 290km comprehensive of all spectral bands will weight around 7.4
GBytes without any compression.
Hence, a data-compression step is provisioned for at the term of the MSI processing cycle.
Nevertheless, data-compression shall be controlled such as to avoid jeopardizing image
quality. To this end, a preliminary trade-off analysis based on the standard JPEG2000
compression algorithm is reported in [RD-30] to support the optimal selection of this
compression step.
4.3.10 CONSTRAINTS AND DEPENDENCIES DRIVEN BY THE CGS
NETWORK
As a matter of fact, the CGS network that will support the PDGS operations is at this stage
not settled. This section reviews the constraints and dependencies associated.
4.3.10.1 Near-Real-Time Data Timeliness
The large MSI mission data-rate and global acquisition plan coupled with the available CGS
network selected to recover the payload data on-ground will constrain the amount and data
that can be provided with NRT timeliness, while ensuring that all data can be provided within
a day.
Based on [AD-03], it is assumed that about 25% of the globally acquired data by one
spacecraft can be downlinked with NRT-compatible timeliness when using a suitable CGS
network of 4 stations. It was also calculated that a payload data recovery plan leading to
100% of NRT-compatible timeliness can be obtained using a network of about 6
complementary ground-stations.
Figure 4-6 extracted from [RD-19] depicts the distribution of the age of the data at the time it
is acquired on-ground, without zone prioritization for NRT playback, and considering a CGS
network composed of the Kiruna, Svalbard, Maslapomas, and Prince-Albert stations. In this
simulation, the globally averaged timeliness amounts to 6.298 hours at data-reception time.
When using the NRT priority downlink functionality outlined in section 4.5.4.2.2 ensuring that
specific zones are acquired with NRT-compatible latencies, the remaining global timeliness
distribution may be altered significantly; Figure 4-7 shows that in the same 4 stations
scenario, prioritizing Europe for NRT timeliness makes the geographical distribution of data
age vary meaningfully with respect to the routine scenario.
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