ENVISAT measurement data can be transmitted to ground stations via the Artemis data relay satellite using the Ka-band. The antenna on the platform is located on the upper payload panel just above SCIAMACHY’s sub-solar port, i.e. the instrument window permitting observations of the sun when it has reached highest elevation each orbit. When the antenna dish is deployed in operating position, it vignettes SCIAMACHY’s sub-solar window. Sub-solar measurements, necessary to monitor the long-term behaviour of optical components, are therefore only possible when the Ka-band antenna has been moved to its parking position.
ENVISAT Ground Segment
All information generated by the instrument, i.e. recorded housekeeping (HK) telemetry reporting the instrument status over the complete orbit and measurement data, is downlinked either via the X-band or the Ka-band link. The X-band link transmits data to the high latitude receiving ground station at Kiruna while the Ka-band link connects ENVISAT via the Artemis data relay satellite with the receiving station at ESRIN. In the early phase of the mission, when Artemis was not operational, only Kiruna and an additional X-band station at Svalbard were used. In the nominal ‘Kiruna-Artemis’ scenario, Global Mission data are provided to Kiruna for 8 orbits per day, to ESRIN for 6 orbits per day. HK telemetry is also sent via the command & control (C&C) S-band link of the Kiruna station and its supplementing Svalbard C&C facility. This telemetry stream spans only the coverage interval of Kiruna or Svalbard visible orbits. Of the 14.3 orbits per day, 9-10 are Kiruna visible orbits with coverage intervals amounting up to 12 min (spacecraft elevation > 5°). In addition, low elevation coverages may also be used. Each coverage in a daily cycle occurs at a different time relative to ascending node crossing (ANX) depending on the actual location of the orbital track.
ENVISAT and its instruments are controlled via the Flight Operation Segment (FOS) and the Payload Data Segment (PDS). FOS handles all command & control issues including flight dynamics aspects. In addition, mission planning is performed in cooperation with the PDS. FOS comprises the ENVISAT Flight Operation Control Centre (FOCC) at ESOC and the S-band stations at Kiruna and Svalbard. The PDS is the responsible entity for ENVISAT measurement data. It is an European wide distributed ground segment.
Central control of the PDS occurs at the Payload Data Control Centre (PDCC) at ESRIN. The Payload Data Handling Stations in Kiruna (PDHS-K) and Frascati (PDHS-E) not only receive X-band or Ka-band data but also process and disseminate data in near-realtime (NRT). For data products in offline (OL) mode processing, archiving and distribution is shared among several facilities. At the Low Rate Reference Archive Centre (LRAC), located at Kiruna, level 0 data are consolidated and archived. From these consolidated products, the Processing and Archiving Centres (PAC) generate, archive and disseminate consolidated level 1b and level 2 products. Also NRT products from the two PDHS are collected at the PACs. Although implemented at national remote sensing facilities, PACs are an integral part of the PDS.