Monitoring weather and climate from space

A-DCS

The Argos Advanced Data Collection System (A-DCS) is a data collection relay system. The primary objective is the collection and dissemination of data for the scientific community, through measurement of temperature, pressure, humidity, sea levels and location. The data are then transmitted to the user community through the Argos network. Flying the Argos system aboard polar-orbiting satellites provides worldwide coverage. Additionally, incorporating the A-DCS instrument on a moving satellite allows for locating an in-situ platform using Doppler shift calculations. This positioning capability permits applications such as monitoring drifting ocean buoys and studying wildlife migration paths.

The system is used with great success in the following applications:

  • Studying oceans and atmospheric conditions.
  • Preserving and monitoring wildlife.
  • Monitoring volcanoes.
  • Monitoring fishing fleets.
  • Monitoring shipments of dangerous goods.
  • Humanitarian applications.
  • Managing water resources.

The A-DCS consists of two units, RPU (Receiver Processing Unit) and TXU (Transmit unit). Both contain a nominal and cold redundant sub-system, the selection is made via an RPU interface card, which has a pair of relays for each unit.

On transmission of an Argos Beacon the signal is received by the RPU via an antenna (401.650 MHz). The RPU, having the ability to process 12 messages simultaneously (three 4,800 bps and nine 400 bps), converts the incoming signal to an IF signal and ascertains the location of each platform using the Doppler Effect. (Accurate timing allows a location accuracy of between 150–1000 m.)

The RPU processing software performs the following signal functions: acquisition of carrier; signal demodulation; bit and frame synchronisation; Doppler counting; time tagging and message formatting. Once the 12 buffers are full, the RPU encoder commands the data to be transferred to memory ready for transfer to the spacecraft. On receipt of a data transfer request from the spacecraft the data is available for real-time transmission or delayed transmission, as determined by the orbit.

Unlike previous generations, the A-DCS instrument is able to send messages to the users' mobile terminals through its UHF transmitter (TXU, 465.9875 MHz bi-phase PM 200 or 400 bps). This provides the ability for message acknowledgement; time sync of the beacon; update of the beacon activity profile and updating of the satellite status/position for the beacon.

The up-link occurs via Argos Master Beacons (Svalbard, Toulouse and Fairbanks). The up-link Argos message is analysed/processed and prepared for down-link by the RPU, which is then transmitted by the UHF transmitter. This transmission is independent of the satellite operations and has no impact on them, since the down-link is permanent — that means that when there is no message, the down-link contains filler code.

The Argos programme is administered under a joint agreement between the National Oceanic and Atmospheric Administration (NOAA) and the French space agency, Centre National d’Etudes Spatiales (CNES). The A-DCS instrument is provided under a cooperative agreement by CNES and was developed by Thales Elancourt. Data processing is performed by CLS Argos, Toulouse and Service Argos Inc, USA.
 
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