Data Acquisition.

Halley station is physically isolated from the outside world for all but 2-3 months in the austral summer. During the summer period, the station can normally expect one or two visits from a BAS ship, and occasional visits by BAS aircraft. For the rest of the year, communications are maintained using the INMARSAT maritime satellite telecommunications system. The latter provides for facsimile, voice, telex, and data communications to and from the BAS headquarters in Cambridge. The data link gives a 9600 baud connection using DECnet/DDCMP between VAX/VMS clusters at Halley and Cambridge. This provides all levels of service, including remote interactive login. However, the service is charged per unit of connect time, and therefore its usage is cost limited. Because of this, the bulk of data collected at Halley is returned by ship to the UK once per year. Data summaries are routinely sent via the data link, as are subsets of the full data sets for periods of particular interest.

Communications to the AGO sites is currently limited to annual visits by BAS aircraft to retrieve the previous year's optical disks and to carry out station maintenance. A very low data rate link is planned (e.g. ARGOS) which will be used to monitor station performance. Since the AGOs are below the horizon for geostationary satellites, the retrieval of scientifically useful amounts of data during the rest of the year awaits the availability of a polar orbiter with an appropriate transponder.

Key parameters are generated, consolidated and compressed at Halley, and transmitted to Cambridge once per week. They are then validated on the BAS Remote Data Analysis Facility (RDAF) and encrypted into common data format before being routed via NSI/SPAN to the Central Data Handling Facility (CDHF). The SESAME RDAF is a VAXstation 3300 connected to the BAS Cluster by a TCP/IP link via the node (COMMS) controlling Antarctic communications. It is a node on NSI with the name `SESAME'.

The Upper Atmospheric Sciences Division of BAS has established an optical disk-based databank and data analysis system called the Geospace Data System (GDS). This already holds AIS, riometer, FPI and magnetometer data from Halley together with key parameter data, and riometer and magnetometer data from AGO A77. It is expected to be extended to accommodate ELF/VLF data and data-sets from the other AGO sites as they come into operation. The HF radar data are dealt with separately, using both optical disk storage and Exabyte tape, to maintain compatibility between the Arctic and Antarctic radars.

For periods which the mission identifies for special study (event data), BAS data will be available on request if they are already held in the GDS or HF radar databanks at Cambridge. For special periods where the data are still in Antarctica, the data link will be used to retrieve data, but this will be subject to cash limits established each year.

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