Data Acquisition

Special hardware and software are needed to acquire data directly from the satellite or via a DOMSAT link. Data acquired by either of these two methods is formatted as "raw" AVHRR data. Refer to the AVHRR Data Characteristics section of the ADAPS Overview for a description of the "raw" AVHRR data.

Direct Receive

The direct receive hardware located at the EROS Data Center (EDC) consists of an automatic tracking antenna, preamplifier, down-converter to an intermediate frequency, amplifier, high density tape recorder (HDT) for off-line acquisitions, bit synchronizer, and frame synchronizer. The antenna tracking system must have the satellite orbit and schedule information, so that it can point the antenna and acquire data from the spacecraft at the proper time. When the antenna system has "locked on" to the spacecraft, the antenna will track it while receiving the data. The maximum time of sight is about 15 minutes. Once the data has been "bit synchronized" where bits are identified as "on" or "off", the frame synchronizer finds a unique pattern in the bit stream (called a frame sync sequence) which signals the beginning of each scan line of data. This sequence is located and checked for errors, then used to decode the rest of the actual data. The output of the frame synchronizer is a sequential bit stream arriving at 1.3 million bits per second (mbps). The acquisition software system accepts this data and fills a series of buffers containing the incoming bit stream. The buffers are then written to disk as a "raw" AVHRR image file, and transferred to the ingest and archive system. The "master schedule" of acquisitions will also be updated to reflect an acquired or missed acquisition. A schedule of predicted passes over a receiving station can be generated using the AVHRR model and ephemeris information.

DOMSAT Receive

Each NOAA polar orbital satellite passes over a NOAA receiving station about twelve times a day: four over Wallops Island, Virginia and eight over Fairbanks, Alaska. These NOAA receiving stations can acquire direct HRPT data and tape-recorded playback data. The tape-recorded data (GAC and LAC) are received at 2.6 mbps and recorded at the stations. The HRPT data is received at 1.3 mbps during the pass and relayed to a commercial communications satellite (DOMSAT) in real time. After the satellite has passed over, the recorded data is played back through the communications satellite (DOMSAT) to NOAA in Suitland, Maryland at half speed (1.3 mbps).

EDC has a stationary 8 meter antenna that also acquires selected HRPT, GAC, and LAC data over the communications satellite, as it is broadcast to NOAA in Suitland. NOAA generates a schedule of recorded data and when it will be played back, available via an electronic catalog bulletin board. This schedule is used to select scenes to be acquired via DOMSAT. The selected scenes are placed in a "master schedule" which is used by the DOMSAT acquisition software to "weed out" non-selected scenes. The same hardware needed for direct acquisition is used to bit and frame synchronize the data. The DOMSAT acquisition software then writes the incoming bit stream to buffers and saves it on disk as a "raw" AVHRR HRPT, GAC, or LAC image file. The AVHRR data is then transferred to the ingest and archive system. The "master schedule" of acquisitions will also be updated to reflect an acquired or missed DOMSAT acquisition.

For a detailed description of the AVHRR data and TIROS-N series satellite instrumentation, refer to the NOAA Technical Memorandum NESS 95, NOAA Technical Memorandum NESS 107, NOAA Polar Orbiter Data User's Guide

Reference:
Acquisition and Preprocessing NOAA HRPT Data
(Jim Ellickson, NESDIS)
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