The following are short descriptions of the types of hardware and peripherals used at EDC to acquire AVHRR data directly and over the DOMSAT communications link.
The High Density Tape (HDT) is used to acquire data offline. Offline recording to HDT is done when the VAX 3800 is down, disks are full, or when nobody is around to monitor the system. An HDT tape contains 14 tracks (seven usable data tracks for recording up to 21 acquisition). The recorded data is a copy of the data stream received from the satellite. The data recorded on HDT must be piped through the frame sync and acquisition process as if it were being directly acquired.
The current acquisition system is a DEC VAX 3800 with one CPU and 16 Meg of memory. Four gigabytes of disk space is reserved for acquisition and transfers to the archive system. The acquired data is transfered from the VAX 3800 to a Silicon Graphics over an ethernet connection.
An NBS time clock is attached to the VAX 3800 system and is used to adjust the acquisition time. The clock on-board the satellite can drift plus or minus one second. This on-board clock time is received as part of the AVHRR data stream and compared to the NBS clock time. The difference between the times is saved in a header associated with the AVHRR data and used to adjust the clock time when the data is processed.
ACQUMAN is the VAX executable which acquires satellite data from the NOAA satellites using the 3 meter tracking dish. ACQUMAN will acquire data from either the dish or from HDT. When acquiring data from the dish, ACQUMAN does not interfere with the Antenna Control Unit (ACU) and therefore divorces itself from the details of tracking. When acquiring from HDT, ACQUMAN must position the HDT to the correct footage reading and channel. The bit synchronizer, frame synchronizer and receiver are controlled by ACQUMAN when receiving from the dish, but only the frame synchronizer is needed when playing back an HDT.
ACQUMAN uses the NBS time clock in order to determine the acquisition start and stop time as well as to correct the satellite times. ACQUMAN also subsamples the incoming data and displays it on the Peritek quicklook display.
Since ACQUMAN shares an output disk as well as the frame synchronizer with the Wallops DOMSAT INHALE process, ACQUMAN will disable Wallops INHALE during local acquisitions by sending messages to a common mailbox.
Since data can acquired from the satellite and relayed from each NOAA receiving station, two acquisition streams (Wallops and Gilmore) are necessary. There is also a conflict between direct receive at EDC and DOMSAT acquisitions. To resolve this the Wallops data stream is shared with the direct receive at EDC. This is because the coverage area of the Wallops receiving station is very similar to the EDC coverage area. When the direct receive at EDC is completed it is switch to Wallops to ensure that the recorded data played back by Wallops is acqured. INHALE takes care of this switching and aquisition of the recorded data from Wallops. INHALE also monitors the Gilmore data stream and acquires this data.
ACQUMAN and INHALE use a "master" schedule to determine which images are to be acquired or saved. After an acquisition the master schedule is updated to indicate it the pass was acquired, missed, or placed on HDT. The next acquisition is also determined at this time. The following is an example of the master schedule:
199 AH14012795175940 N14 286307980 286308880 286307980 286308880 0 N 99 . D W HRPT 0 0399 AH14012795194240 N14 286314160 286314930 286314160 286314930 0 N 99 . D W HRPT 0 0400 AL14012795191137 N14 286315271 286315616 286312297 286312987 1 M 99 . N W LAC_ 0 0399 AH14012795213200 N14 286320720 286321619 286320720 286321619 1 M 99 . D F HRPT 0 0401 AL14012795205342 N14 286322059 286322404 286318422 286319112 1 M 99 . N F LAC_ 0 0
The header record indicates the number of records in the master schedule. Each record in the master schedule contains the scene ID, satellite number, orbit number, local start and end times of acquisition, GMT start and end times of acquisition (both in total seconds since 1987), acquisition operation code (don't acquire, acquire the image to disk, acquire the image to HDT, acquire to both, or pull the image from the HDT), status flag (no acquire, pending, on disk, on tape, missed), tape file number (if the image was acquired to HDT), product flag (. indicates no product) day/night acquisition flag, station id, type of data (HRPT, LAC, or GAC), and a number.