.Satellite direct broadcasting, more commonly called direct readout service, is used to distribute "raw" or minimally preprocessed satellite data to anyone anywhere in the world in real-time. The services from POES are the Automatic Picture Transmission (APT) and the High Resolution Picture Transmission (HRPT) of AVHRR. The HRPT transmissions also include soundings from the TIROS Operational Vertical Sounder (TOVS) instruments.
A satellite passing directly over an antenna site will be within view of the antenna (horizon-to-horizon) for about 15 minutes when the satellite is at 833-km
altitude for about 16 minutes when the satellite is at an altitude of 870 km. Coverage is the area the satellite can see while it can be seen from the ground
station. For an overhead pass of the satellite, this area is ~ 6,200 x 3,200 .
APT services and subsystems were initially designed to broadcast direct readout satellite imagery to low-cost ground receiving equipment. In 1990, according
to NOAA, more than 5000 stations, established in at least 123 countries, received APT data from U.S. and U.S.S.R. satellites. APT signals from U.S. satellites
are transmitted on 137.50 or 137.62 MHz. Basic receivers consist of a low-cost steerable helix, yagi or omni-directional antenna, VHF receivers, and a display
device such as a personal computer, a facsimile receiver, a photographic device or a CRT. APT data are acquired whenever a POES passes within range of a ground
station, at least four times in a 24-hour period. The number of satellite overpasses depends on the latitude of the station; high-latitude stations can receive
far more than four passes a day. Each station receives only while the spacecraft is within range (for about 15 minutes). The data rate is 33,280 bps. Only
two of the five AVHRR channels are transmitted. The currently operating NOAA-11 and NOAA-10 provide Channels 2 and 4 during daylight and Channels 3 and 4 during
the night. NOAA-9, which is currently operating in stand-by status, provides Channels 2 and 4 throughout orbit. The AVHRR channels are described in Table 1.
Besides providing the location of clouds, the data, depending on the presence or absence of clouds, will provide cloud top or sea surface temperature (SST)
to an accuracy of ~2
C with a 4-km resolution.
The HRPT data are digital and include all five channels of AVHRR data as well as TOVS data and information on the satellite's attitude control system. Resolution
is 1 km at nadir and about 0.7C accuracy on sea surface temperature.
Data acquired from TOVS are used to produce atmospheric soundings. Software transforms upwelling infrared and microwave radiant energy into vertical temperature and water vapor profiles. A variety of other parameters, such as total ozone, cloud height and amount, and tropopause temperature and pressure, are also derived.
The data are transmitted to the ground receiving stations by one of three 5.25 Watt, S-Band transmitters coupled to one of three quadraphase antennas. The S-Band, real-time data consist of a split-phase PSK digital bit stream at 665.4K bits per second.
The differences between HRPT and APT transmissions are summarized in Table 2.
The NOAA National Environmental Satellite, Data and Information Service (NESDIS) maintains ground stations to capture POES data both for immediate use and for permanent archiving. The overall coverage for HRPT data from all of the NOAA NESDIS ground stations over the Northern Hemisphere is shown in Figure 2, which is taken from the NOAA Polar Orbiter Data Users Guide. There are no NESDIS ground stations in the Southern Hemisphere.
The 4-km, low-resolution data are known as Global Area Coverage (GAC). An entire orbit of GAC (115 minutes) can be stored by a single recorder. Only 11.5 minutes of high- resolution imagery (1.1-km resolution), known as Local Area Coverage (LAC) can be stored on a single recorder. LAC therefore must be scheduled.
Given advance payment and notice, NESDIS can turn around HRPT, GAC or sounding data in two to three days. The data would be processed to Level 1b. Level 1b means the raw data have been quality controlled, assembled into discrete data sets, and Earth location and calibration information have been appended (but not applied) (Kidwell 1988). Using NESDIS for HRPT data would be appropriate only if coverage exists for the area of interest.
