The satellite sensor is the "Advanced Very High Resolution Radiometer" (AVHRR), which is carried onboard Polar-orbiting Operational Environmental Satellites (POES) like NOAA-15 and NOAA-17. The sensor measures in several spectral bands (AVHRR/3 measures in six bands).

In the APT system an analog signal is transmitted continuously on VHF and can be received in real time. The nominal resolution is 4 km/pixel and the line rate is 120 lines/minute (2 lines/second). This system provides a geometrically-corrected (to reduce the perspective effect due to the Earth's curvature and the satellite altitude) and reduced resolution data stream from the AVHRR/3 instrument. Only two spectral bands are transmitted to the ground at any time: a visible channel (Ch. 2) is used to provide visible APT imagery during daylight, and an IR channel (Ch. 4) is used constantly (day and night). A second IR channel can be scheduled to replace Ch. 2 during the night time portion of the orbit.

An APT line, consisting of one line of Video A and one line of Video B, is output every third AVHRR scan. Ancillary AVHRR data appear on one edge of each line and their 64 second repetition period defines the APT frame lenght. The resulting line rate is 2 per second.

Click on the next picture for a real size and unprocessed NOAA satellite image, where the different parts are described. Only six minutes from an entire satellite pass are shown.

The two AVHRR channels used are identified in the daily TBUS message, and are further classified by the daytime and night-time portion of the orbit. Channel identification is also included as a part of the telemetry frame. Only One Complete APT Frame from an entire received pass is necessary to calibrate the image (see picture). One complete APT frame in both channel A and B has a series of 16 "wedges" used when calibrating the APT image. Each of the wedges are composed of 8 successive video lines. Wedges 1 through 14 (the count starts at the top of the indicated yellow line on the picture) are identical on the images from both channels A and B. Only wedges 15 (the back scan value when one of the IR radiometers "looks" at a blackbody radiator) and 16 (channel identificaton) vary between channel A and B.

The MIRP (Manipulated Information Rate Processor) processes the AVHRR data and outputs the APT format (simultaneosuly with the HRPT, LAC (Local Area Coverage) and GAC (Global Area Coverage) formats). The digitized AVHRR input consists of 10-bit words. The MIRP inserts calibration and telemetry data for each of the selected APT channels being transmitted, and AM modulates the 2400 Hz subcarrier, corresponding to the light and dark areas seen by the instrument, with the 8 Most Significant Bits (MSB) of the 10-bit data. The formatted data passes through the MIRP digital-to-analog converter, is filtered and modulated onto the 2400 Hz carrier. Note that the 8 bit radiometric resolution provides up to 256 different grey levels available for each video channel, A and B. The processing algorithm maintains nearly equal geometric resolution of 4 km along the scan line. This is accomplished by using a separate resolution reduction in each of five regions or zones either side of the nadir.

Equivalent output digital data rate is 4160 words/second (2080 words/line). The number of words/sensor channel (video data) is 909. Each Space Data/Minute Markers have 47 words. Sync A and Sync B have 39 words each one. Each Telemetry data have 45 words. Note that 909x2 + 47x2 + 39x2 + 45x2 = 2080 words. The D/A conversion accuracy is 8 MSB's of each 10 bit word.

• Telemetry frame:
  • Each video channel A and B have their own telemetry frame
  • Each telemetry frames consists of 16 points (wedges): height of 128 video lines
  • Telemetry frame rate is 1 frame per 64 seconds
  • Each telemetry point is repeated on 8 successive APT lines
• Space and minute marker:
  • Time between two successive markers is one minute
  • Minute markers are repeated on 4 successive lines, with 2 lines black and 2 lines white
  • B video is always an IR channel (Ch. 4 usually), so spaces are white and minute markers black
  • When A Video is a visible channel, spaces are black and minute markers white; otherwise it appears like B video
• APT Synchronization:
  • Sync A precedes Channel A data. Is a 1040 Hz square wave - 7 cycles
  • Sync B precedes Channel B data. Is an 832 pps pulse train - 7 pulses