NOAA KLM User's Guide

Section 4.2

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4.2 APT System

4.2.1 General

The Automatic Picture Transmission (APT) system provides a reduced resolution data stream from the AVHRR/3 instrument. Any two of the AVHRR channels can be chosen by ground command for processing and ultimate output to the APT transmitter. A visible channel is used to provide visible APT imagery during daylight, and one IR channel is used constantly (day and night). A second IR channel can be scheduled to replace the visible channel during the nighttime portion of the orbit. The analog APT signal is transmitted continuously and can be received in real time by relatively unsophisticated, inexpensive ground station equipment (a list of equipment manufacturers is available on the NOAASIS Internet site, or by mail. See Internet resources in Appendix E) while the satellite is within radio range. The characteristics of the transmitted signal remain unchanged in the NOAA KLM and NOAA-N,-P satellite series from those in the TIROS-N series (NOAA-8 through NOAA-14), while there is a minor change in the data format to account for the modified channel 3 on the AVHRR instrument.

4.2.2 APT Transmission Characteristics

The processed AVHRR instrument data AM modulates a 2400 Hz subcarrier. The maximum subcarrier modulation is defined as the amplitude of the gray scale wedge number 8 (see Figure 4.2.2-1), producing a modulation index of 87% ±5% (not exceeding 92%). The AM modulated subcarrier is subsequently used to FM modulate the VTX transmitter operating in the 137 - 138 MHz band. Table 4.2.2-1 summarizes the pertinent APT transmission characteristics.

Table 4.2.2-1. APT Transmission Characteristics.
Line Rate 120 lines/min
Data Channels 2 transmitted
6 available
Data Resolution 4.0 km
Carrier Modulation 2.4 KHz AM subcarrier on FM carrier
Transmitter Frequency (MHz) 137.50 or 137.62
Transmitter Power (EOL) 5 W (37dBm)
Radiated Power (dBm, @ 63 degrees) 36.7
Polarization RCP

Figure showing APT Frame format

4.2.3 APT Data Frame Format

The MIRP processes the AVHRR data and outputs the APT format (simultaneously with the HRPT, LAC and GAC formats). All the processing in the MIRP is done in the digital realm. 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.

On the NOAA KLM and NOAA-N,-P series, two of the six possible AVHRR spectral channels are multiplexed so that channel A APT data is obtained from one spectral channel of the first AVHRR scan line, and channel B from another spectral channel contained in the second AVHRR scan line. The third AVHRR scan line is omitted from the APT, and the process is then repeated. It can be seen that the data processing algorithm is designed so that data from every third line from each of the two selected channels of the original high resolution AVHRR output are formatted for each of the two APT channels. The algorithm also 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. The details of this algorithm is shown in Table 4.2.3-1 and Figure 4.2.3-1. The two AVHRR channels used are identified in the daily TBUS message, and are further classified by the daytime and nighttime portion of the orbit. Channel identification is also included as part of the telemetry frame.

Table 4.2.3-2 enumerates the APT format parameters. Figures 4.2.2-1, 4.2.3-2, and 4.2.3-3 illustrate the APT frame format, the video line format and signal synchronization details. Examining the frame format shown in Figure 4.2.2-1, it can be seen that both channel A and B have a series of 16 "wedges" used in calibrating the APT image. Each of the wedges is composed of eight successive video lines. Only the wedges of one frame from an entire, received pass, are needed for calibration. Also note that wedges 1 through 14 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 identification) vary between channel A and B.

The channel identification wedge has changed in the NOAA KLM and NOAA-N,-P series now that there are six possible channels 1, 2, 3A, 3B, 4 or 5. The modulation index of wedge 16 will equal one of the first six grey scale wedges. Wedge 3 will correspond to channel 3A being in use, while wedge 6 will correspond to channel 3B being in use. All other channel numbers will be the same as the number of the corresponding grey scale wedge.

Table 4.2.3-1. APT Linearization Algorithm.
Zone 1
0 -16.98 degrees from nadir
average 4 contiguous samples 628 AVHRR data samples per channel 157 processed APTwords output to D/A converter
Zone 2

16.98 - 34.83 degrees either side of nadir
average 3 contiguous samples, skip 1, repeat 330 AVHRR data samples per channel 110 processed APT words output to D/A converter
Zone 3

34.83 - 43.83 degrees either side of nadir
average 2 contiguous samples 166 AVHRR data samples per channel 83 processed APT words output toD/A converter
Zone 4

43.83 - 48.84 degrees either side of nadir
average 1.5 samples

(A+B/2 and B+C/2)
93 AVHRR data samples per channel 62 processed APT words output toD/A converter
Zone 5

48.84 - 55.4 degrees either side of nadir

retain original resolution 121 AVHRR data samples per channel 121 processed APT words output to D/A converter


Figure contains APT Linearization
Table 4.2.3-2. APT Parameters.
Frame
Rate 1 frame/64 secs
Length 128 lines
Format See Table 4.2.3-1
Line Parameters
Rate 2 lines/sec
Number of words 2080
Number of sensor channels 2
Number of words/sensor channel 909
Format See Figure 4.2.3-1
Line sync format See Figures 4.2.3-2 and 4.2.3-3
Word parameters
Rate 4160 words/sec
D/A conversion accuracy 8 MSB's of each 10 bit word

Figure contains APT Linearization

Figure contains APT Linearization

Amended March 10, 2004


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