TM 11-5841-287-30
Section II. FUNCTION OF ENCODING SUBSYSTEM
case of radar video data, the CPU is bypassed, transfer
2-3.
General
being direct to the output memory The contents of the
output memory are transferred, word-by-word, to the
The encoder receives moving target (MT) and fixed
output format generator for final processing During final
target (FT) radar video data, radar mode data and ADAS
processing, parity and/or ID bits are added to the various
data, which it processes (encodes) into a single serial
data words, as required Program instructions controlling
data stream suitable for downlink transmission to the
input data processing, data transfer, and output
ground station The encoded data is structured in frames
formatting are permanently stored in the output ROM
consisting of 12, 890 bits At a bit rate of 41, 667
(read only memory), while the variable radar video, radar
bits/second, each frame requires 0 309 second to
mode, and ADAS data are stored in output RAM
transmit Because of system bandwidth limitations, video
(random access memory)
data is grouped into three range fields, designated A, B
b. Video Input Data Processing The radar
and C, A-field being closest to the aircraft. Each video
video data signals (MT/FT VIDEO) are processed by
field, in turn, is divided into 560 range bins, which
dividing each video pulse period into 1680 range units
represent the smallest unit distance for which separate
(bins) and generating a digital magnitude word
video data is generated. A-field data is transmitted every
representative of video amplitude for each of the 1680
frame, while B- and C-field data is transmitted during
range bins Magnitude data for each range bin is
alternate frames.
accumulated over 256 radar pulse repetition intervals
(PRI) for A-field and 512 PRI for B- and C-fields An
2-4.
Data Frame Structure
average magnitude value is then computed for each
range bin, expressed as an 8-bit data word As a final
The contents of a typical data frame are illustrated m
step, the data is placed on the data bus for transfer to
the RAM portion of the output memory Synchronization
code, is the first data to be transmitted. Following the
during video data accumulation is derived from an
preamble, an 11-bit radar mode word is transmitted. The
external 5 MHz gated clock signal (GTD CLOCK)
radar mode word contains such information as ADAS
Synchronization during video data transfer is completely
print command, antenna status (left or right), antenna
asynchronous to video data accumulation and is
mode (single or both), and video field status (A/B or A/C)
accomplished using internally generated frequency
Forty-eight 11-bit ADAS data words are transmitted next
references The clock rate during video data transfer is 1
Certain ADAS navigational data words are replaced by
25 MHz
the encoder with equivalent substitute data, which is
c. ADAS Input Data Processing The ADAS
derived from the radar set recorder The substituted data
input data signal (DATA) is received m the form of 48 11-
represents radar range and range delay, aircraft drift
bit data words, each containing two, 4-bit ADAS
angle, and aircraft ground speed The remaining
characters ADAS processing is accomplished simply by
transmitted data consists of 1120, 11-bit words,
loading a serial-ln-parallel-out register with two complete
representing video field data To counteract the effects of
ADAS words (minus parity bits), and then transferring the
a high noise environment, check bits and error correcting
data words (via the data bus) to output RAM under CPU
codes are incorporated within the data structure (see fig
control This cycle is repeated for each of the 48 ADAS
data words Substitute RANGE and RANGE DELAY input
parity bits are provided Protection of FT target data is
data is received in the form of eight binary type signals
accomplished by using ID and parity bits, while MT data
Two of these signals are used for representation of the
protection is accomplished by ID and Hamming code bits
three possible radar ranges (25/501100 km), while the
remaining signals are used for representation of the
seven
possible
range
delay
conditions
2-5.
Overall Functional Description
(0110120130/40/50/60 km) Similar to ADAS data, the
range and range delay data word is transferred (via the
The following paragraphs describe overall functional
data bus) to output RAM under CPU control
operation of the encoding subsystem Refer to figure 2-4
d. Ground Speed Input Data Processing
for applicable block diagram details
Substitute aircraft ground speed data (FILM SPEED) is
a. General Upon entering the encoder, the
received in the form of a variable rate pulse stream The
radar video, radar mode, and ADAS data undergoes
higher the ground speed, the higher the pulse rate
conversion to 8-bit parallel format by the input data
Conversion of the variable-rate pulse to binary code is
processing circuitry After conversion to suitable digital
accomplished by counting a fixed rate clock between two
form, the data is transferred to the output memory under
central processing unit (CPU) control In the
2-3
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