Transportable and mobile tactical radar systems which need to operate with coverage extending over hilly and mountainous terrain have to cope with heavy volumetric clutter even at distant ranges. Under such conditions STC circuit which is widely used to reduce large echoes from close-in clutter will not be effective.
**Hence an adaptive microwave sensitivity control is employed which has the capability to intelligently self-program the receiver sensitivity in each range–azimuth cell in an accurately and optimum fashion.
This is done by deriving a coarse clutter map from a zero-velocity (low-pass) filter, built up over a few scans for each range-azimuth cell, operating on the I & Q channel ADC data. The clutter map is built after applying a constant attenuation of 30dB uniformly in the total range-azimuth plane. Then the relative clutter level w.r.t. the saturation point is computed for each range-azimuth bin and the corresponding attenuation accurately worked out to bring the clutter everywhere into the linear dynamic range. The adaptive attenuation programming is a one time operation initiated under full power transmission by the radar operator with the push of a button. This may be done whenever the radar site is changed or whenever required.
AMSC block is configured as MASTER & SLAVE. AMSC-MASTER is housed in SPU-Rack & AMSC-SLAVE housed in Receiver-Rack. The two are connected through the serial line.
The function of AMSC-MASTER is to derive the clutter map built up over 8 scans from I & Q ADC data and to transfer this map data to AMSC-SLAVE processor through a
serial channel.
AMSC-SLAVE receives the map data, stores in its memory as a replica of map memory of MASTER, transfers the map data from RAM to the EEPROM and starts outputting map values every PRT to RF CONTROL card for generating attenuation values.
The derivation of coarse clutter map from zero-velocity filter is done as follows. In every CPI 210 range samples of I & Q data are taken starting with every PRT.
The range samples are taken at 3 µ sec interval. The I & Q samples for each range cell are integrated over 16 PRTs in a CPI. The magnitude of I & Q data is computed for each range cell using 7/8 L + 1/2 S algorithm and stored in external memory. This way magnitude for all the range azimuth cells in a scan is computed and stored in a memory. The computation and accumulation of magnitude is done for over 8 scans and the action is stopped. Since we have256 CPIs in a scan and 200 range cells per CPI, the number of range azimuth cells per scan will be 256 * 200 =51,200 i.e., 51K of external memory is required for storing the map information.
External RAM used in the circuit is of 128K words capacity and 8 pages are used to store the map information. The processor selects the memory page using the MSB 3 bits of CPI number. The locations in each page are addressed by the processor using LSB 5 bits of CPI number and 8 bits of range cell address.
The data is send to AMSC-SLAVE on a serial port of the processor. The MASTER-to-SLAVE communication is synchronous (same serial clock is used for both the processors).
Mode of communication is duplex mode, where in the word sent by MASTER is echoed back by SLAVE. The MASTER processor checks for the correctness of the received word before sending the next word. If there is any error, the word is repeated. The AMSC block operates in three modes.
MODE # 1: No clutter map generation and no transfer of data in this mode the slave has to only output the map values stored in RAM every PRT. This is normal mode of
operation.
MODE # 2: No clutter map generation, only data stored in AMSC-MASTER EEPROM is transferred to AMSC-SLAVE.
MODE # 3: Clutter map generation and transfer of data to SLAVE by MASTER.
Slave processor has to receive the data and store in its external memory. Once the data transfer is completed, the data is outputted with every PRT.
For each word of data to be transferred, three 16-bit words are sent to slave. First word gives the page number of the memory, second word gives the address of the memory where data has to be stored and the third word is the data which has to be stored in the address location given by the second word. The MSB three bits of each word are used to code the word as page number, address and data. The slave has to decode the three bits and take appropriate action like selecting memory page number or forming address pointer to load the data or load the data into specific location of the memory.
The process of derivation of clutter map has to be done with full transmitter power ON and a 30dB uniform attenuation applied to the front end, which is done by the slave processor.
AMSC action is initiated by INIT switch on the display front panel. AMSC-INIT switch resets both MASTER and SLAVE processors. If AMSC-AMSC-INIT switch is held pressed for one scan, MASTER processor should go in for derivation of clutter map.
If this facility is not given, any accidental pressing of the switch during Radar operation causes 30dB front end attenuation being applied by the SLAVE processor and the detection will suffer for 8 scans.
The hardware in AMSC card senses whether the AMSC-INIT switch is pressed for one scan and set a flag.
After initialization with reset, the MASTER processor waits for one scan time and polls the flag. If the flag is active, it starts with MODE # 3. If not it will go to MODE # 2.
Sampling of data for map generation starts with the first CPI encountered after initialization with reset. This first CPI number read from counter is stored. The functions of processor in each CPI are:
Read the CPI number.
Read current I/Q values of each range cells ; accumulate will previous I/Q values stored in internal memory.
Compute the magnitude of accumulated I and Q values of previous CPI.