CHAPTER 7. TARGET TRACKING
5. LOBE-ON-RECEIVE-ONLY (LORO)
LORO is a mode of radar operation developed as an EP feature for a track-while-scan radar. LORO can be employed by any radar that has the capability to passively track a target. In a LORO mode, the radar transmits a continuous signal from a set of illuminating antennas. This continuous signal hits the target, and the return echo is received by a different set of receive antennas (Figure 7-12).
The receive antennas are passive and generate azimuth and elevation tracking signals by electronically scanning the reflected signal. The tracking signals are sent to the antenna servos to keep the illuminating antennas pointed at the target and centered in the receive antenna tracking area. The range tracking circuit uses the time delay between the transmission and reception of the illuminating antenna signals. A split-gate tracker is used to provide range tracking.
Figure 7-12. LORO Mode
a. The illuminating antennas used in the LORO mode have very narrow beamwidths and transmit at a high power level. This reduces the effectiveness of noise jamming techniques against a radar employing a LORO mode. In addition, the continuous signal from the illuminating antennas negate the effectiveness of most angle deception jamming techniques designed to defeat TWS radars. These specialized jamming techniques exploit the scan rate of TWS antennas. In the LORO mode, the illuminating antennas do not have a scan rate. The limited effectiveness of both noise and deception jamming techniques is the major advantage of the LORO mode.
b. The LORO mode also provides a track-on-jam (TOJ) capability to exploit noise jamming techniques. In a TOJ mode, the receive antennas passively track any detected noise jamming signals. The radar assumes that the most intense jamming signal is the target. The receive antennas process the strongest jamming signal as if it were a target echo from the transmit antenna signal. The receive antennas generate azimuth and elevation tracking signals to keep the jamming signal centered in the tracking area. The TOJ mode does not provide target range.
6. MONOPULSE RADAR
Monopulse radars are among the most complex radar systems. From a single pulse, a monopulse radar can derive all the data needed to update a target’s position. It does this by comparing the relationship of two or more radar beams that are transmitted together from the same antenna but received separately. By comparing the phase or amplitude of the energy in these returned beams, target azimuth and elevation can be found. The speed that a monopulse radar updates the target’s position, coupled with its azimuth/elevation accuracy and resistance to jamming, make this a popular choice amongst many newer TTRs.
a. The Magic T circuit allows monopulse radars to gather and process information from a single pulse that is transmitted and received using separate antennas. Figure 7-13 depicts a four-beam monopulse radar system. The Magic T is a sophisticated wave guide that can separate multiple signals by their phase relationships. This allows the radar tracking computer to compare the signal amplitude from the reflected pulses in several distinct ways. As the reflected energy enters the Magic T, it is separated by phase. The energy in the “H” arm will be in-phase and will exit from ports 1 and 2. The received energies entering the wave guide in the “E” arm exit at the number 1 port. This energy is exactly 180° out-of-phase with energy entering the H arm. This ensures there is no transfer of energy between the E and H plane arms. A typical monopulse radar would have eight Magic T's.
Figure 7-13. Monopulse Magic T
b. The output of a Magic T is the sum and difference of the two signals. These sum and difference values in amplitude or phase are used to generate azimuth and elevation error signals as well as to compute range. Monopulse radars may split the incoming signal as depicted in Figure 7-14. Upper antennas receive the A and B signals. Lower antennas receive the C and D signals. The various combinations of signals are processed and compared by simple addition and subtraction of the signal characteristics. From these steps, azimuth, range and elevation data are computed.
Figure 7-14. Magic T Output Signals
(1) The top equation in Figure 7-15 is used to compute target range. Target range is derived by adding the signal from the A scan to the signal from the B scan. This value is then added to the sum of the C and D scan signals. The output of these combinations is then passed to the range circuit which figures out the range of the target and displays it to the operator. Monopulse range tracking is accomplished by using either a leading-edge or split-gate tracking loop.
(2) Target elevation tracking error is derived using the middle equation from Figure 7-15. The signal from the A scan is added to the signal from the B scan. The signals from the C and D scans are also added. This time, however, the sum of A+B is subtracted from the sum of C+D. This value is then passed to the elevation circuit. Elevation signals are sent to the operator display and the servo mechanism, which corrects to the updated position of the target.
(3) The bottom equation from Figure 7-15 is used to compute the azimuth error. The signal from the A scan is added to the signal from the C scan. The B and D scans are also added together. The sums of these values are subtracted from each other. This difference equals the tracking error in azimuth. The radar system will then position the search beam to even the energy level between the two pairs of sums. When this occurs, the azimuth tracking error is zero.
Figure 7-15. Monopulse Tracking Loops
c. A further illustration of the idea of signal combinations can be seen by referring to the F-16 in Figure 7-16. All the energy is received in the B scan area.
The A scan signal is added to the B scan signal. The signals from the C and D scans are also added. However, the sum of A+B is now subtracted from the sum of C+D. In this case, the values from the A scan and the C scan are zero. This total value of (A+B) - (C+D) is then passed to the elevation circuit.
Figure 7-16. Monopulse Elevation Tracking Error
(1) The comparison in Figure 7-17 shows that all the energy is in the B scan. The radar will reposition the scan vertically to balance the energy between the B and D scans. When the energy level is balanced, the elevation error is zero.
Figure 7-17. Monopulse Elevation Track
(2) Using the azimuth error equation from Figure 7-15, the azimuth tracking loop computes the azimuth error and repositions the antenna to equalize the received energy in all the beams. The monopulse radar has now established a tracking solution (Figure 7-18). All these computations are done instantaneously on a pulse-to-pulse basis.
Figure 7-18. Monopulse Azimuth Track