1、 Rec. ITU-R M.1372-1 1 RECOMMENDATION ITU-R M.1372-1*Efficient use of the radio spectrum by radar stations in the radiodetermination service (Questions ITU-R 35/8 and ITU-R 216/8) (1998-2003) Summary This Recommendation provides some of the methods that can be used to enhance compatibility between r
2、adar systems operating in radiodetermination bands. Several receiver post-detection interference suppression techniques currently used in radionavigation, radiolocation and meteorological radars are addressed along with system performance trade-offs (limitations), associated with the interference su
3、ppression techniques. The ITU Radiocommunication Assembly, considering a) that the radio spectrum for use by the radiodetermination service is limited; b) that the radiodetermination service provides essential functions; c) that the propagation and target detection characteristics to achieve these f
4、unctions are optimum in certain frequency bands; d) that the necessary bandwidth of emissions from radar stations in the radiodetermination service are large compared with emissions from stations in many other services; e) that efficient use of the radio spectrum by radar stations in the radiodeterm
5、ination service can be achieved by reducing transmitter unwanted emissions and utilizing interference suppression techniques; f) that methods to reduce spurious emissions of radar stations operating in the 3 GHz and 5 GHz bands are addressed in Recommendation ITU-R M.1314; g) that the inherent low d
6、uty cycle of radar systems permits the use of interference suppression techniques to enable radar stations in close proximity to use the same frequency, recommends 1 that interference suppression techniques such as, but not limited to, those contained in Annex 1, should be considered in radar statio
7、ns to enhance efficient use of the spectrum by the radiodetermination service. *This Recommendation should be brought to the attention of the International Maritime Organization (IMO), the International Civil Aviation Organization (ICAO), the International Maritime Radio Committee (CIRM), and the Wo
8、rld Meteorological Organization (WMO). 2 Rec. ITU-R M.1372-1 Annex 1 Interference suppression techniques 1 Introduction As spectrum demands for radiodetermination bands increases, new radar systems will need to utilize the spectrum more effectively and efficiently. There will be heavily used areas t
9、hroughout the world where radiodetermination systems will have to operate in high pulse density environments. Therefore, many radar systems may be subjected to pulsed interference in performing their missions. The incorporation of interference suppression circuitry or software in the design of new r
10、adar systems will ensure that system performance requirements can be satisfied in the type of pulsed interference environment anticipated. Interference suppression techniques, are generally classified into three categories: transmitter, antenna, and receiver. Receiver interference suppression techni
11、ques are more widely used. Receiver interference suppression techniques are categorized into predetection, detection and post-detection. The following is a brief discussion of several interference suppression techniques currently used in radionavigation, radiolocation and meteorological radars. Syst
12、em performance trade-offs (limitations), are also addressed for many of the interference suppression techniques. 2 Antenna beam scanning suppression Interactions between two radars of different types almost always involve asynchronism between the scanning of the two antenna beams. Consequently, the
13、situations that are normally of concern are limited to: radar side lobe/back lobe to radar side lobe/back lobe; radar main beam to radar side lobe/back lobe; radar side lobe/back lobe to radar main beam. The antenna side-lobe and back-lobe levels are generally determined by the radar antenna type (e
14、.g. reflector, slotted array, or distributed phased array). Reflector type antennas typically have average antenna back-lobe levels of 10 dBi. Consequently, back-lobe-to-back-lobe coupling is typically 70 to 80 dB weaker than main-beam-to-main-beam coupling. Slotted array antennas and distributed ph
15、ased array antennas can achieve back-lobe levels of approximately 30 to 40 dBi resulting in back-lobe-to-back-lobe coupling typically 90 to 120 dB weaker than main-beam-to-main-beam coupling. The power coupled between two radars (radar 1 and radar 2) is proportional to the sum of the gain of radar 1
16、 antenna in the direction of radar 2 the gain of radar 2 antenna in the direction of radar 1. The sum of the two antenna gains (G1(dBi) + G2(dBi) is commonly referred to as the mutual antenna gain. As the two antennas rotate, the mutual gain fluctuates rapidly by large amounts. Since the rotations o
17、f the two radar antennas are asynchronous, i.e. since their rotation rates are not rationally related, any one point on each radars antennas pattern lies in the direction of the other Rec. ITU-R M.1372-1 3 radar shifts progressively through every point on that other radars pattern. Eventually, the m
18、ain-beam peak of each antenna will point toward the other radar at the same time. However, that event will be exceedingly rare and fleeting. The vast majority of the time, illuminations of each radar by the other radars main beam will occur when the other radar illuminates the weak side lobe of the
19、other radar. This is especially the case when 3-dimensional radars, which use pencil beams scanned in elevation as well as azimuth, interact with 2-dimensional radars, which almost invariably scan only in azimuth. Thus, the pencil beams of 3-dimensional radars normally spend much of the time searchi
20、ng regions above the horizon, where they cannot couple strongly to the surface-based radionavigation radars. Furthermore, some 3-dimensional radars often use electronic steering and scan in deliberately pseudo-random patterns or patterns that are quasi-random because they adapt to the target environ
21、ment. In such cases, the main beam of the 3-dimensional radars revisit the direction of 2-dimensional radars only at irregular intervals instead of periodically. The fact that main beams of all radars are narrow causes the fraction of time during which main-beam-to-main-beam conjunctions prevail to
22、be extremely small. Figure 1 shows a temporal pattern of mutual gain between two planar-array radar antennas with both radar antenna beams scanning the horizon. Figure 2 shows the temporal pattern of mutual gain between two planar-array radars with one of the radars beam scanning 45 above the horizo
23、n. Figure 3 shows a mutual antenna gain distribution for two reflector type antenna radars with gains of 27 dBi on the horizon. The Figure shows that only three per cent of the time the mutual antenna gain exceeds 0 dBi, and fifty per cent of the time the mutual antenna gain is below 19 dBi. Figure
24、3 also shows mutual antenna gain curves for two planar array type antennas with both radar main beams on the horizon, and with one main beam elevated 45. 1372-015040302010010203040500 5 10 15 20 25FIGURE 1Sample of mutual-gain pattern for planar-array RL and RN radar antennas with RL beam on horizon
25、(spans 7 scans of the RL radar antenna)Time (s)Mutualantenna gain(dBi)4 Rec. ITU-R M.1372-1 1372-025040302010010203040500 5 10 15 20 25FIGURE 2Sample of mutual-gain pattern for planar-array RL and RN radar antennas with RL beam elevation 45(spans 7 scans of the RL radar antenna)Time (s)Mutualantenna
26、 gain(dBi)1372-035040302010010203040506060FIGURE 3Per cent of time exceededMutualantenna gain(dBi)0.01 100.1 0.5 1 2 5 30 50 70 90 95 98 99 99.5 99.9 99.99Two reflector type antennasTwo planar-array type antennas with bothmainbeams on the horizonTwo planar-array type antennas with onemainbeam elevat
27、ed at 45Rec. ITU-R M.1372-1 5 3 Integrator The process of summing the echo pulses from a target is called integration. Integrators are generally used in radars for two reasons: to enhance weak desired targets for plan position indicator (PPI) display, to suppress asynchronous pulsed interference. Th
28、e principle of the radar video integrator is that radar signal returns from a point target consist of a series of pulses generated as the radar antenna beam scans past the target, all of which fall in the same range bin in successive periods (synchronous with the radars transmitted pulses). It is th
29、is series of synchronous pulses from a target which permits integration of target returns to enhance the weak signals. The integrator also suppresses asynchronous pulsed interference (pulses that are asynchronous with the radars transmitted pulses) since the interfering pulses will not be separated
30、in time by the radar period, and thus will not occur in the same range bin in successive periods. Therefore, the asynchronous interference will not add-up and can be suppressed. Basically two types of integrators have been used in radar systems. The most common type of integrator is the feedback int
31、egrator shown in Fig. 4. A binary integrator shown in Fig. 5 has also been used in a few radionavigation radars. Figure 6 shows a simulated output for a desired target return (pulse width = 0.6 s, pulse repetition frequency (PRF) = 1 000) without integration for a signal-to-noise ratio, S/N, of 15 d
32、B. Figure 7 shows a simulated output of radar without integration in the presence of the desired signal and three interference sources (interferer 1, pulse width = 1.0 s, PRF = 1 177; interferer 2, pulse width = 0.8 s, PRF = 900; interferer 3, pulse width = 2.0 s, PRF = 280) with interference-to-noi
33、se ratios (I/N) of 10, 15 and 20 dB, respectively. 1372-04InputlimiterOutputlimiterDelayTD= 1/PRFFIGURE 4Feedback integrator block diagramKeineout6 Rec. ITU-R M.1372-1 1372-05FIGURE 5Binary integrator block diagramBinary counteror PROMThresholdcomparatorD/AconverterShift register(range gate)Clock0,
34、1 binaryeineout1372-068765432100 5 10 15 20 25 30 35 40 45 50FIGURE 6Simulated output of radar without integrator forS/N = 15 dB5 ms/cm1 V/cmRec. ITU-R M.1372-1 7 1372-070 5 10 15 20 25 30 35 40 45 50876543210FIGURE 7Simulated output of radar without integrator in presence of interference5 ms/cm1 V/
35、cmDesired S/N = 15 dBInterferer 1 I/N = 10 dBInterferer 2 I/N = 15 dBInterferer 3 I/N = 20 dB3.1 Feedback integrator The feedback integrator shown in Fig. 4 consists of an input limiter, an adder, and a feedback loop with an output limiter and a delay equal to the time between transmitter pulses (1/
36、PRF) in radars using non-staggered pulse trains. The overall gain, K, of the feedback loop is less than unity to prevent instability. The input limiter serves as a video clipping circuit to provide constant level input pulses to the feedback integrator, and is a necessary integrator circuitry elemen
37、t to suppress asynchronous pulsed interference. The input limiter limit level is usually adjustable, and controls the transfer properties of the feedback integrator. Figure 8 shows the radar output for the same interference condition shown in Fig. 7 with feedback integration for an input limit level
38、 setting of 0.34 V. The asynchronous interference has been suppressed by the feedback integrator. 8 Rec. ITU-R M.1372-1 1372-080 5 10 15 20 25 30 35 40 45 50876543210FIGURE 8Simulated output of radar with feedback integrator in presence of interference5 ms/cm1 V/cmDesired S/N = 15 dBInterferer 1 I/N
39、 = 10 dBInterferer 2 I/N = 15 dBInterferer 3 I/N = 20 dB3.2 Binary integrator The binary integrator shown in Fig. 5 consists of a threshold detector or comparator, binary counter or programmable read-only-memory (PROM) logic (adder/subtractor circuit), a multi-bit shift register memory, and a digita
40、l-to-analogue (D/A) converter. Each inter-pulse period is divided into range bins. Each time a pulse of a target return, noise, and/or interference exceeds the comparator threshold level, the binary counter or PROM is bumped up to the next level. For this simulation, a PROM logic with non-linear sta
41、te progressions of 1, 2, 4, 8, 16 and 31 was used. If the successive pulses of the target return pulse train continue above the comparator threshold in the given range bin, the PROM is advance to the next highest programmed state until a maximum integrator level of 31 is reached. If in any PRF perio
42、d the signal fails to exceed the comparator threshold, the PROM logic is bumped down to the next lowest programmed state until a state level of zero is reached. The subtraction provides the target return pulse train signal decay required after the antenna beam has passed the target, and also enables
43、 the suppression of asynchronous interfering signals. The voltage amplitude at the integrator D/A converter output is determined by the binary counter or PROM level (0 to 31) for the particular range bin times 0.125 V. Therefore, for a binary counter level of 31, the maximum enhancer output voltage
44、would be 3.875 V (31 0.125). Figure 9 shows the radar output for the same interference condition shown in Fig. 7 after binary integration. The asynchronous interference has been suppressed by the binary integrator. Rec. ITU-R M.1372-1 9 1372-090 5 10 15 20 25 30 35 40 45 50876543210FIGURE 9Simulated
45、 output of radar with binary integrator in presence of interference5 ms/cm1 V/cmDesired S/N = 15 dBInterferer 1 I/N = 10 dBInterferer 2 I/N = 15 dBInterferer 3 I/N = 20 dB3.3 Trade-offs Target azimuth shift: 0.9 (0.7 beamwidth) for feedback integrator 0.2 (0.2 beamwidth) for binary integrator Angula
46、r Resolution: 1.2 (0.9 beamwidth) for feedback integrator 0 (0 beamwidth) for binary integrator. 3.4 Desired signal sensitivity Approximately 1 dB decreases when the integrator is adjusted to suppress pulsed interference with the normal video mode and with moving target indicator (MTI) mode in the 2
47、 and 3 pulse canceller mode without feedback. However, in the MTI mode with feedback, the sensitivity loss can approach 2 dB due to the need to adjust the integrator input limiter to limit the interference level below the receiver inherent noise level. 10 Rec. ITU-R M.1372-1 4 Double-threshold detec
48、tion The double-threshold detector, sometimes referred to as sequential detection, is a post detection signal processing technique used in radionavigation and search radars. The function of the double-threshold detection circuit is to extract or identify targets from radar target pulse returns. Howe
49、ver, the double-threshold method of detection also has an inherent capability to suppress false alarms caused by asynchronous pulsed interference. Figure 10 shows a simplified block diagram of a double-threshold detector. 1372-10FIGURE 10Double-threshold detector block diagramShift register(sliding windowof N PRIs)FirstthresholdDownUpCounterTargetNo targetSecondthreshold(M out of N)einThe “double-threshold” detector consists of establishing a bias level, T, the “first threshold”, at the output of the radar detector or Doppler filter and then
