1、 Rep. ITU-R M.2032 1 REPORT ITU-R M.2032*Tests illustrating the compatibility between maritime radionavigation radars and emissions from radiolocation radars in the band 2 900-3 100 MHz (2003) 1 Introduction Tests have been performed to assess the effects of emissions representative of radiolocation
2、 radars having a secondary allocation in the 2 900-3 100 MHz band on two representative maritime radionavigation radars having a primary allocation in that band. The maritime radionavigation radars used for these tests are identified as Radars A and B in this Report1. The tests were performed in two
3、 separate efforts. In the first effort, the radiolocation emissions were simulated by means of signal generators, using pulses with no intra-pulse modulation and were roughly representative of emissions from P0N type radiolocation radars described in Recommen-dation ITU-R M.1460 Technical and operat
4、ional characteristics and protection criteria of radiodetermination and meteorological radars in the 2 900-3 100 MHz band. In the second effort, tests were performed with longer pulse width and higher duty cycle P0N type emissions, which are not typical of those radars identified in Recommendation I
5、TU-R M.1460. Analog reconstructions of digitally recorded emissions from a stepped-frequency radiolocation radar that operates with the characteristics and parameters similar to that of Radar 2 in Recommendation ITU-R M.1460 were also used as unwanted stimuli to one of the maritime radars. This Repo
6、rt describes the conduct of these two test efforts and their findings. 2 Objectives The objectives of the testing were: to quantify the capability of representative maritime radionavigation radars interference-rejection processing to mitigate unwanted asynchronous P0N pulses due to emissions from ra
7、diolocation radars as a function of their duty cycle, pulse width, and power level; *This Report is in support of Conference Preparatory Meeting text regarding WRC-03 Agenda item 1.17. 1These tests addressed pulsed maritime radionavigation radars having pulse widths, pulse repetition frequencies (PR
8、Fs), bandwidths, noise figures, and antenna beamwidths typical of those identified in Recommendation ITU-R M.1313. Those radars typically employ interference mitigation techniques/processing methods identified in Recommendation ITU-R M.1372 to allow them to operate in the presence of other radionavi
9、gation and radiolocation radars. Mitigation techniques of that kind are relatively inexpensive to provide now that powerful digital signal processing circuitry is available at low cost and is in wide use for other navigation radar functions. Older and less sophisticated maritime radionavigation rada
10、rs may not have the same level of interference rejection capabilities as those typically provided in the International Maritime Organization (IMO)-category radars identified in Recommendation ITU-R M.1313 Technical characteristics of maritime radionavigation radars. 2 Rep. ITU-R M.2032 to quantify t
11、he capability of representative maritime radionavigation radars interference-rejection processing to mitigate an unwanted stepped frequency radiolocation waveform; to observe and quantify the effectiveness of representative maritime radionavigation radars interference rejection techniques to reduce
12、the number of false targets, whether in the form of radial streaks (strobes), or point-like “speckle”; to observe and quantify the interference mitigating effects of applying antenna pattern modulations on the radiolocation radar emissions. 3 Radars under test Radar A is an older system while Radar
13、B was introduced recently (circa 2000). Nominal values for the principal parameters of the two radars were obtained from regulatory type-approval documents, sales brochures, and technical manuals. These are presented in Tables 1 and 2. TABLE 1 Radar A transmitter and receiver parameters Additional q
14、uantities of interest are the antenna main-beams time-on-target and the associated numbers of pulses-on-target during the main-beam dwell. They are contained in Table 3. For each pulse repetition frequency, these quantities are derived from the parameters listed in Tables 1 and 2. The radars were al
15、igned by technicians prior to commencement of the testing to ensure their optimum performance. Parameter Radar A (older radar) Frequency (MHz) 3 050 30 Pulse power (kW) 60 Range (nmi) 0.25-3 6-12 24-64 Pulse width (s) 0.06 0.50 1.0 PRF (Hz) 3 600 1 800 900 IF bandwidth (MHz) 22 4 4 Spurious response
16、 rejection (dB) 40 System noise figure (dB) 10 RF bandwidth (MHz) 100 Antenna scan rate (rpm) 33 Antenna scan time (s) 1.8 Antenna horizontal beamwidth (degrees) 1.25 Polarization Horizontal Rep. ITU-R M.2032 3 TABLE 2 Radar B transmitter and receiver parameters TABLE 3 Derived parameters of maritim
17、e radionavigation radars under test 3.1 Characteristics common to the radars The two maritime radars are basically similar. Both have magnetron transmitters. Both can transmit pulses with pulse widths ranging from 0.06 (or 0.08) s to 1.0 (or 1.2) s. Both use a number of IF bandwidths, each geared to
18、 a different pulse width. Both radars can operate with range scales as short as a fraction of a nautical mile and as long as 64 to 96 nmi (approximately 118-178 km). Both operate nominally on 3 050 MHz. Both have an antenna scan time close to 2 s and a horizontal beamwidth between 1 and 2. Neither r
19、adar performs moving-target-indication (MTI) or other Doppler-based signal processing. Both radars have a feature that rejects asynchronous pulsed interference. Both radars use logarithmic IF amplifiers and use a.c. coupling in their video signal paths. This is almost universal in maritime navigatio
20、n radars. These design choices are apparently based on a finding, made in 1956, that envelope-detected signal fluctuation due to clutter return having a Rayleigh distribution is essentially independent of the intensity of the clutter (or the effect of Parameter Radar B (newer radar) Frequency (MHz)
21、3 050 30 Pulse power 30 Range (kw) 0.375-1.5 3-6 12 24-96 Pulse width (s) 0.08 0.30 0.60 1.2 PRF (Hz) 2 200 1 028 600 IF bandwidth (MHz) 28 3 3 3 Spurious response rejection (dB) 60(1)System noise figure (dB) 4 RF bandwidth (MHz) Unknown Antenna scan rate (rpm) 26 Antenna scan time (s) 2.31 Antenna
22、horizontal beamwidth (degrees) 1.9 Polarization Horizontal (1)Measurement revealed a spurious response rejected by 44 dB. Parameter Radar A Radar B Time-on-target (ms) 6.3 12 Pulses-on-target 23 11 6 23 13.4 7.3 4 Rep. ITU-R M.2032 range) when the signal is processed in a logarithmic amplifier follo
23、wed by a.c. coupling2. In practice, signal fluctuations of sea and rain clutter return depart somewhat from the Rayleigh model, with the result that the root-mean-square (r.m.s.) fluctuation does vary with clutter intensity and range, but less so than if a linear receiver or a logarithmic receiver w
24、ith d.c. coupling were used. Very significantly, both radars have processing to reject asynchronous pulsed interference. The form of the interference rejection (IR) process in Radar B differs somewhat from that in Radar A, but the process exploits the same principle in both radars. Radar A compares
25、the contents of a given range cell on each pulse repetition interval (PRI) with the contents of that same cell on the previous PRI, and displays a spot (or blip) on the screen only if both cells contain detections. Radar B has a process that notes the signal levels in three consecutive sweeps instea
26、d of two. At any given range, if the signal pulse amplitude exceeds those on previous and following PRIs by an inordinate amount, it replaces that amplitude with a weighted average of the values on the preceding and following PRIs. In the Radar B variant tested in the first effort, the tolerable dis
27、parity between the signal amplitude in the current PRI and the amplitudes in the preceding and following PRIs was adjustable. In the second test effort, the software controlling the IR function had been revised; the operator could only disable it. The IR control enabled is the systems default settin
28、g. Figure 1 illustrates typical occurrences of asynchronous pulses having the width (2 s) used in the current tests as they appear within successive range sweeps of a radionavigation radar similar to Radar A or Radar B when operating on the range scale used in the current tests. The diagram also sho
29、ws some of the pulses that would be returned from a real target at a range (2.37 nmi or 4.39 km) equivalent to a round-trip delay of 29.25 s. (They are shown disproportionately long due to limitations of the software used to generate the diagram; they would actually be only one eighth as long as the
30、y appear.) Under the conditions that prevailed in the tests, a point target would give returns on 23 sweeps within the antennas main beam, only 12 of which appear in the diagram. Since real-target return is synchronous, all returns fall into the same range cell. Both radars have user-selectable sens
31、itivity time control (STC), which attenuates heavy sea clutter return by desensitizing the receiver at short ranges but not at long ranges. Both radars also have a user-selectable fast time constant (FTC), which differentiates the video signal and is used to discriminate against rain clutter. 3.2 Ch
32、aracteristics that differ between Radars A and B 3.2.1 Major differences Radar B contains an RF preamplifier and has a nominal noise figure of 4 dB, while Radar A apparently has no RF preamplifier and has a noise figure between 9.3 dB and 11 dB. Radar B has more extensive signal processing and targe
33、t tracking capabilities, including an adaptive local constant-false-alarm-rate (CFAR) feature and a scan-to-scan correlation feature, which Radar A does not have. The local CFAR (acting within a small fraction of one range sweep) is of a type 2CRONEY, J. April 1956 Clutter on radar displays. Wireles
34、s Eng., p. 83-96. Rep. ITU-R M.2032 5 known as an ordered-statistic CFAR, which is a type that permits the desensitizing effect of interfering pulses to be lessened or avoided. In this type of CFAR, a selectable number of background signal samples (range-bin contents) can be discarded, so that only
35、the remaining ones (and particularly the strongest remaining one) can be used to establish the detection threshold. The process discards the samples having the greatest amplitude, so that as more samples that are discarded, the less influence the high amplitude pulses are likely to have on the sensi
36、tivity of valid target detection. Rap 2032-01403020100FIGURE 1Occurrences of asynchronous 2 s pulses in radar sweeps and range cellsSuccessive radionavigation-radar sweepsTimeequivalentrange(s)withinsweep6 Rep. ITU-R M.2032 Radar B can also perform a scan-to-scan correlation process that provides an
37、 additional means for discriminating between signals that are present consistently, such as a valid target, and signals that appear at random times, such as asynchronous pulsed interference. The more sophisticated signal processing capabilities of Radar B are attributable to the advances in digital
38、microcircuits, including cost reductions, that have been made in the years since Radar A was designed. Implementation of this local CFAR process requires substantial amount of digital memory, which was not available when Radar A was developed. It is expected that future designs of maritime radionavi
39、gation radars will improve these features as well. 3.2.2 Minor differences There are also some more subtle differences between the two radars. While both radars have logarithmic IF amplifiers, Radar A uses diode networks to perform log shaping within the IF amplifier, while Radar B uses a logarithmi
40、c amplifier/detector implementation; i.e. it makes use of several log IF gain stages each with an associated envelope detector. The outputs of the IF amplifiers/detectors are summed to provide a video signal with a logarithmic characteristic. Table 4 summarizes the similarities and differences betwe
41、en the maritime radionavigation Radars A and B. TABLE 4 Similarities and differences between maritime navigation Radars A and B Feature Radar A Radar B Location of transmitter and receiver circuitry Below deck Antenna pedestal IF amplifier type Log amplifier Log amplifier/detector Video coupling a.c
42、. a.c. STC Yes (operator adjustable) Yes (operator adjustable) FTC Yes (operator adjustable) Yes (operator adjustable) Asynchronous pulse rejection (interference rejection) 2 pulse comparison 3-pulse comparison with substitution (see text) Automatic gain control (AGC) Yes (selectable) Yes (selectabl
43、e) Autotuning No Yes RF preamplifier No Yes False-alarm-rate control Manual Adaptive local CFAR (synthetic targets only) Scan-to-scan correlation No Active on synthetic target symbols Display intensity 2 non-zero levels Up to 15 non-zero levels Display type Real-time radial scan Raster scan Persista
44、nce Fixed by cathodic ray tube (CRT) phosphor Variable Rep. ITU-R M.2032 7 3.3 Radar A and B receiver IF bandwidth and noise figure measurements The noise floor of the radar receiver was computed as k T B plus the noise figure, where B represents the radar 3 dB IF bandwidth. 3.3.1 Radar A The measur
45、ed 3 dB IF bandwidth was 21.3 MHz when the radar was set for short-range operation (0.25 to 3 nmi range, or approximately 0.46-5.56 km). This closely corresponds to the specifications contained at one point in the radar technical manuals. For Radar A, the measured receiver noise figure was 11 dB, wh
46、ich is 1 dB higher than the specification in one technical manual (10 dB) and 1.7 dB higher than the specification in another technical manual (9.3 dB). The noise floor of Radar A was calculated to be 90 dBm. 3.3.2 Radar B Additional measurements were performed on Radar B to better characterize its
47、IF response. These measurements included determining its input-output response, measuring the IF selectivity (for a 3 nmi range), and noise figure. As stated previously, the radar uses a multistage logarithmic IF amplifier/detector. The tests showed that the radar has up to 70 dB of rejection at off
48、-tuned frequencies within the 2 900-3 100 MHz band and has a high dynamic range as well. The dynamic range of the radar is shown in Fig. 2 and the response of the IF circuitry at a video output test point with the radar set to a 3 nmi (5.56 km) range is shown in Fig. 3. Rap 2032-02100 90 80 70 60 50
49、 40 30 20 10 01009080706050403020FIGURE 2Radar B dynamic rangeRF power at receiver input port (dBm)Videooutput(mV)8 Rep. ITU-R M.2032 Rap 2032-032 975 3 000 3 025 3 050 3 075 4 00001020304050607080FIGURE 3Radar B IF selectivityFrequency (MHz)Relativegain(dB)Measurements at a video level slightly above the mid-pulse minimum visible signal level revealed a spurious response, visible in Fig. 3, that was suppressed by 44 dB approximately 30 MHz above the tuned frequency. There is no reason to expect that this spurious response had any effect on the
