As detailed last month in Part 1 of this two-part article series, an amplitude monopulse system requires tightly matched amplitude balance between receiver channels to provide good bearing accuracy. Also, minimizing bearing errors depends on minimizing phase and amplitude imbalance in the SBFN and antenna. Differences in insertion loss and gain among these channels result from variations in physical components. The receiver channels also operate over a wide dynamic range, and incorporate amplifiers with high differential gain that is subject to variations with temperature. For good antenna interface matching (return loss greater than 15 dB), variations in the electrical lengths of the cables are not critical because signal magnitudes, rather than phase, are used in am amplitude monopulse system. However, any insertion- loss variations in those cables can be critical for accurate estimation of bearing. According to ref. 9, acceptable TCAS cable insertion loss variance is 2.5 +/- 0.5 dB. Table 1 illustrates Constructed Interferogram Schleiren Shadowgraph (CISS) bearing errors versus cable insertion loss variance and receiver gain variance in four channels [(1) forward, (2) right, (3) aft, and (4) left].

The most critical parameter for amplitude monopulse system bearing accuracy is amplitude imbalance between the four cables and the receive channels. The results of an analysis show that for cable-loss variance of 2.5 +/- 0.5 dB, the (peak and RMS) bearing accuracy is within required system specifications for balanced gain (50 dB) in the four receive channels. For cable-loss variance of 2.5 +/- 0.5 dB and gain variance of 50 +/- 1 dB in the four receive channels, the (peak and RMS) bearing accuracy is still within the required system specification. But for cable-loss variation of 2.5 +/- 0.5 dB and gain variance of 50 +/- 2 dB or worse in the four receive channels, the (peak and RMS) bearing accuracy is out of specification. As the analysis showed, real-time dynamic amplitude calibration of the system’s receiver channels including cables should provide for better than +/-0.5-dB amplitude imbalance.

The bearing accuracy of an amplitude monopulse system suffers as a result of unit-to-unit performance variations of its components due to manufacturing tolerances. Some of these bearing errors can be reduced, but these tolerances must be verified during product design and testing to ensure unit-tounit repeatability in production. Some of the error sources in the CISS antenna cannot be adequately controlled as part of manufacturing processes alone. Fabrication tolerances cause unit-to-unit variations in electromagnetic (EM) field strength (amplitude) antenna patterns and receiver channel amplitude imbalance. In practice, for the 10 antenna modules, the unit-to-unit variance (due to manufacturing tolerances) causes a bearing-error RMS variance of 2.7 deg. The antenna module design should include tolerance analysis to improve performance and eliminate production problems.

The effects of tolerance on antenna module performance can be analyzed using the sensitivity approach.¹⁰ This is the easiest method of predicting a worst-case scenario for change in the most important antenna performance parameters: gain, beamwidth, and sidelobes corresponding to a given set of tolerances. The relative variance of these parameters influences system bearing accuracy. The investigation of the antenna structure (Fig. 3)⁴ showed that the most critical physical dimensions are the feeding, shorting, and center post height tolerances that have the following sensitivity: for omnidirectional gain, 0.03 dB per mil of height change; for omnidirectional gain ripples, 0.01 dB/mil; for beamwidth (directional mode), 0.06 deg/ mil; for sidelobe/backlobe level, 0.03 dB/mil; and for bearing RMS error, 0.05 deg/mil. For the real height mechanical tolerances at +/-10 mils, the effects of the total antenna variations on bearing RMS error is 1.0 deg.

Bearing accuracy also depends on directional antenna pattern performance. For azimuth measurements, it is desirable that all sidelobes/backlobes are compressed to a low level.11 Table 2 shows the results of bearing errors versus sidelobe/backlobe levels for the four-monopole antenna.

The practical airborne antenna sidelobe/backlobe level of about 10 dB cannot be improved sufficiently because of an aircraft antenna aperture limitation. Figure 5 illustrates the correlation between bearing error and sidelobe/backlobe level. The bearing accuracy versus the sector-to-sector gain variance is illustrated by Table 3.

Sector-to-sector gain variances from 0.75 to 0.95 dB cause bearing variations from 0.91 to 3.55 deg. But gain variance of less than 0.75 dB does not cause substantial bearing accuracy variance. The problem with conventional beam-forming techniques arises from beam pattern constraints: there is a trade off between the bearing error on one side and width of the main beam and antenna gain on the other. Table 4 illustrates the correlation between the bearing error and 1090 MHz antenna gain.

For an amplitude monopulse system, bearing accuracy can be improved by increasing the number of antenna monopoles (greater than four monopoles in Fig. 3). But implementation of this antenna requires greater parallel receiving channels, and therefore is costly and of lower reliability. The bearing performance of the amplitude monopulse antenna depends upon the elevation of the intruder. Table 5 illustrates the antenna bearing error versus different elevation angles. The most critical parameter is the elevation angle, the variation of which leads to bearing error peak variances of as much as 3.2 deg.

The LUTs improve bearing accuracy based on the different elevation angles of an intruder. However, sensitivity to fabrication errors for the antenna module terminated by the four cables and transmit/receive (TX/RCV) network must be analyzed since these errors affect the bearing accuracy. Once amplitude and phase variances of the four cables have been calculated, it is possible to determine the effects of these errors on the antenna pattern and, thus, on the bearing estimation accuracy.

The SBFN includes four twobranch hybrids (Fig. 2). The actual parameters of a two-branch hybrid differ from the ideal due to the mismatching of terminations, losses, and discontinuities, as well as manufacturing tolerances. Analysis of these destabilization factors showed12 that the most critical factor is the mismatching of terminations. Deviations of the antenna interface return loss from 16.0 to 10.0 dB cause substantial degradation of antenna beamwidth, omnidirectional gain ripples, and sidelobe/ backlobe level, which in turn lead to an increase in bearing errors. For a narrowband TCAS antenna, the bearing performance depends upon the frequency of the received signals. Frequency variations from 1087 to 1093 MHz cause bearing variances of as much as 1.83 deg.

An aircraft TCAS antenna configuration can be oval or round.4,7 The round antenna is a fully symmetrical design while the oval antenna (Fig. 3) is symmetrical with respect to plane YY (forward-aft direction) only, but has physical and electrical asymmetry with respect to plane XX (right-left direction). Table 6 shows bearing error as a function of the antenna shapes.

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