Analyze Bearing Accuracy Of A Monopulse System

Amplitude monopulse direction- finding (DF) systems depend on tight component tolerances to achieve a high degree of bearing accuracy. By using an amplitude monopulse system with four-monopulse antenna and switched beam-forming network as an example, it is possible to better understand the effects of the RF/microwave components on the overall system peak and rootmean- square (RMS) bearing accuracy. The first portion of this two-part article will show how to analyze the effects of component performance variations on amplitude monopulse systems.

Airborne DF systems rely on three main system approaches: the amplitude-comparison method, the phase-comparison method, and the amplitude-phase method.1-3 The first of these techniques is relatively simple and cost effective, but also relatively inaccurate. The second approach, often referred to as the phase or interferometer method, is more precise. The last method, the amplitudephase approach,3 is a compromise between the simplicity (and inaccuracy) of the first technique and the highly accurate but spaceand cost-inefficient phase method.

Reference 4 summarizes the characteristics of typical airborne amplitude and phase interferometer DF systems. The bearing accuracy of an amplitude monopulse system is typically on the order of 3 to 20 deg. RMS.^{1-3, 5} The bearing accuracy of a phase monopulse system is typically on the order of 0.1 to 0.3 deg. RMS.^1,2 The low accuracy of the amplitude monopulse system is limited due to an inability to accurately measure small differences in amplitude between received signals, inaccuracies in the antenna pattern mathematical models, and other causes that will be addressed shortly. Fortunately, by considering the various causes of poor bearing accuracy in amplitude monopulse systems, it may be possible to make improvements.

The example amplitude monopulse system considered here for analysis includes a four-monopole antenna with switched beam-forming network (SBFN).^4,6Figure 1 shows a block diagram of an amplitude monopulse system4,6 for airborne Traffic Collision Avoidance System (TCAS) applications. The TCAS uses signals from a directional antenna to determine the bearing from the host to another aircraft. The directional antenna module consists of the four-monopole antenna, SBFN, matching network, and interface.4,6 Each SBFN port is coupled through a coaxial cable to a corresponding receiver. The outputs of the receivers are coupled to an analog- to-digital converter (ADC).

An airborne amplitude monopulse system estimates an intruder aircraft's bearing by comparing magnitudes of signals received by the four-monopole directional antenna. The antenna aperture is divided into four sectors (or quadrants). Incoming signals are processed inside the antenna module to produce four discrete electrical signals, such that each signal represents a unique quadrant of the polar coordinate system. A significant improvement in bearing accuracy can be achieved by means of an effective bearing algorithm using all four received signals in the four sectors. By doing so, a bearing may be detected not only by an antenna pattern's main lobe but by information from the sidelobe or backlobe of another antenna beam or beams. This additional information provides greater bearing accuracy and eliminates demanding requirements for sidelobe/backlobe level suppression.

In the example amplitude monopulse system, the L-band antenna module provides directional antenna patterns by using the special SBFN, including a 4 x 4 hybrid matrix and 0/180-deg. switched phase shifter (Fig. 2).⁶ Four 90-deg. hybrids are serially connected to form the 4 x 4 hybrid matrix. Four ports (ports 5, 6, 7, and 8) of the hybrid matrix are connected to four antenna monopoles (A1, A2, A3, and A4, respectively), while the other four ports (ports 1, 2, 3, and 4) are connected to a transmit/receive network by means of coaxial cables (Fig. 1). The eight-port hybrid matrix is used in the SBFN to provide equal amplitudes and specific relative phases for the four antenna monopoles. The directional transmit mode is implemented by alternative activation of one input port (1, 2, 3, or 4) of the SBFN while the switched phase shifter provides 0 deg. phase shift.

Each of the four antenna module inputs corresponds to a beam in one of four directions: front (F), right (R), aft (A), or left (L). During the receive directional mode (which provides bearing measurements), all four antenna connectors are monitored. The relative signal strength from the four SBFN ports 1, 2, 3, and 4 shows the azimuth direction of a selected object according to a special bearing algorithm (index) and antenna lookup tables (LUTs). By comparing the signal intensities in the four receive channels (Fig. 2) electrically coupled to four outputs of the SBFN, it is possible to determine an object's position. Also, the TCAS antenna module should provide an omnidirectional transmit mode when the signal passes through only one input (port 2) of the SBFN while the switched phase shifter is in the 180-deg. phase-shift state. In this case, the four antenna monopoles are activated with equal magnitudes and progressive 90-deg. phase shifts (0, 90, 180, and 270 deg.).⁶

The conventional TCAS directional antenna is a four-element, verticallypolarized, monopole array capable of transmitting four selectable directions at 1030 MHz. The antenna is capable of receiving replies from all directions simultaneously with bearing information at 1090 MHz, using amplituderatio monopulse techniques.⁷ For the omnidirectional mode, the antenna and interface require a complicated amplitude and/or phase calibration network. Figure 3 shows an exploded view of the novel antenna module,4 including single metal base ground plate (base plate) (labeled 1 in Fig. 3), feeding posts (labeled 2), shorting posts (labeled 3) , capacitive hat printed circuit board (PCB) (labeled 4 in Fig. 3), antenna radome (labeled 5), SBFN multilayer card ASSY (labeled 6), four electrical connectors (labeled 7), and parasitic elements (labeled 8, 9, and 10). The profile and the weight of the airplane TCAS antenna should be kept extremely low.

The feeding and shorting posts with capacitive hats form the four low-profile folded monopoles.^4,6 The heights of the feeding posts, shorting posts, and parasitic elements are equal to a fraction of a wavelength, at 0.043?₀. The top capacitive PCB (labeled 4 in Fig. 3) includes a thin dielectric substrate with four coated two-sided metallization copper capacitive hats (labeled 11) and center parasitic decoupling element (labeled 10 in Fig. 3). Each capacitive hat area is equal to a fractional wavelength of 0.027?0². The two sides of each capacitive hat and central element are electrically coupled through a plurality of through-hole metallization (viaholes) (labeled 13 in Fig. 3). Four feeding posts (labeled 2) are electrically coupled to the four capacitive hats (labeled 11) on one side and to the antenna terminals of the SBFN on the other side. The space between the diagonal feeding posts is one-quarter wavelength. The large spacing among the elements (on the order of one-half wavelength (?0/2) leads to reduced gain and significant backlobes. Moreover, further reduction of this spacing to ?0/8 leads to a super-gain array. For a practical design, a spacing of ?0/4 was chosen for the elements. Each shorting post (labeled 3 in Fig. 3) is electrically coupled to one of the capacitive hats (labeled 11) on one side and to the ground plate (labeled 1) on the other side. The multilayer SBFN card assembly (labeled 6) was implemented with a combination of microstrip and stripline.

To minimize the coupling between the antenna monopoles, according to Fig. 3, decoupling (parasitic) elements (8, 9, and 10) are installed between the capacitive hats (11). One central parasitic element (9) is surrounded by four parasitic "skirts" (8) uniformly placed on a circular grid of radius R from antenna central to the skirt centrals. The center grounded post (9) and outside grounded "skirts" can be parts of the antenna metal base plate (1) (if possible) or separate element (as shown in Fig. 3), which must be grounded through the viaholes of the SBFN card assembly (6) to the antenna ground plate (1). The grounded center post (9) can be implemented as a round or square shape (Fig. 3). The decoupling element (10) is implemented as a printed square-shaped two-sided metallization portion on the capacitive hat (12). The decoupling element (10) is grounded to the base plate (1) through the center element (9).

The bearing accuracy of an amplitude monopulse system (Fig. 4) is limited mainly by the inability to accurately measure small differences in amplitude between received signals, but also by frequency variations, manufacturing tolerances, elevation angles, fuselage size, environmental conditions, component time-varying effects, inaccuracies in mathematical models for the antenna patterns, and other factors. According to ref. 8, the bearing error of a complete aircraft TCAS system should not exceed 9 deg. RMS or 27 deg. peak for all azimuth angles and for elevation angles from -10 to +10 deg. For elevation angles greater than +10 to +20 deg., the bearing error should not exceed 15 deg. RMS or 45 deg. peak for all azimuth angles.

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There are many factors that adversely affect the performance of the amplitude monopulse system in estimating the bearing of surrounding aircraft:
1. receive channel imbalance;
2. elevation angle variance;
3. manufacturing tolerances;
4. frequency variations;
5. antenna pattern shape (sidelobe/ backlobe level, sector-to-sector gain variations);
6. variance of coupling between antenna ports;
7. the physical shape of the antenna;
8. cable-loss variations;
9. interface mismatching;
10. different fuselage sizes and shapes;
11. environmental conditions;
12. receiver and digital-signal-processor (DSP) performance;
13. different aircraft structural components; and
14. time-varying component-based errors.

The first six factors can be taken into account by the antenna LUTs. The next five factors (7-11) can be taken into account by special calibration. Factors 12, 13, and 14 can be resolved by optimization of the receiver structure.

Next month, this two-part article series will conclude with a closer look at which of the factors on the above list can be impacted by high-frequency component designers. Such additions as amplitude and phase calibration networks can play a major role in achieving improved measurement accuracy with an amplitude monopulse system. Part 2 will present the equations needed for calculating the effects of different components on system accuracy as well as tabulated data from actual systems. In particular, it will focus on the importance of antenna performance on system accuracy.

See associated table

ACKNOWLEDGMENT:
The author would like to thank his colleagues at Rockwell Collins who helped implement the antenna module.

REFERENCES
1. K. Wiolland, "TCAS Uncovered," Avionics News, July 2006.
2. Microwaves101, "Signal Sorting Methods and Directional Finding," http://www.microwaves101.com/encyclopedia/.
3. M. P. Murphy et al., "Hybrid Amplitude/Phase Comparison Directional Finding System," United States Patent No. 5,541,608, Feb. 28, 1996.
4. L. G. Maloratsky et al., "Aircraft Directional/Omnidirectional Antenna Arrangement," United States Patent No. 7385560, June 10, 2008.
5. C. M. Rose and K. M. Dangle, "Precise Bearing Only Geolocation in Systems with Large Measurements Bias Errors," United States Patent No. 5,526,001, Feb. 1, 1995.
6. L. G. Maloratsky, "Switched Directional/Omnidirectional Antenna Module for AmplitudeMonopulse Systems," IEEE Antenna Magazine, 2008.
7. B. E. Dinsmore and M. D Smith, "Apparatus and Method for an Amplitude Monopulse Directional Antenna," United States Patent No. 5,191,349, March 2, 1993.