DF Techniques Based on Amplitude Measurements

The most basic form of DF is to perform an angular search using a directional antenna whose directional characteristics are known and find the angle at which the received power is either a maximum or a minimum. The choice depends on whether a well-defined maxima or null in the directional response exists.

The antenna can be continuously rotated and a suitable electro-mechanical system used to display the angle that corresponds to the minimum (or maximum) received signal power. One limitation of this scheme concerns the difficulty of measuring the DF of a signal that is present for a short duration. Nevertheless, some DF systems for radar ES are based on the use of a rotating parabolic reflector antenna. The relative simplicity, coupled with the capability against weak signals provided by the high antenna gain, partly compensates for the other limitations.

Amplitude comparison DF is a more sophisticated idea. The desired angular coverage is divided into sectors, and each sector is associated with a directional antenna having a beam width comparable with the angular width of the sector and a receiver that is designed to measure the amplitude of an observed signal. The angle of arrival is determined in two stages. First, the pair of receivers associated with the largest signal power measurements is found.

A coarse estimate of the angle of arrival is defined as the mid-angle between the angles that each of the antennas is pointed. Second, the angle of arrival estimate is refined by computing the ratio of the amplitudes, and using a look-up table, or a calculation based on a model of the directional gain of the antennas, to produce a fine-angle estimate. A trade-off occurs between the number of antennas and the achievable accuracy.

This technique is often used in radar ES systems; it is relatively straightforward to implement, and, for microwave frequencies, the antennas are relatively compact. RWRs used in fighter aircraft often use four antennas to provide 360° angular coverage, whereas ES systems for naval craft often use six or eight antennas.

Many communications ES systems use amplitude- comparison DF techniques based on the Adcock pair antenna. This technique is based on the idea of taking the vector difference of the output signals from two closely spaced vertical monopole or dipole antenna elements. The result is a figure-8 gain pattern with the null occurring for signals that propagate across the baseline of the antenna elements.

The separation of the antenna elements involves a compromise depending on the frequency range to be covered. Too close a spacing-reduces the sensitivity whereas too large a spacing results in a distorted gain pattern. The Watson-Watt DF system, in its simplest form, consists of two Adcock pairs oriented at right angles.

The angle of arrival of a received signal can be directly determined from the ratios of the signal powers measured from the two Adcock antenna pairs. With some additional processing, an unambiguous DF measurement can be obtained. At the cost of increased size and complexity, improved performance and frequency coverage can be obtained by using four Adcock pairs.

Interferometric DF Systems. The basic interferometric DF system consists of a pair of monopole or dipole antenna elements that are separated by less than half a signal wavelength and the means for measuring the phase difference between their output signals.

Using the measured signal frequency, the known signal propagation velocity, and the antenna separation, the signal angle of arrival with respect to the antenna baseline can be computed. The angle of arrival measured for this arrangement is ambiguous; the signal can arrive from either side of the baseline.

This limitation can be resolved by adding one or more antenna elements to form a two-dimensional array for each pair of antenna elements, an angle of arrival estimate relative to the baseline of the antenna pair is obtained. By solving for the result that is most consistent with these measurements, an unambiguous estimate for the angle of arrival is obtained. One implementation uses an array of 5 antennas positioned in a regular pentagon to form 10 antenna pairs, five of which correspond to the faces of the pentagon and the other five to the diagonals (41).

The interferometric DF technique is expensive in hardware. Each antenna in the array requires a dedicated channel from a multichannel receiver that has accurate phase-matching between the channels. Digital signal processing techniques facilitate the implementation of such systems, one point being that phase-matching errors can be corrected by measuring them with a suitable calibration signal, storing their values in a table, and using the stored calibration data to correct subsequent measurements.

The correlative DF techniques used by some systems are another development of this concept. Well-designed interferometric DF systems have a relatively good reputation for accuracy, particularly when a large antenna array is used.

Single-Channel DF Systems. To minimize size, cost, weight, and power consumption, several DF system implementations have been developed that require only a single-channel receiver. The pseudodoppler DF technique is distinguished by the use of a circular array of uniformly spaced antennas with a commutator switch that sequentially connects one antenna in the array at a time to the receiver.

The effect is analogous to moving a single antenna element on a circular track and contributes a sinusoidal phase modulation to the received signal. An estimate of the angle of arrival is obtained by measuring the relative phase shift of this modulation component. The Watson-Watt technique has also been applied successfully to single-channel DF systems.

Single-channel DF techniques are widely used for low- cost portable systems. However, a relatively long observation time is needed compared with the conventional Watson-Watt and interferometric techniques.

Other DF Techniques. Other DF techniques are possible and have some advantages. Circular antenna arrays using the Butler matrix network can provide unambiguous DF with a receiver having as few as two channels. A theoretical comparison of their performance with other techniques is given in Ref. 42. Super-resolution techniques, such as the multiple-signal classification (MUSIC) algorithm (43), have the ability to resolve multiple signal sources in angle, even when their signals overlap in frequency. However, the large antenna arrays and the cost of the associated receiver and processing hardware are difficult to justify for most applications.

Attempts have been made to use power measurements to provide an indication of range. This method presents some difficulties. The actual power radiated by a transmitter is dependent on various factors that include the antenna configuration, height, and the selected transmitter output power (if this functionality is available). Furthermore, in a ground environment, propagation losses depend on the nature of the terrain. The usefulness of power measurements increases if measurements are available from multiple sites.