JED - August 2011 - (Page 37)

TECHNOLOGY SURVEY SIGINT AND DIRECTION FINDING ANTENNAS By Ollie Holt A ntenna directional characteristics and the different properties of an RF signal can be used to determine signal direction of arrival (DOA) or angle of arrival (AOA). These characteristics are: antenna pattern characteristics and signal, amplitude, phase, time and Doppler artifacts. Let’s look at the simplest and oldest form of DF first, which utilizes the directional characteristics of an antenna. Using a simple spinning or pointable antenna that correlates the received signal amplitude with antenna pointing angle, a DF solution can be determined. The reported DOA for the signal would be the pointing angle, where the highest amplitude level was received. This technique requires a relatively narrow beamwidth antenna and a mechanical or human pointing system. The accuracy of the DF measurement depends on the ability to continually know the pointing angle of the antenna to a fine angular resolution and the precision of the amplitude measurement. If these parameters can be held to a high tolerance, the DF accuracy can be very good. This approach works for ground-based applications, but it can be a structural design problem for an airborne platform. An alternative to a mechanical scanned antenna would be an electronically steered antenna array. This consists of an array of at least two or more antenna elements. More elements allow the creation of narrower beamwidths and therefore finer AOA resolution. Using phase and amplitude control of each antenna element allows the generation of a SUM and DELTA pattern. The DELTA pattern forms a null in the center of the SUM beam. By scanning the SUM and DELTA beam through the signal recording, the position of maximum signal amplitude (SUM) and minimum signal amplitude (DELTA) results in a bearing solution. DF performance is a function of the beamwidth (number of elements) and can easily achieve accuracy of 1 degree or better with 16 or more elements. A simpler technique that reduces installation issues on airborne platforms is amplitude DF. Amplitude DF uses antennas that have fixed, overlapping fields of view. The simplest installation is four antennas located perpendicular to each other – usually one located in each quadrant. For a good amplitude system, well characterized antennas are required. The antennas should provide around 180 degrees of coverage overlapping into the adjacent quadrants. The received signal amplitude is measured by two overlapping antennas; the measured amplitudes are normalized and converted into an amplitude ratio. The amplitude ratio is then compared to the antenna characterization data to find a match. The amplitude ratio match then correlates to an AOA solution. Typical amplitude AOA installations can achieve 3 to 10 degrees AOA accuracy. The DF accuracy is very dependent on how well the antennas can be characterized in the installation and amplitude measurement accuracy. Time Difference of Arrival or TDOA uses a similar antenna arrangement as amplitude DF, but the installation and measurement requirements are more critical. For the installation, all the time delays between the antenna element and the measurement device must be known. For installations with minimal antenna separation the measurement resolution requirements need to be in the nanosecond range or less. For a time difference, remember that in 10 nano- seconds the signal would travel about 3 meters, so for antenna installations with element spacing less than 3 meters and allowing for some measurement error, we are looking at some challenging time measurements. Performance of typical TDOA systems is in the 1-degree range for a system capable of measuring time difference accurately to around 1 nanosecond. This technique is better suited to a large platform or multiple platforms that are time synchronized in order to achieve maximum antenna separation. Wider antenna separation reduces the requirement for fine time resolution. Phase difference of arrival or phase interferometry is the measurement of the signal’s electrical phase difference at two or more antenna elements looking in the same field of view. For unambiguous results, at least two of the elements should be spaced less than a half-wavelength apart. The more elements used, the more accurate the results. Typical performance for three or more elements can be 1 degree AOA or better. The only drawback to this technique is the requirement to control phase errors induced between the antenna and the phase measurement device. Doppler DF measures the frequency difference of arrival between two or more antenna elements. Doppler DF requires that either, or both, the signal source and the measurement antennas are moving. Doppler DF also requires very fine frequency measurements in order to achieve single-digit AOA accuracies. This month’s survey covers 80 SIGINT/DF antenna products from 19 companies. JED’s next technology survey, slated for the October issue, will focus on flightline test equipment for EW applications. The Journal of Electronic Defense | August 2011 37

Table of Contents for the Digital Edition of JED - August 2011

The View From Here
Conference Calendar
From the President
The Monitor
Washington Report
World Report
Defining a Career Path in EW
Technology Survey:SIGINT/DF Antennas
Book Review
EW 101
AOC Member Page
Index of Advertisers
JED Quick Look

JED - August 2011