The Doppler shift direction finding technique estimates the angle of arrival of the received signal by measuring the Doppler shift induced by a single moving antenna around a circle. As illustrated in Figure 2, the measured frequency of the received signal reaches its maximum value at point B since it is where the antenna is moving closer to the transmission source at its maximum speed. In contrast, the measured frequency is at its smallest value at point D since the antenna is moving away from the transmitter (see Figure 3). At positions A and C, no Doppler shift happens since the antenna is momentarily stationary relative to the transmitter.
Note that instead of having a single antenna physically moving in a circle, modern approaches use a multi-antenna circular array and sequentially switch between the antennas.
Fig. 2. Doppler shift direction finding
Fig. 3. Doppler shift seen at different positions in the circle (B and D)
The Time Difference of Arrival (TDOA) method consists of deploying three or more receivers at different locations and measuring the time difference of the received signal. An accurate time reference is crucial, often derived from GPS.
The Watson-Watt technique works by measuring the magnitude of the signal along two orthogonal Adcock antenna pairs. The arctangent function of these two measures provides the signal bearing.
The basic principle of the correlative interferometer consists of two steps:
- Compute some phase differences of the signal received at multiple co-located antennas.
- Compare the measured phase differences with a reference data set. The reference is obtained for a DF system of known configuration at a known transmitter angle. Interpolation of the reference table can be used to get better accuracy.
The Advantages of the Correlative Interferometer Method
Correlative interferometry has many inherent performance advantages over the other techniques:
- Typical accuracy is better than 1°. The accuracy is also better than other DF techniques in the presence of multipath fading, inter-channel interference and external noise sources for similar antenna diameters.
- The measurement of the elevation is possible.
- The antennas usually used cover very large frequency ranges.
- The response time is faster than with the Doppler shift system since the antenna outputs are sampled simultaneously rather than sequentially.
Because of these numerous advantages, this is the technique we’ve selected for our FPGA implementation case study.
DF System Overview
The overall DF system is composed of the following elements:
- An N-element antenna. Our implementation study assumes the use of a commercial five-element DF antenna such as those available from Aerosystems International Inc. These wide band antennas often cover from 20 MHz to 3.6 GHz.
- An RF front end responsible for coherently filtering and downsampling the RF signals to the DAQ system.
- An FPGA-based DAQ system that performs the analog to digital conversion (ADC) and the main processing of the algorithms. This is the subject of our second blog post.
- A host PC, used to perform the last steps of the algorithm and to display the results. The host PC might also aggregate the angles of arrival of the received signal computed from different DF systems and perform the triangulation to estimate the location of the transmitter.
In this post, we briefly described the most popular techniques for today’s direction finding applications, with emphasis on the correlative interferometer algorithm. This technique mainly consists of computing the phase differences of the signal received at multiple co-located antennas, and then comparing the measured values with a reference table.
In a subsequent post, we’ll evaluate an FPGA implementation of the correlative interferometer algorithm core.
Denisowski, Paul. 2011. “A comparison of radio direction-finding technologies.” Dept. of Energy Spectrum Management Conference. Las Vegas. http://www.denisowski.org/Articles/Denisowski%20-%20Comparison%20of%20Radio%20Direction-Finding%20Technologies.pdf