Doppler radio direction finding
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Doppler radio direction finding, also known as Doppler DF, is a
The Doppler DF system uses the Doppler effect to determine whether a moving receiver antenna is approaching or receding from the source. Early systems used antennas mounted on spinning disks to create this motion. In modern systems, the antennas are not moving physically, but electrically, by rapidly switching between a set of several antennas. As long as the switching occurs rapidly enough, which is easy to arrange, the Doppler effect will be strong enough to determine the direction of the signal. This variation is known as pseudo-Doppler DF, or sometimes sequential phase DF. This newer technique is so widely used that it is often the Doppler DF seen in most references.[1]
Direction finding
Early
A great advance in RDF technique was introduced in the form of the
Robert Watson-Watt introduced the next major advance in direction finding as the "huff-duff" system, a nickname for high-frequency direction finding. Huff-duff also used crossed antennas, often an Adcock antenna,[7] but sent their output to the two channels of an oscilloscope. The relative strengths and phases of the two signals deflected the X and Y locations of the oscilloscope's electron beam by different amounts, causing an ellipse or figure-8 to appear on the screen, with the long axis indicating the direction of the signal.[8] The readout was essentially instantaneous and proved able to easily detect even short transmissions. Huff-duff was used in about one-quarter of all successful U-boat sinking.[9]
Both of these systems have drawbacks. The Bellini-Tosi system still has moving parts, albeit small ones, but has the more major limitation that it requires the operator to hunt for the signal, which may take several minutes. Huff-duff provides a direct and immediate indication of the signal direction, but only at the cost of requiring an oscilloscope or similar display system with an equally fast response. Both require two closely matched receivers and amplifiers, and often a third for the "sense" antenna if used.[10]
Doppler effect
If one places an antenna on a moving platform like the roof of a truck, the movement of the truck will cause the
The magnitude of the shift is a function of the wavelength of the signal and the angular velocity of the antenna:
- S = r W/λ
Where S is the Doppler shift in frequency (Hz), r is the radius of the circle, W is the angular velocity in radians per second, λ is the target wavelength and c is the speed of light in meters per second.[13] Converting to more common units:
- To convert Hz to radians per second, multiply by 6.28 (2 pi)
- To convert MHz to Hz, multiply by 1 million
- Eliminating the constants gives (6.28 × 1000000) / 300000000 = 1/0.02093... ~= 48
Such that:
- S = r Fr Fc/ 48
Where Fr is the frequency of rotation in Hz and Fc is the target frequency in MHz.[13][a]
Consider the example truck hunting an
- S = 50 × 0.0222... × 101.8/ 48 = 2.4 Hz
This amount of frequency shift is too small to be accurately measured. To improve the detection odds, the product r W must be increased. For this reason, Doppler DF systems normally mount their antennas on a small disk that is spun at high speed using an electric motor. Performing the same calculation using an antenna mounted to a 50 centimetres (20 in) diameter disk spinning at 1000 Hz results in:
- S = .25 × 1000 × 101.8/ 48 = 530 Hz
Which is easily detected. Nevertheless, such a rotation speed, 60,000 rpm, demands precision systems. Because the antennas have to move at very high speeds, this technique is only really useful for higher frequency signals where the antennas can be shorter[b] and the higher Fc produces a larger dividend.[13]
Early examples of Doppler DF systems date to at least 1941,
A significant advantage of this technique is that it requires only a single receiver, amplifier, and the appropriate FM demodulator. In contrast, huff-duff and B-T systems require two closely matched receivers, one for each antenna pair, and often a third for a sense channel.
Pseudo-Doppler
To further simplify the system, it is possible to simulate the movement of the antenna with a small amount of additional electronics. This is the pseudo-Doppler direction finding technique.[16]
Consider a pair of omnidirectional antennas receiving a signal from a target transmitter. As the signal propagates past the receiver, the amplitude of the signal at the antennas rises and falls. At long distances from the transmitter, well into the "far field", the wavefronts can be considered to be parallel.[17] If the two antennas are arranged perpendicular to the line to the target, the phase difference between them is zero, whereas if they are arranged parallel to the line, the phase difference will be a function of the distance between them and the wavelength of the signal.[17]
For this example, consider the two antennas to be located 1⁄4 of the target wavelength apart and aligned parallel to it. If the two antennas were sampled instantaneously, the difference in phase between them would always be the same, 90 °. But if one instead switches the input from one antenna to the other, there will always be some inherent delay during which time the signal continues to move past the two antennas. In this case, if the original sample was taken when the peak of the wavefront was at the nearer antenna and the system then switched to the farther one, the phase would not be 90 ° but somewhat smaller, because the wavefront approached the second antenna during that time.[13]
Now consider a series of such antennas arranged around the circumference of a circle, and a switch that connects to the antennas in turn in a clockwise fashion. If the target signal is at the midnight position, then the phase shift will be increased when the switching is moving "forward" between the 7 and 11 o'clock positions and reduced when moving "away", between 1 and 5. When switching between antennas perpendicular to the line to the signal, 11 to 1 and 5 to 7, the shift will be a constant value.[13]
The signal from the antennas is sent into a single receiver, resulting in a series of pulses whose amplitude depends on the phase at the instant of sampling. That signal is then smoothed to produce a sine wave.[18] That sine wave is modulated exactly as it would be in the case of a single moving antenna. In the case of the moving antenna, the frequency shifts because the antenna is moving through the wavefront as it passes, whereas, in the pseudo-Doppler case, this is accomplished by timing the samples to simulate the movement of a single antenna. The direction to the target transmitter can then be determined in the same fashion as the moving-antenna case, by comparing the phase of this signal to a reference signal. In this case, the reference is the clock signal triggering the switch.[13]
Because it has no moving parts and can be built using simple electronics, the pseudo-Doppler technique is very popular. Whilst not quite as fast as to measure the huff-duff system, in modern systems the measurement is so rapid that there is little practical difference between the two concepts. Pseudo-Doppler has a significant advantage in that the antenna system is much simpler, using monopole antennas, and if the switching system is located on the antenna, only a single wire runs back to the receiver and thus only one amplifier is required.[16] Because this technique is so widely used it is often referred to simply as Doppler DF, the "pseudo" rarely being added.[13]
The main disadvantage of the technique is the requirement for more signal processing. Because the "movement" in pseudo-Doppler proceeds in steps, the resulting signal is not as smooth as it is in the case of a moving antenna. This results in a signal with considerable numbers of sidebands that have to be filtered out. The switching system also introduces electronic noise, further confusing the output.[19] Modern signal processing can easily reduce these effects to insignificance.[16]
Notes
References
Citations
- ^ "Pseudo-Doppler Direction Finder Amanda Ke, Melissa Li, Jimmy Mawdsley" (PDF).
- ^ Army 1977, p. 3.3.
- ^ Sadler 2010, p. 4.
- ^ Yeang 2013, p. 188.
- ^ Moell 1987, p. 28.
- ^ Army 1977, p. 3.17.
- ^ a b Sadler 2010, p. 6.
- ^ Army 1977, p. 3.36.
- ^ Bauer, Arthur O. (27 December 2004). "HF/DF An Allied Weapon against German U-Boats 1939–1945" (PDF). p. 1. Retrieved 2008-01-26.
- ^ Sadler 2010, pp. 5–6.
- ^ a b c Army 1977, p. 3.26.
- ^ Moell 1987, p. 123.
- ^ a b c d e f g h Moell 1987, p. 121.
- ^ US Expired 2414798, Horace Budenbom, "Direction finder", published 28 January 1947, assigned to Bell Labs
- ^ Rembovsky et al. 2009, p. 21.
- ^ a b c Sadler 2010, p. 7.
- ^ a b Army 1977, p. 3.27.
- ^ Poisel 2012, p. 376.
- ^ Moell 1987, p. 137.
Bibliography
- Sadler, David (25 February 2010). HF Radio Direction Finding (PDF) (Technical report). Roke Manor Research. Archived from the original (PDF) on 9 August 2017.
- Moell, Joseph (1987). Transmitter Hunting: Radio Direction Finding Simplified. TAB Books. ISBN 9780830627011.
- Radio Direction Finding. United States Army. 1977.
- Rembovsky, Anatoly; Ashikhmin, Alexander; Kozmin, Vladimir; Smolskiy, Sergey (2009). Radio Monitoring: Problems, Methods and Equipment. Springer. ISBN 9780387981000.
- Poisel, Richard (2012). Antenna Systems and Electronic Warfare Applications. Artech House. ISBN 9781608074846.
- Yeang, Chen-Pang (2013). Probing the Sky with Radio Waves. University of Chicago Press. ISBN 9780226015194.