Various radar-like techniques can be used to measure the time-of-flight of radio signals. Essentially this approach uses a timing reference (maybe GPS) to control a pulsed or chirped transmitter, and in some manner synchronizes the receiver to the transmitter. From an Amateur point of view, limited to low power and narrow bandwidth transmissions, this technology is in its infancy, but there are several useful applications already available.
There are several different techniques used, although there is some cross-over between them. There is much an Amateur can do to study the ionosphere using ranging, even without transmitting a signal.
Conventional Ionosondes use swept pulse transmissions, where the receiver is broadband and tracks the transmitter. The delay in reflection from the ionosphere causes the received signal to be shifted in pitch, and this pitch determines the apparent height of the reflection, while the signal level determines the received strength. This type of sounder requires high power and causes considerable interference. There are not so many of them now as there once was.
As an example, Canterbury University operate a conventional research ionosonde at Birdling's Flat. Daily soundings are usually available.
A second type of swept ionosonde, the Chirp Sounder, is now more popular. Because the transmitter and receiver use direct digital synthesis and DSP techniques, tracking is very accurate, bandwidth is much lower, and the transmitter power can be much lower (250W as opposed to 10kW for a conventional sounder). Further, the Chirp Sounder is typically operated from a precise timing reference, often GPS locked, and so the receiver can be (and usually is) at a different site to the transmitter. This type is widely used by military communications networks, as it allows field stations to receive the soundings from base and use them to predict the best frequencies to use. This arrangement is called Oblique Sounding.
For more information about Amateur passive sounding (i.e. using transmissions from other stations) see the Chirp Sounding page.
PN-Sequence Coding is used by many high-speed HF modems, especially those in military use. A special pseudo-random coded sequence in the transmission which allows the received data to be very accurately timed. This permits the receiver to compensate for time and frequency shifts in the received signal. These signals can be used for ranging. A correlator is set to detect the special sequence, and the time delay (or several different path delays) can then be measured and plotted. This works best if the ground-wave signal is available in addition to the sky-wave, so that the absolute time delay can be measured.
This technique typically has a delay range of about 8-10 ms (good for short-path DX) and a resolution of about 500µs.
Peter G3PLX has software available which will track the frequency shift and time delay of STANAG 4285 signals, which are quite common on HF. Con ZL2AFP offers a system which (in addition to tracking STANAG 4285 signals) has a transmitter program which sends a simple ID message with the same PN sequence. A further advantage of the ZL2AFP system is that it can operate at 2000 bps as well as the military standard 2400 bps. At 2000 bps the bandwidth of the signal is reduced to about 2.4 kHz, so it can be used with a conventional SSB transmitter and receiver. This technique is very sensitive, and only requires low power. It is also real-time, and you can see variations on a second-by-second basis.
See Introduction to ZL2AFP Sounder for more information.
Over-the-Horizon Radar (OTHR or 'Woodpeckers') is a combined technique, where conventional radar pulses are coded with phase modulated sequences. While OTHR is generally used to detect large objects at very long range (ships, missiles, icebergs), it can also be used to study propagation and ocean wave heights. No known Amateur designs are know, although ZL2AFP and ZL1BPU did briefly experiment with a simple ASK pulsed system with Hellschreiber ID.
Copyright © Murray Greenman 1997-2009. All rights reserved. Contact the author before using any of this material.