Doppler example

As with any Doppler process; a relative velocity between the source and observer will cause an additional frequency change as the signal is transmitted and received. It is also a well known fact that the amount of frequency
change is proportional to the magnitude of the relative velocity. However the problem of ‘unravelling’ the frequency change is more complex when the frequency of the original signal is constantly changing – as is the case in
our FMCW radar.

The method of extracting the Doppler signal from an FMCW radar signal has been well tested and documented by the likes of Strauch, R.G. (1976) and Barrick, D. E. (1973). It involves the processing of multiple sweeps of modulation.
As a result of the moving target, the phase between each modulation sweep will change at a constant rate (given a constant velocity), and by cross processing multiple modulation sweeps, the constant phase change or frequency can be extracted using conventional
Fourier analysis. By incorporating this cross processing technique, AVTIS 2/3 will be able to detect the Doppler signal from a single, or distributed targets to allow a strong mapping of the motion of active regions of the volcano.

Below is an image that shows one of the tests that were carried out, to test the cross processing technique. The image shows a car (target) and the SAFIRE radar, which was used for the testing.

SAFIRE setup for Doppler processing test

One of the major constraints of doing such techniques with a 94GHz radar, stems from the relation that dictates the maximum velocity that can be detected, which is set by the sweep modulation time. For a 94 GHz radar, this results in problems that; for a velocity of 6 m/s (∼ 13 MPH)
the modulation sweep time must be of the order of 128 microseconds! This puts major constraints on the hardware used to sample the radar IF, and how to control the modulation such that it is swept quickly and linearly. It was evident that AVTIS 1, in its current configuration could not acheive such a fast sweep time, and thus lead to the use of the SAFIRE radar.

3D Doppler scan, illustrating target postion (from radar) and instantaneous velocity