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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 1 - GENERAL SYSTEM DESCRIPTION 1-39
fore the Drift technique described here is a tremendous savings in system complexity. The Drift mode concept
appears at first glance to be similar to the beamforming technique, but it is a fundamentally different process.
The Drift mode depends on a single echo source being isolated such that its phase is not contaminated
by another echo (from a different direction but possibly arriving with the same time delay). This
technique works amazingly well because at a given time, the overhead ionosphere tends to drift uni-
formly in the same direction with the same velocity. This means that each off-vertical echo will have
a Doppler shift proportional to the radial velocity of the reflecting plasma and to cos a where a is the
angle between the position vector (radial vector from the observation site to the plasma structure) and
velocity vector of the plasma structure, as presented in Equation 1-14.
1:82. Both isolating the sources of different radial velocities and resolving echoes having different ranges
(into 10 km height bins), results in very effective isolation of multiple sources into separate range/Doppler bins.
If multiple sources exist at the same height they are usually resolved in the Doppler spectrum computed for that
height, because of the sorting effect which the uniform motion has on the radial velocities. If the resolution is
sufficient that a range/Doppler bin holds signal energy from only one source, the phase information in this
Doppler line can be treated as a sample of the phase front of a plane wave. Even though many coherent echoes
have been received from different points in the sky, the energy from these other points is not represented in the
complex amplitude of the Doppler line being processed. This is important because the angle of arrival calcula-
tion is accomplished with standard interferometry (i.e., solving Equation 1-24 for ), which assumes no multi-
ple wave interference (i.e., a perfect plane wave).
1:83. A fundamental distinction between the Drift mode and beamforming mode is that in the Drift mode the
angle of arrival calculation is applied for each Doppler line in each spectrum at each height sampled, not just at
the maximum amplitude Doppler line. A data dependent threshold is applied to try to avoid solving for loca-
tions represented by Doppler lines that contain only noise, but even with the threshold applied the resulting an-
gle of arrival map may be filled with echo locations which result from echoes much weaker than the peak Dop-
pler line amplitudes. In beamforming, only the echoes representing the dominant source at each height are
stored, therefore no other source echoes are recoverable from the recorded data.
1:84. It has been found that vertical velocities are roughly 1/10th the magnitude of horizontal velocities [Rei-
nisch et al, 1991]. Since the horizontal velocities from echoes directly overhead result in zero radial velocity to
the station, the Drift technique works best in a very rough, or non-uniform ionosphere, such as that found in the
polar cap regions or the equatorial regions, because they provide many off-vertical echoes.
1:85. For a smooth spherically concentric (with the surface of the earth) ionosphere all the echoes will arrive
from directly overhead and the resulting Drift skymaps will show a single source location at zenith angle = 0.
For horizontal gradients or tilts within that spherically concentric uniform ionosphere however, the single
source point would move in the direction of the N/N (N as in Equation 1-1) gradient (the local electron densi-
ty gradient), one degree per degree of tilt, so the Drift measurement can provide a straightforward measurement
of ionospheric tilt.
1:86. Resolution of source components by first isolating multiple echoes in range then in Doppler spread (ve-
locity distribution) combined with interferometer principles is a powerful technique in determining the angle of
arrival of superimposed multipath signals.
Two Frequency Precision Ranging Mode
1:87. The phase of an echo from a target, or the phase of a signal after passing through a propagation medi-
um is dependent on three things:
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