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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
1-38 SECTION 1 - GENERAL SYSTEM DESCRIPTION
Example A.:
Given the antenna geometry shown in Figure 1-22, at an operating frequency of 4.33 MHz (l =
69.28 m), a beam in the eastward direction and 30
off vertical would, according to Equation 1-24,
require a phase shift of 90
on antenna 4, –45
on antennas 2 and 3, and 0
on antenna 1. If an echo
is received from that direction it would be received on the four antennas as four complex amplitudes at
the height corresponding to the height (or more precisely, the range, since there may be a horizontal
component to this distance) of the reflecting source feature. Therefore, a single number per antenna
can be analyzed by treating one echo height at a time, and by selecting only one (the maximum) com-
plex Doppler line at that height and that antenna. Assume that the following four complex amplitudes
have been receive on a DPS system at, for instance, a height of 250 km. This is represented (in polar
notation) as:
Antenna 1: 830
135
Antenna 2: 838
42
Antenna 3: 832
182
Antenna 4: 827
179
To these sampled values add the +90
and –45
phase corrections mentioned above producing:
Antenna 1: 830
135
or –586 + j586
Antenna 2: 838
132
or –561 + j623
Antenna 3: 832
137
or
–608 + j567
Antenna 4: 827
134
or –574 + j594
East Beam (sum of above) = –2329+j2370 (3329
134.5
in polar form)
Since the sum is roughly four times the signal amplitude on each antenna there has been a coherent
signal enhancement for this received echo because it arrived from the direction of the beam. It is in-
teresting to note here, that these same four amplitudes could have been phase shifted corresponding to
another beam direction in which case they would not add up in-phase. The DPS does this seven times
at each height, using the same four samples, then detects which beam results in the greatest amplitude
at that height. Of course at a different height another source may appear in a different beam, so the
beamforming must be computed independently at each height.
Drift Mode – Super-Resolution Direction Finding
1:81. By analyzing the spatial variation of phase across the receiver aperture, using Equation 1-24, the two-
dimensional angle of arrival (zenith angle and azimuth angle) of a plane wave can be determined precisely us-
ing only three antennas. The term super-resolution applies to the ability to resolve distinct closely spaced
points when the physical dimensions (in this case, the 60 m length of one side of the triangular array) of the
aperture used is insufficient to resolve them (from a geometric optics standpoint). Therefore, the use of inter-
ferometry provides super resolution. This is required for the Drift measurements because the beam resolution
achievable with a 60 m aperture at 5 MHz is about 60, while 5 or better is required to measure plasma veloci-
ties accurately. Using beamforming to achieve a 5 angular resolution at 5 MHz would require an aperture di-
mension of 600 m, which would have to be filled with on the order of 100 receiving antenna elements. There-
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