Aerospace and Electronic Systems Magazine December 2017 - 65

Frazer
reducing clutter and hence in clutter limited cases it may improve
target detectability in which case the penalty PM is of no concern.
During MSE we examined the energy budget of the one-way
transmission system. In the example reported here we measured
the SNR of a preserved mode (σpm = 40 dB) in the MISO case for a
total transmitter power summed across all power amplifiers of Pt =
378.5W. We then applied the computed beamformer weights at the
transmitter location to form a conventional SISO transmit beam
and re-measured the preserved mode SNR (σps = 43 dB) and total
transmitter power (Pt = 61.8 W). The relative sensitivity between
the SISO, PS, and MISO, PM, cases is directly computed as
SS/ M = 10log(378.5 / 61.8) + (43 − 40) = 10.9 dB

(11)

which corresponds well with the expected relative sensitivity of
PM = 10.8 dB.

MODE SELECTION ON RECEIVE
While the Mode Selection Experiment was concerned with demonstrating mode-selectivity on transmit we took the opportunity of
the installed experimental equipment to confirm established results
for mode-selectivity on receive [50]. When implementing modeselectivity on receive one does not require a multiple waveform set
on transmission so only a single waveform was transmitted from
the Elliott transmitter. The mode-selectivity on receive case corresponds to the target to receiver part of the two-way OTHR situation.
For a limited period prior to the beginning of the MSE-II campaign the transmit system at Coondambo was replaced with a multichannel receiver connected one channel per element to the 12
element minimum redundancy array. The down-range Elliott site
was configured to transmit a single radar waveform.
A representative example of the measured data is shown in Figure 11 which shows the single channel received signal in Figure 11a
and the output of a MVDR mode-selective beamformer designed to
retain the E-layer propagation mode in Figure 14b. All modes other
than 1E are rejected to the system noise floor. The signal at a range
of 1,480 km and Doppler offset of 1.5 Hz was a local-to-the-receiver injected calibration signal and should be ignored. Figure 11a
shows the single channel result with 1E, 1F2l, 1F2h-o, and 1F2h-x
propagation modes (ordered in increasing range) together with a
local to the receiver injected calibration signal at 1.5 Hz Doppler
and 1,480 km range. Mode-selection was applied using the MVDR
mode-selective beamformer with the 1E propagation preserved and
all other modes rejected. This is shown in Figure 11b.
The results confirmed expectation that mode-selection on receive is achievable provided the receiver aperture is sufficient.

MODE-SELECTIVE OTHR

Range-Doppler maps showing the results of the mode-selection demonstration on receive experiment. In both cases, the units are Doppler (Hz)
on the horizontal axis and radar range (km) on the vertical axis, and the
intensity units dBJu are dB Joules uncalibrated.

CONCEPT
The concept of MS-OTHR is to isolate and process in parallel each
ionospheric propagation mode that is suitable for target detection
whilst simultaneously rejecting unsuitable propagation modes and
hence maximize target detection and tracking performance. This
requires elevation filtering jointly at both the transmitter and the
receiver and where this filtering differs for each range of interest
in the radar range gate.2 To understand the challenge in this goal,
reconsider the example provided earlier in Figure 3. In the case of
both E and F layer propagation support, the diagram in Figure 3
2

In [27] we introduced the concept of Mode-Selective OTHR (MSOTHR) and discussed how such a system might be employed. We
have constructed an experimental system at a site in Western Australia and, in the following, provide some insight into results from
that system. This is the two-way backscatter case and extends the
detailed results of the one-way case in the previous section.
DECEMBER 2017

Figure 11.

Classical OTHR are usually bistatic with physically separate
transmit and receive systems that permit continuous waveforms
without receiver overload. This is to maximize energy-on-target.
They typically have spatial selectivity in azimuth but not in elevation. One exception to both these common attributes is the
Nostradamus OTHR [51]. We note that while Nostradamus is
spatially selective in elevation it is unable to generate multiple
simultaneous range-dependent elevation beams on transmit.

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine December 2017