Using a DPS as a Coherent Scatter HF Radar

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Using a DPS as a Coherent Scatter HF Radar

• Lindsay Magnus
• Lee-Anne McKinnell
• Hermanus Magnetic ObservatoryHermanus, South
  Africa

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Outline

• The types of scatter/reflections that are
  discussed in this paper
• A description of the operation of the Lowell
  Digisonde
• Experimental setup
• Multiple frequency Drift Ionogram
• Fixed frequency Drift Ionograms
• Spectra from the different scattering regions
• Suggestions for future work.

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Ionospheric reflection
A vertically propagating radio wave will continue
to pass through the ionosphere, with increasing
electron density, until such time as the radio
probing frequency is equal to the plasma
frequency of the surrounding ionosphere. At this
point the radio energy is reflected and will
return to the transmitter. This is an Ionospheric
Reflection
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An off vertical ray path
ionosphere
Antenna beam pattern
HF ray path
ground
HF signals transmitted above foF2 and that are
radiated off-vertical may be totally internally
refracted in the ionosphere and come back to
Earth at some distance away from the
transmitter. At this point the ray can be
reflected on further or be scattered back to the
transmitter. This is known as Ground Scatter.
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Magnetic field added
Magnetic field
If the ionosphere is permeated by a magnetic
field then under certain conditions field aligned
irregularities can form in the ionosphere. As the
ray passes through the ionosphere it will pass
through the irregularities.
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Ionospheric coherent scatter
Irregularities in the ionosphere tend to form
along magnetic field lines. Their spatial
structure can be Fourier decomposed. If there is
a component of the spatial spectrum that has a
separation that is half the wavelength of the
probing wave then coherent scatter can occur.
The scatter from each of the irregularities will
form a coherent wave-front along the line AB
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Ionospheric backscatter
As an HF ray passes through the ionosphere it is
continuously refracted. The coherent scatter from
parts of the path that are NOT orthogonal to the
irregularities are lost (A and C in the
figure). At B the ray path is orthogonal and the
signal will return along its incident path to the
transmitter. This is Ionospheric Backscatter
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The Lowell Digisonde
The Lowell Digisonde provides Vertical incidence
ionograms that are used to determine ionospheric
electron density profiles. These profiles are
created from the Routine Scientific Format which
provides an amplitude for every sampled range.
Ranges with significantly larger amplitudes are
designated as reflections from ionospheric layers
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How does the Digisonde work?
The Digisonde transmits a series of pulses,
samples each range in quadrature and then
performs an FFT for each range. This provides a
Doppler spectrum for each range
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This Experiment
The Digisonde was configured to try and observe
Ionospheric Backscatter. The Digisondes form part
of a South African network to provide ionospheric
scaled parameters for direction finding
applications. The Digisonde was first configured
to determine if there was any scatter above foF2
(the maximum frequency reflected from electron
density layers). If any scatter was observed, the
Digisonde was parked at a fixed frequency to
get better temporal variations in the scatter.
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A Drift Ionogram
The Drift mode allows the user to store the full
Doppler spectrum for certain ranges rather than
just the amplitudes for all the ranges.
In this drift ionogram one can see the typical
virtual height profile that shows foF2 to be
7.5MHz yet there is scatter at frequencies larger
than foF2, the question is what type of scatter
is this?
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Drift Backscatter two hours
During these 4 minute fixed frequency Drift scans
at 9MHz, there are two distinct regions of
scatter, those from above and below 500km
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Drift Back-scatter four minutes
A zoomed in version of the fixed frequency Drift
scan made at 9MHz at 0704UT. This single Drift
file was unpacked to show the full Doppler
spectrum for each data point
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The Spectra
Unlike the scatter from above 500km, the scatter
from below 500km exhibits a distinct Doppler
shift indicating that this is most likely
ionospheric scatter and not ground scatter
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Ray Tracing
Using a ray-tracing tool and an ionosphere for
Grahamstown, it is clear that it is not possible
to get ground scatter from 210km when sounding at
9MHz.
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So what is it?

• We know that it is definitely not an ionospheric
  reflection as we are sounding way above foF2
• It is not ground scatter as the range and Doppler
  spectra are not consistent with ground scatter
• Possible coherent ionospheric backscatter

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Way forward

• Digisondes that are collocated with coherent
  scatter radars (SuperDARN) should make Drift
  Soundings at frequencies above foF2.
• This data can then be correlated with SuperDARN
  scatter characteristic to confirm if it is indeed
  backscatter
• If it is indeed backscatter then Digisondes can
  be configured at all latitudes to observe
  ionospheric flow dynamics and convection
  coupling.