Using a DPS as a Coherent Scatter HF Radar

Lindsay MagnusLee-Anne McKinnell

Hermanus Magnetic ObservatoryHermanus, South Africa

Slide 2

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.

Slide 3

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

Slide 4

An off vertical ray path

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.

ionosphere

ground

HF ray pathAntenna beampattern

Slide 5

Magnetic field added

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.

Magnetic field

Slide 6

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

Slide 7

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

Slide 8

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

Slide 9

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

Slide 10

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.

Slide 11

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?”

Slide 12

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

Slide 13

Drift Back-scatter – four minutes

A zoomed in version of the fixed frequency Drift scan made at 9MHz at 07:04UT. This single Drift file was unpacked to show the full Doppler spectrum for each data point

Slide 14

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

Slide 15

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.

Slide 16

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

Slide 17

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.