TDoA with propagation delays based on IRI2016 electron densities

This is a follow-up of this blog post where the method referred to there is applied to TDoA maps. The first example is a STANAG 4285 signal (FUO Toulon) centered on 8436.4 kHz (USB: 8432.6 kHz)

TDoA maps using ground-wave propagation delays

TDoA using propagation delays based on IRI2016

The maps using propagation delays based on IRI2016 electron densities match better the (assumed) true position of the transmitter. It is a small effect but noticeable, and can be seen better in the side-by-side comparison below:

TDoA comparison

Another example are the TDoA maps from the last blog post where the TDoA maps based on more realistic propagation delays are more consistent between the two measurements; both indicate a position on the South coast of Cornwall:

TDoA comparison

TDoA comparison

TDoA code update

The TDoA code now does not explicitly use gnuplot any more. All plotting is done in octave. This will it make easier to extend the code (work in progress).

Recently some interesting signals have been independently observed by several people, see, e.g., tweet. These are frequency hopping signals.

Tuning a KiwiSDR to 7812 kHz two of the signal fit into its passband. Below are TDoA maps for these signals. Note that these maps are likely biased by the fact that some of the KiwiSDRs might receive the ground-wave signal and others a sky-wave signal.

TDoA maps

TDoA cross correlations

TDoA maps

TDoA cross corrlelations

Towards ionospheric ray-tracing

For the work on HF TDoA a realistic method for the signal delay is needed. Up to now, the great-circle distance divided by the speed of light is used which of course is incredibly naive. Nevertheless it seems to work, at least to some degree.

Rather than attempting a full-fledged ray-tracing simulation one can use the theorems of Breit and Tuve and Maryn's theorem for obtaining the signal propagation delay from virtual vertical height profiles, see, e.g, arXix:1104.2248 and here.

In order to evaluate this technique let us use the oblique ionograms measured by the University of Twente web SDR (thanks to PA3FWM for making these available! and for him and G3PLX for explanations):

The plots below show the time delay vs. UTC for two days in June at two different frequencies. Overlaid are time delays obtained by

• VOACAP: all propagation modes with probability>0.3 are shown,
• simulations explained above using electron density profiles obtained from the international reference ionosphere model IRI2016. 

(For this comparison the oblique ionograms have been corrected for a time offset of ~0.3ms whose origin is not yet clear)

All plots are generated using octave and this octave binding of the IRI2016 model.

As the IRI model is a climatological model (as is VOACAP), the agreement between model calculation and data is not expected to be perfect. Only the general features of these backscatter ionograms are reproduced, such as the 2-hop E-layer reflection for lower frequencies during mid-day but, e.g., not sporatic E-layer reflections in the late afternoon/evenings.

Note also that this simple method does not take into account magneto-ionic effects which become important for lower frequencies (and at local night-times).