Monday, May 21, 2018

TDoA STANAG 4285

This post is for people interested in ham band intruders. In this case it is a STANAG4285 signal on 5361.8 kHz. The frequency shown on the plots is the center frequency of the modulation (5363.6 kHz = 5361.8 kHz + 1.8 kHz).

For the TDoA analysis, IQ recordings with GNSS time stamps from four KiwiSDRs located in Europe were used. The cross correlations below show at least two different modes of propagation:
Cross-correlations

Only the points marked in red which were selected by hand are used for the time-difference of arrival analysis. The location of this signal likely is Frederikshavn, Denmark: (see also this UDXF post)
TDoA


Let's now have a closer look at the signals themselves: the modulation is STANAG 4285 with 600 bps (PSK after descrambling) using the long interleaver. In the figures below one can see the HF channel response in taps of the adaptive the channel equalization filter. The bit stream is encrypted and consists of 7-bit frames marked by a LFSR sequence (format F1 in this blog post).
STANAG 4285: top - descrambled symbols; bottom: decision-feedback channel equalizer taps
STANAG 4285: top - descrambled symbols; bottom: decision-feedback channel equalizer taps
STANAG 4285: top - descrambled symbols; bottom: decision-feedback channel equalizer taps
STANAG 4285: top - descrambled symbols; bottom: decision-feedback channel equalizer taps
The code used for this analysis can be found on GitHub: https://github.com/hcab14/TDoA, and https://github.com/hcab14/signal-analysis.


This is how the cross-correlations looked a few hours later and it serves to illustrate that the ionosphere is far from being a perfect mirror:
Cross-correlations

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