In order to see if there is any additional ISAR signatures in the ISS reflections, I thought it would be a good idea to Doppler correct the data. This was easily done using the Doppler correction GnuRadio block I've already written for my GroundStation software I've described previously (and which is available
here). I applied 2 corrections - one for the transmit path from Dijon and one for the receive path to my home. After applying these corrections I get the following waterfall plot:
It can be seen that compared to the 'raw' data (shown in
here) there is a very significant reduction in the frequency shift of the signal. There is still some residual frequency fluctuations which I suspect are due to
- The poor stability of the receiver - its quite temperature sensitve
- Orbital modelling inaccuracies - I'm using a TLE from today and predicting where it was a week ago
- Timing - I don't have a time index into the raw data I record
- Atmospheric propagation
I don't think item 4 will have much of an effect. I roughly optimised the timing to minimise any drift - I should improve the capture flowgraph to improve its metadata capture (although I'm unsure how to determine latencies or if I even need to).
I think the bottom line is that I can do a reasonable job of performing the Doppler corrections, but there are no other signatures to observe. I should now do the calculations to see if its even possible to observe them!
For interest, below is a picture of my antenna set up - as you can see its hardly sophisticated!
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