ISS/GRAVES, X-ray telescopes & things......

One quick observation I've noticed over the last week or so of measuring the returns from GRAVES via the ISS is that to get a strong signal I need ISS to be at 20 degree elevation angle. The pass yestersay at around 19:20 (local) I only got the following short detection:

In order to investigate why ISS was only detectable in a small amount of the pass I made the following graph:

The areas in blue are outside of the nominal azimuth & elevation pattern of the transmitter at GRAVES. Hence it is quite obvious why only a small part of the pass I managed to detect ISS. There was an earlier pass which had an elevation of 5 degrees to me here, which I didn't detect at all. If I make a plot for that pass then I get the following:

From the graph it is clear that ISS would have only been illuminated briefly and when combined with the extreme range (and local trees/houses/etc) would make it difficult to detect it.

The things I intend to investigate further are:

1. If H-polarisation improves the signal-to-noise (ideally I'd do this using 2 identical sets of antennas and receivers but that would have to wait a while).
2. Create a program which analyses the data collected and detects all the meteors/planes/etc and saves the 'interesting' data to be further analysed.
3. Use the program I've developed to make the graphs above, be able to search for suitable times for me to attempt to detect the moon.

As an aside I've ordered the parts to make the frequency doubler which is necessary to improve the frequency stability of the RTLSDR dongles and hopefully allow me build a 2 (or maybe more) channel coherent receiver.

One of the other projects I'm currently looking at is building an X-ray telescope to be launched on a high altitude balloon. To be honest calling it an X-ray telescope is a bit grandiose, it'll be a 1 pixel detector! Anyway the first stage of the design process is to see what the atmospheric attenuation of X-rays is like. The graph below shows the altitude at which 0.5, 0.1 and 0.01 of the incident radiation is left.

The X-ray absorption data (for O2, N2, Ar) is from "AD-A278 139, NBS Circular 583" and the atmospheric model is from the C implementation of NRLMSISE-00 which returns the number density for O2, N2 & Ar. In order to check that I've not made any blunders, I compared with the only other X-ray atmospheric graph I could find here in fig 2.4, page 46. Although the line for 0.5 agrees well, the other lines do not. In order to double check my answers I tried using the classic 1976 atmospheric model (which just returns the density of air) and the X-ray absorption data for air. Using these new set of data I get a very similar graph to that above (and get very good agreement at sea level which I also check with NIST). I'm unsure how to proceed at the moment with this, but will start to look at some of the other issues, such as energy resolution, collimation, weight, power, etc.

Doppler corrections to the ISS/GRAVES data

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

1. The poor stability of the receiver - its quite temperature sensitve
2. Orbital modelling inaccuracies - I'm using a TLE from today and predicting where it was a week ago
3. Timing - I don't have a time index into the raw data I record
4. 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!

Reception of ISS from GRAVES radar

GRAVES is a CW radar transmitter located in Dijon, France. Most of its energy is transmitted in a southerly direction and at an angle of around 20 degrees above the horizon. To listen to the echos from Graves I point my 4 element yagi in roughly a south east direction, I rotate it to use V-polarisation (the general consensus is that V-pol is better than H-pol - at least for meteors) and then feed that into my rtlsdr receiver (with a pre-amp to get over the cable run). The antenna is balanced on my wheelie-bin to give it around 1.5m above the ground - hardly an optimum antenna position. The receiver is tuned to 143.05MHz and a waterfall plot is created showing Doppler against time. Most of the time only noise is recorded, however about every 30 seconds or so a 'ping' will be heard from a meteor. Last night I listened when ISS was over Spain/France which meant that it should be illuminated by GRAVES and recorded the signal, shown below:

I've added some scruffy annotations below:

The Doppler signature from ISS is clearly visible along with a number of meteors and possibly a plane. (I'm not totally convinced that its a plane - the rate of change of Doppler is a lot to be explained by just a change in geometry). Anyway, some interesting results and a number of avenues for improvement:

1. Change RTLSDR to use a TCXO to have some confidence in the absolute Doppler values measured.
2. Use of multiple RTLSDR to measure returns from both polarisations to determine optimum - for all scatterers observed.
3. Improve antenna location - move antenna to my roof when best polarisation determined.

Another object that I'm interested in receiving via the GRAVES transmitter is the moon - but I don't know how difficult that will be from my location and with my current antenna position.