Phase 10: Antenna, Complex Receiver, Digital Receiver Integration Tests (30 October 2001)

The aim of this phase was to perform some integration tests of the hardware built over the past two months - and hopefully prove that the system is capable of performing astronomical operations. Components which are integrated into the system are listed below:

The following two photographs illustrate the system components. The spiral antenna is located on the ESL roof top and connected via a 20metre length of RG9U cable. The received signal is first bandlimited to 200MHz (centred around 1420MHz) and then input to the complex receiver. The LVDS output is then fed to the parallel port LVDS receiver. Here blocks of up to 256k samples can be captured and downloaded to a host PC.

Within the complex receivers 1-2GHz band lie two known signals of interest:

GPS Signal Results

The GPS signals are significantly stronger than the Hydrogen line, and thus studied first. With very short integration times the GPS signal could be detected. The graphs below illustrate the received signals. Here a length 256k point FFT was integrated for 100 integrations and the comb like structure of GPS can be seen. The GPS signal is a psuedo random sequence with a symbol rate of around 1MHz (1024 symbols per sequence) which give rise to peaks occuring at around 1kHz in the spectral domain. Also note that the receiver also generates its own spectral garbage - usually occuring on even 1 and 2MHz frequency bins. These spectral bins cannot be used. They only occupy one spectral bin.

Well its one thing to see spectra - but were we actually receiving GPS? Steve decided to further analyze the collected data to see if we could extract which GPS satellites we were receiving at that instant. There are only 32 possible C/A codes - each containing a stream of 1024 symbols. Since the satellites are moving with respect to the receiver we also tested for doppler shifts in the range of plus/minus 4.5kHz (in 100Hz steps). The results of the best correlations are shown in the graph below. The results indicate that we were receiving signals from GPS satellites 1, 3, 20, 22, 25 and 30. I also made a photograph of my GPS receiver minutes after recording the data set. The two satellites which are overhead (1 and 22) have the strongest correlations and the remainder also have matching, but weaker correlations. A very good result considering the antenna has 100 degree field of view, and the aircondition unit was partially obscurring the field of view on one side.

Hydrogen Line Emission Results

Secondly, consider the Hydrogen emission line located at 1420.4MHz. This astronomical source is significantly weaker and requires much more integration to see. The graphs below illustrate the results of one nights integration. The duty cycle of the system is poor (no receiver calibration, small collecting area, etc.) result in about a duty cycle of around 0.4% - or about 20 seconds of data. The duty cycle of the next system will be atleast an order of magnitude faster. However, with this against us we still managed to get a small detection of 5 sigma. My first radio astronomy experiment. Well, it was exciting for a short time!

With the success of these measurements an eight antenna system will constructed in November. Stay tuned for more astronomical results!

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