Hi Jesus, If you are interested, you can see a video of a three-antenna, single-pol correlator in action doing exactly this, at http://www.ovsa.njit.edu/expansion/assets/IMG_0485.MOV (this is just a movie capture with my iPhone). The bandwidth of the correlator is 500 MHz, but in the video the display is zoomed in to the GPS L2 band. During the video, we are changing the nominal delays (you can see slight, abrupt phase jumps every time the delay changes by 1 ns), and can also see the phases sweep due to the fact that we are not correcting for fringe rotation (we do that in software). We are also not using fine-delay corrections in the correlator, since this can also be done in software after recording the data. We can share this design with you, if it will help get you started.
On another topic, Andrew, you state that we can calculate the delays since we know the position of the antennas and source. Does anyone know an accurate way to find the location of GPS satellites? I wrote software to calculate this, nominally to an accuracy of a few meters for the satellites, but I never succeed in getting steady phases after correction for the calculated position, indicating that my calculations are not correct to better than about 30 arcminutes. My calculations are based on the so-called "SP3" files kept at ftp://igscb.jpl.nasa.gov/. If anyone knows of suitable software for getting local pointing coordinates of GPS satellites to arcsec accuracy, please let me know. Thanks, Dale ________________________________ From: [email protected] [[email protected]] On Behalf Of Andrew Martens [[email protected]] Sent: Tuesday, June 19, 2012 9:37 AM To: Jesús García LLedó Cc: casper list Subject: Re: [casper] Some questions Hi Jesus I am including the CASPER list in my reply as your questions are probably shared by almost anyone developing correlators/beamformers. On Mon, 2012-06-18 at 14:57 +0200, Jesús García LLedó wrote: > Hi Andrew, > > I write you in order to speak you about a problem I am dealing with. Firstly > I would like to tell you some details of my problem and I will try to avoid > boring you. > > The idea is to implement an Antenna Array to receive signals from > geostationary satellites. These kind of satellites can be seen a signal > source fixed in the sky. > Now, to make things simpler, we only want to implement a two antenna > correlator. > The signal from the Satellite arrives to each of the antennas, but as these > antennas are separate in the space, we have some delay between these two > signals. > So what we have to do is just to correct this delay, and them sum these two > signals. Making this we would improve the Signal to Noise ratio of the > resultant signal in 3dBs. > We want to make this with broadband communication signals. At least with a > bandwidth of 100MHz. > > I think this is more or less the working philosophy of CASPER, so we could > use some of the tools and blocks which are already implemented.To do this I > also > think that with a RoACH board, a KatAdc Board and a DAC board it will be > enough. > > I write you to ask you about the blocks that I would need to solve this > problem. You will only need the DAC if you want to transmit. A katADC will give you two inputs, I am assuming that you have only a single polarisation per antenna? The iADC will also be fine if you don't need the variable gain and switch to ground offered by the katADC. > I think in CASPER, time delay between the signals is measured and corrected > in the frequency domain. It is more accurate to say that our correlators are FX correlators so we try to do things in the frequency domain where possible. However, this could be done in the time-domain, we just don't have as much support for it. There are plans at Manchester to develop a programmable group delay FIR filter block so that fractional (less than one ADC sample) delay can be done in the time domain. > I suppose that the PFB + FFT block divides the spectrum of the signal into > several bins, then for each bin the phase difference from the two antenna > signals is measured > (but I don't know with wich block...) and afterwards I guess that phase > difference is corrected, the two signals are summed and then a synthesis > filter (the inverse of the > PFB + FFT block) is necessary to return to the time domain.... The process in our systems is as follows; 1. The signal is sampled by the ADC 2. The signal can be delayed by multiples of one sample in a block called a 'coarse delay' block (delay_wideband_prog in Delays subsection of library). This allows most of the delay to be removed. 3. The PFB/FFT divides the signal into bins. 4. The final fraction of delay can be removed using complex multiplication to rotate the phase of the data in the FFT bins. This is done in a block called the 'fine delay' block. I can send the one we use to you as it is not yet in the library. If you are not using baseband signals you will need to do fringe stopping at this point too. 5. We add the signals from different antennas and transmit the the result. Note a few things; 1. We don't go back to the 'time-domain' i.e no synthesis filter. 2. We don't measure the phase difference in the FPGA. We use software to calculate the delays in advance given that we know the position of the antennas and source. Attached is an image showing the data flow in our system to help explain. Regards Andrew
