| Ruoyu, Buying a commercial Iridium antenna is, I'm sure, an ideal solution for your work. If the antenna is entirely passive (i.e it has no LNA and filter built in) then it will still pick up frequencies outside the band of interest. And, though attenuation will increase as these environmental signals get further from the Iridium frequency band, strong local signals will still likely be received at a signal level much stronger than the Iridium signal because their path loss is so much smaller. I suggest to you that you take a tool like gqrx or a grc flow graph and just move an FFT through all the spectrum supported by your daughter card, you'll get a basic feel for what your local RF environment looks like through your antenna of choice. I suspect that as soon as you get into 1.8GHz you will see a lot of strong broadband signals from cellular equipment. I know I also see some narrow band terrestrial signals in the 1.6-1.7GHz band locally in the US.
Filters will all to some degree attenuate your signal of interest, but by how much varies dramatically depending on the type and design of the filter, it could be 0.5dB or 20dB, but the point is that it attenuates potential interferers and noise by a great amount. An LNA cascaded into a bandpass filter as close as possible to the antenna is generally an ideal setup for this type of weak signal work.
-Ian
Hi, Ian, I carefully read your email, it is very informative and helpful. Thank so much for your suggestions. Actually, we have ordered a helix iridium antenna, which should arrive in 2 weeks. And we have decided to purchase another two LNAs with narrower band. As to the bandpass filters, if we have the dedicated antenna with has exactly same bandwidth as real satellite, is the filters still necessary ? Will the filters attenuate the signal ? Thanks.
From: Ian Buckley [address@hidden]
Sent: Thursday, September 05, 2013 12:33 AM
To: Chi
Cc: address@hidden; Li Ruoyu
Subject: Re: [Discuss-gnuradio] How to use USRP to detect and collect weak satellite signals
Ruoyu, First off, Singapore is a very noisy place (From a radio perspective), so you are going to have to pair any external low noise amplifiers with appropriate filters for your signals of interest. It is vital that none of the components in the radio signal chain receivers too much power which can lead to non-linearity and ultimately to damage to the radio. Placing a very high gain amplifier such as you have, in conjunction with a wide band antenna is not a safe proposition with a USRP daughter board, other loud and local signals will overwhelm the initial analog stages of the radio. I find even when using a dish pointed skywards in an urban area that introducing a wideband LNA without a filter causes other local cellular (etc) signals to saturate my USRP frontend.
When I'm working with satellite signals close in frequency the Iridium signals you are interested in (~1.6GHz) I use a combination of a Minicircuits ZX60-242GLN-S+ LNA and a NBP-1560+ bandpass filter with good results with a variety of USRP's/daughter boards. For weather satellites in the 137MHz band (which is the same band as the Orbcomm downlink) I use a custom LNA+filter from SSB in Germany. In both cases gain is approximately 30dB and the noise figures very low, typical numbers for LNA's ideal for satellite use are 0.4-0.8dB NF.
You should rethink your antenna(s) completely, Orbcomm use a right hand circular polarized signal for their downlink and you could thus use any antenna design that was intended for use to receive NOAA's APT weather satellite signal …you will find many references on how to build these if you google "APT antenna"….here's one incredibly simple example of a less than ideal, but simple solution that worked fine: http://websterling.com/tsro/apt/. In fact listening to the APT signal from NOAA-16, NOAA-18 and NOAA-19 may be a good starting point for you to develop your skills.
Iridium is also a RHCP signal, and in those bands people typically use a patch or helix antenna for these types of signal, both can be built quite easily and at low cost.
You should also familiarize your self with the open source software, "predict" and "gpredict" (There are others but these are recommended), as it is important to know when (and where with a directional antenna) your signal of interest is actually visible in the sky.
And lastly GPS L1…..in the last few weeks I happen to have been listing to this with Balint from Ettus whilst we have been testing some antennas and other hardware. A USRP nor any other radio is going to see the raw signal above the noise floor with an omnidirectional antenna, the magic of GPS is all down to spread-specturm processing gain and is an interesting study if you have the time. One quick trick is to use autocorrelation to look for the signals, since the L1 C/A spreading code repeats every 1mS, it's relatively easy to prove it's there even though you can not see it in the FFT. And if your curious about what it looks like when you work a little harder to listen to it, then a USRP (SBX in this case) and a 1 meter dish prove more than sufficient: http://ionconcepts.com/ionconcepts/signals/L1-signal.png
-Ian
Hi, All I am working on a project which requires to receive signals from LEO satellites like orbcomm and iridium. I tried to use USRP N210 to collect the data, but I found the signal may be too weak to be observed. Stuffs I used: 1. USRP N210, DBSRX2 800-2400MHz 2. Minicircuit cable amplifier, provides 37 dB gain at 15V/0.68A supply input. To ensure signal is enough enhanced, I used two of them in recent experiments. 3. Horn antenna with frequency range from 0.8 - 18 GHz. beamwidth 60 degree. After connected all things and warming up, I run "uhd_fft" to check if signals can be seen in frequency domain. I expected a peak around the center frequency. However, it is just noise. But if I turned center frequency to GSM band, it showed signal clearly. And then I turned it to 1.57542 GHz, which is the L1 band of GPS, it also shows nothing but noise. So, I am wondering if it is because the signal it too weak to be detected in that way. Have anyone ever done weak signal detection and collection before with USRP ? not only satellite, any weak signals is fine. If you have done similar project before, could you please tell me how you know signal is there if it cannot be seen by FFT. Any other function can help ? Many thanks ahead.
Best regards, Ruoyu
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