GPS L1 SIgnals at 1575.42 GHZ?

[henry zuckerman]

I've seen this suggested as a possbile beacon/test signal for the upper end of the rtl-dongles' frequency range.  I've not had any luck finding anything at or around this freq. Tried a simple vertical antenna (1/4 wave monopole over a ground plane & an 8-turn helical (RHCP) antenna, both mounted outdoors and fed with 9913 coax.  Has anyone been able to find the L1 signal(s) with their rtl-dongle?  If so:

1. What sort of antenna did you use?
2. How far "off" (frequency offset) was the signal?

Tnx

[henry zuckerman]

Umm, that would be 1575.42 Mhz - It's getting late, here.

[Guido]

Henry,

I have been able to receive he L1 signals with a standard GPS patch antenna with a built in LNA (normally used as external antenna with GPS devices in cars). Initial tests with just the standard DVB-T antenna failed. It seems the LNA is required to imrpove the overall noise figure required for GPS decoding. Decoding was also only successfull when placing the antenna in open field or directly under the roof of the house (with degraded performance). You may also receive other nearby Inmarsat, Iridium signals nearby the L1 carrier with the GPS patch antenna. My Hama Nano dongle had a 50kHz offsetfrom the L1 carrier.

73,

Guido

[imo]

How the L1 signal looks like?  imo/

On Monday, May 28, 2012 10:01:51 AM UTC+2, threeme3 wrote:

Henry,

I have been able to receive he L1 signals with a standard GPS patch antenna with a built in LNA (normally used as external antenna with GPS devices in cars)...

[henry zuckerman]

Most excellent.  I had suspected that an LNA near the antenna would be needed at those frequencies.  I will give my helical ant a try later with a MGA-86576 preamp at the feed.

[DJ0MY]

Henry,

 

here is a video I made some time ago with the Funcubedongle....(just as a reference of what you should see)...

 

Unfortunately, I do not have a video made with the RTL SDR, but since tuner chip is the same you should see the same as the performance is nearly identical....

(note that it is a wideband signal and there is not much you will hear)

 

http://www.youtube.com/watch?v=350rCn365j8

 

You should be able to pick something up with your helical.

 

If possible (for first tests) connect the dongle directly to the antenna and use 20-25dB gain setting....

 

Regards,

 

DJ0MY

[Miguel A. Vallejo]

I found on my junkbox a damaged GPS path antenna with LNA. I'll try to
recover it and test if the DVB-T dongle can see anything and how does it
like.

Keep tuned!

[henry zuckerman]

Good video - looks like the Inmarsat signals might be more useful (allow narrower channel bandwidth).  I haven't had enough time to set up the antenna & LNA outdoors, yet.

[Guido]

Imo,

The L1 is a C/N spread spectrum signal with a chip rate in the order of one 1MHz. The signal looks like a broadband noise hump centered around the carrier frequency. With the GPS tools it is possible to correlate this spreaded spectrum with one of the possible C/N codes, resulting in a sharp spike at a given moment in time. In fact this time is the basis for position determining. I understand that also the phase information may be used for sub-meter precision.

73, Guido

[imo]

Thanks for the info. A database for various signal shapes (with pictures) would be nice to have :). imo

[Guido]

Hi Imo,

No database required. Here is the link of michele's blog where he write about his initial rtl-sdr/gps experiment. It contains a link to the acquisition script that may be used to acquire the correlations. A file called Codegen_GPS generates one of the 32 possible C/A codes.In his blog you see a picture of correlations with 8 satellites acquired at the same time.

Here is the link: http://michelebavaro.blogspot.com/2012/04/spring-news-in-gnss-and-sdr-domain.html 

73, Guido

[Random Walk]

In order to make that work, you'll need to supply a voltage of approximately +3.3 - 5 volts relative to the coaxial ground on the center conductor of the coax to the antenna, without sending it to your dongle. (or it could partially burn out the protection diode) The professional way to do that would be to use a bias tee, or power injector. Alternatively, you can build one yourself in a metal project box. Use a low value (pf range) capacitor to block dc from getting to the receiver. Make sure to mark the crucial connections, to antenna, to receiver, + power, - ground clearly! Also filter the power input connection as best as you can because that is an ideal path for RFI to mess up your reception. Use a good rf choke. Don't use a standard binding post as that looks like an antenna to microwaves. I would recommend using a small coaxial connector of a kind you don't use for other stuff for the power, then put both a choke and  capacitor and perhaps a current limiting resistor on that using as short leads as possible. Depending on how careful you are on the outside you also might want to put a diode to prevent inadvertent application of the wrong polarity voltage.

The LNAs in GPS antennas don't require a lot of current, so you can safely limit the current with a variable resistor, experiment a bit to get the appropriate value.