GPS frequency

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walrusgumboot
Posts: 21
Joined: Thu Mar 09, 2017 7:04 pm

GPS frequency

Post by walrusgumboot » Fri Mar 10, 2017 11:01 pm

I set the Nongle N3 rx to look for the GPS signal at 1.57542 GHz.

As the screen grab attached shows, I could detect only one signal at 1.574.984 GHz.

Do I have a calibration error here, or is the 1.574.984 GHz signal a spurious emission of some kind.

Can anyone else see the GPS signal at 1.57542 GHz?

All replies welcome. :)
Attachments
GPS frequency.JPG
Screen grab sdrsharp Nongle N3
GPS frequency.JPG (183.07 KiB) Viewed 3435 times

rtlsdrblog
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Re: GPS frequency

Post by rtlsdrblog » Sat Mar 11, 2017 2:48 am

What antenna are you using? You'll need a GPS antenna to receive it. Doesn't look like much signal is there on your screenshot, that spike is probably just a spur.

Take a look at this tutorial http://www.rtl-sdr.com/rtl-sdr-tutorial ... -plotting/.

walrusgumboot
Posts: 21
Joined: Thu Mar 09, 2017 7:04 pm

Re: GPS frequency

Post by walrusgumboot » Sat Mar 11, 2017 4:55 am

rtlsdrblog wrote:What antenna are you using? You'll need a GPS antenna to receive it.
I am using a DIY groundplane antenna with vertical and radials of 4.5 cm made from silver solder brazing rods at 45° for a nominal 50Ω impedance.

I chose the groundplane not knowing what direction the gps satellite might be at.

Should I be using an antenna with more gain - multi-element Yagi, corner reflector, dish etc?

Thanks for your input.

Walrus.

rtlsdrblog
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Re: GPS frequency

Post by rtlsdrblog » Sun Mar 12, 2017 2:11 am

This is for GPS satellites right?

A ground plane antenna is designed for terrestrial signals, you won't receive much from the sky.

You need a non directional antenna which points most of the radiation pattern towards the sky since GPS satellites move all over the sky. So a Yagi, dish etc are not good ideas. A patch antenna or ceramic patch is though. You'll probably also need an LNA to reduce the noise figure.

A simple active patch antenna from eBay or aliexpress will work very well for GPS. These have built in LNAs. But you'll need to make a bias tee to power it as the nongles n3 doesn't have a bias tee built in.

walrusgumboot
Posts: 21
Joined: Thu Mar 09, 2017 7:04 pm

Re: GPS frequency

Post by walrusgumboot » Sun Mar 12, 2017 8:06 pm

There's not much in it really. Patch antennas typically have gain of the order of 6 - 9 dBi.

A half-wave dipole has a gain of 2.19 dBi and a radiation resistance of 73 ohms, so a quarter-wave monopole has a gain of 2.19 + 3 = 5.19 dBi.

I chose the groundplane because it is omnidirectional, and I didn't know where the satellites might be at, or what their azimuth and elevation might be. Clearly, the ground plane antenna wouldn't be of any use when the satellite was directly overhead, but that is not often the case
azimuth-and-elevation-angles-of-GPS-satellites..png
azimuth-and-elevation-angles-of-GPS-satellites..png (16.53 KiB) Viewed 3368 times
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rtlsdrblog
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Re: GPS frequency

Post by rtlsdrblog » Mon Mar 13, 2017 12:43 pm

A groundplane is omnidirectional towards the horizon only. You can get the satellites on the horizon, but your missing out on the main large area of the sky, where most will be. Additionally you'll be picking up interference from terrestrial sources more strongly with the GP antenna.

You didn't mention if you have an LNA and/or filter? The noise figure of the RTL is not great at L-band, so a good low noise figure LNA is needed to bring this down. Also, if you have interferers in your area and filter might be needed too. This is why those cheap $3 GPS antennas are great. They have an LNA and filter built in, and a right hand circularly polarized ceramic patch antenna tuned to the GPS frequencies.

With the LNA you might be able to see the GPS hump with your GP antenna.

Also, what type of coax and length are you using? At 1.5 GHz coax will be quite lossy, so you'll need to use high quality coax and short runs only. Or place the LNA by the antenna to overcome the coax loss.

parabolix
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Joined: Mon Jul 31, 2017 6:24 am

Re: GPS frequency

Post by parabolix » Mon Jul 31, 2017 6:46 am

You won't be able to see the gps birds as you would other carriers; they operate well under the noise floor and use noise correlation to lock/decode. from memory, each bird has it's own pseudo random noise frames, which helps with what would be mutual interference.

rtlsdrblog
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Re: GPS frequency

Post by rtlsdrblog » Tue Aug 01, 2017 4:11 am

parabolix wrote:You won't be able to see the gps birds as you would other carriers; they operate well under the noise floor and use noise correlation to lock/decode. from memory, each bird has it's own pseudo random noise frames, which helps with what would be mutual interference.
You can actually see a wide but weak 'lump' at the GPS frequencie if the antenna is good enough. Listening in USB gives a digital sound. Enough to confirm that you are receiving it.

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