[time-nuts] Low SNR GPS reception and cheap LNAs

Chris Albertson albertson.chris at gmail.com
Fri Apr 25 15:42:16 UTC 2014

On Fri, Apr 25, 2014 at 6:28 AM, Attila Kinali <attila at kinali.ch> wrote:

> Hi,
> I recently bought a bladeRF[1] to experiment a bit with GPS decoding.
> I tried to get GNSS-SDR[2] which seems quite good, but has its flaws.
> One of the things was that i cannot seem to get a fix in my environment.
> One of the problems seems that my antenna position is far from optimal.
> Aparently, GNSS-SDR uses only a very rudimentary acquisition technique
> (at least so i have been told). Now i wonder what techniques for low SNR
> acquisition are around. Would someone be so kind and give me some key
> words to google for?
> I also am looking to add an LNA to my reception chain, which is a
> mix of a 50R antenna with 75R Coaxcable (sat coax stuff is just a lot
> cheaper :-). Has anyone a recomendation for a good LNA that can be used
> in a flying construction (soldering onto two back-to-back glued
> connectors)?
> Ie it shouldnt be a QFN or BGA. DFN works but i'd rather have something
> with pins, like SC-70/SOT-323 or similar/larger.

You best bet is to change out the antenna.  You can buy them with a higher
built-in gain up to about 40dB.   My understanding is that designing a GOOD
LNA is not so easy as little things like the exact layout of the PCB and
how the PCB transitions to connectors matters a lot.  But you can buy these
ready made for cheap.  I've seen complete LNAs in an enclosure with
connectors at good prices on eBay.    The user manuals I have say using 75R
cables with compression type F connects is OK.    I doubt the cheaper type
f-connectors would work well.

I have a good high quality Tremble in-line amplifier with N-connector and
the ability to pass DC.   In my experiment I place the antenna indoor and
use amplifier and then outdoors with no amplifier.  I get MUCH better
results with my 26dB gain antenna on the roof and 25 feet of cable than
with indoor amplified antenna with short cable.      My un-scientific
conclusion was that amplified noise is still noise.

>                         Attila Kinali
> [1] www.nuand.com/bladeRF
> [2] www.gnss-sdr.org
> --
> The trouble with you, Shev, is you don't say anything until you've saved
> up a whole truckload of damned heavy brick arguments and then you dump
> them all out and never look at the bleeding body mangled beneath the heap
>                 -- Tirin, The Dispossessed, U. Le Guin
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Chris Albertson
Redondo Beach, California

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