[time-nuts] GPS SDR

Jim Lux jimlux at earthlink.net
Thu Feb 2 05:40:58 UTC 2012

On 2/1/12 9:27 AM, Chris Albertson wrote:
> On Wed, Feb 1, 2012 at 8:19 AM, Attila Kinali<attila at kinali.ch>  wrote:
>> On Wed, 01 Feb 2012 09:07:23 -0500
>> John Ackermann N8UR<jra at febo.com>  wrote:
>>> There've been numerous threads on the Gnuradio mailing list about code
>>> to receive GPS using the Ettus Research USRP hardware.  I don't know
>>> whether anyone has actually made it work, but it appears that it's been
>>> the subject of quite a few academic projects.
> That is the problem with an academic projects typically something like
> this would be part of a Master's theses or a senior project.  and then
> the student graduates and was no more interrest in supporting it.
> I thought it might be interresting but then found out you need to buy
> $2,000+ worth of hardware for even start experimenting.    Open Source
> SDR needs to run on a common affordable platform or it will never gain
> the critical mass of users that it take to make the project live
> longer then a few months.
> I think the way to go is to find a commercial GPS chip that has a low
> level interface and then build the uP controller using a common
> development system.   Both the chip and the uP board need to be,
> common, well documented and cheap.   Then with this you build an open
> source thunderbolt type device.     An SDR that samples the microwave
> RF is going to be un-affordable, even mixing and down converting
> microwaves is not so simple as doing the same on HF ham bands.  But
> there might be low level GPS chip available for cheap.

Actually, most of the JPL GPS receivers do direct sampling from RF with 
a single bit converter. You need about 100dB of gain from the antenna, 
with some filtering (to get L1 by itself), and then you run it into a 
limiter, and just sample the output at around 40 MHz, run it into an 
FPGA, and do your stuff.

No superhet, no mixers, no nothing.  It's only 1.5 GHz.. these days, 
that's not particularly exotic.

gives some hints on the hardware side.

>> From my experience the only way projects like this get started is one
> guy works until he has a demo of a proof of concept and can say "Hey
> look this sort of works and can do simple things" and then others join


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