[time-nuts] GPS SDR

Jim Lux jimlux at earthlink.net
Thu Feb 2 05:47:50 UTC 2012

On 2/1/12 10:12 AM, Attila Kinali wrote:
> On Wed, 1 Feb 2012 09:27:30 -0800
> Chris Albertson<albertson.chris at gmail.com>  wrote:
>> 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.
> That's because the URSP is a general purpose system. It is designed
> to do many things. That makes it expensive. And being expensive,
> it has a low production volume, which makes it even more expensive.
> I think, a specialized GPS SDR can be build for less than 500 USD
> in low (a dozen at max) volumes.
> I guestimate, that the RF/ADC part would cost somewhere between
> 100 to 200 USD in parts. The big uncertainty here is the FPGA.
> I have no clue how much logic space for a GPS SDR would be needed
> at minimum and how much would be desirable. Hence i have no guess
> what the FPGA would cost (could be anything from a cheap 20USD
> FPGA to a 300 USD one).

how many channels do you want to track at once?
I can tell you that you can track at least 12 channels simultaneously 
with single bit ADC sampling at around 40 MHz in a pair of  Virtex II 
3000 parts. You might be able to do better (I don't know how full the 
two FPGAs are).  (that's the published spec for the radio we're flying 
on the SCAN Testbed on ISS)  It's actually a L1,L2,L5 receiver, but, of 
course, the tracking loop is the same for all frequencies, whether you 
track 12 S/Vs in L1 or 4 in all three channels.. it's all the same.

You'll have to do the nav solution in some sort of other processor.. 
that's just running the tracking loops and generating raw observables.

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