[time-nuts] how good an oscillator do you need for a GPS simulator

Azelio Boriani azelio.boriani at screen.it
Fri Dec 16 10:46:38 EST 2011


A used Spirent is only 26K to 37K. Interesting: playing back bits from
RAM... can it be that simple? Obvious: a DAC is required.

On Fri, Dec 16, 2011 at 3:47 PM, Jim Lux <jimlux at earthlink.net> wrote:

> On 12/15/11 10:25 PM, Chris Albertson wrote:
>
>> For testing, I'd assume the gps simulator only needs to be good enough
>> that the receiver will detect the signal.   There is some Doppler
>> shift so the receiver must have to look over a wider range of
>> frequencies so if the simulator was inside that range it could work.
>>  Light travels at about one foot per nanosecond.   so your simulator
>> should need to know the time to within a few tens of nanoseconds.
>> Receivers can deal with not-perfect signal.  Multipath and refraction
>> are common.
>>
>>
> Not just functional test, but to verify the "added noise" from the
> receiver.
>
> TO just see if it can acquire and track, pretty crummy would work, because
> it's just like the horrible signals it's seeing "off the air"
>
>
>  You GPS simulator would likely have a GPS receiver inside of it and
>> sync to a real GPS.
>>
>
> Not necessarily.  You might be testing in a screen room with no external
> signals available.
>
> Clearly, one can go out and spend 500k on a nice Spirent, but I'm looking
> at what is the few hundred dollar solution.  A reasonably quiet oscillator
> driving an FPGA or playing back bits from RAM
>
>
>
>  On Thu, Dec 15, 2011 at 9:37 PM, Jim Lux<jimlux at earthlink.net>  wrote:
>>
>>> Say you want a quik n easy n cheap GPS simulator to test a GPS timing
>>> receiver.  How good does the oscillator (presumably some nice multiple of
>>> the chip rate) have to be?
>>>
>>
>
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