[time-nuts] Question on GPS receiver selection for 1pps

[paul swed]

OK interesting but the question was what is the need for a reasonable 1 pps
for gpsdo locking. The first comment on this is anything below 30 ns.

On Thu, Nov 18, 2010 at 10:17 AM, jimlux <jimlux at earthlink.net> wrote:

> Magnus Danielson wrote:
>
>> On 11/18/2010 02:16 AM, SAIDJACK at aol.com wrote:
>>
>>> Hi Paul,
>>>
>>> "good enough" is in the eye of the beholder..
>>>
>>> The M12M receiver can achieve 2ns rms. So 300ns seems very bad.
>>>
>>> Some early units had up to 1000ns error, so 300ns is quite good in that
>>> context.
>>>
>>
>> The 1 us number was never their actual performance, but the published
>> number from the ICD-200 spec, and it was often given for the receivers. It
>> was later reduced to about 340 ns with SA on... what the same old receivers
>> give as performance in todays spec-manship needs to be measured. So the
>> improvements may not have been that great actually.
>>
>
>
> 1 us is "one chip" in the C/A code
>
> System performances in direct sequence spread spectrum schemes typically
> assumed that a receiver could only track the PN sequence to 1 chip. That 1
> us (approximately) corresponded to a positioning uncertainty of 300m, which
> was deemed "good enough for civilian use, but not good enough for mid-course
> and terminal guidance of ICBM warheads", for which you'd need  something
> better (e.g. the 30m, 10.23MHz P/Y code).  I've heard stories about how good
> your accuracy needs to be to make sure you destroy hardened silos in a
> "first strike" scenario.
>
> There was a great recap of the history of GPS in GPS World a few months
> ago, and I think some of this was discussed in there (certainly the whole
> idea of why PN codes were selected, and the various architectures proposed
> is in the recap..)
>
>
> Back in the 70s and early 80s, tracking to a single chip was doing fairly
> well.  Most receivers of that day were procured in the context of a systems
> engineering effort that flowed down a "shall have position to 500 meters"
> kind of requirement to a "receiver shall have timing no worse than 1
> microsecond" hardware requirement.  (and the high level requirement was
> almost certainly flowed back up from a "how good can we do" expectation)
>
> The idea of FFT based acquisition, and stuff similar to that, was a
> "wouldn't it be nice",but when I was doing this stuff in the 80s, a 10MHz
> bandwidth real time FFT box was a pretty big unit with a whole bunch of big
> boards with pipelined hardware multipliers, discrete counters, ram chips,
> addressing logic, etc.  (the 16x16 bit multiplier accumulator ASIC had just
> come out).  Maybe 50-100kg, and 50x50x80 cm, and kilowatts of power (that
> straight Schottky or AS TTL draws a lot of power)
>
> But these days, we have signal processing that's many orders of mangitude
> better (thank you Moore's law), so it's pretty easy to track to a fraction
> of a chip, which observation led to deliberately adding errors to the C/A
> code (selective availablity).  And then, when signal processing got even
> better, and people figured out how to do code-less carrier phase tracking,
> etc.. even SA became obsolete.
>
>
>
>
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