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 <jim...@earthlink.net> wrote: > Magnus Danielson wrote: > >> On 11/18/2010 02:16 AM, saidj...@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. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
