Ahh. Very interesting explanation. So is it somewhat correct to assume (yes, I know) that for a stationary (non-mobile) environment, these extra sats dont make much difference? This seems to be what the explanation is saying.
Ok. So let me see. For a frequency standard for use in lab equipment, it appears that short term, phase noise and other sources of noise are the things to be concerned with to get better results. These seem to really be accomplished with a good oxco. However, if I want a very accurate time-of-day clock for long periods of time, then I need long term stability which is where the GPS comes in. Do I have this right? So if I want a really souped-up freq standard for my lab, then I should concentrate on finding the best oxco I can (which may be disciplined by the GPS or manually occasionally calibrated to GPS), and use the best power supply I can find. These seem to be what I should concentrate on rather than more channels. I do believe that I read some stuff on the internet that the HP GPS DO's do seem to have very good power supplies (or converters) which contribute to low spurs. So it seems the HP's do have a real advantage (not just the name). Thanks to everyone for the help (hope I am getting the idea here!) 73 Eugene W2HX -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, November 26, 2010 4:48 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] ok, newbie questions > Yes I agree a newer thunderbolt would surely suffice for me and probably > also the ocxo in my 8662A synth > > But I am still academically curious about the impact of more channels of > satellites? What is the value of these extra sats? > Thanks! In a mobile scenario, you need measurements to four or more satellites to solve for x,y,z and time. A ground vehicle travelling in areas with lot of antenna masking - high buildings close to the road or trees - blocking line of sight from antenna to the satellites, will want to track all satellites in view. And since especially low elevation satellites will come and go, you want to pick these up really quick once visable. Thus you want as many channels (correlators) as possible. For a stationary timing scenario, (x,y,z) might be known - or averaged over many measurements (site survey). This gives that in a timing mode the receiver only needs to track one satellite. The GPS receiver evolution moved from 1-channel multiplexing, to 6 channels, and 8, 12 and even more. While there might be timing variants of generic navigation receiver, the core correlator chipsets are shared. So even if the timing receiver does not benefit that much from 8+ channels, once the navigation receivers got more channels, the timing versions had to follow. Modern timing receiver have better hardware to put the 1PPS signal where intended. Se http://www.leapsecond.com/pages/vp/sawtooth.htm for a comparison of semi modern receiver and an old one. This might have a bigger impact than the number of channels. -- Björn _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
