Hi Magnus, > Hi Skip and Tom, > Yes... almost. The thing is that the GPS orbits is a few minutes shy of 12 > hours
Right. I think that's why he picked the subject line: "12 hours (-2 minutes)". Since you're interested in this level of detail, there are papers about GPS orbits, repeat times, sidereal time, and orbital maneuvers: http://www.kristinelarson.net/wp-content/uploads/2015/10/ig0806_gnss-solutions.pdf http://www.kristinelarson.net/wp-content/uploads/2015/10/gpsrep.pdf These details turn out to be more important in the geodetic community than the T&F community. We tend to average a lot, but they have embraced high-rate kinematic GPS receivers as zero-drift seismometers. Some more papers: http://xenon.colorado.edu/igs5_revised.pdf ftp://ftp.ngs.noaa.gov/pub/abilich/papers/Bilich2008_Denali.pdf This issue of "just a bit less than 12 hours" caught my eye because I ran across it in ADEV plots. See: http://leapsecond.com/pages/sidereal/index.htm http://leapsecond.com/pages/sidereal/sv.htm http://leapsecond.com/pages/sidereal/14years.htm The 14years.htm page nicely shows the occasional "station keeping" orbital maneuvers of each GPS SV. /tvb ----- Original Message ----- From: "Magnus Danielson" <mag...@rubidium.dyndns.org> To: <time-nuts@febo.com> Cc: <mag...@rubidium.se> Sent: Monday, May 23, 2016 11:57 PM Subject: Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes) > Hi Skip and Tom, > > Yes... almost. The thing is that the GPS orbits is a few minutes shy of > 12 hours, since they is aligned to sidereal time, so the pattern shift > on the sky and it takes half a year to repeat exactly... if it where not > for orbital changes. > > Cheers, > Magnus > > On 05/24/2016 02:01 AM, Tom Van Baak wrote: >> Hi Skip, >> >>> Any help in understanding this behavior? Thanks in advance. >> >> Yes, GPS satellites do repeat every ~12 hours in orbit around the mass of >> the earth -- but -- you and the earth turns 180 degrees during those 12 >> hours. So you're no longer where you should be when the 1st repeat occurs. >> Instead you have to wait yet another 12 hours for the earth to get back to >> the place where you were, in time to see the 2nd repeat. Now when you hear >> "get back, get back, get back to where you once belonged", you'll think of >> GPS satellites instead of the Beatles. >> >> So the LH plots are correct. Here's another take: >> >> 1) Say it's 6 PM MDT in Denver at lat/lon +39/-104 and you see a pattern of >> N satellites in the sky. >> 2) Tomorrow morning at 6 AM MDT that same pattern will be in the sky -- not >> for you -- but for some guy at 6 PM lost in Inner Mongolia at lat/lon >> +39/+104. >> 3) Tomorrow evening at 6 PM MDT that same pattern will again be in the sky, >> this time for you in Denver. >> >> /tvb >> >> ----- Original Message ----- >> From: "Skip Withrow" <skip.with...@gmail.com> >> To: "time-nuts" <time-nuts@febo.com> >> Sent: Monday, May 23, 2016 3:43 PM >> Subject: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes) >> >> >>> Hello Nuts, >>> >>> I am attaching a capture from Lady Heather of a 3-day run. You can see the >>> temperature vary by 7C over each day. The TB is being run open loop and >>> another GPSDO 10MHz input to the unit instead of the unit's oscillator. >>> >>> I expected the purple line to repeat every 12 hours based on the GPS >>> constellation being the same (which maybe it kind of does), but there is >>> definitely a 24 hour repeat. What is really weird is that the number of >>> satellites that LH sees also repeats on a 24 hour cycle, not 12 (bottom >>> trace). >>> >>> Any help in understanding this behavior? Thanks in advance. >>> >>> Skip Withrow >>> >> > _______________________________________________ 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.
