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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What time is it, really?
From: Greg R_
Date: 2008 Jul 17, 12:09 -0700
From: Greg R_
Date: 2008 Jul 17, 12:09 -0700
--- Billwrote: > What time is it, really? I believe the musical group Chicago answered that question back in the late '60s... ;-) > A while ago there was a thread on time and the affect of dropping > leap seconds on cel nav. Don't think I was on the list for that thread, but as I understand it leap seconds are added to UTC as needed to keep it within 0.9 seconds of astronomical time. The rule that I remember from back when I was first learning celnav was that your observation time had to be accurate within 4 seconds, otherwise your LOP could be off by up to 1 NM just from that error alone (I interpret that to mean +/- 2 seconds). So I would say that unless you need exceptional accuracy with your celnav sights you're probably OK just ignoring the leap seconds. -- GregR --- Bill wrote: > > A while ago there was a thread on time and the affect of dropping > leap > seconds on cel nav. > > I asked a question regarding US Navy computer time lagging behind my > GPS > time (a Garmin 76). The answer passed my common-sense > test--server/router > lag. > > Recently I obtained a clock that polls the Colorado, USA broadcast > time > signal. (Radio Shack, so perhaps not the optimal atomic-based > timepiece.) My > understanding, the clock should give me UTC (factoring in the time > lag of a > broadcast traveling at 186,000 miles per second). It seems to to run > nominally a half second faster than my GPS (within minutes after the > clock > has polled the Colorado signal and reset). > > If I understand correctly, the Nautical Almanac is based on "UT", > which > Dutton's claims is UT1. This may differ from UTC. > > Searching the web, the articles on GPS time versus UTC are confusing > to me > (perhaps the date of the articles are part of the problem.) > > I am informed: > > * GPS time will always lag behind UTC. > * GPS time will always be ahead of UTC. > * GPS satellite clocks do factor in leap seconds so can run > 19 seconds or more behind UTC. > * GPS time is "steered" to approximate UTC. (Meaning?) > > Dutton's also provided two URL's for UTC/UT1 corrections (DUT1) which > proved > to be dead ends. I finally found a March posting with a correction > factor of > -0.4 seconds, which seems to sync up with my observations. > > My questions: > > Does anyone know of a URL for up-to-date UTC/UT1 corrections (DUT1)? > > How does GPS time relate to UTC or UT1? Is the relationship somewhat > dependent on the make/model/age/firmware of the GPS unit? > > Can a person with two watches (clocks) ever know what time it is? > > Thanks > > Bill B. > > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---