To get through this never ending winter I recently took a celestial navigation course. The instructor was very knowledgable with a lot of practical experience but one thing he said has me puzzled. He said that you of course must maintain accurate time to do accurate celestial navigation but you should not use the time provided from a GPS as it could be off by as much as several seconds from actual UTC time. I find this hard to believe. Any comments on this? Paul
Celestial Navigation and GPS Time
- Thread starter Phiggins
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GPS leap second
I did a quick search on the web and this what i found. Since 1972 precision clocks around the world have ticked using atomic seconds, but earth rotation is slowing down. Leap seconds have been inserted in order to keep noon happening at noon, The GPS system time (as used by the system itself) does not include leap seconds, but the difference between GPS time and UTC is included in the data sent by the satellites, so receivers can (and most navigation receivers do) display current UTC or zone time, rather than the GPS system time. it can be off by several seconds.
I did a quick search on the web and this what i found. Since 1972 precision clocks around the world have ticked using atomic seconds, but earth rotation is slowing down. Leap seconds have been inserted in order to keep noon happening at noon, The GPS system time (as used by the system itself) does not include leap seconds, but the difference between GPS time and UTC is included in the data sent by the satellites, so receivers can (and most navigation receivers do) display current UTC or zone time, rather than the GPS system time. it can be off by several seconds.
Uh, you must be precise.
Are you talking about UTC(USNO) or UTC(PTB)? In any event, I do not know the answer to your question. I just skimmed the article from March GPS World to wit: Interoperability on Time. It is a great example of too many acronyms but it does illustrate the need for precision in re to time and navigation. GNSS, Galileo, GPS, GLONASS, GGTO, GST, UTC, UTC(USNO), UTC(PTB), TAI, ESA, SISNet, EGNOS, WAAS, MOPS, PTF, HPE, GPS and Galileo SIS, TWSTFT, EU, IGS, and various combinations of these acronyms are replete throughout the article. If anyone here can accurately define half of these acronyms, he is truely a nerd and needs to do more sailing. The article is a discussion of timing signal differences of nanosecond and fractions thereof and ways to combine the various satellite signals for greater accuracy in determining GPS position. I barely understand what the writer is trying to say and he concludes with a psychological interpretation which is likewise over my head. Questions for you: How accurately can you read the time displayed on your GPS? A few seconds (or in the terms of the article, a few billion nanoseconds)? If you have a GPS, why do you want to use CN or is it CNav or CelNav or what? If your GPS fails, where will you get time signals then? Why not use that time signal in the first place?
Are you talking about UTC(USNO) or UTC(PTB)? In any event, I do not know the answer to your question. I just skimmed the article from March GPS World to wit: Interoperability on Time. It is a great example of too many acronyms but it does illustrate the need for precision in re to time and navigation. GNSS, Galileo, GPS, GLONASS, GGTO, GST, UTC, UTC(USNO), UTC(PTB), TAI, ESA, SISNet, EGNOS, WAAS, MOPS, PTF, HPE, GPS and Galileo SIS, TWSTFT, EU, IGS, and various combinations of these acronyms are replete throughout the article. If anyone here can accurately define half of these acronyms, he is truely a nerd and needs to do more sailing. The article is a discussion of timing signal differences of nanosecond and fractions thereof and ways to combine the various satellite signals for greater accuracy in determining GPS position. I barely understand what the writer is trying to say and he concludes with a psychological interpretation which is likewise over my head. Questions for you: How accurately can you read the time displayed on your GPS? A few seconds (or in the terms of the article, a few billion nanoseconds)? If you have a GPS, why do you want to use CN or is it CNav or CelNav or what? If your GPS fails, where will you get time signals then? Why not use that time signal in the first place?
UTC
Back when I was a Submarine Radioman we used to use GPS to "Cal" our atomic clock that we used as a reference for our navigation. I believe the time code coming from the GPS is very very acurate and should be sufficant. If you lose that time reference you can always go to your HF radio and get the time tick from WWV.
Back when I was a Submarine Radioman we used to use GPS to "Cal" our atomic clock that we used as a reference for our navigation. I believe the time code coming from the GPS is very very acurate and should be sufficant. If you lose that time reference you can always go to your HF radio and get the time tick from WWV.
And before WWW and radios
people actually used clocks. Can you imagine? The book by Dana Sobel a few years ago about the development of the chronometer was very interesting. Called "Longitude" I believe. Stu
people actually used clocks. Can you imagine? The book by Dana Sobel a few years ago about the development of the chronometer was very interesting. Called "Longitude" I believe. Stu
Before the WWW?
If I recall correctly, before the www we were living in caves and Hinckley was still hollowing out logs.
If I recall correctly, before the www we were living in caves and Hinckley was still hollowing out logs.
gps time
Scott's right, the time signal from the gps is extremely accurate-far more accurate than you need for celestial nav. Remember, the gps system calculates the propagation time from the satellites to triangulate your position-at 186000 miles per second, your distance from the sats is a really small time interval!
Scott's right, the time signal from the gps is extremely accurate-far more accurate than you need for celestial nav. Remember, the gps system calculates the propagation time from the satellites to triangulate your position-at 186000 miles per second, your distance from the sats is a really small time interval!
GPS time is accurate, but....
The GPS internal clock is extremely accurate, but the time displayed on the GPS can lag up to a few seconds, depending on the unit. You can check your GPS readout against WWV to see if this is an issue with your GPS.
The GPS internal clock is extremely accurate, but the time displayed on the GPS can lag up to a few seconds, depending on the unit. You can check your GPS readout against WWV to see if this is an issue with your GPS.
GPS doesn't get any more accurate!
The time stamp is sent directly from an atomic clock on a sattelite and is sent every few seconds (or minutes), that updates the time on your GPS. Considering early mariners used wind up clocks that spent weeks without being updated to do the same navigation, it's like saying a sledgehammer is too small to drive a thumbtack! I suspect your instructor has a certain disregard for devices that take away from his/her livelyhood...(That darn GPS!)
The time stamp is sent directly from an atomic clock on a sattelite and is sent every few seconds (or minutes), that updates the time on your GPS. Considering early mariners used wind up clocks that spent weeks without being updated to do the same navigation, it's like saying a sledgehammer is too small to drive a thumbtack! I suspect your instructor has a certain disregard for devices that take away from his/her livelyhood...(That darn GPS!)
Several Seconds?
Would probably equate to several yards, just how accurate do you need to be when your in the middle of an ocean? I'm happy if what ever method I'm using puts me in sight of my destination. I smell B/S, I think that Todd hit the nail on the head with his evaluation of the instructors motives.
Would probably equate to several yards, just how accurate do you need to be when your in the middle of an ocean? I'm happy if what ever method I'm using puts me in sight of my destination. I smell B/S, I think that Todd hit the nail on the head with his evaluation of the instructors motives.
What's the beef?
The accuracy of the time tick is nowhere near as close as you're going to get with a cocked hat triangulation with celestial navigation. It just doesn't matter. Phil's right.
Stu
The accuracy of the time tick is nowhere near as close as you're going to get with a cocked hat triangulation with celestial navigation. It just doesn't matter. Phil's right.
StuA decent quartz watch
Will keep time so much more accurately than anything used when celestial was the only way to navigate. And you will be using that sextant when the batteries in the GPS die...or after the lightning strike when all electronics are toast (yes even the handhelds).
Will keep time so much more accurately than anything used when celestial was the only way to navigate. And you will be using that sextant when the batteries in the GPS die...or after the lightning strike when all electronics are toast (yes even the handhelds).
Celestial for romantics
having spent 20 years as an AF navigator I think I'll just stick with basic DR and redundant GPS units. And, I agree the GPS time is more than sufficient/accurate.
having spent 20 years as an AF navigator I think I'll just stick with basic DR and redundant GPS units. And, I agree the GPS time is more than sufficient/accurate.
it's accurate enough for the navy
The GPS is used to synch the navigation and strategic fire control (targetting) clocks on nuclear submarines. The time provided is good enough for the fire control system to use a celestial fix to lob a weapon several thousand nautical miles and have it hit within the infield if aiming at the pitcher's mound. If there is a discrepency in your time display it is between the signal as sent and the time as displayed on the unit.
The GPS is used to synch the navigation and strategic fire control (targetting) clocks on nuclear submarines. The time provided is good enough for the fire control system to use a celestial fix to lob a weapon several thousand nautical miles and have it hit within the infield if aiming at the pitcher's mound. If there is a discrepency in your time display it is between the signal as sent and the time as displayed on the unit.
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