alant
Well-Known Member
Maybe, but how are you accessing or listening to TIM?
If via any device likely to delay signal, then isn't accurate time.
Use GPS time to set watch for astro navigation?
- Thread starter Prasutigus
- Start date
Maybe, but how are you accessing or listening to TIM?
If via any device likely to delay signal, then isn't accurate time.
Prasutigus
Well-Known Member
Mobile phone. Now I'm going to compare the mobile TIM with the landline TIM..
right, done that, the mobile is about 0.2 secs after the landline!
laika
Well-Known Member
The key phrase in AntarcticPilot's post is "up to". It'll probably be better than that. A couple of years ago I built a time server using a raspberry pi and a gps receiver (an adafruit ultimate gps). The nmea output from the gps receiver averaged about .6s delay with a whole load of jitter. For any kind of accuracy you need to synchronise the time using the PPS ("pulse per second") signal which cheaper receivers don't output (but the adafruit ultimate does). Then of course there may be some non-deterministic delay in actually displaying the data.
A few years back I tried to "Ask Raymarine" whether the GPS date on their plotters utilised a PPS signal and whether they'd measured its accuracy. The best they could say was that it should be accurate to within "a couple of seconds".
Frankly if you check your GPS against TIM (as you have) and the analogue pips and it sounds close enough then it's close enough
alant
Well-Known Member
Have you got a landline offshore then?
mjcoon
Well-Known Member
Prasutigus
Well-Known Member
Not yet... but it's in the pipe-line!
Prasutigus
Well-Known Member
Thanks again for the tsunami of erudition, in answer to what may have been a slightly ignorant question!
Uricanejack
Well-Known Member
Yes. it should work.
Although GPS time is out by 18 seconds the signal includes a correction the GPS display unit applies.
I would recommend checking it against a good watch daily.
WWV still transmits a UTC time signal which can be picked up by a short wave radio.
Occasionally I have seen my display give the time incorrect by a minute. I have no clue why. The only reason I noticed. I do a daily time check.
Although GPS time is out by 18 seconds the signal includes a correction the GPS display unit applies.
I would recommend checking it against a good watch daily.
WWV still transmits a UTC time signal which can be picked up by a short wave radio.
Occasionally I have seen my display give the time incorrect by a minute. I have no clue why. The only reason I noticed. I do a daily time check.
MM5AHO
Well-Known Member
There's confusion in this thread between "accurate" and "precise".
Kerenza
Well-Known Member
And as accuracy has been redefined by the ISO, at least in scientific circles, there's no point in being pedantic when we all know what people mean.
K
GHA
Well-Known Member
Has anyone actually managed to receive that in europe? I've never had any luck.
Jonas
Well-Known Member
All of this makes me wonder just how accurate a position line is expected. Could be a little disappointing for someone...
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Prasutigus
Well-Known Member
No, tried once with a long aerial, top of hill, zilch.. but in London though.
JumbleDuck
Well-Known Member
"Bang Goes The Theory" sent Dallas Campbell around the world by plane with one of a pair of atomic clocks. After 80 hours he had aged by 230ns less than he would have if he had stayed on the ground.
https://www3.open.ac.uk/media/fullstory.aspx?id=19636
st599
Well-Known Member
No, GPS time is not UTC. It hasn't been adjusted for the 17 leap seconds that have occurred since launch.
JumbleDuck
Well-Known Member
Except for the leap second offset, GPS time stays within 1 microsecond of UTC and the deviation is broadcast to a precision of 90ns or better. How much better than that does an astronavigator need? Mind you, using a GPS receiver for astronavigation timing seems a bit like using a diesel generator and an array of lamps to illuminate solar cells to recharge a battery, but each to his own.
https://ilrs.cddis.eosdis.nasa.gov/docs/timing/gpsrole.pdf
maby
Well-Known Member
The confusion is much more fundamental. Saying that GPS time is in some way "inaccurate" or "imprecise" is pretty much the same as describing a clock made for Mars as "inaccurate". GPS time is extremely accurate, but makes no claim to reflect time on the Earth. The fact that GPS time and Solar Time were once synchronised and that GPS time runs at pretty much the same rate as Solar Time has led to the confusion - but these were relatively unimportant design decisions that were made when the GPS system was first developed and initialized. All that actually matters for GPS to work is that all the satellites and the ground stations agree on the clock that they are using and that their internal clocks are synchronised pretty accurately to each other. If the GPS system keeps running long enough, then the discrepancy from Solar Time will become very large - but that discrepancy will still be accurately predictable such that it will be easily possible to derive an accurate solar time from a GPS time.
Prasutigus
Well-Known Member
Greetings, it's all perfectly logical.. the GPS time will be used to keep a regular check on the chronometer(s) on board; thus if the GPS does die, (like all the Ocean Theory tutors say it will), the GPS will have contributed (from the grave so to speak..) its atomic accuracy to the £5 Casio watches subsequently used for astro
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AntarcticPilot
Well-Known Member
People are missing a point. GPS time is exceedingly accurate - BUT the readout of that time on a navigational GPS is not necessarily as accurate, because of intrinsic delays between the receipt of the signal and its display. As the update frequency of the GPS is about every 1-2 seconds, the time can be out by that much. GPS used to provide accuratetime signals use a different processing path, and are exceedingingly accurate - but they aren't navigational systems.
Lon nan Gruagach
Well-Known Member
No, we dont all know what people mean, because people dont know what they mean.
To answer the op (pedantically) you can calibrate your watch by using GPS. GPS will deliver a second, a minute, a day a week as accurately as you could wish for. You will know precisely how fast or slow your watch is. What you cannot do with GPS time is set your watch (precisely)
Having said that, if anyone here claims to be able to astro navigate with a precision that shows a difference in sightings taken 17 seconds apart, well, theres language in The Lounge for that, but suffice it to say I might call them ambitious.
