BS: Leap Seconds, and GPS longtitude

Subject: BS: Leap Seconds, and GPS longtitude
From: Mr Red
Date: 23 Dec 16 - 06:51 AM

BBC prog; the Museum of Curiosity A factual comedy show.
In it, former director of the Greenwich Observatory Dr Kristin Lippincott, pointed out that due to the Earth's wobble & slowing down, they have to alter GMT by leap seconds just to make the GPS come back to Greenwich at the observatory.

The zero meridian is not as important, but it means all SatNavs will be out by as much as 100 metres, one second before the leap second kicks in. Which is a somewhat variable occurrence, for the general public at least. Unless you have a really swanky SatNav that is in communication with a phone network and is designed to compensate. (I know of none such, but haven't investigated)

Of course if you don't set the SatNav to roads (fastest or shortest) it may send you down a footpath. 100 metres would be damn close in that scenario!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 23 Dec 16 - 07:00 AM

That is not right.
The leap second just relates to Earths rotation.
GPS satellites are within that rotation.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Mr Red
Date: 23 Dec 16 - 08:08 AM

tell that to Dr Kristin Lippincott, I was just quoting her! She was not joking either!

FWIW I have heard about GPS satellites using cesium and rubidium clocks to keep time in the contest of the leap second, now why would that be? see the Wiki: Atomic clocks & picture labelled Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.

As an aside the New Scientist did report that there are moves afoot to separate sidereal time from that which the scientific community needs to express data & theories etc.
Because clocks are so accurate, and technology relies on ever smaller margins (eg microwave data conduits returning to be faster than fiber-optics in money trading! think micro-seconds!)

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Nigel Parsons
Date: 23 Dec 16 - 08:38 AM

If a fog comes down on a mountain I wouldn't like to rely on a navigation method that could be 100m out!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Howard Jones
Date: 23 Dec 16 - 09:28 AM

On a mountain you shouldn't be relying solely on GPS, you should always have map and compass as well, and sometimes they are more reliable.

GPS can be wildly inaccurate in some circumstances. In mountains the signals can sometimes be deflected by big cliffs - I tracked my ascent of Tower Ridge on Ben Nevis which showed a number of significant deviations to either side, which if they were to be believed would have placed me several hundred feet in the air above the neighbouring corrie.

However, at the scales used for leisure purposes the Ordnance Survey maps are not precise, due to scaling, projection and other differences, and printing inaccuracies. Navigation ultimately relies on relating whatever you can see around you to information your various tools can provide.

The leap second arises because we now want to measure time far more precisely than the rotation of the earth allows. GPS is just part of this.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 23 Dec 16 - 09:37 AM

"Printing inaccuracies" in maps: back in the eighties I was doing a lot of detailed work with maps of all kinds. Quite a few of the inaccuracies are deliberate. For example it is very typical in the real world to have a railway and road alongside a river. If their widths were drawn at the true scale on a map they would be on top of each other and too fine, or otherwise illegible. Very frequently, then, they are strategically moved in the interests of useability rather than accuracy.

You have similar things on remote islands. The maps may never have been updated since say 1920, so the map itself is accurate rwlative to landmarks but the whole island may be several miles away from its true latitude and longitude.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Iains
Date: 23 Dec 16 - 11:30 AM

An explanation.
I will not try to paraphrase otherwise the resident troll/s will attack


https://confluence.qps.nl/display/KBE/UTC+to+GPS+Time+Correction

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Donuel
Date: 23 Dec 16 - 12:08 PM

Iains I wonder if ship Captains have to know this complex stuff for their systems or if they call expert technicians.

All I know is that the great tsunami quake sped up Earth rotation by a red **** hair.
Without Einstein, GPS wouldn't work for obvious reasons.

Mr Red does anyone really know what time it is?

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DaveRo
Date: 23 Dec 16 - 01:07 PM

DMcG wrote: ... the map itself is accurate relative to landmarks but the whole island may be several miles away from its true latitude and longitude

This effect is alive and well in these days of electronic charts:
Behind the lighthouse

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 23 Dec 16 - 01:10 PM

Either that or the captain needs a refresher course on steering!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Iains
Date: 23 Dec 16 - 01:11 PM

Donuel.
I have no idea. I do know that GPS accuracy depends on what device you are using. A total station is far more accurate than a mobile phone and I believe a mobile phone accuracy is far greater when using satellite positioning rather than cell towers. Having been on a mountain in thick fog with a compass and 6" map my suggestion is move very slowly and very carefully.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Mrrzy
Date: 23 Dec 16 - 04:17 PM

WHILE we're on the topic of seconds, how could they (as in, how dare they) redefine the second so it no longer relates to the seconds of arc in a setting sun? The minute is supposed to be the time it takes a solar radius to pass the horizon! A pox on Cesium and the horse it rode in on!
Sorry.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 23 Dec 16 - 07:24 PM

Well this resident troll couldn't read the piece without signing up or logging into something or other, so a paraphrase would be good. Do it carefully if you do it at all, and stop calling people trolls just because they can see right through you.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 23 Dec 16 - 07:38 PM

By the way, Keith, when you tell us that we're wrong about something, you unfailingly resort to "higher authorities" (rather selectively) in order to make your case. On this occasion you appear to be dissing Dr Lippincott, an eminent expert in this field. Well I've checked, and I find that she's not a historian who's both still alive and been active as a historian in the last thirty years. Worse, she has no history book on the shelves of Waterstones. So I suppose that makes you right...

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Mr Red
Date: 24 Dec 16 - 03:41 AM

Mr Red does anyone really know what time it is?

Well I miss the bus in about 10% of trying so the Rouge Towers clock is permanently 5 minutes fast. And even then...........

Science tells us there is no mathematical reason time is unidirectional. Not that it helps my travel arrangements!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 24 Dec 16 - 03:56 AM

Well, I know what the time is, Donuel. But as you said Einstein explained why there is no universal time, so if I told you I am afraid it wouldn't help...

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Mr Red
Date: 31 Dec 16 - 01:34 PM

On December 31, 2016, a "leap second" will be added to the world's clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), the US Naval Observatory said in a statement.

Don't start singing Auld Lang Syne too soon now!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 31 Dec 16 - 02:45 PM

Steve,
If the Earth was alone in space and not rotating at all, GPS would still work just fine.
The leap second is needed because Earth's rotation is slowing making our days slightly longer by the sun.
GPS satellites each have their own atomic clocks and do not care when the sun or a star crosses anyone's meridian.

I am quite sure that Dr L knows all that very well and was just being flippant, which is what panellists do on that programme.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 31 Dec 16 - 03:37 PM

I have done rather more fighting with times than i would wish, but what is happening here is an attempt to use two different labelling systems for time. The first is to pick a 'zero' point and then count seconds from it, like unix time for those who know about that. So you might hav 10,000,000,000 seconds after the chosen origin. The second scheme is to measure time based on cycles round the sun. So the TIME does not need to be altered but the LABELS we use do.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 31 Dec 16 - 07:40 PM

Dammit. I forgot and tried to let in the New Year one second early. Fortunately, Mrs Steve was so taken with Jools Holland that she let me in one second late so all's well!

It's a lump of coal, piece of bread, pinch of salt and a silver coin this end. Back door to be kept open until I come in through the front door. I used to be tall, dark and handsome. I expect to be greeted with a glass of the finest malt. That could be slightly at odds with the purists but, dammit, I try!

Just a second...

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Rob Naylor
Date: 01 Jan 17 - 04:42 AM

GPS time is NOT kept in phase with UTC, so when a second is added to UTC, it is NOT added to GPS time. So there is no glitch at all in GPS positions when a leap second is added to UTC.

GPS time was zero at 00 00.0000000 on 6th January 1980 and is now 18 seconds ahead of UTC.

So as I'm typing this:

UTC time was: 2017-01-01 09:33:23
GPS time was: 2017-01-01 09:33:41

People who use GPS at a professional level, which would include masters of ships (other than perhaps recreational sailors) as well as surveyors, geodesists and many engineers, are well aware of this.

Interestingly (or not?), when I re-triangulated part of northern Brazil in 1985 using Block I GPS, I found a 55m bust in their primary triangulation network around Macapa....which IGBE (Brazilian equivalent of OS in UK) took some convincing was real.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Rob Naylor
Date: 01 Jan 17 - 04:56 AM

Also, although long term, the earth is slowing down, the reason for the addition of leap seconds is the same as the reason for the addition of a day during a leap year....ie the length of a solar day is not precisely 84,600 seconds, just as the year is not precisely 365 days long.

The slowdown/ wobble is the reason that leap seconds are not added predictably (or not very predictably....they're usually agreed 6 months in advance). If you look at a graph of the earth's rotation you'll see that recently it's actually sped up a tad....that just increases the interval required between adding of leap seconds.

So Dr L was not being entirely accurate (or perhaps was simplifying for the sound-bite?).

I do have some background in this, SS, my first degree being in Astrophysics with a postgraduate degree in Geodesy. I was on the first GPS training course held in Europe, in, I think, 1984 and have been working professionally with GPS since 1985.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Rob Naylor
Date: 01 Jan 17 - 05:03 AM

And, maybe even more confusingly, the international scientific time standard is not GMT, but International Atomic Time (IAT, or TIA if you're Francophone). This keeps a running count of the (atomic-clock-defined) SI seconds and is always 19 seconds ahead of GPS time.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: David Carter (UK)
Date: 01 Jan 17 - 07:41 AM

TAI is based upon an ensemble average of atomic clocks in national laboratories. UTC (which is used for civil timekeeping and astronomical calculations, so I would say is the primary standard). As lots of people have already said, UTC is TAI corrected to mean solar time, which itself is not all that useful as there is annual variation of the difference between mean solar time and apparant solar time of up to 16 seconds. Apparant solar time is what a sundial measures.

Rob is right that the reason we need leap seconds is that the length of the day is not exactly 84,600 seconds, but the reason for this is that the rotation rate of the earth is in fact slowing down. Sure that slowing is subject to variations due to geological effects, and as Rob says this is the reason that the insertion of leap seconds is irregular. But in the long term the earth's rotation is slowing down, as tidal friction transfers angular momentum from the rotation into the orbit of the moon. As a consequence, the moon is getting further away. The rate of the slowdown has been determined by F.R Stephenson and L.V. Morrison to be an increase of the length of the solar day of 1.7 milliseconds per century. If we continue to insert leap seconds they will get more frequent in future millenia.

And as Rob says, GPS time is not affected by any of this, it is its own atomic standard, similar to but offset from TAI. Kristin Lippincott, who despite what Steve says is an historian (of art and ancient manuscripts) seems to have strayed a little far from her area of expertise here.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 01 Jan 17 - 08:45 AM

Such knowledge on display here. Makes the average biologist (me) feel quite 'umble. I ask meself what makes these people tick... 😉

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: David Carter (UK)
Date: 01 Jan 17 - 04:10 PM

Wikipedia to some extent Steve. Even if you have done stuff, even taken exams in stuff, several decades ago, it helps to be able to look it up easily.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 01 Jan 17 - 07:25 PM

Er, that was a joke, David. Think seconds, tick... never mind. The moment has passed. And you're not even a yank. I'm fine with wiki, by the way. I gave Jimmy ten quid only last month.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: David Carter (UK)
Date: 02 Jan 17 - 04:06 AM

Whoosh, missed that one.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 02 Jan 17 - 12:06 PM

Steve,

By the way, Keith, when you tell us that we're wrong about something, you unfailingly resort to "higher authorities" (rather selectively) in order to make your case. On this occasion you appear to be dissing Dr Lippincott, an eminent expert in this field.

I did not diss the Dr, but I was right though Steve.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 02 Jan 17 - 12:16 PM

Of course, Keith. Or shall we call you God.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 02 Jan 17 - 12:30 PM

No. Just stop attacking me at every opportunity.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: David Carter (UK)
Date: 02 Jan 17 - 01:13 PM

Steve, maybe you could acknowledge that on this occasion Keith's two posts were entirely right.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 02 Jan 17 - 01:33 PM

I'm not keen on being beaten around the head with his handbag, David.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Nigel Parsons
Date: 03 Jan 17 - 10:11 AM

I didn't see this one at the time:
From: Mrrzy - PM
Date: 23 Dec 16 - 04:17 PM

WHILE we're on the topic of seconds, how could they (as in, how dare they) redefine the second so it no longer relates to the seconds of arc in a setting sun? The minute is supposed to be the time it takes a solar radius to pass the horizon! A pox on Cesium and the horse it rode in on!
Sorry.
Surely this was never the case. The Sun's apparent diameter changes with the Earth's distance from it. So a time unit defined in this way would change with the time of year.

Cheers
Nigel

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Steve Shaw
Date: 03 Jan 17 - 10:31 AM

Apart from that, there's a fair amount of optical trickery attached to a sunset!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: David Carter (UK)
Date: 03 Jan 17 - 10:53 AM

Hmmm... the idea of 24 equal periods in a day appears to be Egyptian, and the division of an hour into 60 minutes and a minute into 60 seconds appears to be because the late Babylonians used sexagesimal arithmetic in their astronomical calculations. From here.

There may be people reading this who know more though.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 03 Jan 17 - 11:07 AM

The distance to the sun does not change enough for its diameter to change visibly, but the angle of setting varies with latitude and with season, and that does alter the time taken to cross the horizon.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 03 Jan 17 - 01:27 PM

IT is a source of some irritation that I know Lewis Carroll wrote a short piece on the need for an international dateline, which clearly must precede the 1884 establishment of the time zones, but can I find when he wrote it? I cannot.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Nigel Parsons
Date: 04 Jan 17 - 03:27 AM

DMcG.
Some info Here (Highlighted) and Here (third paragraph from bottom.

If further searching needed it is better to search with his real name 'Dodgeson' rather than the pen name used for childrens' books.

Cheers
Nigel

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Nigel Parsons
Date: 04 Jan 17 - 03:51 AM

Of course, between Dodgeson's 'thought experiment' (1860) and the establishment of the International Dateline (1884) we have Jules Verne's 'Around The World in Eighty Days' (1873) which deals with a similar anomaly.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 04 Jan 17 - 04:18 AM

Thnks Nigel. In fact, i didn't google at all, i used my bookshelves. I have two 'complete works of' for him. One includes all his work under both names, the other only works published as Lewis Carroll (which i regard as a bit of a cheat on behaalf of the publishers). Bookshelves being what the are, i could only find the wrong copy.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Mr Red
Date: 04 Jan 17 - 05:01 AM

One includes all his work under both names

where to place it? Under C for Charles/Carroll, L for Lewis, D for Dodgeson, R for reverence, M for Maths? or even b for bibliography !

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: DMcG
Date: 04 Jan 17 - 05:17 AM

No idea - maybe that's why I couldn't find it!

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: clueless don
Date: 04 Jan 17 - 09:05 AM

on 31 Dec 16 - 02:45 PM, Keith A of Hertford wrote, in part:

Steve,
If the Earth was alone in space and not rotating at all, GPS would still work just fine.

If the Earth was (were?) not rotating at all, wouldn't it be rather hard for the GPS satellites to maintain geosynchronous orbit?

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 04 Jan 17 - 09:17 AM

No, because they are in polar not geosynchronous orbits.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: David Carter (UK)
Date: 04 Jan 17 - 03:17 PM

I don't think they are Keith. They seem to be on highly inclined (about 55 degrees) but not polar orbits. See this site, and select "GPS Operational" from "Satellites in Orbit". They aren't polar but they are certainly not geosynchronous.

There is a system of satellites for a Motorola satellite phone system called Iridium. These are indeed in polar orbits. They are notable as the shape of the satellite reflective antennae means that at certain points in their orbit they focus sunlight on the earth. So for a very short time, a few seconds, they can be very bright indeed, as bright as magnitude -8, so around 50 times as bright as the planet Venus. You can get a prediction of when and where you can see them from this site, enter your own location and it will tell you the times of the bright Iridium flares, and other interesting things such as passes of the International Space Station. You need a clear sky about an hour after sunset or an hour before sunrise, less than this and the sky is too bright, more and the sun isn't shining on teh satellite as it is in the earth's shadow.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 05 Jan 17 - 04:28 AM

Thanks David. I have been on that site for years and love watching bright Iridium flares. You can really impress people by "predicting" their appearance.

In summer they can appear longer after sunset than in winter.

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 05 Jan 17 - 05:00 AM

Rather short thread on Iridium flares,
http://www.mudcat.org/thread.cfm?threadid=89716&messages=3

Subject: RE: BS: Leap Seconds, and GPS longtitude
From: Keith A of Hertford
Date: 05 Jan 17 - 05:02 AM

thread.cfm?threadid=89716&messages=3