Jump to content
Sign in to follow this  
Followers 0
Guest WrongWay

GPS Time

Recommended Posts

Guest WrongWay

How accurate is the time/date reporting on the receiver ?

 

What I'm trying to say is that the time/date reporting on the GPS receiver is ultra accurate due to the accuracy required of the GP System.

 

The GPS unit gets the time/date data from the constellation, right ?

 

Anyone ?

 

thanks,

 

Jim

Share this post


Link to post
Guest vadim

absolutely right - it is ultra accurate as soon as it starts receive satellite signals

 

but indoors after a while it may accumulate a significant error

Share this post


Link to post
Guest WrongWay

Ok, I can understand that. The reason I asked is because I was scrolling through some waypoints I set on Saturday (10 Feb 01), and the time/date stamp on them was off by about 5 hours (advanced 5 hours). This was in the open, more than 3 sats at all times. What would cause this ?

Share this post


Link to post
Guest Hamster

Your unit may be set to the GMT timezone which is 5 hours ahead of eastern time

Share this post


Link to post
Guest Mike_Teague

yea...

 

the accuracy of the time is superb... internally, I think its around 10 or 20 microseconds or something like that.. What you see on teh actual display though can lag , etc. though. But I still set all my clocks with it...

Share this post


Link to post
Guest auximage

Strange. My friend and I were compairing readings on our two different GPS's, and found that not only the long/lat were different, but also the time was off by about 2-3 mins. Could a battery that is slightly used make a difference on this? I wouldn't think so, as to make such a large gap.. Ideas?? Comments?

Share this post


Link to post
Guest makaio

On the subject of time, I've got my Garmin 12XL set with an offset of -08:00 (PST), but there doesn't appear to be a way to make it aware of daylight savings. I.e., twice a year I have to toggle the offset (from -07:00 to -08:00) to reflect the proper time. Anyone else find this a pain or do your units have a DST option?

Share this post


Link to post
Guest WrongWay

I've checked the settings...I'm located in AZ and have the offset at -7.00 I haven't touched that since init'ing it in 97'. It's currently reading the correct time, and was last night.

 

Help ! What short of 2.21 million gigawatts could have cause this ?

Share this post


Link to post
Guest makaio

But you guys don't do DST down there do ya? The in-laws live in Tempe and I don't believe they do DST, but I could be wrong.

Share this post


Link to post
Guest WrongWay

No DST. Even if there was DST I forgot to adjust, it would still only be one hour, I had a 5 hour delta. My receiver was reporting 5 hours ahead. I guess this doesn't seem to be that big of a deal, other than the waypoint in question should be suspect. How can the Lon be accurate if the time is off ? Longitude is a function of time, right ? Maybe I've been watching too much PBS.

Share this post


Link to post
Guest spigot

I read somewhere (looking for where it was) that GPS time was something like 13 seconds off from absolute atomic time. I can't remember why, though. I think it just has to do with the system when it was sent up and since that time the 'atomic time people' have included leap seconds which haven't been incorporated into the system.

 

Here ya go: http://www.oc.nps.navy.mil/~jclynch/timsys.html

Share this post


Link to post
Guest Mike_Teague

yeah, it's technically off of UTC(GMT), but the handy-dandy satellites give the correction right in the almanac data... So everyyone's unit should show real UTC.

Share this post


Link to post
Guest peter

quote:
Originally posted by WrongWay:

No DST. Even if there was DST I forgot to adjust, it would still only be one hour, I had a 5 hour delta. My receiver was reporting 5 hours ahead.


 

The timestamps for waypoints and the tracklog are always reported in UTC, not local time. That way the time recorded is unambiguous and cannot be modified by any user setting.

Share this post


Link to post
Guest WrongWay

Mike's right, as I'm starting to find out.

 

"GPS Time has no leap seconds and is ahead of UTC by several seconds. "

 

Here's the site that came from:

http://www.colorado.Edu/geography/gcraft/notes/gps/gps_f.html

 

On that site it talks about GPS Error Sources: Noise, Bias, and Blunders.

 

I'm starting to think the 5 hour delta falls in the last catagory. But I have no explaination as to why. All parameters on the receiver seem to be set correctly. If people are unable to locate the caches we'll know there was a blunder of some magnitude. Now if I can just remember where I put them...

Share this post


Link to post
Guest WrongWay

Peter's right. I screwed up on more than one thing. First, the delta was probably 7 hours not 5, and my offset is -7. Second, I should have RTFM...in all honesty I did, but it was years ago when I bought this unit, and of course, I knew it all. Ha !

Share this post


Link to post

quote:
Originally posted by spigot:

I read somewhere (looking for where it was) that GPS time was something like 13 seconds off from absolute atomic time. I can't remember why, though. I think it just has to do with the system when it was sent up and since that time the 'atomic time people' have included leap seconds which haven't been incorporated into the system.

 

Here ya go: http://www.oc.nps.navy.mil/~jclynch/timsys.html

 

So that explains why the Vista shows different times if you use it normally (Normal mode or Battery save mode) compared to when it's used in GPS Demo mode. If I change from one setting to the other, the time displayed in the system page jumps by 13 seconds.

 

Anders

Share this post


Link to post

I did some non-scientific tests. The NMEA time that comes out of my GPS is roughly equal to the time on my PC when synced with an NTP clock over the internet, which is roughly equal to the time on my cell phone, which is roughly equal to the time on my cable box!

 

However, the time DISPLAYED on my gps is about 10 seconds faster!

 

So to recap, if T=current time:

 

GPS NMEA = PC NTP = CDMA Cellphone = Cable Box = T

 

GPS display time = T+10s

 

Rob

Mobile Cache Command

Share this post


Link to post

quote:
Originally posted by mrcpu:

I did some non-scientific tests.


 

Nice, but I bet the time on your VCR is flashing 12:00 icon_wink.gif

 

Me Fail English? That's Unpossible!

Share this post


Link to post

quote:
Originally posted by mrcpu:

I did some non-scientific tests.


 

Nice, but I bet the time on your VCR is flashing 12:00 icon_wink.gif

 

Me Fail English? That's Unpossible!

Share this post


Link to post

From the National Institute of Standards and Technology:

 

"NIST Radio Station WWVB and the telephone voice announcements are two examples of one-way time transfer systems. The Global Positioning System (GPS) also provides time information via the one-way technique and is the most accurate worldwide one-way time transfer system.

 

GPS satellites broadcast a timing signal (a tick of the clock) on a phase modulated L-band carrier along with information identifying the time for which the tick corresponds. The satellites broadcast a time code referenced to the clock on the satellite, but there is also information enabling the user to obtain an estimate of GPS system time as well as UTC(USNO). The user's receiver may then compare the arrival time of the GPS signal to the local clock with a Time Interval Counter (TIC). The major challenge here is to account for the propagation delay. The geometrical delay is obtained from the receiver’s location (which can be obtained from the GPS system) and from the broadcast satellite positions. Major sources of timing error are the geometrical delay, the effect of the ionosphere and troposphere on the propagation time, multipath, and hardware delays. Currently, the GPS system provides time to the general public with uncertainties measured in nanoseconds. With a well-designed receiver system the user can obtain the time to better than 100 ns in a few minutes, and to about +/- 10 ns with a 24 hour average (and a good local clock)."

 

Yes, there is a difference in GPS time and UTC because of leap seconds introduced since GPS has been it operation, but your GPS adjusts for this.

 

4497_300.jpg

Share this post


Link to post

The Global Positioning System (GPS) epoch is January 6, 1980 and is synchronized to UTC. GPS is NOT adjusted for leap seconds.

 

As of 1 January 1999,

TAI is ahead of UTC by 32 seconds.

TAI is ahead of GPS by 19 seconds.

GPS is ahead of UTC by 13 seconds.

 

TIA is International Atomic Time and began Jan 1 1958. It does not include Leap seconds and is used by systems that cannot handle leap seconds.

 

UTC is Universal Coordinated Time and IS the standard around the world! It does use leap seconds to adjust time when UTC gets out of sync with UT1 (time based on astronomical position).

 

Atomic time is the official time now, being more uniform than planetary movements, but it does need to match up with the the dips and bobs of the Earths movements every now and than.

Share this post


Link to post

quote:
Originally posted by st_richardson:

Atomic clock: 9:43:05

GPSr: 9:43:05


 

Looks like you're about 10 nanseconds slow there icon_wink.gif

Share this post


Link to post

quote:
Originally posted by st_richardson:

Atomic clock: 9:43:05

GPSr: 9:43:05


 

Looks like you're about 10 nanseconds slow there icon_wink.gif

Share this post


Link to post

GPS V has automatic DST setting. It will adjust for you by itself or leave you alone if you live in Arizona.

 

nscaler

"Anyone not here, raise your hand!".

Share this post


Link to post

I have noticed with some of the older gps's you have to make sure the time is entered by hand correctly. Once I entered the pm instead am (over a year ago, yeah that's the ticket) and the gps didn't know where it was. This last weekend I was comparing a Globalmap 100 with an etrex. The etrex sets it's own time. When I checked the coords for a cache I was placing and compared them they were way off. I checked the times and the Globalmap time was off by 1 hour. Once it was reset they agreed on the coordinates. Am I wrong? or did I just stumble onto something?

 

[This message was edited by Jeo and Mc5 on August 25, 2002 at 08:36 PM.]

Share this post


Link to post

That would be strange if a GPS was set up like that. Your time display uses the sat signals to set itself accurately, but the display should play no part in figuring out the coordinates. When you adjust the display to, say, another time zone, or DST, all it is doing is adjusting the display only. The internal timing (or really the time broadcast by the satellites) is still using UTC (or GPS) time to triangulate satellites. Adjusting to another time zone just displays time as displayed in another zone, and should have no effect on your coordinates.

 

I suppose that some units may rely on displayed Time zones to try and aquire the needed almanac data from the satellites. In other words, to try and firgure out what satellites might be overhead so they can aquire data and figure out where you are.

Share this post


Link to post

but it worked!!!

and it makes sense if you think about it. The sats are not in geosynchronis (I hope I spelled that right) orbits. They are constantly changing their relationships to the earth and one another. The gps only measures the doppler effect of the signals and the newer ones know when and where they are supposed to be according to a time schedule which is inputted into a computer program. If the program is old ie; a sat fails or the timing is wrong and it doesn't know the time wouldn't that throw everything off?

 

[This message was edited by Jeo and Mc5 on August 25, 2002 at 09:10 PM.]

 

[This message was edited by Jeo and Mc5 on August 25, 2002 at 09:12 PM.]

Share this post


Link to post

Timing is everything. The Gps does more than measure the doppler effect, and the satellites are sending alot more info than just the tick of a clock. Every 12 mins the satellites send a whole slue of new data, including where the other satellites should be. This program that you are speaking of, is not something installed in the GPS from the factory. It comes from the satellites every 12 mins. If you move 500 miles, or have not used the GPS for some time, your GPS will go through a cold start process of trying to look for satellites to get some clue as to where it is and aquire some data (and a new almanac). Often in this situation, your GPS may ask you a question like ''What country are we in?'', or you can let it figure it out for itself. What I think your old gps was doing was trying to use the time setting that you had in it to get an initial idea of where in the heck to start from. BUT once locked on to the satellites it can figure the rest out for itself.

 

I don't claim to know it all...just willing to learn it all.

Share this post


Link to post

Here... do a little test. Take your GPS's outside (your old one too if you still have it) let it aquire a fix. Mark your waypoint. Then go in to your settings and change your time zone or whatever it is willing to let you change in the time setup, and go back to the main screen and see if your waypoint is not still right next to you, right where you left it. If it isn't I'll have crow for dinner.

Share this post


Link to post

If this is not just a program for the gps, how come there are updates that you can download to to your gps to make it work better? I have had several gpss' and they all work a little different. Every company that makes gpss' had a team of programmers that they thought knew it all and yet they all work different. So which one is right? All I'm saying is double check the data, and There is no one answer that fits all solutions.

Share this post


Link to post

I just did my test and I was still in the same place. I think you'll find that you can only change the hours and nothing else.

 

I just looked at the manual for the Globalmap 100 and you have a feature called PCF (position correction factor) whereby you can artificially (where is that spell check?) alter your position to match, say, a chart. It is like 'User defined datums' in other units. Anyway, if you had this thing going it certainly could have thrown your position off.

 

Also, if your unit was off an hour and it 'could' affect your position, then you would be WAAYY!! off! Because one nanosecond of error = one foot in postion. One nanosecond is one billionth of a second.

 

Of course there are program upgrades, but what we are talking about here comes from the satellites data itself. It is called the satellite almanac.

 

4497_300.jpg

Share this post


Link to post

Well I'll be a monkey's uncle!?

I guess that by the time I got around to setting the time it finally locked on or something like that. The test I did was with a globalmap 100 with the updated drivers. v 1.8.

Share this post


Link to post

My old(er) Magellan MAP 410 required you to set the clock after loading the batteries the first time, but once it had a signal it corrected it.

When I'm not using it as a backup GPS, it's hooked up to a Linux PC on my LAN that keeps all my computers' clocks synched up using the NTP daemon.

You can also pull a time signal from the internet, but having the GPS there seems so much cooler. icon_smile.gif

Share this post


Link to post

My old(er) Magellan MAP 410 required you to set the clock after loading the batteries the first time, but once it had a signal it corrected it.

When I'm not using it as a backup GPS, it's hooked up to a Linux PC on my LAN that keeps all my computers' clocks synched up using the NTP daemon.

You can also pull a time signal from the internet, but having the GPS there seems so much cooler. icon_smile.gif

Share this post


Link to post

Sounds pretty techy... but if you are actually pulling in a satellites signal with your 410 while doing this, then it is my understanding that you are also much more accurate than any other method. The atomic clock on the internet usually says it is accurate to about 0.9 seconds or so. GPS is maybe 100ns. But, hey, we won't be able to see the difference anyhow unless you can get behind the display.

Share this post


Link to post

quote:
Originally posted by Jeo and Mc5:

If this is not just a program for the gps, how come there are updates that you can download to to your gps to make it work better?


 

For the same reason any other software is updated: add features, fix bugs, and generally make it work better. The data being sent out by the GPS satellites is only data, some program need to interpret it and use it to calculate position. This is the software that GPS recievers run and what is sometimes updated. Though generally updates are more to fix errors with other parts of the coding. Look back through an update history to see the types of things that are changed.

 

I'm not lost!

I just don't know where I am.

Share this post


Link to post

quote:
Originally posted by EraSeek:

The Global Positioning System (GPS) epoch is January 6, 1980 and is synchronized to UTC. GPS is NOT adjusted for leap seconds.

 

As of 1 January 1999,

TAI is ahead of UTC by 32 seconds.

TAI is ahead of GPS by 19 seconds.

GPS is ahead of UTC by 13 seconds.

 

TIA is International Atomic Time and began Jan 1 1958. It does not include Leap seconds and is used by systems that cannot handle leap seconds.

 

UTC is Universal Coordinated Time and IS the standard around the world! It does use leap seconds to adjust time when UTC gets out of sync with UT1 (time based on astronomical position).

 

Atomic time is the official time now, being more uniform than planetary movements, but it does need to match up with the the dips and bobs of the Earths movements every now and than.


 

I found a web site http://tycho.usno.navy.mil/gpstt.html

describing this but then implying that every 12.5 minutes the leap second correction is transmitted to the GPS. Also, the Casio GPS-Watch manual also implies this correction.

 

So, is GPS time and UTC time the same (it seems to be from the later posts)???

Share this post


Link to post

quote:
Originally posted by crr003:

So, is GPS time and UTC time the same (it seems to be from the later posts)???


 

No, GPS time and UTC time are not the same (now) but differ by the appropriate number of leap seconds as adopted from time to time. The leap second correction can increase or decrease but to this point in time it has always been on the increase (world is slowing down).

 

Basically GPS time was set equal to UTC time when the GPS system was born (5/6 Jan, 1980) but effectively the earth is slowing down which has changed/affected UTC time but GPS time has remained fixed. The current difference is 13 seconds and this leap second value is transmitted as part of the nav message so that GPS time can be corrected to current UTC time from which we can apply a zone correction to get our local time.

 

That 12.5 minutes is roughly the time it takes to transmit a full almanac in which things like the leap second correction and all that sort of stuff is contained.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

quote:
Originally posted by crr003:

So, is GPS time and UTC time the same (it seems to be from the later posts)???


 

No, GPS time and UTC time are not the same (now) but differ by the appropriate number of leap seconds as adopted from time to time. The leap second correction can increase or decrease but to this point in time it has always been on the increase (world is slowing down).

 

Basically GPS time was set equal to UTC time when the GPS system was born (5/6 Jan, 1980) but effectively the earth is slowing down which has changed/affected UTC time but GPS time has remained fixed. The current difference is 13 seconds and this leap second value is transmitted as part of the nav message so that GPS time can be corrected to current UTC time from which we can apply a zone correction to get our local time.

 

That 12.5 minutes is roughly the time it takes to transmit a full almanac in which things like the leap second correction and all that sort of stuff is contained.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

thanks - I guess it would be more correct to say then that absolute GPS time does not equal UTC time, but the displayed GPS time equals UTC time (assuming current data including leap second info are being received)?

 

quote by EraSeek

 

The Global Positioning System (GPS) epoch is January 6, 1980 and is synchronized to UTC. GPS is NOT adjusted for leap seconds.

 

As of 1 January 1999,

TAI is ahead of UTC by 32 seconds.

TAI is ahead of GPS by 19 seconds.

GPS is ahead of UTC by 13 seconds.

 

quote by st_richardson

 

My current local time via...

 

Atomic clock: 9:43:05

GPSr: 9:43:05

Share this post


Link to post

Yes, sorry if I wasn't clear on that. I tried to state it on an above post:

''Yes, there is a difference in GPS time and UTC because of leap seconds introduced since GPS has been it operation, but your GPS adjusts for this.''

 

GPS Time is the system time. The time displayed, and I believe used, BY your GPSR is UTC (universal coordinated time) which does in fact include the leap seconds.

Share this post


Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  
Followers 0

×
×
  • Create New...