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All twelve GPS satellites, all the time.
kft
It must be the winter weather, or perhaps the area we went to (in the winter?) but on our trip to Lancaster PA this past weekend we had all twelve GPS satellites everytime I looked with only number twelve being different on the signal strenth but only by the smallest amount.

The season, the location, the time of day I looked, the rooftop antenna? or all of the preceeding??, whatever the reason(s) I thought it was neat.

<Edited by Marvin Hlavac; Reason: picture added>
Marvin Hlavac

kft, thanks for posting your observation. I just quickly checked here on the parking lot, and I too see 12 GPS satellites, even though I'm under a tree, and my SiRFstarIII GPS receiver (BU-353) is not even on the roof (it just sits here on the dashboard).

I wonder if GPS satellites transmit these days more power than they used to just few years ago. Even though I no longer pay much attention to how many satellite connections I have, each time I look the number is 10, 11, or 12. I don't think it was this good a couple of years ago, even though I've been using SiRFstarIII GPS receivers ever since they appeared in 2005.
insanitor
Might I ask what program are you guys using to look at these sattelites?
Marvin Hlavac
insanitor, the first picture I took the liberty of adding to kft's posting is a screenshot of DeLorme Street Atlas, one of kft's favorite GPS software. The second one is from Garmin Mobile PC.
kft
Marvin, thanks for the graphics they do add alot when trying to make a point. Up until this past weekend we had not seen all twelve satellites and I do check gps status frequently (I also set all our clocks and watches to the NIST atomic clock every Sunday. My wifes 32 year old Seiko loses about 6 seconds per WEEK, my old Accutron tuning fork watch about 2 1/2 minutes per week, and the pricy Rolex Oyster about a minute per DAY, heck our Seth Thomas wall clock keeps to withing 2 minutes per week!) forgot where I was going with this, oh, like i said I am a bit anal which is why I check the satellite status alot. I do remember from my shortwave radio days that with the cold weather came much improved reception so whether it is this or some change in the power output of the satellites I can only ponder.

If anyone else HAS to know the correct time and does not know the way to the atomic clock here it is:

http://www.time.gov/

its an offical US Govt site so if you have to activate or download something I can't imagine there being a problem.
insanitor
Interesting.

I used to be a watchmaker for Seiko. I could probably fix your watch or at least tell you what is wrong with it.

P.S.

I also live two blocks away from the best Accutron man alive. My teacher, Mr Ruppert, who was hired by the Bulova School of Watchmaking as a teacher for the repair of the Accutron.

I was one of his students but I did not learn how to fix the Accutron. It's very hard to find people who can fix that thing.
kft
Time fancinates me. Seiko I have to admire for accuracy, Rolex for its craftmanship and the Accutron, is just, well---cool love listening to the tuning fork vibrate..

The time loss with the Rolex is consistent with the other mechanical watches I have, all Seiko. I would have liked to have done your trade; tnks..
Ken in Regina
Naw, you don't even need to do that. Just fire up your GPS. Once your GPS has a solid fix, you'll get the same accuracy as NIST (Stratum 1). Both use cesium atom decay for timing and each GPS sat has three onboard.

If the navigation software you are using on your laptop doesn't show you the satellite information (satellite signal strengths, time, accuracy, etc.) you can get utility programs that will report that information from your GPS receiver.

If you like using NIST, download NISTime to your PC. You can probably also get it at your favorite trusted download site.

You can run it manually any time you want to update the time on your PC .. only takes a second or two and saves waiting for the browser to load, checking the NIST site and manually resetting the time. Or you can just leave it running all the time and let it update the PC's time every hour or so. I have it on both my desktops (XP) and my laptop (Vista 32-bit). You can select other sites if you ever have problems with it accessing the NIST site. The linked page above explains it all and has versions for various computer operating systems.

Speaking of time, it's way past my bedtime.

...ken...
kft
Ken, would the pathways both have the same inaccuracy (delay)?
Clock-satellite-satellite-gps antenna/receiver


clock-internet


Or are you saying that each satellite has three atomic clocks ?
kft
Ken you are correct.

http://www8.garmin.com/aboutGPS/
Marvin Hlavac
* Re: Time -- Windows XP and Windows Vista can synchronise computer time once each 24 hours via Internet connection. That's what I use. Prior XP I used a little utility software designed for this purpose (I don't recall it's name now).


* Re: All twelve GPS satellites, all the time... -- I asked about this yesterday in sci.geo.satellite.nav group, and I was reminded the fact that there are now more GPS satellites orbiting the earth. A few years back there were just 27 of them (24 in operation and three extras in case one fails):


Image from wikepedia.com


Now, according to Wikipedia, there are 31 birds in the sky:

Quote:
"... As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements..."
kft
Marvin, when I select the gps status on either DeLorme (8) or CoPilot (10) the screen is limited to a maximum of 12 satellites, do newer versions allow for the receiving of more than 12?

I have found more info on my question. DeLorme's LT20 is listed as a 12 channel receiver and the LT40 a 16 channel receiver. So the MR350 we are using has to be a 12 channel antenna/receiver and I would guess that the driver for the product is what contains the status window?

Now the queston for me is, is there a maximum number of satellites used in navigation and is that 12 or under??
I am hoping that the answer is yes and that the reason for the total satellite inventory in orbit is an attempt to assure area reception or backups OR perhaps the total includes military use only..
Ken in Regina
Quote:
Originally Posted by kft
Now the queston for me is, is there a maximum number of satellites used in navigation and is that 12 or under??
I am hoping that the answer is yes and that the reason for the total satellite inventory in orbit is an attempt to assure area reception or backups OR perhaps the total includes military use only..
As usual, the answer is, "It depends...". As Marvin mentioned, one limitation is in the number of receiving channels in your GPS receiver. Another is in the navigation software you are using. Regardless of the number of channels in your receiver, the nav software may only use a maximum of 12. I don't know of any nav software that uses more than 12. (That doesn't mean there is none out there .. just that I don't know of any.)

The configuration of the satellites your GPS can "see" is far more important than how many it sees, anyway. That is, if the satellites it sees are all lined up in a row, you won't get a very good location fix no matter how many satellites it sees. The more they are scattered around the sky overhead, the better triangulation you get, and therefore the more accurate the location fix.

You only need three satellites to get good enough triangulation for normal navigation (automobile or walking) if those satellites are in a good configuration. 12 channels just gives you a better chance of getting a few satellites that are in a good configuration.

To answer your concern, Yes, the total number of satellites zipping around up there are for coverage and redundancy (backup).

...ken...
Ken in Regina
Quote:
Originally Posted by Marvin Hlavac
* Re: Time -- Windows XP and Windows Vista can synchronise computer time once each 24 hours via Internet connection.
Hmmm... My XP Home/SP2 only checks once a week and there's no obvious way to change that. I can force it to check manually any time I want, so I guess there's no need for an extra utility. I've just been using NISTime for so long that it's become habit. Plus I have more choices about which sites I can synchronize from.

One of my desktops keeps pretty good time. Weekly updates are just fine for it. The other is terrible. It can drift as much as three or four seconds a day. I haven't paid enough attention to my laptop to have a solid feel ... but now that I think about it, the fact that I haven't seen any big discrepancies when I have synchronized it means it must be pretty good.

...ken...
Ken in Regina
Quote:
Originally Posted by kft
Ken, would the pathways both have the same inaccuracy (delay)?
Clock-satellite-satellite-gps antenna/receiver


clock-internet


Or are you saying that each satellite has three atomic clocks ?
Hi kft,

I'm not sure if you found your answers or not.

Yes, each GPS sat has three cesium "clocks" onboard, for redundancy. If all are working and there are any discrepancies they use a "majority rules" method to decide the right "time".

As for latency - the delays in the transmission paths - for practical purposes it's probably about a tie between satellite-to-GPS-receiver versus NIST's clock-to-server-to-PC.

The actual latency would come down in favour of the satellite downlink. Any time you have a long internet path (multiple routers between the NIST server and your PC) it will introduce nearly as much delay at each routing/switching point as in the satellite downlink. But that's time measured in microseconds or milliseconds. Very few of us do anything that requires that sort of accuracy.

Most of our devices with digital clock displays on them can only be set to the nearest minute. Only a few can be set to the nearest second. The clocks and watches we use, including the ones in our computers are only accurate to a second, or even worse, over the course of a day.

So, it's really interesting to talk about and understand this stuff (it was part of my job for one small chapter). But for practical purposes, anything that keeps our various clocks within a couple of seconds of the "real" time is more than good enough most of the "time".

...ken...
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