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How many locked satellites?

 
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spinowner
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Joined: 25 Nov 2004

Posts: 585

PostPosted: Sun May 01, 2005 6:15 pm    Post subject: How many locked satellites? Reply with quote

Turned on my receiver on my way home from work today just for the heck of it, and at one point I noticed it was locked in on 10 satellites. Has anyone ever had more?
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Marsha and Silent Bob
Past MnGCA President


Joined: 02 Sep 2003

Posts: 6261

PostPosted: Sun May 01, 2005 6:22 pm    Post subject: Re: How many locked satellites? Reply with quote

spinowner wrote:
Turned on my receiver on my way home from work today just for the heck of it, and at one point I noticed it was locked in on 10 satellites. Has anyone ever had more?

I never counted but all the bars have been on the screen (I assume that means the max of 12) usually when I'm on an Interstate.
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Pear Head
Past MnGCA President


Joined: 04 Apr 2004

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PostPosted: Sun May 01, 2005 6:37 pm    Post subject: Reply with quote

I'm sure someone knows more than I about this, but I know that your GPS doesn't actually USE all of the sats at once. It only uses the 4 best ones I think (best being best geometery, not best signal).

I know there is a LOT of information out on the web that goes into detail about how they work. We did a presentation last year and touched a little bit on how GPS works - it's interesting but can get quite complicated.
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jaywc7
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Joined: 13 Sep 2004

Posts: 360

PostPosted: Sun May 01, 2005 8:33 pm    Post subject: Reply with quote

Used to sell handhelds,,, and I work with high grade aviation GPS on a daily basis. Even so - I'm sure there's much more then I know...

There are 24 satelites - 12 in each hemisphere at any given time. There are always 5 satellites in horizon position for a GPS reciever nomatter where you are on Earth. When one passes from north to south hemisphere, another passes from south to north. The satelites transmit a signal that basically just says "Here's where I am, and here's the time I was here". The GPS doesn't transmit, and the satellites don't recieve. Ususally in handhelds and such, you need 3 satellites to get a 2 dimensional lock. You need 4 to get a 3 dimensional lock. I suggest never marking a cache with 3 satellites. It is likely way off. I'm pretty sure that a GPS uses any available signals it can get, but not sure on that. Pearhead may be right that it only takes what it needs. I do know that aviation GPS usually require 5 satellites, and use whatever they can get. Aviation GPS's have what is called RAIM. This stands for Reciever Autonomous Integrity Monitoring. Basically, the GPS monitors the satellites it has, and requires a good signal to fly certian approaches. When your GPS shows your accuracy, this is basically RAIM in your handheld more or less. GPS provides two levels of service: Standard Positioning Service and Precise Positioning Service to all users, horizontal positioning accuracy of 100 meters withi a probility of 95% and 300 meters, or less with probability of 99%. The satellites also transmit what is called "ephemeris, which is the exact orbital location. Then of course, you've got WAAS, which is Wide Area Augmentation System. This basically consists of two gigantic towers on each side of the country. WAAS is available in the US only. The WAAS towers measure the satellite signal, and are extremely accurate. The reason being is that they are already in a predetermined position, which does not change. What happens is that the satellite signal gets distorted as it comes through the atmosphere. The WAAS towers calculate that degree of error and send it back to the satellites. The satellites then transmit that to your GPS if it is available. WAAS was developed primarily for aircraft using GPS for precision instrument approaches into airports.

There is a whole load of other information about GPS as well, which I could go on and on for hours about. If anyone is interested in more details, just let me know.

As far as satellites go, I think the most I've had is 10 or 11. Somewhere in there. GPS is an amazing tool for sure.

Does anyone remember what a compass looks like??? Laughing
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Pear Head
Past MnGCA President


Joined: 04 Apr 2004

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PostPosted: Sun May 01, 2005 9:03 pm    Post subject: Reply with quote

Here's an article that starts off kind of slow but gets more and more interesting:

http://electronics.howstuffworks.com/gps.htm

Also, your description of WAAS is a little misleading..

Garmin wrote:
WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.


Also, I don't believe that as each satellite passes into a hemisphere another one comes out of that hemisphere.. There ARE 24 sats, with 3 in reserve in case on fails. I may be wrong on this, but it seems that if there are 3 in reserve, they can never know which one is going to fail next, so how can you get it into the correct spot at the correct time. Correct me if I'm wrong....
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jaywc7
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Joined: 13 Sep 2004

Posts: 360

PostPosted: Mon May 02, 2005 12:24 am    Post subject: Reply with quote

ok - well hey - know that I might not have the best explanation, but I do my best... If you can find other info, that's great. I know what I know, but am always open to new perspectives, or better info... I looked through the federal aviation regulations and aeronatical information manual for most of that info, but sorta described WAAS as it was described to me, so I may have been wrong. I can look it back over though...

I prefer to either know things well, or not speak at all, so I'll review it. Sorry for any wrong information I supplied...

The bottom line for me is : I do my best to understand the way things work, but regardless of my understanding, I can fly the approaches never the less...
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Marsha and Silent Bob
Past MnGCA President


Joined: 02 Sep 2003

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PostPosted: Mon May 02, 2005 6:41 pm    Post subject: Reply with quote

Off-topic discussion about "Things that make you go Hmmm...." was split off and moved Arrow here.
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miles58
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Joined: 07 Mar 2005

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PostPosted: Tue May 17, 2005 9:03 pm    Post subject: Reply with quote

Odd thing.

Sunday, because we were supposed to be having a bad reception day I turned the GPS on in the car and kept an eye on it. Connected to an external powered antenna I consistently had 10 sats + WAAS and twenty foot EPE readings.
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KC0GRN
Past MnGCA Board


Joined: 22 Feb 2004

Posts: 1424

PostPosted: Wed May 18, 2005 12:55 am    Post subject: Reply with quote

ahhh yeah the solar winds. I was expecting more problems this last weekend than I had with reception. Probably they just state worst case stuff when talking about it. Usually I blame solar flares for computer problems, only because it's a convenient thing to blame stuff on Laughing
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ArmyCrayzee
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Joined: 19 Aug 2005

Posts: 16

PostPosted: Fri Aug 19, 2005 3:41 pm    Post subject: Reply with quote

jaywc7 wrote:
Does anyone remember what a compass looks like??? Laughing


US Army will not allow a soldier to use a GPS for land navigation courses. Some schools go as far as passing soldiers down before letting them get on the course. There is no difference in the reading betwen a GPS and a compass although a GPS can't get screwed up by things like your quick-draw knife, weapons magazines, cell phone, or two-way radio. Laughing
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Pear Head
Past MnGCA President


Joined: 04 Apr 2004

Posts: 5694

PostPosted: Sun Aug 21, 2005 3:10 pm    Post subject: Reply with quote

ArmyCrayzee wrote:
jaywc7 wrote:
Does anyone remember what a compass looks like??? Laughing


US Army will not allow a soldier to use a GPS for land navigation courses. Some schools go as far as passing soldiers down before letting them get on the course. There is no difference in the reading betwen a GPS and a compass although a GPS can't get screwed up by things like your quick-draw knife, weapons magazines, cell phone, or two-way radio. Laughing


No, but it can get screwed up by things like water, a drop, or dead batteries.

I will mark my automobile when heading into some caches so I know where it is, but at the same time I look at my compass to see which way I am heading so that I can use the compass to find my way out if my GPS fails.
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