GPS

[DHD]

OK !

We need someone that ready knows about the GPS

I have a 5 channel and most of the time it locks on to at least 4-5. The question is, If I lock on to the Sat's with 4 Will I be JUST AS ACCUATE as a person that uses 10 channels ?

Thanks

[Kevin A.]

I looked into this question a while back when I bought a Lowrance Global MapSport for around half price. I wondered "why half price" ($600+ to $329, Reed's had them)and as it turned out this was when Lowrance changed the unit from 5 channel to a 12 channel receiver (Jan-Feb 1998). The unit is now called the GlobalMap 12. If I remember right, GPS needs a minumum of 4 sats to tell you where you are. Three to find your lat/lon and one to tell you altitude. The 10 or 12 channel receivers are an "improvement" in that the unit knows where the other sats are, even though it doesn't really need them at any one time. It tracks the additional sats and switches to them if the lock is lost on one of the sats that it was using to determine your location. How does it lose it's lock?...you walk under a tree. If it can switch to another sat when the tree blocks one of it's locks then the unit can still tell you where you are and service goes on uninterupted. If it didn't have the other sat to switch to you would lose your position & the unit would go into tilt mode until you walked out into the open and the unit re-locked. Why 10 or 12 sats...probably first of all inexpensive enough receivers can be built to track that many and second, 10-12 should be plenty to insure you don't lose lock even in the woods (more or less). When I got mine I jumped at the 5 channel unit as I use it only on the water...there are darn few trees overhead out in Lake Michigan, though sometimes they would be welcome.

I'm pretty sure the extra 1 (for you) or 6 or 8 sats for someone else (beyond the basic 4) does nothing to improve accuracy. Maybe something has changed, but I think that is the case.

[Matt]

I have a 5 channel GPS (Eagle AccuMap)and when the 12's came out I did a lot of reading. The thing I came away with is a 5 channel is just a accurate as a 12. The twelve just has more options for keeping track of your position. I paid particular attention to the drawbacks of 5 channels. I personally did not/have not noticed any of the drawbacks that everyone was talking about. I think because my unit is a high-end unit. I have a friend with a 12 and he briefly looses position as often as I do. So, I think if you have a quality unit you will be just fine, 5 or 12.

[DHD]

THANKS

I really like my unit,and I also use it on the water. No trees !!so has long as there just as accuate ,I will keep mind. Beside the software I have to use I also like. Thanks again

[DHD]

THANKS

I really like my unit,and I also use it on the water. No trees !!so has long as there just as accuate ,I will keep mind. Beside the software I have to use I also like. Thanks again

[Grayson]

Most unit are capable of better accuracy than you will be able to achieve because of the select availablitly built in by Uncle Sam. This is a error that is always in use by the gov. They de-scramble this with their units. You should be able to get somewhere around 50 to 150 ft with most any unit on the market today. Thing is some are not very user friendly. Remember that the pixel takes up a lot of space, so the higher pixel count screen will be more accurate. If you learn to read the numbers this will help also. North and west the numbers get higher. I have tested a lot of units and most will get to about 50ft most days of your target, using the numbers. 150-300 ft using icons, because they use up to much space on the screen. You never know just where you are. Try this in your yard, sit up a course and use the numbers and you will see what I mean . Good Luck

[Stinky]

Boy you guys really don't have a clue. Check out www.garmin.com for the real info. The satellites are above the earth in a geosynchronous orbit. Depending on where you are you may get more or less of a reading from one satellite than another. It is typical to only get 5-6 good sat readings but they all have some weight on the descision of the readings. It is all time based , how long it takes to read a sat sig. from one to the other is what actually pin points your location. The GPS receivers only recevie they don't communicate back up to the sat's. BTW: They just lowered the SA variance from a coulpe hundred yards to 60 yards max. I guess Europeans were going to send up their own system so Uncle sam changed the SA to ease the pain. They can change it back at any time. I think the US gov. is worried that it if someone else has their own system it will be out of their control and they don't want that to happen. Enough said check out Garmin's web site for the real info.

[bob oh]

Stinky, "they" didn't lower SA they turned it off.
Bob

[Kevin A]

Stink...rather a bold statement with regard to being clueless. Read the original posts & you may notice that all the pearls of wisdom you've offered weren't really on the table.

Your right...GPS is based on time...just like Loran that came before it...which, until 9 days ago, still had greater absolute accuracy than the old SA-GPS. It appears the non-SA GPS now has a slight edge on accuracy.

As for someone else putting up a system...I believe EU has had an equivalent system to Loran for many years and still can & do use it. The Russians have had the GLONASS sat system for many years. Here's a comparison of the two systems: #####://www.oso.chalmers.se/~geo/gg_comp.html

Here's a little more light reading for you: #####://www.navcen.uscg.mil/systems/Default.htm

Thanks for cluing us in with a new heading.

[Hans]

The satellites are NOT geosynchronous.


Hans

--
"There is nothing; absolutely nothing; half so much worth doing,
as simply messing about in boats." :-)

[ufda]

Is differential GPS still more accurate now that SA has been turned off?
Thanks,
ufda

[Capt_Paul]

Yes it is.

[Kevin A.]

DGPS accuracy is a function of distance from the correction broadcast beacon. Actual tests from a few guys off the east coast have been around 1-3 meters (repeated trolling around a fixed marker from various directions). Std GPS (non-SA) tests have been around 10-20 meters, some as close as 7-8. This will vary a little day by day, but we're miles from where we were two weeks ago.

[Smokey]

I'll try to be nicer about this than some others. Most 12 channel units are a bit more accurate than most 1, 3, 5 or 8-channel units but the reason doesn't have much to do with the number of channels the unit monitors - they are simply newer and improved units that were produced later. In order to get a fix on the surface you need three satalites, if you want elevation too you need a fourth. The main advantage of 12 channel receivers is they can hold satalites that aren't being used in reserve, so to speak, in case something blocks the signal path for any of the ones being used. This is a particularly handy feature if you happen to be using a unit in the woods or in a city where trees or buildings might obstuct some of the satalites. As to SA, it was turned off at midnight eastern time on 5/1. Accuracy now is typically around 20 or so meters for typical units. This is enough for a lot of folks to feel they don't need differential receivers. But you should keep in mind that you will still get a much improved positon indication if you do choose to use differential - and positions locations using differentialy can now be had down to 1 or 2 meters but you can figure on a consistant 10 meters.

As to the basi question asked the simple fact is that GPS accuracy changes every second but once the minimum number of satalites is in view the accuracy you achieve is not a matter of how many additional channels you are able to monitor. Its a function of satalite geometery at the moment (not being geosyncronis it changes all the time), distortion in the atmosphere, clock error in the satlaites being monitored, empherious error in the predicted satalite orbits, and finally the simple quality of the machine you have either in your hand or on the dash. One of the nice things about the removal of SA is that now we will be able to see, for the first time, which machines actually have the quality that gives accuracy and which don't. SA used to mask that so we had no way of knowing. I hope this helps you out - Oh, almost forgot something - later model units with more channels tend to lock on faster than older units with fewer channels - that is a real difference.

S.

[LM Jones]

From what I've learned, you're right. The newer 12 channels have the possibility (read possibility) of being more accurate. This is in part thanks to newer better programming but also to the fact that the unit doesn't always select the 3 or 4 satellites with the strongest signal. The unit will use the 4 satellites which are in position to give it the best position fix. Think of it this way, if you were to 'fix' your position (at your computer) and you did so by measuring the distance from the three brightest light sources in the room, you would be able to 'fix' your position with some accuracy. Now, if you 'fixed' your position by measuring the distance from four light sources that are all 90 degrees apart, you would be able to 'fix' your position with even greater accuracy. This is what the 12 channel GPS should be capable of doing. But beware, not all GPS that receive 12 channels have this capability! I know mine does.
Another thing you have to look at is "how accurate does the GPS unit display my position (Lat./Lon.)?" There are a lot of GPS units that will only display to 1/100th of a degree. Than there are those that can display to 1/1,000th of a degree. Which would you rather have? I spend enough time finding and marking my positions that I don't want to spend any more time trying to find them again! Now, if I could just talk my wife into that CSB DGPS receiver!

[DHD]

Thanks for the info folks

The real reason I was asking for is

1. I would like to buy a new GPS and really understand which one " really " is a better unit or at least is in the top 5.

2. I have a software program that you can scan in fishing maps (FHS )and as long as you know 3 waypoints ,the map becomes digital and than you can download w/p's and see were fishing ,but it is only as accurate as the w/p's.