GPS Accuracy

[RB]

How accurate do you find your handheld GPS? I've used mine this winter to locate where I had previously been ice fishing. It doesn't get me as close as I would like it to. How accurate should I expect it to be? Also, I thought the government altered a system that would make our gps's more accurate.

[Jeremy]

After the gov turned of SA(last spring), GPS accuracy was supposed to be within 30 ft, my garmin 12 when I get 5 or 6 satellites locked in claims to be within 17 ft

[Catch37]

RB: You should expect to see something in the 20'-30' range with your GPS.You should have your plotter screen on the smallest range possible ( 1/10 mile) when trying to locate a spot.

[MedicineMan]

What about DGPS? Is that system operational and is there units available for the public to use?

[Mayfly]

Last week I was headed north on a state highway that I had not traveled since November. I also had not wiped off the track I made then. On the smallest scale (Garmin 12 - 2/10 of mile) I was making a double line which corresponded to either lane of the highway. I.E. , in November I was southbound and last week I was northbound. The double lines would represent basically, the width of the 2 - lane road. I think that's mind-boggling!
Paul in Appletown

[ND]

Raytheon makes a DGPS. I use their 420D chartplotter.
Acuracy to 15ft or less and works great.

[BobB(MI)]

RB,

You raised a good question about GPS accuracy. The other posts bring up some important points about GPS use as well. Let me give you two answers to your accuracy question:

1. The Short Version: Within 30 to 60 feet, 95% of the time.

2. The Long Version: Why this accuracy? In order to fully explain, I'll ramble on a bit and discuss a little history and background. GPS uses 3 satellites to determine a 2 dimension position (lat and long). It uses a 4th satellite to determine a position in 3 dimensions (adding altitude). With that being said, what's the big deal about 8-channel or 12-channel receivers you hear about? Here is where some accuracy starts to come in - even though only four satellites are needed, not all the satellites are positioned to return the most accurate information. Today, most GPS units are 12-channel, parallel recievers and they are able to constantly determine the best 4 satellites to use, then recalulate your position using the others to "fine tune". When the government, Department of Defense, used Selective Availability (SA) to derate the accuracy of civilian GPS units, we were guaranteed to be within 100m (300'), 95% of the time. Since May 2000, SA has been turned off and accuracy is now at 10-20m, 95% of the time. Many sales staff do not understand how GPS works and sell some units as more "accurate" than others. In reality, some have more features and abilities - ie: if you compare any unit to another if they lock onto the same satellites, you are still going to be within 10-20m, 95% of the time. There are some inherent differences as to how some process signals and re-calculate positions.

Want more accuracy? As one post discussed, there is Differential GPS (DGPS). DGPS was originally developed by the USCG and uses fixed, land based receivers with known positions that can transmit an error correction to your GPS receiver. In addition to the USCG (free) there are also 3rd party receivers which transmit on FM to a pager sized add-on to your GPS. These are subscription services for the most part. Accuracy is generally improved by 1/2 over GPS w/o SA - 5-10m, 95% of the time.

Still want more? Raytheon is operating Wide Area Augmentation System (WAAS) for the Dept of Transportation - FAA. Similar to DGPS, it uses a land based network of receivers and 2 different satellites and has shown accuracy to 3m, or about 1/2 of DGPS.

One interesting way to check your accuracy at a point in time is to leave your GPS on in a fixed place for about an hour. Make sure the plotter, or map screen, is set you 0.1mi or less and that your "position pinning" feature is turned off - if you have one. After an hour or less, you may see a random trail - your accuracy is the outside limits of that trail. It used to vary all over the place with S/A!

With all that said, you can usually divide the above accuracy numbers by 2 and say that is your accuracy 50% of the time. In case you're wondering, part of my "real" job deals with some of the above. I don't work for any of the manufacturers or get money from them. I personnaly use Lowrance products, but have seen and used Garmin, Trimble, Raytheon, etc. They all work well and it is just what features you get used to....

Thanks for letting me ramble on a long note.

Bob

[JKJ]

You don't mention the brand of GPS you have. On the Lowrance Global Map 100, there is a feature called "Waypoint Averageing".

When this function is turned on, and the unit is left in one spot for an hour or more, that waypoint is pinned down.

For instance, stand in your driveway and save a waypoint. Come back later to that spot and select a "Go To" that waypoint. your unit will indicate you are anywhere from Arrived to .01, .02 or 03 miles off. After Waypoint Averaging that spot, you will always be right on.

Hope that helps.

[MedicineMan]

So DGPS uses only land based towers and not satellites? Is
reception then a problem - you are limited where DGPS is useable
vs satellites working everywhere?

[BobB(MI)]

MedicineMan,

Good question - I re-read my post and realized there may be a bit of confusion on the DGPS. DGPS does use satellites; however they are the same 24 satellites that orbit Earth for everyone's GPS use. If you receive a DGPS signal, you are getting a signal from the land based receivers (exact known position)that is transmitting the "error" or correction factor to your unit based on the signal from the satellites it is locked on to. They are telling it where it is, but it already knows where it is - the difference is an "error". Kind of think of it as a big triangle with the land based receiver as an anchor point.

One problem with DGPS is that there are not free land stations around the country - only areas where the USCG is - typically. As I mentioned, there are 3rd party groups who have installed fixed receivers, but they charge a fee and it is not real common to use them for recreational GPS use.

Now one of the things that makes the WAAS system different is that they use their own 2 satellites that communicate with their own grid of land stations. The principle is similar and as stated, accuracy, as well as coverage, is a bit better with WAAS. It is a DOT-FAA run program that Raytheon is operating. Kind of a next generation after the Department of Defense established GPS.

Hope that clears it up a bit. Maybe muddied the waters more?

Bob

[Rich Il.]

Hello Bob,
just read your interesting article on gps.I just received a magellan 300 as a christmas gift from my daughter and son in law,Iwould like to know,your thoughts on it.Iknow it is not the best, but for a starter, it should be ok.Will be taking it to canada this summer,fishing separation lake.Please send your thoughts.Have a great fishing season.
thanks,
Rich

[BobB(MI)]

Rich,

Your Magellan unit will work great for you, specially if you are new to GPS. Sounds like you have a considerate daughter and son-in-law, too.

I have run one Magellan in the past, but I don't remember the exact model number. I'm assuming the 300 is a handheld which is always a good choice for traveling fisherman.

Out in a lake you will usually always get good reception and a good, consistent lock on satellites. As stated before, you will have accuracy within 300' most all the time and more than likely 150' better than 1/2 the time - all will. The largest, most important issue is that you experiment/play with it BEFORE you get to unfamiliar territory. I had an experience to fish in a 3 day tournament that saw fog for all 3 days. On day 2, there were many new GPS units wich people bought the night before. As boats were launching, people were on the docks and in the parking lot trying to learn the features of the units - timing is everything.

Maybe others who have the same model would have some operating tips? Might be worth a post "Help with a Magellan 300..." or something - your call.

When you really think about it, the ability to locate yourself within 300' pretty much anywhere in the world is in your hand...kind of amazing. Go out and play - walk around the yard with the instructions, keep it on in your vehicle while driving around, etc. Take some extra batteries (a 12v plug in adaptor is handy) and enjoy the fishing. Hope you have a great trip.

Bob