DGPS - Differential Global Positioning System

[sandcruiser]

Not to piss on a beehive or anytyhing,

but for general offroad use, when is it that you find better than 10-meters to be important?

In my limited experience, there have been times when I've used the gps to find my way in everything from downtown Guatemala City to the plains of Idaho to the mountains of Costa Rica. But I've never found myself thinking "I think I'll keep going west because the GPS says I'm still 20 feet from the river"

Again, I'm not trying to belittle your position. I remember getting in an argument with a backpacker in Monteverde, Costa Rica seven years who thought I was effete to even have a GPS unit instead of just relying on a map and compass. I just haven't ever seen the need for greater accuracy while driving and am wondering what sort of situations you have found that do require/benefit from the increase in resolution.

[LandCruisers4Life]

Under 'normal' circumstances, regular GPS is fine. However under circumstances where weather, ionic disturbances etc occur, the DGPS system helps to keep you within normal bounds by correcting for errors/abnormalities..... that is where I am coming from.

[Capt. Jim]

I have been lightly following this thread and agree that the enhanced accuracy of DGPS is not needed in off-road or recreational boating. The high accuracy has been developed for use in surveying and vessels that rely on GPS for dynamic positioning. I'm sure the military has a stake in it too.

GPS is fun and easy, but try not to rely too heavy on it. I have been aboard vessels on more than one occasion where the old LORAN or GPS signals we received were inaccurate, mostly because of equipment failure on the vessel. All GPS systems warn you at start up not to rely solely on their device.

[LandCruisers4Life]

Hence.. 2-3 GPS units in your vehicle

[NorCal Cruiser]

Originally Posted by LandCruisers4Life

Hence.. 2-3 GPS units in your vehicle

Or you could learn how to read a map?

[LandCruisers4Life]

Some people can read maps, and some can't. I can read a map just fine. This thread isn't about MAPS, so either stay on subject or go post somewhere else.

[sandcruiser]

dgps is starting to sound a little bit like MIBS

telling people to scram when they point out a perfectly valid point isn't friendly.

While traveling all over Central America, I considered our gps to be almost a necessity... I don't know if I relied on it, per se, but I certainly learned to enjoy it. Having said that, every day before starting out I'd look at the best map I had of the area where I thought we would be driving, and every day after we arrived (almost every day, anyway) I'd mark on the map where we had driven.

All that mapping was redundant to my gps/computer... but I just like to know where I was w/regards to big landmarks, the ocean, towns, etc so that I didn't rely to heavily on the gps. Besides, maps are pretty and I like to run my finger along a given route, or jump miles and miles ahead to see something else. I've never enjoyed the experience as much with screen. It's a little bit like reading a book vs. reading online. Online is fine for a forum, for example, but give me a paperback (please!) if it is over 2 pages.

For me, personally, carrying a second (or third?) gps isn't cost effective. I'd prefer a map and compass for that, as they are less fallible than a 2nd gps (and what if the problem is the satellites or the system, and not my gps unit? ).

I suppose that if the price of the gps was lower (or my discretionary gadget budget higher) I could justify one full-color, snazzy gps unit, hardmounted to the rig and another, handheld unit for when I'm not in the truck.

Anyway, just thinking out loud 'cuz I really don't feel like attacking the stuff on my desk right now.

[jasonmt]

I have seen some pretty impressive dual frequency GPS kit from Leica Geosystems for surveying but I am not up enough on the technology to know if it is a subscription based service or something that would be applicable to this scenario. However being able to derive your correction from a satellite based signal seems like it would have a much wider application than ground based stations.

For land navigation with visible landmarks and the option of waiting out inclement weather I see no argument for "needing" a highly accurate GPS location as it is a solution to a non existent problem. For sea and air navigation where there may not be visible landmarks and no option to wait out inclement weather a higher level of accuracy is needed and as already posted the ground based DGPS systems that are in place for sea navigation seem to be fully funded. Additionally I am sure that advances in technology are going to render the ground based DGPS system as relevant to GNSS as wooden block printing presses are to publishing today.

In the end you can have a GPS system that is accurate to +/- a few centimetres and a map that is using a different geodetic reference system than your GPS system and that +/- a few centimetres accuracy is useless anyways. At that point it is up to the user to determine if they are going to rely on a map or the GPS system.

[LandCruisers4Life]

You should always carry a map, or maps, and compass. I only said what I did earlier to NorCal Cruiser because of the way he came off being sarcastic.

[LandCruisers4Life]

Originally Posted by jasonmt

In the end you can have a GPS system that is accurate to +/- a few centimetres and a map that is using a different geodetic reference system than your GPS system and that +/- a few centimetres accuracy is useless anyways. At that point it is up to the user to determine if they are going to rely on a map or the GPS system.

Might be useless, unless you have been to where you want to go and have your own coordinates etc. Sometimes we have to make our own corrections on maps according to what our own information tells us.

Valid points however, thanks for contributing.

[NorCal Cruiser]

Originally Posted by LandCruisers4Life

Some people can read maps, and some can't. I can read a map just fine. This thread isn't about MAPS, so either stay on subject or go post somewhere else.

Oh, sorry, did I forget to add my smiley as you did with your "Hence.. 2-3 GPS units in your vehicle " comment? Personally I don't I don't think it's worth buying multiple GPS units for casual use. If you want to spend your cash buying 2 or 3 GPS units for yourself more power to you. But as Capt. Jim pointed out GPS units can fail and even GPS units caution users to not solely rely on the GPS.

[LandCruisers4Life]

That is definitely true. However, tech junkies like me, just MIGHT have several GPS units..... I have two now, and a 3rd hand held for getting out of the cruiser to hike off somewhere if need be.

Off topic, but I saw a ham guy on MUD post a picture of the inside of his 40, and it had about 6 radios, laptop and probably more..... so I aint THAT sick!!

[Capt. Jim]

Originally Posted by LandCruisers4Life

tech junkies like me, just MIGHT have several GPS units..... I have two now, and a 3rd hand held for getting out of the cruiser to hike off somewhere if need be.

Fire all three GPS units and compare positions. It will be interesting to see how similar, or dis-similar they are. I have been on boats with two GPS units, and each one gave a different position. The difference was greater than the separation distance between the two GPS antennas.

[Flyingdog5000]

I've been following this thread for a bit and have enjoyed the discussion. I would like to add my two cents worth and my suggestions for land nav toys to have with you. GPS is a cool tool and I wouldn't be in the woods without one, but as a search and rescue coordinator for our local sheriff's office I've seen a few common errors that people make with them.

1) Don't know how to use it. This isn't just about turning them on and reading your coordinates or adding waypoints, but understanding such terms as "map datum" and how to interpret what the unit is saying to you. I can't tell you how many times I've asked someone for a position report and they give me a Lat/Lon. I then ask what datum they are using and the reply is something like, "Datawhat??"

2) Over reliance on the GPS unit. This is about people who think the GPS unit is going to give them turn by turn instructions to camp. While this is probably coming in much the same way it has in street mapping, its not here yet.

We had a rescue on Mt. Hood a few years ago where a lost climber and his buddy were stuck in whiteout conditions up on the mountain. They had a GPS, but the batteries died (they also didn't know how to use it, which by that time was sort of moot). They didn't have a map or compass.

Had they had a working GPS they could have plotted their position, even in the whiteout. Had they a map, they could have seen where they are in relation to the lodge and the warm fire and determined a bearing to follow. Had they a compass, they could have shot the bearing the map gave them and walked to the lodge. Had they only a map and compass, they could have made an educated guess as to their position (remember, they are on a somewhat narrow climbing route) and still made it out. Instead, we had to wake up a bunch of people and go rescue them.

3) Lack of basic map and compass skills. Learning to navigate by dead-reckoning without a compass is a vital skill, as is learning how to quickly and efficiently shoot a bearing, take a bearing and determine your location using a map and compass together. This teaches terrain awareness and provides a useful skill for use when the batteries go dead.

I won't argue about DGPS or WAAS other than to say I've never needed accuracy any better than what my standard GPS provided. There really are very few maps that have usable resolution down to less than 10 meters anyway. I live in the Pacific NW and tree canopy and weather are continual gremlins when using GPS, but I can usually walk a few meters in one direction or another and get a lock. I don't rely on my GPS for altitude data, instead relying on the barometric altimeter in my watch.

As far as essentials, I have two GPS units with me most of the time. One is a handheld (Garmin Etrex)that stays in my pack (along with spare batteries) and the other is hard mounted to the mother board of my laptop, which stays in my truck.
I always carry a map of the area that I'm traveling through. This is usually a 1:24,000 USGS Quad Map that I printed out using software. Occasionally, I will also have a forestry map if one is available. I carry a Silva Ranger Compass that has a sighting mirror and built in grid reader for accurately plotting my position. I wear a Suunto Vector watch, which provides a pretty accurate altimeter and a backup electronic compass, should I drop the Silva somewhere. (The watch is kind of a luxury, but I like it) Along with a pencil to write on the map, I'm pretty well covered with that set of equipment.

Doug

[LandCruisers4Life]

FlyingDog5000 you're right - everyone who really does back country travel should be versed in map and compass work as well as being able to use and understand a GPS. I was a navigation man in the service and my basis was map & compass. All other navigational aids were built on top of that firm grasp of how to find out where I was, where I was going etc. When you use a GPS with that kind of background, you use it differently in my opinion. The GPS plots a direct line, unless you have a waypoint by waypoint trail you're following for your turns etc. If it plots a straight line, 99% of the time it's not a straight line to the object, and it is essential to have a map of the area to see what obstacles you will need to work around in the terrain. GPS is another tool in the toolbox. DGPS is added upon that tool, to help keep your accuracy up even under foul conditions. The more tools I have in my navigational toolbox, the better. Just like my Land Cruiser with traction devices. The more I have the better - good tires, traction control, lockers, gears etc... all that adds up to a better chance at success in the end. In navigation, it's the same.

[LandCruisers4Life]

A quote from Lowrance:

Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense now has the technology to localize the control system to deny GPS signals to selected areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, aviators, drivers, hikers, hunters, and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area. A Lowrance GPS receiver in combination with advanced technology of today's GPS management will take you anywhere you want to go.

The decision to allow civilians so much accuracy in location information was finally made because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.

Differential GPS (DGPS)

Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver – located at an accurately surveyed site – to the other GPS components. This non-moving DGPS reference station knows where the satellites are located in space at any given moment, as well as its own exact location. This allows the station to compute theoretical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the error in the satellite's signal. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements and calculations.

The two most common sources of corrective DGPS signals currently are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive DGPS correction data from Coast Guard beacon transmitters, a mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data. Most Lowrance GPS receivers are DGPS ready.

Wide Area Augmentation System (WAAS)

GPS is plenty accurate for route navigation, but the U.S. Federal Aviation Administration has special need for aircraft traffic control that go beyond basic GPS. The FAA has a plan under way to boost GPS performance even further with its Wide Area Augmentation System, or WAAS. This GPS add-on will include a time control element that will help airliners fly closer together while avoiding collisions. In addition to carefully spacing airplanes along travel corridors, WAAS will eventually make instrument landings and takeoffs more accurate as it replaces existing aviation navigation systems.

Non aviators can use WAAS signals to make their GPS navigation even more accurate. However, WAAS has some limits you should know about.

First, the U.S. government has not completed construction of the WAAS system, so it is not yet fully operational. The ground stations are in place, but only a few of the needed WAAS satellites have been launched.

WAAS can boost the accuracy of land GPS navigation, but the system is designed for aircraft. The satellites are in a fixed orbit around the Equator, so they appear very low in the sky to someone on the ground in North America. Aircraft and vessels on open water can get consistently good WAAS reception, but terrain, foliage or even large man-made structures frequently block the WAAS signal from ground receivers.

You'll find that using your GPS receiver without WAAS is both easy and amazingly accurate. It's easily the most accurate method of electronic navigation available to the general public today. Remember, however, that this receiver is only a tool. Always have another method of navigation available, such as a map or chart and a compass.

[bhmmapping]

how old is that lowrance ad?

[LandCruisers4Life]

It came off the current Lowrance web site.

[Capt. Jim]

BTW, did a short delivery of a 75' Hatteras yesterday and the GPS took a crap enroute.

[LandCruisers4Life]

Dig out the maps.....

Which brings me to something. I have used my Global Map 100 for several years without problem, but they don't make this model anymore which is very unfortunate. So I was browsing CRAIGS LIST one day and saw one... excellent condition, all the papers, box etc for $50! So I contacted the guy right away and bought it. So I now have two of my favorite GPS, and two DGPS antennas. Too bad I cannot find a spare beacon receiver. Lowrance stopped building them and they're scarce.

[Waorani]

I use DGPS on a daily basis and have been since the original Coast Guard Beacon stations went into service. Wouldn't be without it - when I'm mapping features to submeter accuracy!

For use in a vehicle/on trail - you're wasting your time and most modern WAAS capable receivers are better than the map data you are trying route on.

However, if you really want to do it right - go to H-Star!

[LandCruisers4Life]

The whole point of this thread was to introduce non-users to DGPS, as well as alert that the national DGPS system is in threat of decomission. Just in case anyone is interested in preserving this resource. The US Coast guard transmitters would remain, just the national inland service (which I enjoy having) is threatened.