Differential Global Positioning System

[LC/LR4Life]

EGN,

With due respect, I can tell you that the only time I have had inaccuracies as such have been with solar interference. The unit that I use is extremely accurate and doesn't jump because someone is standing nearby.

I disagree with your statement that DGPS only works while standing still. It is used by the Coast Guard for navigation, and of all things, boats do not stand still !! They are in constant motion. Another, is that while off road, it is not uncommon to crawl along between 2-10 mph at most.

Why do you need accuracy within a few feet? To help you decide which turn to make if you have to of course!
:26_16_2:

 

[egn]

I disagree with your statement that DGPS only works while standing still. It is used by the Coast Guard for navigation, and of all things, boats do not stand still !! They are in constant motion. Another, is that while off road, it is not uncommon to crawl along between 2-10 mph at most.

Sorry, I didn't mean that DGPS doesn't work when moving. I just wanted to stress the point that DGPS doesn't help against errors through multipath reception and in some applications like surveying this problem is fixed by staying a long time at the same position. Then you get together with DGPS a accuracy well below 1 feet.

I think in order to make your point you should also provide an answer why WAAS is no good replacement for DGPS. In principle the accuracy of WAAS is nearly as good as DGPS. But the problem seems to be that the reception of the WAAS signal isn't as good as the DGPS signal in most areas because of the satellite signal is blocked by trees and mountains in a lot of offroad areas.

So this may be an important point in favor of your call.

Why do you need accuracy within a few feet? To help you decide which turn to make if you have to of course!

But this seems to be only important to me when you would like to follow a recorded GPS tracks. Maps are often not so accurate, especially in offroad country.

If you follow a recorded track from other people you cannot be sure that the data is so good as yours.

 

[Lynn]

. But the problem seems to be that the reception of the WAAS signal isn't as good as the DGPS signal in most areas because of the satellite signal is blocked by trees and mountains in a lot of offroad areas.

I've only used my WAAS-enable GPSr a few times, back in Colorado, and I have yet to see the 'WAAS Correction' indicator come on.

I know that the WAAS satellites are intended for airline use, and are in geosynchronous orbit (fixed position over the equator), and over the horizon for most of the US. The only time that you would get a WAAS correction signal is if you are a) in the air or on the proper slope of a high mountain, or possibly b) on the coast.

So my theory about WAAS and handheld or car-mounted GPSrs is that it is a marketing scam.

Do you get a regular indicator that WAAS is in use when you are inland, on the ground? I want hard data, not just "I have a WAAS-enabled GPSr and it works great." I want to know if you are actually getting an indication that WAAS is available.

Plus, WAAS and the USCG DGPS are US only, so no use for overlanding outside the US.

Oh, and for what it's worth, there are several different methods of providing Differential GPS. Many years ago I remember there was one system that used a combination of GPS and Loran. I also read about a DGPS system that used a combination US GPS and Russian GPS. Some land survey systems use GPS receivers set at a fixed (surveyed) point to calculate the differentials, then broadcast them to portable survey instruments in the area.

 

[LC/LR4Life]

Hey Lynn, that is just the point I like to make about WAAS. Exactly, it isn't really useable to us folks here on the ground.

EGN,
Lynn already picked up my reason WHY WAAS is no good to replace DGPS. I think I stated my reason in my first post no?

 

[egn]

Like Lynn I would really like to know how good WAAS/EGNOS works on the ground. As I noted above I have a GPS tracker with WAAS/EGNOS support. It is a blackbox and I have no other indication beside the quality of the position recording. And I must say that when I compare the position against very good maps (1:25.000) the recorded positions are most of the time perfect.

Yes, WAAS/EGNOS is designed for aircraft use and the geostationary satellites are not optimal for land use, especially if you go further north. But at a place where it is possible to do an Inmarsat satellite call (I own a old one) it should be possible to receive the WAAS/EGNOS signal. That will not be possible in dense forrest and not in deep canyons, but at a lot of other places. I have used Inmarsat near the polar circle and it worked fine.

Beside that it is the question how the use of WAAS/EGNOS is implemented in the receivers. It won't be necessary to receive the signal constantly if the received correction grid is stored for some time. It can be updated when the satellite comes in sight again.

I don't argue against regular DGPS. It is a nice system and if it is provided it is of great help if you own the necessary hardware. But WAAS/EGNOS is not so bad either and you can use it now on most receivers if the conditions are right. And with SA disabled the GPS positions are good enough for most private use anyway.

Anyway, I think your call will not find much resonance because there are just not to many people that own the necessary equipment.