DGPS - Differential Global Positioning System

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For those who enjoy GPS, you might like to know that DGPS is a system to add to regular GPS which accurizes your position. When satellite signals get distorted because of sun flares, ionic atmpspheric conditions etc, you will suffer a great error in your normal GPS operation.

WAAS is new, it is touted by many GPS companies to be the latest in error correction since sliced bread. However, WAAS operates as well from Satellites. These WAAS satellites are in orbit around the equator, and were designed for AIRCRAFT use, not for us, who are on the ground.

The call for DGPS.
What is DGPS, and how does it work?
DGPS is the Differential Global Positioning System, operated by the US Coast Guard. However, there has been a program whereby a National DGPS system has been put into place. This national DGPS system is under threat of being decomissioned. DGPS works with regular GPS receivers in this way: Your GPS in your Land Cruiser receives satellite signals with a certain degree of accuracy. However in inclement weather, or under certain ionic atmospheric conditions, the satellite signal will degrade tremendously, causing you to possibly become lost (remember, WAAS signals are also satellite driven). When you couple a regular GPS with a DGPS Beacon Receiver on your Land Cruiser, your position will be corrected. Why...? Differential GPS signals come from the Earth. There are hundreds of DGPS transmission stations all over the United States, which constantly transmit to your DGPS enabled receiver. The GPS receiver takes the incorrect signals from the satellite, and the signals from the DGPS transmitter and after some calculations by the GPS, it arrives at the correct location- where you really are in other words.

The National DGPS website:

Nationwide DGPS - USCG Navigation Center

National DGPS Steering Council:

NDGPS - USCG Navigation Center
 

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In real world use what difference does it make if you have 5 m, 10 m, or even 30 m of error? Unless you are using your GPS for surveying I don't think it would be worth the cost.

Here's some more info regarding DGPS from the place where I buy my GPS units for work- DGPS Services

DGPS Services

Several of the GPS receivers available today uses real-time differential solutions typically providing sub-meter to five meter accuracies. This technology allows the use of the Coast Guard radio-navigational beacons and/or satellite differential providers for differential correction. The Coast Guard beacons are used free of charge and are limited to the range of a specific beacon signal. Most Coast Guard stations are currently located along major navigable waterways.
Satellite differential services are subject to an annual subscription fee, typically $800/year, but the range is not limited. Performance of the satellite differential system is not going to vary whether you are in Washington today or Florida tomorrow.


WAAS
The Wide Area Augmentation System (WAAS) has been created by the Federal Aviation Administration (FAA) to augment GPS with additional signals for increasing the reliability, integrity, accuracy and availability of GPS for aviation users in the United States. WAAS consists of a network of 25 ground reference stations and a number of geo-stationary satellites broadcasting a signal in the GPS band. The WAAS signals contain information including differential corrections and GPS satellite health status.
A complimentary system called the Local Area Augmentation System (LAAS) has been created to allow GPS to be used for landing airplanes. LAAS is installed at individual airports and is effective over just a short range, with an accuracy better than one meter in all dimensions.

The primary WAAS benefit to mapping and agricultural users of GPS is another source of free differential satellite corrections and additional robustness of service with the addition of one or more ranging signals. LAAS will have minimal impact on mapping and agricultural users.
 
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I think some people want the best accuracy possible. Also, the US Coast Guard doesn't just operate DGPS near waterways, they also are part of the free National DGPS program which I spoke of in my original post. Here are a couple of maps:

SiteLocation.jpg



CurrentCoverage.jpg


The little red markers in the first map are the transmitters which are part of just the Coast Guard program. The others are part of the NDGPS program. Areas covered in yellow by the NDGPS transmitters, and threatened with decomission are in the second map and are part of the NDGPS coverage across the US.
 
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You need to be a couple of hundred miles from a DGPS station that sees the same constellation of satellites.


You then need to be able to get the time log of your data off your mobile unit.

You then take the time correction info from the the DGPS station and apply that to the time data from your mobile unit.

This corrects for the unavoidable errors of atmospherics, satellites, etc. It doesn't correct for the systemic error's associated with the unit.

You can create your own DGPS station by setting up a unit at a know location.

I've never had to do this so this is just the basic work flow as colleagues have described it to me.
 

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I think some people want the best accuracy possible.

While I agree that some people will want the best accuracy possible you will quickly become more accurate than your map. The acceptable error for points such as the intersection of roads on a 1:24k map is 1/30 of an inch- about 20 meters on the ground. On a 1:100k map this 1/30 of an inch error is about 86 meters on the ground.
 
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You need to be a couple of hundred miles from a DGPS station that sees the same constellation of satellites.


You then need to be able to get the time log of your data off your mobile unit.

You then take the time correction info from the the DGPS station and apply that to the time data from your mobile unit.

This corrects for the unavoidable errors of atmospherics, satellites, etc. It doesn't correct for the systemic error's associated with the unit.

You can create your own DGPS station by setting up a unit at a know location.

I've never had to do this so this is just the basic work flow as colleagues have described it to me.

Where did you get that information? It's totally false. A DGPS ready GPS unit doesn't need an operator to do ANYTHING. It also doesn't need to be a couple hundred miles away.

A DGPS ready GPS unit has a connection for a DGPS Beacon Receiver. This Beacon Receiver (mounted on your 4wd) takes in signals from a DGPS Transmitter. The GPS then takes the information from both the satellites in its view, and the information from the DGPS Transmitter that it gets through the DGPS Beacon Receiver, does the calculations (itself), and then gives you your current location.
 
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While I agree that some people will want the best accuracy possible you will quickly become more accurate than your map. The acceptable error for points such as the intersection of roads on a 1:24k map is 1/30 of an inch- about 20 meters on the ground. On a 1:100k map this 1/30 of an inch error is about 86 meters on the ground.

I can agree with that. Part of my point though, is to help save a valuable resource for us, the 4 wheel drive community which is facing being decomissioned.

I have it for my Land Cruiser, and I am sure there are others out there who do as well.
 
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Where did you get that information? It's totally false. A DGPS ready GPS unit doesn't need an operator to do ANYTHING. It also doesn't need to be a couple hundred miles away.

A DGPS ready GPS unit has a connection for a DGPS Beacon Receiver. This Beacon Receiver (mounted on your 4wd) takes in signals from a DGPS Transmitter. The GPS then takes the information from both the satellites in its view, and the information from the DGPS Transmitter that it gets through the DGPS Beacon Receiver, does the calculations (itself), and then gives you your current location.


Whoa dude, relax.

I see now that what we call/called the WAAS is now generically referred to as DGPS. I apologize for the confusion.


I don't want to get into a pissing match, but re-read my post.


You need to be within a couple hundred miles of the DGPS base station that you use for correction. Otherwise there are errors, some can lead to little or no correction. It is about both receivers seeing the same satellites and the same conditions.

I also just realized you are interested in the consumer level of GPS. The method I outlined if for much better accuracy.
 
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Ok ok.... WITHIN a couple hundred miles or so, you're right on that. However WAAS and DGPS is not the same thing, even if people think it is, it isn't. WAAS, as posted earlier was develeped for Aircraft. DGPS used in conjunction with a regular GPS unit that is DGPS capable can get you within 3-9 ft of where you want to be.
 

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Ok ok.... WITHIN a couple hundred miles or so, you're right on that. However WAAS and DGPS is not the same thing, even if people think it is, it isn't. WAAS, as posted earlier was develeped for Aircraft.
WAAS is a form of DGPS. Don't even try to say it isn't. The source and handling of the correction data is different, but it is still a DGPS system. Doesn't matter that it was developed for aircraft, it is still usable. In fact it didn't meet the FAA's needs so they developed LAAS which is the way I thought they should have gone in the first place.

DGPS used in conjunction with a regular GPS unit that is DGPS capable can get you within 3-9 ft of where you want to be.

Many of the government DGPS transmissions are not receivable deep in mountain or canyon country because the signal is blocked by geography.

Just get a WAAS enabled GPS and turn WAAS on. When it is not blocked by geography you get higher resolution. No need for a separate radio receiver to get the DGPS correction signal.

A receiver with a high accuracy clock will do more for accuracy than having a WAAS or DGPS signal. Log the raw data, then post processes it against a base station. Few cm accuracy is possible that way without spending on a survey unit. The key is that accurate clock and the right raw output.
 
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Hi,

I think a couple of things need to be cleared up:

1) WAAS is not DGPS. WAAS is a space based augmentation system that corrects primarily clock and ephemeris (similar to almanac) errors, as well as some ionospheric errors over a wide area (hence the name Wide Areas Augmentation System). WAAS is delivered to receivers on the L1 carrier, so if you have a WAAS capable receiver all you need to do is enable the correction and if you are within the WAAS "model" area you will benefit from the augmentation.

2) DGPS is differential GPS. The key bit here is "difference." The USCG operates a DGPS system and in Europe they have EGNOS. What is differenced is the codes from the satellites. As was pointed out, you need to be within a couple of hundred kilometers from a reference station and have your mobile unit tracking a subset of the satellites that the reference (aka base station) sees and have a coomunications link from the reference station to your mobile unit to receiver the differences between reported satellite positions and "actual" postitions. If you use DGPS on your rig you will need a "beacon" receiver or other device. Sometimes you can use GSM or GPRS in some areas.

2) Surveyors use carrier phase differencing which can simplistically be considered a variant of #2 above.

:)
 
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:confused:


My memory is that the first WAAS was a ground based system that was deployed along the coast concentrated near ports.

We always called it a type of DGPS because it used a correction.


Oh well.
 
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:confused:


My memory is that the first WAAS was a ground based system that was deployed along the coast concentrated near ports.

We always called it a type of DGPS because it used a correction.


Oh well.

nah... coast guard beacon is ground based radio signals. waas is only two sats helping the east and west coast.

dgps is just a second radio receiver attached to a gps. there is a black box and a extra antenna... sometimes the antenna is built into the gps antenna.. just depends on the equipment.

"dgps" just means differential gps.. loosely used term.
 
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WAAS uses ground correction stations as well and is more than accurate enough for the average wheeler. Several years ago WAAS was unreliable and new and DGPS was required to get an accurate readings. I can remember going and setting up the GPS receiver for 10 minutes at a time and DGPS postprocessing to get reasonable point collections(within a meter) - now I use a consumer grade bluetooth GPS(earthmate) and even on a moving collection I see a mean PDOP of around 4 with most clear readings less than 2. As far as WAAS being for aircraft use only, my understanding was that the higher accuracy was required for landing, so it def. would be applicable for ground use.
 
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WAAS was developed for Aircraft, hence the FAA involvement. It is not DGPS, and others as well as myself have already pointed out the differences here.

One of my major points to this whole thread was to bring to the attention of users of the National DGPS system is that the government is looking at decomissioning it. Like other resources available to 4 wheelers that are going away slowly but surely, this would be yet another. I for one prefer DGPS to WAAS. I would encourage other wheelers to register as a free member of those concerned for the NDGPS program on that US Coast Guard site I posted above on. If not for yourself, for other wheelers.
 
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...

One of my major points to this whole thread was to bring to the attention of users of the National DGPS system is that the government is looking at decomissioning it. Like other resources available to 4 wheelers that are going away slowly but surely, this would be yet another. I for one prefer DGPS to WAAS. I would encourage other wheelers to register as a free member of those concerned for the NDGPS program on that US Coast Guard site I posted above on. If not for yourself, for other wheelers.

This is absolutely correct. One point of clarification is that the USCG system is not going away, and in fact they have recapitalized their system and are in a modernization phase with new reference receivers and operational SW. The NDGPS system is at risk. This system generally covers areas where these are not harbor approaches or navigable waterways. So as LandCruisers4Life has stated, you need to get involved if you want this service. Otherwise, if you want better than autonomous accuracies you have to use WAAS or a fee based space based augmentation system (SBAS) like Omnistar or for truly high precision something like this* (where available).

*-This is a shameless plug as I was the guy that started these systems;)
 
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I'm not really sure what your agenda is here, but I don't think a full DGPS system is necessary for most, if any, recreational wheelers. WAAS is not really new anymore and a lot of the info on that site you linked is based on nearly 10 year old information. I would personally rather see what little federal money is devoted to geospatial activities go to increase publicly accessible remote sensing data, but that is just my opinion and is probably not worth much to most people on here. edit - too slow, comment was directed at OP not mendocino
 
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I'm not really sure what your agenda is here, but I don't think a full DGPS system is necessary for most, if any, recreational wheelers. WAAS is not really new anymore and a lot of the info on that site you linked is based on nearly 10 year old information. I would personally rather see what little federal money is devoted to geospatial activities go to increase publicly accessible remote sensing data, but that is just my opinion and is probably not worth much to most people on here. edit - too slow, comment was directed at OP not mendocino

If you look at many wheelers rigs in remoter areas of the world such as Australia, they have their own DGPS going on. Ever wonder what all those antennas were for on their bull bars? Not just radios that's for sure.

My 'agenda' is to keep this going as there are other wheelers out here who DO use the system and like having better accuracy. I for one have money invested in my DGPS rig and feel that we as consumers should be able to utilize that equipment and not have to toss it just because someone feels they want to decomission a program that many have become used to using. What if they got rid of regular GPS for civilians? I bet you'd be screaming off the hook then wouldn't you.
 
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What if they got rid of regular GPS for civilians? I bet you'd be screaming off the hook then wouldn't you.

that is not a good analogy. If they replaced regular GPS for civilians with something as or more accurate that was even easier to use and required less investment, then I would be mad if they kept pushing funding for the regular gps over the new and better alternative.

I can understand if you have an investment in DGPS, but I don't think there is any advantage to it over WAAS for inland wheeling purposes and I wouldn't want someone to go out and spend the extra money on a DGPS system when they don't need it.
 

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