Dear Capt. Paul:

I am now running my Global Positioning System unit mated to a DGPS receiver while in offshore locations, but I am considering getting another GPS unit that I would be using in interior lakes away from the coastal waters. I am also considering one of the current handheld units offered.

I would like to see a comparison/discussion on DGPS vs. WAAS accuracy, and is WAAS available for all of Louisiana Mississippi and Texas? As you know a separate DGPS receiver can cost more than what several standard GPS unit would cost. Plus, the connecting cables make it a little inconvenient.

I would like to avoid that situation, if WAAS is as accurate.

Thank you, 

James W.

Response by Capt. Paul:

The Differential Global Positioning System

When first activated, GPS accuracy was stated to be within 100 meters with the deliberate degradation of the GPS signals by selective availability (SA) program.

Fortunately for the civilian GPS market, SA degradation was discontinued on May 2, 2000. Without selective availability, the worst-case average of standard GPS signals is considered to be within 15 meters (20 meters in extreme latitudes).

Not satisfied with the inaccuracies, the Coast Guard developed a differential system to supplement GPS. The Differential Global Positioning System is a differential enhancement to the Global Positioning System that provides improved location accuracy to about 1 meter.

DGPS is designed as an aid to  navigation, where critical accuracy is needed, such as in coastal harbors and channels. There was no need to provide a precise accuracy capability in the open sea, so DGPS does not operate for great distances offshore or away from major commercial inland waterways. 

DGPS uses a network of fixed, ground-based reference stations — much as the older LORAN system did — to broadcast the difference between the positions indicated by the GPS satellite systems and known ground fixed positions. These stations broadcast those differences, with the digital correction signal typically broadcast locally over ground-based transmitters of shorter range.

In the United States, the USCG’s DGPS system has been named NDGPS (Nationwide DGPS), and is now jointly administered by the Coast Guard and the U.S. Department of Transportation’s Federal Highway Administration. The USCG broadcasts the corrections on long-wave radio frequencies between 285 kHz and 320 kHz near major waterways and harbors throughout the United States. 

To achieve this better accuracy, the USGS installed GPS receiving stations at precisely placed locations near the coastlines. These stations receive normal GPS signals, determine stated GPS stated positions as compared to their actual position and broadcast corrections.

The correction broadcast is transmitted on a different frequency range from satellite GPS signals and requires differential receivers. These differential signals are transmitted from towers located along the coastline, with their signals designed to provide slightly overlapping coverage areas.

In order to take advantage DGPS signal, you need to have a GPS unit with DGPS capability, which can be connected to another receiver; the differential receiver must have a separate antenna and power supply.

Once connected, the DGPS receiver will indicate the differential signal is being received and will display the corrected position. Receivers near transmitting towers will generally receive a more-accurate position than those farther away from the transmitters, as the corrected signal is for the transmitter/receiving station site. In addition, weather conditions can cause inaccuracies and/or loss of the differential signal.

The cost of the differential receiver is generally more than that of a decent GPS unit.

DGPS positions are considered to be within a typical position accuracy of 3 to 5 meters. Most GPS units can accept and convert the DGPS signals and integrate it in the unit displays. This is a far cry from the 15 meters without it.


Then, along came the Federal Aviation Administration with a need of accuracy in GPS positions away from the U.S. coastline. They designed and implemented a system called Wide Area Augmentation (WAAS).

The FAA wanted to allow GPS to be used for aircraft navigation and as landing aids. They realized the USCG costal transmitting towers would not achieve their goals.

To provide correction coverage, they installed 25 ground-monitoring stations at precise locations. Designed to serve major airports across the U.S., master control stations on either coast collect the data from the monitoring stations and interpolate a corrected GPS signal.

This correction accounts for GPS satellite orbit inaccuracies, internal satellite clock drift, plus signal delays caused by the atmosphere and ionosphere. This is much as the DGPS system, but the signals are satellite based.

So far, so good. Now, here comes the big difference. 

WAAS-corrected signals are sent to transmitters installed in two satellites in geostationary orbit above the U.S. Rather than using land-based transmitting towers with limited range, the satellite transmitter broadcasts cover the entire U.S. 

And here is the kicker: The WAAS signal is in the same frequency range and is compatible within the basic GPS signals. That means a GPS manufacturer has to only allot two additional receiving frequencies in its computer chip inside the GPS unit to accommodate the WAAS signal.

So any WAAS-enabled GPS receiver can read the signal without any other costly modifications or added material. The price increase in the overall GPS-WAAS configuration has been very small. 

Although the entire FAA system is not operational at this time, most GPS receivers with WAAS can receive the signals 95 percent of the time.

Down time is taking place on the satellites as a testing procedure until it is officially designated as operational.

Most WAAS users should be able to receive the signals in all of the U.S. except for the testing down time. The operations plan is to have the system fully operational by August 2018.

This will cover nearly a year of training in the aircraft industry. The satellites are in place and have been transmitting since 1999, and most of the control stations are operable.

The typical WAAS position accuracy is stated to be less than 3 meters (9.8 feet). Some units have reported less than a meter.

One of our web posters has listed his GPS/WAAS figure as less than 6 feet with a handheld Garmin eTrex.

Remember, however, that the entire system is based on orbiting satellites. Always bring a compass and map as a back up to the system. 

I rely on the WAAS differential, as it is more than acceptable. But you still can use both differential systems if you so choose. 

Capt. Paul