By Larry Klementowski
If you’ve researched GPS receivers in the past few years, you’ve probably seen the acronym “WAAS” in the specifications. You may also have seen the term “DGPS.” What do these letters stand for, and are they features you need?
In this two-part article, I will assume that you have some familiarity with the Global Positioning System
(GPS). You probably even own (or at least have used) a GPS receiver (GPSr). But you may not know about the systems already available to make your GPS readings more accurate, plus other systems that are either partially deployed or in the planning stages. This article discusses those systems, tells you where they’re usable, and helps you decide which will be of benefit to you.
Part I: DGPS and WAAS
Let’s start with some terminology and definitions. I’ll try hard to keep all the abbreviations and acronyms straight, and I’ll always define them on first use. But it’s impossible to address this subject seriously without using lots of them, so hang in there!
When we use the term GPS, we are really referring to the United States government’s current satellite navigation system, more properly named NAVSTAR GPS. In more general terms, any such system is referred to as a global navigation satellite system (GNSS).
So, what are GPS augmentations?
In the most general terms, anything that improves the accuracy or reliability of the Global Positioning System augments (improves) the performance of that system. Many of you have used GPS augmentation already, whether you know it or not! Or at least you may have heard about it.
I knew about many of the existing NAVSTAR GPS augmentation systems already, and have used several, but in researching this article, I learned about a few new ones myself! I’ll talk about them in more detail later, but for now, I’ll just mention that there are ground-based systems such as the U.S. Coast Guard’s Differential GPS (DGPS) service that send out terrestrial radio signals with the augmentation data; satellite-based systems such as the U.S. Federal Aviation Administration’s Wide Area Augmentation System (WAAS); network-based systems such as CORS; and hybrid systems. Some augmentation services are even operated by private companies, such as John Deere’s StarFire system for agriculture.
Note that the terminology around augmentation systems is not hard and fast. For example, WAAS and DGPS are very different in function and implementation. Whether both of those systems are considered under the umbrella of “DGPS” or not depends on who is doing the writing. Under currently accepted international conventions, they are both GPS augmentations—but common usage is something else entirely. If someone says they are using “Differential GPS” or had a “differential fix,” it might pay to ask what kind they mean. After you read the rest of this article, I hope that you will understand some of the different types of augmentation systems that exist.
Differential GPS (DGPS)
is a method of improving the accuracy of a mobile GPS receiver by using additional information about the differences in signal reception at various locations. See where the name “differential”comes from? The improvement in accuracy when using a DGPS system is obtained by using difference data from a known, fixed-reference location when you are at a different location with a mobile or handheld unit.
The U.S. Coast Guard’s DGPS system was the first one widely deployed, and may be what people are talking about if they just say “DGPS.” (Although see my earlier note about terminology confusion regarding GPS augmentation systems.)
If you have a boat with a GPS receiver, you may already know about DGPS. The fixed (stationary) portion of the system consists of a very accurate GPS receiver in an accurately known (within small
fractions of a inch), well surveyed location,
plus a radio transmitter (the same ones used for marine radio navigation beacons in the 250–350KHz frequency range). The broadcast signal contains the differential data, which consists of the error between the actual location at the site, and where the fixed GPS receiver thinks
it is located. That error data is received by a DGPS receiver, which is either built into the GPSr, or connected to it via a wired or wireless connection. Your GPSr then knows the error being seen at that moment in the GPS signal at the radio beacon site, and can correct for it. Assuming that you are not too far from the radio beacon station, this process results in a much more accurate location for your DGPS receiver—typically from 1 to 3 meters (about 3.3 to 10 feet), but sometimes even better.
DGPS station in Cardinal, Ontario. (Copyright 2005, Alex Wiecek. Used with permission.)
Because DGPS hardware needs a radio as well as a GPS receiver, it tends to be a bit larger than practical for handheld use. From my research, it seems like the small all-in-one units that were sold at one time may no longer be available. Portable beacon receivers
are available, and can be integrated with industrial mapping units
and data post-processing to provide sub-meter accuracies for a system cost of about $2,500. Not exactly consumer level, but not the mega-bucks I expected.
The U.S. Coast Guard maintains a network of DGPS transmitter stations, and so do many other countries around the world. Sounds like something only for boaters, or maybe folks who live near the coastline, right? Nope. Under the umbrella of the U.S. Nationwide DGPS (NDGPS)
system, the number of DGPS stations has been greatly expanded inland, and now covers much of the U.S. Take a look at the nationwide coverage map
. Or, you can select your state or region
for a closer look. Note that not just the yellow areas are covered; yellow indicates an area that is in range of at least two DGPS stations, but all the gray areas are covered by one station. Not too many areas are still uncovered, but expansion continues to fill in the gaps; see the discussion of NDGPS in a later part of this article.
Similar DGPS stations are maintained by many other countries around the world, using the same maritime radio beacon frequencies to transmit the correction signals. At present, the DGPS system has much better worldwide coverage than other augmentation systems, such as WAAS (North America only), or the WAAS “cousins” in Europe, Japan, and India, which are still partly in development or testing phases. Do you need better than “plain vanilla” un-augmented GPS accuracy outside of the U.S., Canada, and Mexico? DGPS is probably all that you have for now.
I haven’t located a government-sponsored list of worldwide DGPS beacon stations, but GPS receiver manufacturer Trimble maintains a list
that currently shows nearly 200 stations in 38 countries. Typical beacon station ranges are 150 to 300 miles, depending on local topography and station transmitter power. A point to remember is that these radio beacon signals do not need line of sight. They can penetrate even heavy tree cover, or a reasonable amount of buildings or other objects. By contrast, WAAS signals do require line of sight (to equatorial geostationary orbits), and tree cover is often a problem. Let’s talk a bit more about WAAS now.
Wide Area Augmentation System (WAAS)
Much has been written on WAAS,
and if you have a consumer handheld GPSr, you likely have a pretty good explanation of it in your user manual. The WAAS system was actually developed by the Federal Aviation Administration (FAA) for aviation navigation applications. The FAA’s WAAS page
and related pages (see links on the left side of that page), have very good explanations of how WAAS works—of course, with an emphasis on aviation applications.
(Courtesy Federal Aviation Administration.)
At the risk of over-simplifying the WAAS system, here is my “low-tech” explanation:
At about 27 ground stations in North America, data is continuously gathered about the quality of the received signal from each GPS satellite that is in view of the station. The data gathered is not just differential position data, but also contains details of the satellite’s actual location versus its predicted orbit, several types of atmospheric correction data, satellite atomic clock variations from perfect timing, and a few other esoteric pieces of data.
All of this data is sent to several locations in the U.S. (for redundancy), consolidated and massaged, then transmitted up to one or both of the WAAS satellites. These satellites are in geostationary (so-called geosynchronous) orbits over the equator, meaning that they stay fixed in one location in the sky, unlike the normal GPS satellites. (The WAAS “satellites” are actually special hardware and software units installed on satellites used for relaying TV, radio, and telephone data.) From these WAAS satellites, the data is broadcast down to your GPS receiver, which uses it to give you more accurate position information. The WAAS satellites can also “pretend” they are regular GPS satellites, thus adding to the total number of GPS satellites you are receiving and further helping your position accuracy.
(A note to you more technical types out there: The FAA website
has all the technical details on WAAS you would probably ever want. However, you can find even more information by using a web search engine such as Google.)
So how accurate is a handheld GPS receiver with WAAS? No one number can describe the accuracy. Be suspicious of anyone (or any company) that gives you a number. So much depends on where you are on the earth, how “healthy” the satellites are that your GPSr is seeing, whether there are any obstacles near you, current atmospheric conditions, etc. etc. That being said, with really good satellite visibility (for example, if you are on a mountain top), and the GPS and WAAS systems working within specified tolerances, the FAA says you can expect accuracy of 1.5 to 2 meters (4.5 to 6 feet). Now, go into downtown Big City USA, or into that creek bottom with some tree cover, and could it drop to 10m (30-ft.) accuracy? You bet! Not only is poor line of sight a WAAS issue, it rapidly becomes a regular GPS satellite reception issue as well.
The “error” number your GPSr calculates and displays to you (variously called EPE, GPS accuracy, or some other term, depending on the manufacturer) is equally suspect. It is only a general guide. No one outside the GPS manufacturer’s engineering department knows how it is calculated, and they aren’t talking. I have sometimes thought that the manufacturer’s marketing department had a hand in the formula used to calculate the number. Maybe or maybe not—but take the error number with a very large grain of salt!
WAAS satellite not being received; GPS position not being WAAS corrected.
WAAS satellite #138 being received but not yet locked for navigation; WAAS data being downloaded; GPS position not being corrected.
WAAS satellites being received and GPS position being WAAS corrected.
The screen shots above were taken from a Delorme PN-20, which displays “WAAS” on the screen when WAAS correction is active. Other GPS receivers vary in how they indicate the presence of DGPS correction. Some even use the erroneous term DGPS (or put a “D” on satellite bars) when referring to WAAS.
An interesting personal observation about WAAS
Does WAAS always help? I don’t believe so. In ideal or near ideal conditions, WAAS certainly improves GPS accuracy. In less ideal conditions, maybe not. When my wife Julie and I created our Disneyland Resort virtual geocaches, we had to record GPS positions for over 30 locations in the parks, and 15 or so Disney benchmarks. I wanted to make my coordinates as accurate as feasible with the equipment I had, but many were difficult locations, screened from WAAS satellites by buildings. I found that when using WAAS on my Magellan Meridian Platinum and Garmin eTrex Venture, I got results on different days that varied up to hundreds of feet. Not good!
I finally discovered that with WAAS turned off (via a “secret” menu on the Magellan), my results steadied out, and varied only perhaps 10 to 15 feet—just what one might expect from a consumer GPSr. Why the difference? When I was using WAAS, some days I was able to get a good WAAS lock in the parking lot before entering the park. Other days, I couldn’t. With WAAS still turned on, I believe the unit was continuing to use WAAS corrections from the previous day—or even longer ago than that. But much had changed (atmospheric conditions, orbital effects, etc.), so the “corrections” were now creating errors!
Perhaps the problem was unique to those models of GPS receivers. I don’t know. I do know that turning WAAS off gave me better (more repeatable, reliable) results. Because of that experience, I recommend caution when trying to determine accurate locations in areas where WAAS reception is blocked by terrain or buildings. Be sure to get a good WAAS lock no more than a few hours before you take your readings—and then don’t turn the unit off! If you can’t do that, then leave WAAS turned off entirely.
In the next issue of Caching Now: DGPS expands beyond America’s coastlines, and what’s happening with GPS augmentations in the rest of the world.