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GPS Augmentation: Do you Need It? Part II
 
By Larry Klementowski
 
 
 
Part II: NDGPS, LAAS, and worldwide augmentations
In the previous issue of Caching Now, I discussed Differential GPS (DGPS) and the Wide Area Augmentation System (WAAS). This time, we look at the expansion of DGPS, another augmentation project by the FAA, plus initiatives to implement additional GPS augmentation systems—and even separate satellite navigation systems—around the world.
 
The U.S. Nationwide DGPS system
As the Coast Guard’s DGPS system evolved in the U.S. and more stations were added, it became obvious to various U.S. government agencies that nationwide coverage was a possibility. So other agencies soon got on the Coast Guard’s bandwagon. They included the U.S. Air Force, the U.S. Army Corps of Engineers, the Federal Highway Administration, the National Oceanic and Atmospheric Administration, and the Department of Transportation. A new overall project manager was created at the Research Innovative Technology Administration (RITA) under the Department of Transportation (DOT). Got all that? I used to be a program manager for some of those folks, yet that group even set me back a little. That’s a lot of horsepower and potential budget!
 
This group has continued to expand the number of DGPS ground stations, under the program name Nationwide Differential Global Positioning System (NDGPS).  NDGPS is currently operated by the U.S. Coast Guard (USCG) for the Department of Transportation (DOT). As of December 2007, 38 DOT-owned NDGPS stations were active. Together with the USCG-owned Maritime DGPS stations, there are a total of 86 active DGPS stations active nationwide as of December 2007. The DOT is also spearheading an upgrade to the system, called High Accuracy Nationwide DGPS (HA-NDGPS).  A handful of test sites  have been enhanced with the additional equipment needed to broadcast HA-NDGPS signals
 
(Courtesy U.S. Coast Guard.)
 
However, as NDGPS was ramping up, WAAS went operational, and suddenly some smart agency managers realized, “Holy cow! We’ve created two GPS augmentation systems! How did that happen?” So, for the U.S. government’s fiscal years 2007 and 2008 (FY08 began on October 1, 2007), the NDGPS budget (both for basic and high-accuracy stations) has been cut way back. Although funds remain from prior years for some new DGPS sites, construction for those is on hold as of December 2007. What will happen? Hard to predict. The whole situation seems to be caught up in politics and budgets. Both NDGPS and WAAS are operational, and will likely stay that way. There is a slight possibility that some inland DGPS stations could even be decommissioned if they are determined not to have a large user base. Note that neither system is primarily intended forrecreational users with handheld GPS receivers. We are secondary users of both, so decisions about them will likely be made on the basis of their importance to maritime and aviation users.
 
What are the trade-offs on the two systems? Here is a comparison I prepared that might be helpful. (considering North America only):
 

Feature
WAAS
NDGPS
Augmentation type
Satellite (SBAS)
Ground (GBAS)
Primary purpose
Aviation
Maritime
Signal locations
2 (in geostationary orbit)
86 and growing (nationwide)
Signal frequency
1.575 GHz (like GPS)
250–350 KHz (like beacons)
Types of errors corrected
Differential and other errors
Differential only
Signal viewing
Line of sight only
Not line of sight
Useful locations
U.S., Mexico, and southern Canada
Within 150–300 mi of station
Terrain masking
Approx. 30 deg. elevation at the U.S./Canada border
Not applicable within range of station
Tree cover effects
Can be significant if trees are leafed out
Not significant
Accuracy
(GPS alone <10 meters)
< 3 meters typical (95% probability)
1–5 meters (depending on distance from DGPS station)

 
 
WAAS “Cousins” Worldwide
With the U.S. WAAS successfully operational, other space-capable and scientifically advanced countries and regional coalitions (such as the European Union) wanted to have similar systems. Remember, WAAS is for North America only (yes, Canada and Mexico kicked in with the U.S., funding their own WAAS ground stations). Generically, these other initiatives are known as satellite-based augmentation systems (SBAS). As with WAAS, the primary purpose of these systems is for aviation navigation, with other uses (handheld, mobile, etc.) being secondary.So, you now have the following WAAS-like systems in various stages of development and testing:
 
  • European Geostationary Navigation Overlay System  (EGNOS). A good number of ground stations in Europe are operational, the three EGNOS satellites are up and running, and they are in the pre-operational testing phase.  When the whole system is operational, I believe that the EGNOS satellites will look just like WAAS satellites to your WAAS-capable GPSr. If you visit Europe in the near future and your GPSr receives EGNOS augmentation signals, you can use them, but because they are still being tested, don’t depend on them.
  • Japan’s Multifunction Transport Satellite (MTSAT) Satellite Augmentation System  (MSAS). MSAS is now operational. There have been some recent problems with the newer MTSAT-2 satellite, which might now be resolved. Your WAAS capable GPSr should receive valid augmentation signals in Japan, although the Japanese Civil Aviation Bureau says, “Please also note that non-aviation receivers using MSAS signals may NOT provide better positioning performance.” Not sure what to make of that; maybe legal issues…. In Asia outside of Japan, if you receive corrections, they will not be valid because of the lack of ground stations; keep WAAS turned off in that situation. For aviation aficionados, MSAS has some other interesting communications capabilities. (See bottom of that page.)
  • India’s GPS and Geo-Augmented Navigation System (GAGAN). Information is a little sparse, but this GPS augmentation system is supposed to be in testing phase now, and was at one time was supposed to be operational by the end of 2007 or 2008. By the lack of information on Indian government aviation and space websites, I tend to infer that it has been delayed.
 
A final word on WAAS outside of North America and its near off-shore areas: Unless you are sure that you can use one of the above WAAS “cousins,” and you are in its ground coverage area, you need to turn off the WAAS function of your GPS receiver. For example, if you are in South America (say, Brazil), you will have good line of sight to one or both of the WAAS satellites. However,there are presently no WAAS ground reference stations in South America, so the correction data you receive will not be valid for your location. In theory, your GPSr would know that, and not use the invalid WAAS data. But, on the several occasions I have had to try WAAS outside of North America (Japan pre-MSAS, Europe pre-EGNOS, and South America), the two GPS receivers I had with me (Garmin Etrex and Magellan Meridian) showed wildly different (by hundreds of feet) positions depending on whether WAAS turned on or off. I concluded that the position with WAAS off must be correct, and post-trip mapping proved that assumption to be correct. I wish I’d had a handheld GPSr with DGPS capability to compare, because I was often in range of maritime beacons.
 
Local Area Augmentation System (LAAS) is a ground-based augmentation system (GBAS) that the U.S. Federal Aviation Administration (FAA) is in the process of developing. If and when LAAS is completed, it will provide very accurate GPS-based guidance at selected commercial airports. These signals will allow airliners to land in very poor weather conditions, even including hands-off “auto-land” capability for newer aircraft. The capabilities of LAAS are being designed to complement and go far beyond that of WAAS. Even if you were within the 20- to 30-mile projected range of LAAS, it seems unlikely that handheld and recreational GPS users will ever have the need or capability to use it. If you are a high-end private or commercial pilot, you may get to use it someday, federal funding and politics allowing.
 
 
(Courtesy Federal Aviation Administration.)
 
Other GPS Augmentation Systems
Although there are quite a few other types of GPS augmentation systems, they are not very relevant to recreational users of GPS receivers. Many are strictly professional systems, often so costly ($30,000+) that only industrial or government users would ever consider them. Some also require paid subscriptions to use. A full discussion of the pros and cons of survey-grade augmentation systems is beyond the scope of this article, but would be welcome here in the future from a surveyor.
 
Here’s some information on a few current and planned systems.
 
  • Canada-Wide DGPS (CDGPS). This augmentation system is available free over all of North America,  courtesy of Natural Resources Canada and many provincial governments. Augmentation signals are transmitted over two geostationary satellites owned and operated by Mobile Satellite Ventures.  The technology is considerably better than WAAS, with sub-meter accuracy possible. It is not compatible with WAAS-capable GPS receivers, and requires either a separate L-band receiver, or a compatible GPSr with the proper receiver built in. Hardware looks scarce, and tends toward midrange mapping and survey-grade receivers that are very likely beyond the budgets of recreational users.
  • The Canadian Coast Guard’s Canadian Marine Differential Global Positioning System is, as its name implies, a DGPS system for coastal areas and major inland waterways. It is maritime beacon based.
  • The Australian Maritime Safety Authority (AMSA) has deployed a DGPS network of radio beacons around selected areas of Australia’s coast. You can view a coverage map on that same website. It is based on maritime beacons.
  • John Deere’s agricultural augmentation systems. These are very important to U.S. farmers, allowing accurate field plowing and harvesting, and even auto-steering of farming vehicles. Specific hardware is needed, and a paid subscription is required for some levels of service.
  • Real Time Kinematic (RTK) systems. These systems are currently available from John Deere,  Trimble, Septentrio, and Magellan. Strictly speaking, RTK is not exactly an augmentation system or signal. Rather it is a newer, much more complex method of using the signals from the GPS satellites to improve accuracy to potentially 5cm (2 inches), depending on the system used. Wikipedia has a good explanation of how RTK works, so I won’t try to do better here. Usually a survey or mapping team will set up an RTK base station that transmits signals to mobile handheld units, then use the handhelds to do the field survey work. In John Deere’s application, the mobile units are on tractors.
 
Next, here are some systems designed specifically for surveying and mapping that our surveyors and benchmark hunting folks might recognize. These systems augment the determination of a 3D GPS position by post-processing the data. That is, you gather readings from the location, then run them through special software that applies the augmentation data to them to make them more accurate.
 
  • Continuously Operating Reference Systems (CORS). Operated by the National Geodetic Survey, the CORS system provides data to professional GPS users that allows survey companies to post-process their field data. In a nutshell, a survey or mapping team gathers data at a fixed location for perhaps 24 hours, then downloads correction data from the CORS system. Using sophisticated software, the data is all “crunched” together, and the location being surveyed can be determined as accurately as 2cm (0.8 inches). Surveyors can get a “quick look” position after applying CORS corrections for a day or so, but for the best accuracy, CORS data from a full week is needed, so this process has delays built into it.
(Courtesy National Geodetic Survey.)
 
  • Online Positioning User Service (OPUS). Relatively new, this is an extension to CORS that allows users to submit their gathered GPS data to NGS for processing. Users then get their data back all “crunched” and ready to use. So OPUS makes CORS-level accuracy to smaller survey organizations that don’t have the necessary software to process CORS data.
 
Augmentation for Other Than NAVSTAR GPS?
Although NAVSTAR GPS is now arguably the only fully functional global navigation satellite system (GNSS), there have been other systems in the past, and newer ones are planned. The Soviet Union had a fully operational system at one time called GLONASS (it’s a Russian acronym I won’t even attempt to explain!), but after the fall of the USSR, the Russians had budget problems keeping it going. It is partially operable now, with about fifteen satellites,  although that number changes frequently. In partnership with India, there are plans to bring GLONASS back up to full capability. When it’s fully operable, GLONASS will be comparable to the U.S. GPS system. You can buy GLONASS receivers, and even combined GPS and GLONASS receivers.  Don’t expect a small inexpensive one, however, as they are generally larger professional-grade units, not inexpensive handhelds. Is there such a thing as Differential GLONASS? You bet!  Just don’t look for a GPSr with “DGLONASS” capability anytime soon from Garmin.
 
The Future of Global Navigation Satellite Systems
What is the future of satellite-based navigation systems? The United Nations Office for Outer Space Affairs (bet you didn’t even know that existed!) sponsors an organization called the International Committee on GNSS (ICG). A 2005 resolution by the U.N. General Assembly summarized the ICG’s goals as “to promote cooperation, as appropriate, on matters of mutual interest related to civil satellite-based positioning, navigation, timing and value-added services, as well as the compatibility and interoperability of global navigation satellite systems, while increasing their use to support sustainable development, particularly in developing countries.”
 
The initial members of the ICG, and their current and future systems, include:
 
Summary
So, now that you know more about GPS augmentation systems, do you need the ability to use one?
 
WAAS: If you have a handheld GPS receiver, it is very likely that you are already using an augmentation system—namely, WAAS. Hopefully you now know a bit more about WAAS, its capabilities, and its limitations. Use it wisely. If you travel outside of North America, I recommend that you keep up to date on the WAAS-like systems becoming available, and try them out if available. If not, don’t forget to turn off the WAAS in your GPSr when you’re outside North America.
 

DGPS: If you have an installed GPS receiver on your boat, you either already have a DGPS beacon receiver, or should consider adding one. Because of their cost and size, I suspect that beacon-based DGPS may be relegated to marine, mapping, survey, and similar industrial and vehicle applications, rather than recreational uses. (Garmin, for instance, recently discontinued its DGPS products in favor of WAAS capable units). But that could change in the future, so stay tuned!

Larry Klementowski (aka Klemmer or Klem) is an ex-USAF pilot, now part owner of Sekai Electronics, and is involved in various high technology video related projects, including some specialized GPS receivers for aircraft applications. He can often be found out geocaching (590 found, 46 placed) or bench mark hunting (278 found), usually while hiking or mountain biking.  Larry and his wife Julie are also dedicated Disney fans, often walking at Disneyland to check on their Downtown Disney and DCA virtual geocaches, and Berntsen's Disney bench marks!

Originally published on May 8, 2008

 

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