IMPROVING GPS TRACKING PRECISION USING WAAS

INTRODUCTION:

WASS (Wide Area Augmentation System) is the next generation standard for implementing higher accuracy GPS systems. As demand for GPS continues to grow, the US government has implemented a more robust system to meet the needs of next generation tracking and improved reliability while still maintaining standard equipment in the field. Raveon Technologies has taken advantage of this new standard by adding WAAS to both their M7 GPS transponders and their industry leading RavTrack precision tracking solution.  

BRIEF HISTORY OF LORANC/GPS/DGPS/WAAS:

In an effort to support military and commercial aircraft, LoranC was the first implementation of modern GPS technology. Employing multiple ground based radar towers (sometimes compared to as bowling pins) typically near airports and larger cities, LoranC provided excellent positioning for its time as a method to keep track and position aircraft. The downside of LoranC was that it was less available in rural areas and completely unavailable over large bodies of water. LoranC was also not accurate enough to support extremely bad weather and non visual approaches.

GPS technology was next on the scene providing a 10x and greater improvement of positioning while solving the aforementioned approach problems. Using satellites and a low cost receiver approach, GPS took tracking from an expensive professional solution to a system that is now available in most cars, watercraft, and personal equipment.

Adding a differential mathematical approach, a specialized receiver, and land based FM towers, DGPS further improved positioning by another 10x. The downside of the approach is that it relies on line-of-site (to the towers) and provides spotty coverage in smaller towns and rural areas that don’t have the luxury of piggybacking on towers of local FM radio stations. Also of note is that a DGPS receiver is not compatible with a off the shelf consumer GPS unit.

WAAS is the latest approach and by far the most accurate. Using a combination of GPS satellites, ground stations, and geo-stationary satellites (see below for greater detail), accuracies up to less than 1M can be attained using standard off the shelf GPS equipment. With this type of spec, WAAS allows for new markets and solutions requiring a higher level of accuracy previously unattainable with existing solutions.

Some of these markets that Raveon Technologies WAAS enabled GPS transponders now support include vehicle collision avoidance, precision vehicle tracking and navigation, tighter and more specific geo-fencing, more accurate personal tracking, precision agriculture plotting, along with several other position dependent solutions.

TOPOGRAPHYOF A WAAS SYSTEM AND HOW IT WORKS:

WAAS is a combination of ground base stations and satellite transceivers used to greatly improve the accuracy and performance of conventional GPS. Conventional GPS relies on using satellite transceivers “only” which are subject to ionosphere disturbances (billows), satellite orbit errors, timing, and clocking errors. By augmenting ground based stations, the errors associated with GPS are greatly minimized if not eliminated while also adding monitoring and real time adjustment of the GPS satellites themselves.

The system installed in North America is as follows: 35 plus ground base stations called Wide area Reference Stations (WRS) along with the existing satellites provide correlation data that is sent to 2 Wide area Master stations (WMS) on each coast to create a corrected signal. This corrected signal is then retransmitted by a geo-stationary satellite called a Ground Uplink Station  (GUS) that is fixed at the equator.

The system provides for guaranteed accuracy of  3 meters or less with typical accuracies approaching sub 1 meter (multiple samples may be accumulated to achieve even better specs). Adding to the accuracy improvement, uptime approaches 99.999% with a downtime of 5 minutes per year while conventional GPS can only guarantee 4 days per year of downtime.     

SPECIFICATIONS OF WAAS VERSES STANDARD GSP, DGPS, AND LORANC:

The following is a table of specifications and observed lab measurements of the various positional methods used in the US. 

TYPE Horiz. Vert. Comments
       
WAAS 7.6M 7.6M Specification
  .9M 1.25M As measured
       
DGPS 10M 10M Specification
  1.6M 2.1M As measured
       
GPS 100M 50M Specification
  2.5M 4.8M As measured
       
LORAN C 460M 460M Specification
  55M 55M As measured

As can be seen from the table, Raveon Technologies WAAS enabled transponders provide the most accurate tracking and positioning available while employing standard GPS gear. DGPS comes in at second, but as discussed, is only employed and is dependable in cities and urban areas where FM towers are available and line of site can be attained using the specialized DGPS gear. Standard GPS will continue its success in supporting the large consumer but lower precision auto, marine, and personal tracker market. LoranC, although the least precise, has a large installed base in the personal and small aircraft segment. 

Below is a comparison of the position error of a GPS tracking system using WAAS augmentation and DGPS.  In this test, the horizontal accuracy was very good with each method. The WAAS system had much better verticle accuracy. 

CONCLUSION:

By implementing a WAAS GPS M7 or RavTrack system from Raveon Technologies, precision accuracy approaching sub 1 meter is attainable and repeatable using existing GPS receiver systems allowing for new applications and end-products in the positioning and tracking space.