WAAS is an acronym for Wide Area Augmentation System which is an aid to GPS navigation for aircraft in the USA implemented by the Federal Aviation Authority. A general description of WAAS can be found at these web sites: Garmin: What is WAAS? and Wikipedia: Wide Area Augmentation System. WAAS improves accuracy by correcting for errors in the satellite's clock and orbit, and for ionospheric (upper atmosphere) errors.
WAAS In Canada
As WAAS is designed for aviation use in the US it's ground performance in Canada and elsewhere is limited by certain factors. The position of the geostationary satellites over the equator is problematic in Canada as this limits their reception in the higher latitudes. The original WAAS satellites were visible only be a few degrees above the horizon where topography and vegetation can easily block the signal reception. Additional geostationary satellites put into service in 2007 improved coverage in the higher latitudes.
Another factor is the distance from the ground reference stations which collect the data used to generate the corrections transmitted by the WAAS satellites. The relationship between distance from a ground reference station and being able to get valid WAAS corrections is not well documented on the web. The corrections for ionospheric error are dependent on distance from a ground reference station but exact distances are not stated.
In April 2003 the FAA and Canadian officials drafted an agreement calling for the placement of up to four WRS sites within Canada, which would expand WAAS service across most of Canada and is expected to be complete by 2005 (source FAA SatNav News). The expansion in Canada would see the installation and operation of WAAS stations at Winnipeg, Goose Bay, Gander and Iqaluit. With the current stations in the lower 48 and Alaska, WAAS could serve most of the western provinces, southern Ontario and a small area in south-western Quebec. The addition of the 4 stations in Canada extended coverage to the eastern provinces to about 55N latitude while the addition of more stations in Alaska extended coverage into the Yukon and western part of the NorthWest Territories.
The new stations were integrated into the WAAS system on 2007/09/28 (source FAA) (also FAA SatNav News PDF). Their locations are indicated in this figure showing the WAAS Wide-Area Reference Stations (WRS) Locations and are summarized here.
In the fall of 2005 two additional geostationary satellites were launched which will transmit WAAS correction signals. One of these is Galaxy 15 initially at 133.0°W(LyngSat SatTracker) which is seen as 48 on Garmin receivers. The second of the new satellites is Anik F1R at 107.3°W (LyngSat SatTracker) which is seen as 51 on Garmin receivers. Both are much higher in sky than the previous WAAS satellites (27° vs 6° at my location).
In addition WAAS satellite AOR-W (Garmin #35)(LyngSat SatTracker) was moved 54°W to 142°W over February and March 2006. This move left the northeast USA and eastern Canada temporarily without WAAS coverage in 2006.
The status of the new GEO satellites is detailed in this file New_WAAS_Geo_Status.pdf. The final update (13 July 2007) is shown below.
New WAAS GEO Status as of 7/13/07 (final update because GEO are operational)
INTELSAT(PanAmSat), Galaxy-15, W133 deg, PRN -135 (48)
PRN-135 was taken out of "Test Mode" and placed in normal mode at 08:00 UTC on 11/9/06. PRN-135 ranging quality is set to "not monitored". PRN-135 ranging integrity will be improved to NPA quality, then PA quality as operational experience is gained and planned WAAS software upgrades are fielded during the 2nd half of 2007
Telesat, ANIK-1fR, W107.3 deg, PRN -138 (51)
PRN-138 was taken out of "Test Mode" and placed in normal mode at 15:21:00 UTC on 7/13/07. PRN-138 is operating as a NPA quality ranging source. PRN-135 ranging integrity will be improved to PA quality as planned WAAS software upgrades are fielded during the 2nd half of 2007.
It appears that the northeastern US and the maritimes in Canada are totally covered now that Telesat, ANIK-1fR, W107.3 deg, PRN -138 (51) is operational as shown on the WAAS GEO Footprint map, the FAA Non-Precision Approach (NPA) Navigation Service map and the WAAS Satellite Status map.
Another display of the Vertical Protection Level also includes most of Canada in the service area. Note that the vertical accuracy of a GPS fix is commonly stated as half the horizontal (vertical error is double)
The image to the right is a screen shot of my Garmin MAP76C on 2006/05/06 showing WAAS satellites 48 and 51. Elevations were 27° and 28°, respectively and neither were transmitting ranging signals at the time as indicated by their hollow signal strength bars. The "D"s on the signal strength bars indicate that correction signals were being received from one or both of these WAAS satellites.
The increased elevation and signal strength of these two new geo's bode well for improved WAAS reception at the higher latitudes.
Useful WAAS status links: Service Monitoring Subsystem - FAA Tech Center, WAAS Satellite Status map, WAAS Satellite Corrections, WAAS GEO Footprint map and the Satellite-Based Augmentation System Office News (link).
Note that there are two numerical designations for each WAAS satellite. PRN numbers identify the code transmitted by the satellite while the NMEA designation is simply the PRN minus 87. Garmin uses the NMEA number.
The current WAAS satellites servicing Canada are summarized in the table below and in this GoogleMap showing elevations and azimuths to the satellites from selected cities across Canada.
| WAAS /
|PRN #||Longitude||Satellite Name|
|** 46||133||98.0°W||Inmarsat 4F3|
* WAAS satellites 35 and 47 were taken out of service on 2007/07/30 (link)
** WAAS satellite 46 became operational in mid November 2010 (link)
GPSr Firmware and WAAS
The WAAS system generates ionospheric corrections for points on a grid covering most of the surface of the earth. A WAAS enabled GPS receiver receives this data and interpolates the appropriate correction based on it's location in relation to the nearest grid points. This figure shows the global ionospheric grid points while this FAA link shows the current grid point status for North America.
Each WAAS satellite only transmits iono corrections for a part of the earth's surface. The latest firmware on some recreational GPSr's excludes using WAAS corrections when outside of a WAAS satellite's service area. To quote Garmin's software update page " A unit will not use a WAAS/EGNOS satellite if the unit's current position is outside of a given WAAS/EGNOS satellite's service volume." In Dale DePriest's DGPS and WAAS Information Page he states "If you receive a satellite but do not have any ionospheric data for your area the Garmin receivers with the latest firmware do detect this condition. They will not lock on a WAAS SV outside its coverage area. I think they should lock if no other WAAS capable SV covers the area and use the corrections for clock and ephemeris available while applying the internal ionospheric correction algorithm in this case but they don't seem to be this sophisticated." This means that when iono corrections are not available the clock and ephemeris corrections are also not applied.
Dale's site also has a good description of how the total WAAS system works and how to setup Garmin receivers to use it.
Garmin and Magellan also differ in how they operate after a loss of signal from the WAAS satellites. Garmin appears to only apply corrections for a couple of minutes while Magellan will keep applying the corrections for 50 minutes or longer. Which of the two methods is correct is open to debate. To quote Dale again "Clock errors can change rapidly so this data is updated every minute if required, ephemeris errors and ionosphere errors don't change nearly so fast so they are only updated every 2 minutes and can be generally be considered valid for up to 3 times that period of time. Even this time is very conservative in practice."
There are a lot of accuracy claims from users in the newsgroups and web forums based solely on their unit's EPE (Estimated Position Error) or Accuracy readings which are not really valid. A GPS receiver can only estimate the error based on a calculation that is usually biased by the manufacturer to appear better than it actually is. To get a true measure of how accurate a receiver is it has to be measured against a known point. In the survey world these known points are called benchmarks.
The tables below show the actual errors of my Garmin GPS76 when collecting waypoints on survey benchmarks. The waypoints were not averaged. The second table shows the best results obtained during each visit.
Errors were calculated by comparing the coordinates of the benchmark to my waypoints using FizzyCalc. The latest version of FizzyCalc (January 2006) will also give you the azimuth and elevation above the horizon of the WAAS satellites.
Please note that this data should not be considered as representative of normal field conditions. In my experience WAAS is only usable with a clear open view of the sky to south so that a GPSr can receive the correction signals from the geostationary satellites in orbit over the equator.
Average Horizontal and Vertical Errors in Metres GPSr Waypoints GPSr IA H Error V Error GPS 76 50 3.5m 2.0m <4m
Minimums of Horizontal and Vertical Errors in Metres yymmdd GPSr Benchmark* D GRS W Sat(s) #G Sats GPSr IA H Error V Error 031026 GPS 76 CBN A351148 742 km 35 & 47 ? 2.6m 0.9m <1m 031107 GPS 76 CBN A351148 742 km 35 & 47 8 2.8m 0.9m <4m 031222 GPS 76 CBN A351148 742 km 35 & 47 9 2.8m 2.2m <3m 031222 GPS 76 53116-253 743 km 35 & 47 8 3.7m 0.0m <1m 040120 GPS 76 53118-73 755 km 35 & 47 10 2.9m 1.9m <4m 040212 GPS 76 CBN A351148 742 km 35 9 2.7m 1.0m <3m 040217 GPS 76 CBN A351148 742 km 47 8 5.0m 1.3m <3m 040223 GPS 76 CBN A351148 742 km 35 10 2.7m 1.0m <1m 040329 GPS 76 CBN A351148 742 km 47 8 3.6m 0.6m <2m
yymmdd - Date as Year Month Day
GPSr - Model of GPS Receiver
Benchmark - Name of Survey Benchmark
D GRS km - Distance from the Nearest WAAS Ground Reference Station
W Sat(s) - WAAS Satellites
#G Sats - Number GPS Satellites
GPSr IA - Accuracy Indicated By the GPS Receiver or EPE (Estimated Position Error)
H Error - Calculated Horizontal Error or Difference between the Waypoint and the actual position
V Error - Calculated Vertical Error or Difference between the Waypoint and the actual position
*Benchmark data in Canada is available at NRC - Geodetic Survey Division - Online Data.
WAAS Information Links
FAA Web Site - WAAS FAQs
FAA Satellite Position and WAAS Status
FAA WAAS Satellite Corrections
FAA WAAS GEO Footprint
FAA Non-Precision Approach (NPA) Navigation Service Display
FAA Vertical Protection Level Display
Garmin: What is WAAS?
Trimble: Advanced Concepts - Augmented GPS
GPS Information.net - WAAS and its Relation to Enabled Hand-Held GPS Receivers
GPS Information.net - GPS WAAS Operation:Your questions Answered.
Dale DePriest's DGPS and WAAS Information Page
Wikipedia: Wide Area Augmentation System
SXBlueGPS.com: SBAS (Space Based Augmentation Systems) Made Easy
PRN 122 NEMA 35 AOR-W-move.1.mov (cylindrical)
PRN 122 NEMA 35 AOR-W-move.2.mov (spherical)
...and that's all