GPS WAAS Operation:  Your questions Answered.
by Joe Mehaffey
(revised: 28 July 2002)

My GPS has WAAS  (Wide Area Augmentation Service) but it does not seem to be improving accuracy.  What gives?
Like many engineering projects in the development phase,  the current state of WAAS (with only two fully operational, and as yet still experimental, signals available) is in a state of flux and will be for many years to come.  There are many things to consider with the present system:

1) WAAS ground stations send GPS tracking information to a master control site.  The master control site uses this information to compute
        a)      Long term satellite position error
        b)      Short term and long term satellite clock errors
        c)      IONO correction grid
        d)      Integrity information

The above information can only be computed by the master control site based on all of the observations from many ground reference stations.  The information is relayed to the WAAS GEO satellites and is re-broadcast to user receivers.  Note: The WAAS correction information is very different than RTCM corrections because WAAS decomposes the errors into their primary elements (position, clock, & IONO).  RTCM, (DGPS) on the other hand, broadcasts pseudorange corrections which are the sum of all error sources as observed by the RTCM reference station.  This is why spatial deceleration is such a big factor for RTCM, but is not for WAAS (thus the reason it is "wide area" augmentation).

The WAAS integrity information is available for any GPS receiver to use.  It includes integrity information for the WAAS system, individual GPS satellites, and various error estimates & degradation factors for the correction information broadcast by WAAS.

2) For GPS position to be improved by WAAS signals,  a handheld GPS must first be able to receive one of the WAAS satellites with excellent signal quality as a lot of data must be regularly received by the handheld from the WAAS satellite.

3) For GPS position to be improved by WAAS signals,  a handheld must be within a region serviced by one of the correction stations being monitored by the master control site.  This might be somewhere in the range of 500+ miles from one of the correction stations.

 Garmin says:
"The IONO information transmitted by the WAAS system is much more accurate than the basic GPS IONO model. Also, the WAAS system will generally be more accurate than beacon based DGPS because of the way the corrections are rendered by the WAAS system and applied by the GPS receiver.  The primary factor is spatial deceleration, which is the degradation of corrections due to separation from the reference station.  RTCM based DGPS corrections suffer from spatial deceleration to a large degree, but WAAS corrections to a much smaller degree."  (Note:  Spatial Deceleration means:  If you go "far" from the reference station, your correction information is no longer useful.

This does NOT mean that you can be in England, Mexico,  Canada or any other location  very distant from the WAAS correction stations  and expect the USA WAAS corrections to be useful even if you can receive the WAAS satellites.  In fact,  if you are in Europe,  Africa,  Australia or anywhere more than perhaps 500 miles away from the shores of the USA,  the use of WAAS signals may well REDUCE the accuracy of your GPS fixes.  Europe's EGNOS system (compatible with WAAS but for operation in Europe) is in the test phase now and is transmitting a "do not use" flag.  Magellan GPS receivers ignore this flag and use the signal anyway (for what it is worth),  but Garmin receivers honor the flag's intent and will not use the EGNOS data as long as the "do not use" flag is set.


OK:  But if I can hear the satellites and the correction station is too far away to give good corrections,  Why does the GPS not "know" this and just not let WAAS work?
Answer:  With just two satellites,  one could say:  a) If you can hear the signal you just about HAVE to use it if you want to use WAAS at all and b) Since there are many areas of the USA which (at best) have intermittent signals,  the USER must use some intelligence as to if WAAS is useful to him at all.  Perhaps when the WAAS system is out of the development and experimental stage and many more WAAS satellites and correction stations are installed,  it will be possible for a GPS to "know" when a WAAS (or EGNOS) correction signal is useful.  But that is not the present situation.

We must realize that WAAS was not designed for consumer GPS users,  rather it was designed for aircraft use.  The aviation receivers will have a LOT of integrity monitoring logic which may never be implemented into handhelds because of the complexity and cost.


My GPS has WAAS but it will not work when I operate the GPS in "power save" mode.  Why is that?
Answer:  WAAS requires a lot of digital data be received by your GPS.  To do this,  the RF receiver section of your GPS must be operating full time.  Power save mode turns off power to your RF receiver for about 4 seconds out of 5 when GPS signals are stable and WAAS cannot function with this limited receiver ON time.

How much improvement in accuracy can I expect when WAAS corrections are working properly?
Answer:  This is difficult to say with any certainty.  We have observed that MOST OF THE TIME WAAS appears to improve averaged accuracy at a fixed location for maybe 5 minutes averaging from about 7 meters to perhaps 3 meters.  Sometimes there has been no improvement and a few times (when WAAS signals were not of good quality or when we were far from a WAAS correction station) the measurements have actually had more error with WAAS than without.

The upshot of this is:  WAAS is a tool for casual GPS users.  It is not guaranteed to work,  it is not guaranteed to increase accuracy,  it is not really able to tell you when it will improve accuracy,  when it will have no effect,  or when it will make for less accuracy than the normal GPS signal. Under good conditions when YOU ARE OUT IN THE CLEAR AND RECEIVING A GOOD SIGNAL FROM THE WAAS SATELLITE,  you should get both improved accuracy and improved position stability.  YOU must insure that you have the proper conditions so as to experience the improvement at YOUR location.

This is just the way it is and remember:  If you do not like the current limitations of WAAS there is a very simple option  -Turn it off.


Where can I find information on the USA ground stations used for WAAS (WEDGES) corrections?  Answer:  Stanford University.

What sort of Corrections are applied via the WAAS signals to my handheld GPS?
Answer: The satellite broadcasts 3 kinds of corrections.  Clock corrections, ephemeris corrections, and ionosphere corrections.  Only the last one is dependent of where you ground stations are but  this is the largest source of error that the WAAS ground station corrects for. 


OTHER ARTICLES ABOUT WAAS AND HANDHELD GPS RECEIVERS

WAAS and its Relation to Enabled Hand-Held GPS Receivers

GPS Augmentation:    WAAS and LAAS... How does all this work?

Wide Area Differential GPS Laboratory at Stanford University

Dale's DGPS and WAAS tutorial


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