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.
"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.
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.
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
What sort of Corrections are applied via the WAAS signals to my handheld
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.
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
and WAAS tutorial