Article by Mike Barrett and European Space Agency

Introduction

I have now completed a couple of reviews recently with a few in progress with GPS receivers that are WAAS capable.  In particular the Garmin GPSMAP 76 and the Emtac BT GPS (work in progress).   There is currently a lot of controversy over the usefulness of EGNOS in Europe and the validity of using WAAS capable receivers in this mode.

Having scanned the Internet and not found a definitive answer I decided to put the question to the team at the European Space Agency and find out if using EGNOS, when available, is a valid thing to do.

What is WAAS ?
Firstly for those of you who are not clued up yet, WAAS stands for Wide Area Augmentation System.  It's a system of geostationary satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy than currently available with the standard Global Positioning Systems.

How much better?  Try an average of up to five times better.  A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time, and even better you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS if your receiver already supports it. 

You can find out more about WAAS at http://www.garmin.com/aboutGPS/waas.html

OK so WAAS is a good thing, however currently it does not come configured as standard.  You need to go into the configuration section of your receiver and enable WAAS.

Once enabled you need to start the receiver with a good view of the sky and you should see your GPS fixes change into Differential fixes.  These may not be reported in some software packages or tools, for example Tom Tom GPS status screen does not provide this info, but OziExplorerCE does, and so does the Satellite page on the Garmin GPSMAP76.

Where does it Work?
This is the question causing a little controversy at the moment.  WAAS definitely works in North America and has been live now for a while.

If you live in Europe the implementation of WAAS is known as EGNOS (European Geostationary Navigation Overlay Service ).  EGNOS is currently in test mode with live operation planned from April 2004.

So what does this mean to all of us who have WAAS capable receivers?  Do we get better fixes if we use EGNOS? Is EGNOS reliable?  If we get a fix using EGNOS is it trustworthy?  What is the coverage of EGNOS?  I put these questions to the EGNOS Test Bed team at the European Space Agency in an effort to determine the facts of the matter. This is their response:

Dear Mr. Barrett,

 

Thank you very much for contacting the ESTB Helpsdek E-mail . Please note the following information .

 

As you probably know, today we are transmitting a pre-operational EGNOS signal through the EGNOS test bed (ESTB).  This signal is a test signal so it is not a guaranteed service signal.  In reality, however, the performance of this test bed is quite good achieving accuracies of the order of 1-2m in core of Europe. You can verify the cover area on the web page :

 

http://www.esa.int/export/esaEG/ASEJA9UG0SC_estb_1.html#subhead2 or in the attached document showing EGNOS service area ( ECAC area ) .

 

Both EGNOS and WAAS systems are designed against the same International ICAO standards and RTCA MOPS.  When operational, both WAAS and EGNOS receivers will be interoperable, and, therefore, a receiver able to process WAAS will also process EGNOS and vice versa.

 

Today, however, EGNOS test bed (EGNOS mock up, the ESTB) and WAAS systems are currently under testing and as such are forced to transmit the message type 0.  There are certain basic differences in the way message type 0 (again a mandatory message while testing) is generated in EGNOS or WAAS, which have resulted in a number of manufacturers not implementing both solutions, which are, according to RTCA MOPS DO 229C, possible (see note below).

 

This introduces some differences in receivers behaviour and performance.  When the systems become operational for safety of life, message type 0 is not sent anymore and this difference will disappear.

 

EGNOS Operational readiness review is today planned to take place in April 2004.  After this date, EGNOS will conform to EGNOS specifications and will start nominal operations.  ESA and the European Commission are currently discussing the potential future operator so that early introduction of the EGNOS service is efficiently performed.

 

For further information please do not hesitate to contact the ESTB Helpdesk .

 

Best Regards

 

European Space Agency
ESTB Helpdesk

 

This is the coverage area for the EGNOS system at the moment.

The effective area is the entire Western Europe and some of the western parts of Eastern Europe.

Click on the Map for a larger version of the graphic. Graphic provided courteousy of the European Space Agency.

Conclusion
It looks as if the EGNOS system is valid to be used for correcting GPS signals, but until April 2004 there is no guarantee as to the availability of these signals.

If the EGNOS data is available then it will refine your positional data and potentially refine your position to within 1 to 2 metres.  However, until 2004, you must not rely on EGNOS data being available.

References

ESA Website

http://www.esa.int

Pocket GPS Contributor

Mike Barrett

Pocket GPS Contributor Website

http://www.bwww.co.uk

Comments ?

Have any comments about this review ?  Post them here.