Survey Perspectives – July 2007

July 30, 2007  - By 0 Comments

EGNOS Embraces Ground Users

The European Geostationary Navigation Overlay Service (EGNOS) is Europe’s version of the U.S. Wide Area Augmentation System (WAAS) with essentially the same mission. It offers GPS users in Europe more accurate and reliably GPS positioning. Like WAAS, EGNOS focuses on improving the integrity and accuracy of autonomous GPS positioning primarily by modeling refraction caused by the ionosphere. Also, EGNOS was designed to be compatible with WAAS so your WAAS-enabled receiver will work with EGNOS just fine.

The system is similar to WAAS in that it’s an augmentation to GPS. It primarily consists of a network of ground reference stations (RIMS) and three broadcasting satellites.

This is done via the network of ground stations spread out over Europe as follows:

EGNOS

RIMS = Ranging and Integrity Monitoring Stations
MCC = Master Control Stations
NLES = Navigation Land Earth Station
PAFC = Performance and Assessment Check-out Platform
ASQF = Application Specific Qualification Facility

In a quick sentence, the MCCs process the data collected by the RIMSs and the NLESs send the processed data to the three geostationary satellites that rebroadcast to the users. Got that?

As I mentioned, there are currently three EGNOS broadcasting satellites:

-PRN 120 (Inmarsat 3 F2) located at 15.5 West Longitude.
-PRN 126 (Inmarsat 3 F5) located at 25.0 East Longitude.
-PRN 124 (Artemis) located at 21.5 East Longitude.

PRN 120 and PRN 126 are to be used by the general public.

PRN 124 is available to the public but designed for use by companies to “perform various tests on the system” according to the European Space Agency (ESA).

Generally speaking, the user needs line-of-sight visibility to either PRN 120 or PRN 126 in order to use EGNOS (there are a couple of exceptions I’ll mention later). There is a nice online tool at http://www.lyngsat.com you can use to determine approximately how far above the horizon PRN 120 and PRN 126 are in your area:

PRN 120 – http://www.lyngsat.com/tracker/inmar3f2.html

PRN 126 – http://www.lyngsat.com/tracker/inmar3f5.html

One major exception to the line-of-sight rule is something that significantly differentiates EGNOS from WAAS. The line-of-sight rule has been the major inhibitor for WAAS ground users. Some companies have used innovative methods to overcome this limitation, but EGNOS has taken it to another level.

EGNOS has embraced the ground users — something that WAAS has done poorly.

To address the needs of the ground users (engineering and scientific community), EGNOS developed Signal-in-Space through Internet (SISNeT). SISNeT is a method of distributing EGNOS corrections over the Internet instead of requiring line-of-sight visibility to PRN 120 or PRN 126.

What a great idea! WAAS really missed the boat on this one.

All users need is a mobile phone with a data plan (like network RTK) to connect to their GPS receiver and they don’t have to worry about EGNOS satellite visibility. Granted, if you’re using a consumer-grade receiver, this doesn’t make much sense. But if you’re using high performance receivers (1-2 meters) for mapping and you’re in an area where you won’t have consistent visibility to PRN 120 or PRN 126, this makes a lot of sense.

Like WAAS, EGNOS accuracy claims (1-2 meters) are a little ambiguous and generally on the conservative side.

Like WAAS, you need to be working inside the EGNOS IGP (Ionospheric Grid Point) area to take full advantage of EGNOS accuracy. Note that some receivers are designed to exploit EGNOS outside of the IGP area with some accuracy degradation, but your average consumer GPS unit won’t do this. The following graphic defines that area:

A year ago, the ESA declared that EGNOS was declared “fully-deployed” for “non-safety of life” applications such as mapping. This is the so-called Initial Operations phase of EGNOS.

Safety-of-life applications using EGNOS such as aviation navigation aren’t approved yet, but the news seems to be good. On June 28, 2007, the African aviation safety agency (ASECNA) signed up to cooperate with ESA to improve aviation traffic safety over the African continent.

The ESA has done a really good job of publishing detailed information on EGNOS. There is even a website referred to as EGNOS for Professionals that gets as technical as most people would like.

EGNOS web site – http://www.esa.int/esaNA/egnos.html

EGNOS For Professionals web site – http://www.egnos-pro.esa.int/

EGNOS SISNeT web site – http://www.egnos-pro.esa.int/sisnet/index.html

Lastly, I’ll leave you with some data collected last month. It’s always good to get a real field perspective. Through my grapevine, I was able to obtain data collected in Italy (June 2007) using EGNOS corrections. Enjoy.

Here is the graphic plot:

EGNOS

Here is the tabular data:

EGNOS EGNOS

This article is tagged with and posted in Newsletter Editorials, Opinions, Survey, Survey Scene
GPS World staff

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