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I've put together a resource website for UK and Ireland cachers. It includes a page about WAAS/EGNOS and GPS's. Designed to be easy to read and not too technical I hope it explains how things work. Please note it is written for the UK/Europe region, in the USA it works the same way but of course with different WAAS geostationary satellites.




Volunteer UK Reviewer


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Sorry to bring up this old thread but I have a question that is relevant to it.


Regarding WAAS. Is it normal that my Colorado tries to lock to sat #33 and that I live in New Brunswick Canada?


For looking at some webpages SAT 33 seems to be an EGNO rather than WAAS sat, but the fact is that eventually the other listed sat's will get the little "D" on their graph with sat#33.

So I am confused with this?!


Now the funny part is that if I go into the menu-setup-system and change the gps mode from WAAS to normal to Waas again then the unit will try to lock to sat#51 (which seems to happen faster as well)

It's weird.

Edited by ZeMartelo
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If you have WAAS/EGNOS enabled the satellite on the far right of the satellite display will be the "WAAS/EGNOS" satellite. It should be one of the number above, which if you are in Utah is probably going to be 48.




In this shot I'm using 51, typical for the east coast.



48 and 51 WAAS sats both cover nearly all of the US as shown in this real time footprint (48 in black, 51 in brown)


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I would also like to add why no one ever mentions the ground stations for WAAS?


From Garmin



You've heard the term WAAS, seen it on packaging and ads for Garmin® products, and maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. 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 you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.


The origins of WAAS


The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.


How it Works


WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.


Who benefits from WAAS?


Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.


Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.


It just keeps getting better




100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.


15 meters: Typical GPS position accuracy without SA.


3-5 meters: Typical differential GPS (DGPS) position accuracy.


< 3 meters: Typical WAAS position accuracy.

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Here is the PRN/Satellite ID information for WAAS and EGNOS

NOTE: The satellites IDs are the PRN numbers less 87.


The following PRNs have been allocated to the WAAS system:

Geostationary PRN NMEA Satellite ID


AOR-W 122 35@

Anik 138 51*

POR 134 47@

PanAm 135 48*


The following PRNs have been allocated to the EGNOS system:

AOR-E 120 33

Artemis 124 37

IOR-W 126 39

IOR-E 131 44


The following PRNs have been allocated to the MSAS system:

MTSAT-1 129 42

MTSAT-2 137 50


@Phased out July 30th, 2007

*New WAAS satellite


One can readily determine the elevation and bearing of these satellites from your location from these URLs:

The below are listed from west to east.


Name GPS Series Location

POR #47 3F3 Pacific Ocean at 178.0°E@



AOR-W #35 3F4 Pacific Ocean at 142.0°W@



PanAm #48 Galaxy 15 Pacific Ocean at 133.0°W*



Anik #51 F1R Pacific Ocean at 107.3°W*



Inmarsat #34 4F2 Atlantic Ocean at 53.0°W



AOR-E #33 3F2 West of Africa at 015.5°W



IOR-W #39 3F1 Indian Ocean at 064.0°E



I do not know why it is trying to lock on to that EGNOS 33 but here is a list of what it is.

Here is the link to the above.

WAAS Description



I did a little more checking on 33 and it appears from where you are that it is the one that can be seen above the horizon.

The linked maps are interactive and you can click on them to set your location for each bird to see if it is visible to your area.


Hope that helps.

Edited by GEO*Trailblazer 1
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