DIFFERENTIAL GPS

(...continued)

Magellan use of WAAS

Magellan uses WAAS in a similar way to the description of Garmin use given above. However they do not have a menu item to turn WAAS on or off. There is a secret menu entry that can do this. Turning off WAAS frees up some channels for regular satellite use and should certainly be performed if you are outside a WADGPS coverage area. As already mentioned Magellan does not display the WAAS satellites on the satellite status screen so there is no indication on the unit whether WAAS is turned on or off unless you are actually receiving WAAS signals. When you are receiving WAAS the epe indication changes to show WAAS and when averaging a position the WAAS indication is present. Note that the unit will continue to use WAAS corrections long after the signal is lost. Some report that corrections will be used for 50 minutes or longer as indicated by the WAAS averging indication. (It is not clear from the report that WAAS data is still being used or it is just a software bug.) If you power the unit down and back up the use of this stale data will be discontinued.

Internet Corrections

WAAS/EGNOS systems rely on geostationary satellites to supply the correction information. Unfortunately these satellites are not always visible or usable to GPS receivers in urban canyons or high latitudes. To solve this problem the egnos group has developed an internet solution called SisNet which provides the correction information via the internet. A wireless GPS system such as one that is attached to a wireless enabled PDA could collect the information from the internet and then supply it to a regular GPS using standard DGPS correction techniques. In this use a WAAS/EGNOS capable receiver is not required but a specific software implementation would be needed.

For standard beacon receiver corrections you might be able to use Wolfgang's DGPS or for a commercial solution check with Networked Transport. This NTRIP product offers both server and client software for various platforms.

Local Area Augmentation System

LAAS is similar to both the beacon approach and the WAAS approach. It uses a separate transmitter that operates in VHF frequencies and then sends the update data to a compatible gps receiver. It includes both increased accuracy and increased error detection over that obtained from either the gps receiver or WAAS and is accurate enough to provide data for use in landing a plane. Unlike the other systems mentioned the very local nature of LAAS means it is likely only going to benefit aircraft and will not be effective for other users.

Since the LAAS reference station is expected to be close to the plane it can correct for most gps error sources. It even tackles the issue of multipath errors by integrating the data from several (up to 4) ground stations and providing a single correction package for pseudorange data. Troposhperic errors should be common between the site and the plan so corrections include this data. Similar to the methods used by WAAS the LAAS system has built in error detection as well as correction and can reject a solution from a bad satellite.

The focus of LAAS is to provide a standalone system that is separate from the gps receiver (but likely sharing the same external antenna). It has a display that mimics the atandard aircraft display and it is focused on landing data. However there is no reason the LAAS data could not be reduced to pseudorange correction data in a format identical to the beacon receiver format and then used with a standard gps receiver identically to the way dgps corrections are done today. To make this work the gps would need to allow the RTCM correction data to override the WAAS data. It will be very accurate correction data and provide a backup to the LAAS hardware.

Commercial Solutions

In addition to the Augmentation systems listed above there are also systems designed and built by commercial vendors. These are usually referred to as WADGPS systems. These vendors are not just retransmitting government correction signals but have their own ground stations and satellite links very much like the WAAS/EGNOS system. However the corrections generated are more like the beacon dgps system described above in that they send full corrections for pseudo range data. Customers include farmers needing precision positioning to provide tractor guidance, crop dusting, and some surveying activities.

One such system is Omnistar. This system can provide dgps correction for a standard GPS receiver but for highest accuracy they require that you use a full standalone GPS implementation. They, of course, make and sell such a system but systems can also be obtained from NovAtel.

Another competing system is from NavCom using the John Deere Starfire system.

The information in the WAAS section is derived from the FAA technical specification, available from the FAA web site, the European EGNOS sites, other sources, and personal observation. The US Coast Guard web site provided some of the information in the beacon section. Other sites used for reference are indicated in the text. The LAAS data is also from the FAA site. I have no direct experience on LAAS.

by Dale DePriest - all rights reserved

Preliminary post 01/04/24
Updated slightly 01/04/30.
A few more updates on 01/05/16
Some more LAAS stuff. 01/6/16
added US and Europe Maps and updates some data 02/1/12
added some Garmin observations 02/07/18
added some commerial links 03/04/04
added more EGNOS data 03/06/27
updated EGNOS map and WAAS discussion based on latest Garmin firmware release 03/08/08
added some Magellan stuff 03/11/22
added Garmin - satellite table 04/02/14
added internet section - 04/02/28

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