Recreational boaters today in North America typically have GPS receivers that can utilize augmentation signals provided by the Wide Area Augmentation System (WAAS). Use of WAAS for enhanced position fix precision is very common in recreational marine chart plotters with GPS receivers, but the details of the system are not particularly well known. This article attempts to provide more information about WAAS for the recreational boater who would like to have further understanding of what it does, how it does it, and who provides it.
WAAS was really designed for use by aircraft and was created by the Federal Aviation Administration as a means of allowing aircraft to best utilize GPS signals for navigation. At the outset of the system design there were three primary concerns:
To have integrity, the system must warn users when its data are not usable. To have reliability, the system must be available at all times. To have accuracy, the system must enhance the performance of the GPS system accuracy.
WAAS signals were designed to be very compatible with normal GPS ranging signals, and are transmitted on the same frequencies. This simplified the design of GPS receivers that can utilize WAAS signals. WAAS signals were designed to:
WAAS SBAS signals are sent from satellites in Geostationary Earth Orbit (GEO). These orbits have much higher orbital altitude, so the satellites are farther away than the usual Medium Earth Orbit (MEO) satellites in the GPS constellation.
The signals are created by specialized "hosted payloads", added to a space vehicle of a GEO satellite operator who will launch the satellite into a favorable locations to provide coverage. The use of hosted payloads is common in modern satellite business. The term is explained as follows:
A hosted payload is a portion of a satellite, such as a sensor, instrument or a set of communications transponders that are owned by an organization or agency other than the primary satellite operator. The hosted portion of the satellite operates independently of the main spacecraft, but shares the satellite's power supply, transponders, and in some cases, ground systems. (Cf.: http://www.hostedpayloadalliance.org/)
For example, the WAAS SBAS signal with PRN 138 is being send from a hosted payload on an Astrium Eurostar-3000S platform space vehicle. It was launched from the BAIKONUR COSMODROME in Kazakhstan on September 9, 2005, and was carried into orbit by a Russian-built Proton Breeze M launch vehicle. The launch services provider was International Launch Services, a joint venture of Proton builder Khrunichev State Research and Production Space Center, and Lockheed Martin, builder of the Atlas launcher. ILS is based in McLean, Virginia. The space vehicle was maneuvered into a GEO at 107.3-degrees West longitude, and has an expected lifespan of 15 years. Its operator is Telesat, headquartered in Ottawa, Ontario, Canada, and they refer to it as ANIK F1R. "Anik" means little brother in Inuit. But since none of this is of any importance in terms of its role in the WAAS SBAS, in GPS and WAAS technology the satellite is just referred to by its PRN (138) and, secondarily, by its longitude (107.3 West). A GPS user with WAAS never sees anything about the satellite other than its PRN, its look angle, and perhaps its signal strength.
The actual device aboard the space vehicle providing the WAAS signal is a special transponder that typically operates in what is called "bent-pipe mode." (Bent-pipe refers to a technique used in satellite communication in which signals are relayed by a satellite in a manner which is like sending the signal through a bent pipe: the signal, which normally would travel in a straight line, is received by the satellite and then retransmitted from the satellite in a new direction.) The actual signal to be sent from space is uplinked to the satellite from a ground station at C-band, then frequency translated by the transponder to the L-band frequency of the GPS system (1575.42-MHz). (It is also possible that the transponder could have its own atomic clock and operate in "autonomous mode", generating its own ranging signals.)
The power of the WAAS signals at a typical location will generally be at a lower signal level than GPS signals. The expected WAAS signal strength will be from -157 dBm to -161 dBm. The GPS signal is presently specified to range from -153-dBm to -160 dBm. The look angle to a WAAS satellite will vary inversely with latitude; users at high latitudes, such as in Alaska, will have very low look angles.
Since GEO satellites remain always in view, adding ranging signals to the WAAS satellites creates more signal sources for GPS ranging solutions which will (nearly) always be available. This tends to improves the HDOP of the position solution for most users. These extra ranging signals themselves also tend to enhance the precision of the position solution.
The correction data consists of three fundamental sets of information: corrections carrying the quickly changing pseudorange errors for GPS satellites, generally clock error, and sent every six to ten seconds; slower changing corrections, mostly ephemeris errors, sent less frequently; and a grid of ionospheric delay estimates.
WAAS only covers the area of North America. It is a wide-area system, not a global system. Although signals from the WAAS SBAS satellite sources may be in view to users outside of North America, use of WAAS may not ensure enhanced precision for their position solutions.
On some GPS receivers, signal identity data is sent using NMEA protocols. Early NMEA protocols only provided for two-digits to identify the source, so PRN codes with three digits are often translated by subtracting 87 from the PRN code. The translation of the PRN to NMEA number is
PRN NMEA Number 133 = 46 135 = 48 138 = 51
Some GPS receivers will show the PRN codes on their satellites in view page, and others use the NMEA numbers. More information on NMEA and PRN numbers is available in a separate article.
If this article has raised any questions or if you have a comment, feel free to post it to a discussion thread reserved for that purpose.
Copyright © 2017 by James W. Hebert. Unauthorized reproduction prohibited.
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Author: James W. Hebert
This article first appeared December 7, 2017.
