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Is there much WAAS in Minnesota?

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ice tres

Joined: 16 Aug 2005

Posts: 97

PostPosted: Mon Aug 22, 2005 8:46 pm    Post subject: Is there much WAAS in Minnesota? Reply with quote


I read that there are only 2 satellites that send WAAS signals and that they are on the equator off the east and west coasts and both are very difficult to receive from this area. Does anyone know if this is true?

Ice Tres
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Past MnGCA Board

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PostPosted: Mon Aug 22, 2005 11:42 pm    Post subject: Reply with quote

In answer to your question. Yes Minnesota has full WASS coverage. We have a correction station here in Minneapolis. However, generally you need to be in open unobstructed terrain to receive a good signal. WASS was really intended for aviation use not ground use.

Here's a little instruction from the GPS Knowledge Challenge Professor

Lesson 1 - What is WAAS?

Wide Area Augmentation System. 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.

Lesson 2 - Who is the primary benefactor 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. 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.

Lesson 3 - How does it work?

The WAAS provides augmentation information to GPS receivers to enhance the accuracy and reliability of position estimates.

The signals from GPS satellites are received across the at many widely-spaced Wide Area Reference Stations (WRS) sites. The WRS locations are precisely surveyed so that any errors in the received GPS signals can be detected.

The GPS information collected by the WRS sites is forwarded to the WAAS Master Station (WMS) via a terrestrial communications network (Land Line - Fiber Optic). At the WMS, the WAAS augmentation messages are generated. These messages contain information that allows GPS receivers to remove errors in the GPS signal, allowing for a significant increase in location accuracy and reliability.

The augmentation messages are sent from the WMS to uplink stations to be transmitted to navigation payloads on Geostationary communications satellites.

The navigation payloads broadcast the augmentation messages on a GPS-like signal. The GPS/WAAS receiver processes the WAAS augmentation message as part of estimating position. The GPS-like signal from the navigation transponder can also be used by the receiver as an additional source for calculation of the user’s position.

WAAS also provides indications to GPS/WAAS receivers of where the GPS system is unusable due to system errors or other effects. Further, the WAAS system was designed to the strictest of safety standards – users are notified within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate.

WAAS is based on a network of approximately 25 ground reference stations that covers a very large service area. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist.

These WRSs are linked to form the U.S. WAAS network. Each WRS in the network relays the data to the wide area master station (WMS) where correction information is computed. The WMS calculates correction algorithms and assesses the integrity of the system. A correction message is prepared and uplinked to a geosynchronous satellite via a ground uplink system (GUS). The message is then broadcast from the satellite on the same frequency as GPS (L1, 1575.42MHz) to receivers on board aircraft (or hand-held receivers) which are within the broadcast coverage area of the WAAS. These comunications satellites also act as additional navigation satellites for the aircraft, thus, providing additional navigation signals for position determination.

Lesson 4 - My GPS has WAAS (Wide Area Augmentation Service) but it does not seem to be improving accuracy. What gives?

Like many technical projects in the development phase, the current state of WAAS (with only two fully operational signals available) is in a state of flux and will be for many years to come. There are many things to consider with the present system:

1) WAAS ground stations send GPS tracking information to a master control site. The master control site uses this information to compute
a) Long term satellite position error
b) Short term and long term satellite clock errors
c) IONO correction grid
d) Integrity information

The above information can only be computed by the master control site based on all of the observations from many ground reference stations. The information is relayed to the WAAS GEO satellites and is re-broadcast to user receivers. Note: The WAAS correction information is very different than RTCM corrections because WAAS decomposes the errors into their primary elements (position, clock, & IONO). RTCM, (DGPS) on the other hand, broadcasts pseudorange corrections which are the sum of all error sources as observed by the RTCM reference station. This is why spatial deceleration is such a big factor for RTCM, but is not for WAAS (thus the reason it is "wide area" augmentation).

The WAAS integrity information is available for any GPS receiver to use. It includes integrity information for the WAAS system, individual GPS satellites, and various error estimates & degradation factors for the correction information broadcast by WAAS.

2) For GPS position to be improved by WAAS signals, a handheld GPS must first be able to receive one of the WAAS satellites with excellent signal quality as a lot of data must be regularly received by the handheld from the WAAS satellite.

3) For GPS position to be improved by WAAS signals, a handheld must be within a region serviced by one of the correction stations being monitored by the master control site. This might be somewhere in the range of 500+ miles from one of the correction stations.

Lesson 5- OK: But if I can hear the satellites and the correction station is too far away to give good corrections, Why does the GPS not "know" this and just not let WAAS work?

Answer: With just two satellites, one could say: a) If you can hear the signal you just about HAVE to use it if you want to use WAAS at all and b) Since there are many areas of the USA which (at best) have intermittent signals, the USER must use some intelligence as to if WAAS is useful to him at all. Perhaps when the WAAS system is out of the development and experimental stage and many more WAAS satellites and correction stations are installed, it will be possible for a GPS to "know" when a WAAS (or EGNOS) correction signal is useful. But that is not the present situation.

We must realize that WAAS was not designed for consumer GPS users, rather it was designed for aircraft use. The aviation receivers will have a LOT of integrity monitoring logic which may never be implemented into handhelds because of the complexity and cost.

Lesson 6- How much improvement in accuracy can I expect when WAAS corrections are working properly?

Answer: This is difficult to say with any certainty. We have observed that MOST OF THE TIME WAAS appears to improve averaged accuracy at a fixed location for maybe 5 minutes averaging from about 7 meters to perhaps 3 meters. Sometimes there has been no improvement and a few times (when WAAS signals were not of good quality or when we were far from a WAAS correction station) the measurements have actually had more error with WAAS than without.

The upshot of this is: WAAS is a tool for casual GPS users. It is not guaranteed to work, it is not guaranteed to increase accuracy, it is not really able to tell you when it will improve accuracy, when it will have no effect, or when it will make for less accuracy than the normal GPS signal. Under good conditions when YOU ARE OUT IN THE CLEAR AND RECEIVING A GOOD SIGNAL FROM THE WAAS SATELLITE, you should get both improved accuracy and improved position stability. YOU must insure that you have the proper conditions so as to experience the improvement at YOUR location.

This is just the way it is and remember: If you do not like the current limitations of WAAS there is a very simple option -Turn it off.

edit by SB: Removed size 18 tag
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ice tres

Joined: 16 Aug 2005

Posts: 97

PostPosted: Tue Aug 23, 2005 8:42 am    Post subject: Reply with quote

THanks for the info, but I did know how WAAS works and that there is a nearby ground station. However, the ground station does not help with reception but only with accuracy once a signal is received.

But as the article you posted states, many areas of the country receive at best intermittant signals from the satellites. I forget now which numbered satellites are the WAAS ones, but when I first got my Etrex I looked for reception from those specific satellites and my unit was never able to tune them in.
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Pear Head
Past MnGCA President

Joined: 04 Apr 2004

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PostPosted: Tue Aug 23, 2005 5:21 pm    Post subject: Reply with quote

I believe in Minnesota we see sat 36 (or that's how it's labeled on my Garmin Geko). I recently put my Geko into WAAS mode (I haven't used it for over a year). It "searched" for the WAAS sats and missed it the first couple times around, searching all the way up to something like 50.

I can get the WAAS sat if the Geko is in the clear in a farm field and I'm not standing in the way. Anything in the way and the performance goes WAY down. I never use it because the GPS never sees the sat and it just sucks down the batteries.
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