Thread: About 37xx converted to 34xx satellites reception accuracy

@tom1807

Could you do a test on your 3790? Check accuracy with WAAS/MSAS on and off, and report.

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For others, maybe as unsure as i am about the 'WAAS-type' capabilities of 3xxx:

[First, a Terms explanation in case the several terms are confusing: MSAS is the Asian equivalent of NA's WAAS and EU's EGNOS. All these systems are compatible and collectively called SBAS, the correct (but rarely used) generic term for such systems. That is, a WAAS capable unit receives MSAS and EGNOS and vice versa. I'll just use the term WAAS here, it's easier.]
WAAS was first developed for Aviation, particularly for instrument approaches (non-visual condition) and other precision uses. My opinion is that it's not necessary for safe and normal operation of civilian ground vehicle navigation as we just don't need that 'precision' in a car GPS unit, but i've seen first hand the difference WAAS makes in the air and it could be a 'life or death' difference there.

Ground devices like some Garmins have 'piggy-backed' for a free ride by obtaining the signals from the WAAS sats to increase their accuracy. It's maybe not available to 3xxx units, but i don't know one way or the other.

In regard to the capabilities of WAAS-type systems to minimise error. The ground stations (precisely fixed coordinates) receive data from Sats, compare the sat's info given on the station's location to it's actual location, calculate the error if any, then transmit that back to a sat which rebroadcasts the correction to WAAS capable receivers, but not to non-WAAS receivers of course. So in that sense, in a perfect world with no sat being 'off', the accuracy of a unit with WAAS off would be the same as when it's on, as stated in the info quoted by Caty.

In Post #37 giuseppe66 said his Nuvi 765 EU version has accuracy of 6-7m, however his Nuvi 765 TW version gets 3-4m with WAAS on test at same time. With WAAS off he gets same precision in both. That would seem to indicate that 765 EU has WAAS capabilities because his 765 TW is a convert from EU, not an original.

To settle this, is WAAS available to US/EU 37xx/34xx units? It clearly is available in at least some Nuvi units other than 3xx/6xx. i.e. Tom's 3790TW and (apparently) in giuseppe66's 765 EU>TW convert. The info from Caty is helpful but somewhat dated as she points out. These 3xxx units certainly weren't around in 2008, or even in 2010 were they? Aren't they 2011 and 2012 releases?

The other consideration is how 'truthful' is the accuracy reading on the original US/EU 3790's anyway? Mine seems to always read 3m when there is a clear or only partly cloudy sky. I'm getting suspicious that it actually can constantly maintain that degree of accuracy in practice, even if it is WAAS capable. Is it pre-programmed in the fw to show an artificially 'good' reading?

Originally Posted by tom1807

Sorry for my late feedback.
I tested with Standard GPS and WAAS (switched between both).
In both cases my accuracy was 4-5m.
No difference. I guess it has something to do that I am just 25° north of the equator.
I am next week in Europe, will test again

About SBAS Waas/Egnos..
I have researched and made ​​measurements in gps of the SBAS satellites that are tuned in the southern hemisphere. One thing I am absolutely sure: my nuvi 3790/3490 (8GB V8 035 GM OF TC, GPS FW 3:04:03) not tunes WAAS / EGNOS / MSAS .. because I did a lot of measurements side by side with an Etrex 30 and Oregon 450 that officially supports WAAS / EGNOS. At the same time, side by side, while the Etrex and Oregon tuned some SBAS satellite, the 3790 never had the least hint tuning these satellites, which shows very clearly that it is not compatible to this standard. Although it has been said in previous posts that this is a firmware feature'm not sure that is so, because it seems to be mainly a feature of hardware, software secondarily. First, because the 3790/3490 of Neil tunes SBAS satellites and no other 3790 that I have ever seen, and that use the same firmware, can tune SBAS. Second, because the tune of SBAS satellites in gps is in different frequency (slightly higher than 1600 mhz). The standard of our civilian GPS is 1575.42 MHz (L1 frequency). Despite of the Asiatic versions, as of the tom1807's and unusual as of the Neil's case, everything indicates that tuning capacity of WAAS / EGNOS of the 3790 is not available in Western version.

A question: The SBAS / WAAS can improve or worsen the precision of GPS? It depends, check this:
“If the receiver is outside of the area where valid TEC map data are provided, the receiver should use the build in standard ionospheric correction and then there should be no difference between WAAS/EGNOS switched on or off in the receiver setup menu in these cases. But since most manufacturers recommend switching WAAS/EGNOS off if not within the area where the signals are provided for, it may be assumed that the receivers do not make full use of all information provided and do not check whether the correction should be used or not. That way, the position may be even worse when WAAS/EGNOS is switched on when no proper correction signal is provided.” (kowoma.de link)


Another problem already raised: How to know if the SBAS correction system is working?
According to the Garmin website to improved the accuracy must be below 3 meters, and this is what actually happens with the Etrex30 and Oregon 450, but it is important to note that values ​​are for static or low motion. In Gpsreview forum, indicated by Catymag in post 9, the author convincingly explains the uselessness of WAAS / EGNOS for vehicles in motion, subject to be discussed. Notice in the image, in which the satellites are made corrections appears the letter D of differential correction, which does not occur in the case of Neil, but let's await the images of tom1807.
Another detail is that the SBAS satellites in the GPS Garmin obey the NMEA standard, in which the numbers of slots PRN satellites going 33-64 (conventer to simply subtract PRN - 87). Interesting is that in the case of Neil appears the
normal number PRN 193, of the SBAS QZSS japanese.

List, not complete, of SBAS satellites that I could compile:
- PRN 122, 125, 133, 134, 135, 138 - USA WAAS (Wide Area Augmentation System)
- PRN 120, 124, 126, 131- EGNOS (European Geostationary Navigation Overlay Service)
- PRN 129, 137 - MSAS (Multi-functional Satellite Augmentation System) - Japan
- PRN 193- 197 - QZSS (Quasi-Zenith Satellite System) – Japan

So far we are using the empirical method to solve the problem of poor accuracy of converted 3790, the trial and error method that sometimes works and sometimes not. The ideal is to seek to understand why, the cause, for thereafter, more safely be able to find solutions. One explanation for the previously reported rapid loss of accuracy after applying Loly’s method (Predicted Eph, Cold Start) can be: “.. ephemeris information is highly detailed and considered valid for no more than four hours.. The orbital position data, or [Only registered and activated users can see links. ], from the navigation message is used to calculate precisely where the satellite was at the start of the message..” (Wikipedia)

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Voyager, interesting that you experienced a decrease in accuracy with WAAS on, regardless of its signal being absent or too weak. Why do you think that was? Do you still have the 350?
EDIT: Do you mean the measurement decreases (e.g. 6m to 4m) or it's accuracy does (e.g. 4m to 6m)?

Originally Posted by Neil

Voyager, interesting that you experienced a decrease in accuracy with WAAS on, regardless of its signal being absent or too weak. Why do you think that was?

I have the facts of decrease in accuracy with WAAS on in Russia. Accuracy on satellite page was be greater - even 2 meters. But real position was so far of the truth - even about a few ten of meter. It was not surprise. The closest ground station was in thousand of km in absolutely different day time zone, atmospheric and weather conditions. Nevertheless GPS catched signal from nearest WAAS broadcasts sat zone (stationary orbit - Inmarsat probably).

So we was strong recommended to disable WAAS in our locations!

Usually I could see a new satellites with number higher then 32 on satellite page of compatible devices with WASS enabled! Anybody see something like on our disputed devices?

Originally Posted by Giomen

Usually I could see a new satellites with number higher then 32 on satellite page of compatible devices with WASS enabled! Anybody see something like on our disputed devices?

See this pic, from my post [Only registered and activated users can see links. ], 3490 convert is showing sat 153:



Does that mean anything special?

At next time please make retouch of view in windscreen. In this case I will not be admire a nature and I will see only on sat numbers

It is mean what you receive SBAS - but no info about 153 PRN now. I find PRN 155 only:
http://www.gps-forum.ru/cgi-bin/forum/showpost.pl?Board=gpsgeneral&Number=139008&page=0&view=expanded&sb=5