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

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)

HTML Code:

Please Login or Register to see the links

[Only registered and activated users can see links. ]