I just came across your GPS accuracy test, and thought that I should warn you: The A-GPS function allows the GPS satelite almanac to be pre-loaded via Internet. There may also be an update to the real-time-clock (using NTP via Internet, this is optional).

The net result is that the GPS receiver will be able to immediately interpret the signal from any satelites that it can receive, knowing already there exact positioning information. So that there is no need to first download this “satelite almanac” information (from the best available signal) – which is the normal “cold start” procedure. Having a (reasonably) accurate time can also help, marginally, although of course the GPS receiver will end up calculating the really-really-really-accurate time during its “fix” procedure.

So the bottom line is: using A-GPS should drastically reduce the time-to-first-fix, meaning that the time-to-fix will be both short and predictable. Thats all. The accuracy is really effected, at least not once the fix has maintained for a short time already.

Other systems allow to improve on GPS accuracy, both by correcting actual positioning errors of the GPS satelites themselves, and also with some information about environmental propogation (air conditions), although this is locally variable and so the relevance of those propgation informations depends on proximity to the correction source.

DGPS (Differential GPS) uses ground based long-wave transmitters to broadcast correction information around a local area (ocean coastline typically), meaning that a seperate and very different antenna is required (large physically, due to “long” radio waves, as there are no miracles involved).

WAAS (Wide Area Argmentation System => North American) and EGNOS (European Geostationary Navigation Overlay Service => European, like it says) relay the correction information via geo-synchronous satelites, and thus also use similar frequency bands to the GPS satelites themselves, meaning that WAAS-enabled and/or EGNOS-enabled GPS receivers receive the correction information with the same antenna, but need to dedicate some reception channels just for that (such fancy GPS receivers may advertise 50-60 channels, or more, not 8-10!!!). And of course it must be purpose-built for this functionality – apparently the SirfStarIII chipset has it built-in, for example. And the software must manage all of that, of course!

I have also heard that the WAAS/EGNOS correction information can also be downloaded via Internet. This is similar in principal to A-GPS, but especially for correction information, and not the basic GPS satelite almanac. Of course, those functions could be cumulative, with both the almanac (A-GPS) and also correction information (EGNOS, but via “SISNet” infortaion via Internet, and via satelite). I am speculating here, as I have not yet found any device that uses these 2 possibilities together.

Hope that helps!

Please, if you find any information about EGNOS-enabled Android devices, either “standard” via satelite or “cheating” via Internet: please let me know. I’m hoping to find a reasonably priced solution, with good accuracy, so that I can do good mapping with just my own telephone (which is always in my pocket, and so always available…).

Best regards,
Neil Dewhurst.

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I put the link to my digital microscope website in my signature if you would like more info

thanks jim