AGPS or: Global Locate's Marketing debunked

Marcus Bauer marcus.bauer at
Tue Dec 26 16:36:28 CET 2006

The neo1973 will come with a Global Locate GPS chip. In the following I
will try to explain the disadvantages of that situation.

First thing to keep in mind is that all the tomtom PDAs do pretty well
without any "A-GPS" at all!!

So what is that A-GPS good for?

There are two points:

      * TTFF - time to first fix after cold or warm start
      * precision of position during the first minutes of navigation

How is that achieved?

This is achieved simply by preloading the GPS-chip with the ephemeris
data, meaning that a satellite can immediately be used for getting a fix
at the moment it comes in sight instead of waiting 30secs while
downloading the data.

The ephemeris data contains the precise position of a satellite and
outdates within two hours. However it is only 1500 bits long (times 12
satellites over your head) so can be quickly downloaded via i.e. GPRS.

The ephemeris data is available here:

The SiRF chipsets do allow for preloading the data and the protocol
specs are published by SiRF, however nobody has even bothered yet to
write the few lines of code to extend the gpsd ( 

Now what's the problem with Global Locate?

The problem is that the protocol specs for communicating with the GPS
chip are only available under NDA. Their whole marketing is based on
this and they never mention "ephemeris data". This allows them to
paraphrase it over and over again and pretend they have something

Some examples (taken from ):

	Full GPS-constellation data 
	Satellite assistance data provided for 100% of satellites.

-> means: they have ephemeris data

	Strategically located 
	Each GPS satellite is monitored by several RSs at all times to ensure no coverage gaps.

-> means: nothing. just marketing blurb. Their 'WWRN' of RSs (worldwide
network of reference stations) is what others call CORS (continously
operating reference stations)

	No missing satellites 
	Unlike regional RS solutions that leave the handset with a
	critical shortage of assistance data, WWRN data includes
	assistance data for rising satellites.

-> means: they have ephemeris data (we heard that already, didn't we?)

	Highest likelihood of E911/E112 location fix 
	Important in the typical obstructed environment, handset is
	assured of having assistance data for all visible satellites.

-> means: they have ephemeris data (we heard that already, didn't we?)

	Assistance data always current 
	Changes in orbital navigation data or health status broadcast by
	the satellites is processed in real-time by the WWRN.

-> means: they have ephemeris data (oh, really?)

	Minimizes requests to A-GPS server 
	No additional network assistance required as new satellites come
	into view.

-> means: they have ephemeris data (it starts to get boring)

	Shared infrastructure 
	Full GPS-constellation data enables single A-GPS server to
	support handsets in multiple networks worldwide.

-> means: they have ephemeris data - yaaawwnnn

and on and on and on like this, paraphrasing over and over again that
they have ephemeris data. They are selling H20 in bottles.

* Conclusion *

Global Locate: You should just publish the specs for the binary protocal
of your chip. We do the rest for ourselves.

If they don't do that, please Sean, follow your Mantra for an open
plattform and substitute their chip with a more open one.

A word of advice for Global Locate: actually the OSS community likes
underdogs and compared to the market leader SiRF you are exactely that.
But don't show up as arrogant jerks on mailing lists pretending to be an
engineer but talking like a marketing droid.


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