During the last two semesters of my master study in radio frequency systems, I got interested in the performance of satellite navigation, and some of the non-academic notes and web references are put here
How much improved accuracy can you expect by exchanging a GPS-only receiver with one that uses both GPS and GLONASS? 15 to 30 percent lower "dilution of precision", depending on where on the globe you use it. It is well explained in this article from GPS World .
As especially the Galileo system is under development, mostly settled in 2010, several design design decisions has been changed up to year 2010. This means that many books, articles and web sites have outdated information in them regarding modulation types, time schedules etc.
For Galileo, the documentation of "Signals In Space" for the Open service (free for all to use, unencrypted data) was updated February 2010. It is no longer considered a draft, but still "subject to change".
The released document: http://ec.europa.eu/enterprise/policies/space/galileo/files/galileo_os_sis_icd_revised_2_en.pdf
Now the EU hopes to get Galileo partly operational by early 2014 with 14 satellites, as stated in this press release and the EC web site here. And "Full Operation Capability" two years later with 12 to 14 additional satellites.
The impact of modernized GPS: http://www.gpsworld.com/gis/gss-weekly/the-dawn-a-new-era-gps-accuracy-10016
The simple official intro http://www.esa.int/esaNA/GGGMX650NDC_galileo_0.html
The "old" Galileo Mission High Level Definition v. 3 from 2002.
The magazine GPS world has mentioned various incidents of GPS receivers that get jammed by interference from other devices that transmit radio noise in the GPS L1 frequency band.
For Europe or the EU, estimates has been made on how much revenue or GDP is dependent on a GNSS. The last number I read of was from this interview http://www.gpsworld.com/gnss-system/galileo-top-interview-with-ecs-paul-verhoef-10746 with this citation: "Just to give you some figures on that, at the moment, 6 to 7 percent of the European Union GDP is directly dependent on the availability of GPS. This is a GDP value of around 800 billion euros, this is more than 1,000 billion dollars.". As the EU GDP in 2009 was around 11.8*10^12 €, the 800 billion euros is probably the 7 % value (not the total GDP).
Written about the sales value of GNSS-related products: ". . . when a spokesperson for Galileo Services put forth the assertion that, currently, European industry holds a market share of around 20 percent of global GNSS hardware, software, and services, a market size he estimated at 180 billion euros, or $230 billion." Here billion is10⁹, citation from this opinion http://www.gpsworld.com/gnss-system/out-front-one-and-one-10804 .
Machine Control Using GNSS typically requires "sub-meter accuracy", and is said to be a growing market [http://www.gpsworld.com/gis/downloads/gita-conference-presentation-machine-control-using-gnss-april-25-2010-10306 , cited source is "(World-wide Precision Positioning Markets 2008-2012, Lorimer/Gakstatter)"]