More info: Search "Digital Angel" a company that has designed and released self-sustaining micro-chip implants, ostensibly to track kidnap victims, and high-risk government officials. Oracle was pursuing a similar design.
The Digital Angel corporation had set their goal to increase the scanning/tracking capability of their implants in both RANGE and ACCURACY, by using GPS.
There is a similar program being developed by the US military, for real-time battlefield assessment, using implantable chips within the rank and file troops. Again, this technology uses GPS. Have not found much information on the project.
Also, you may find interesting the mobile phone network that was being established by Motorolla several years ago... I do not recall the project name, but the system used a network of several hundered LEO satellites (to the best of my recollection). These units were very small, approx 1 meter square, orbiting at about 400 miles. LEO allows for more precise tracking of targets, and is less prone to space-junk/micro meteor damage. I do not know if the system is operational, or whether the satellites were allowed to begin orbital decay when Motorolla signed-off the project.
Iridium. There were (and are) about 66 satellites. Motorola conceived the project, got a syndicate together, and served as prime contractor for the phones and the satellites.
The system went into operation but never came close to meeting the marketing projections, so the partners wrote off their investment and made plans to de-orbit all the satellites. This was the status 2 or 3 years ago.
Then at the last minute, two things happened. A group of investors with a new business model stepped up and was able to purchase the facilities at a few cents on the dollar, and they snagged a big contract from the US military which helped to meet expenses. So the system is still in operation, but marketed in a different way. As an example, they now have an Iridium phone package for corporate aircraft.
They can continue to buy satellites from Motorola, which established a production line for them. The satellites are small and cheap compared to a mainline comm satellite; of course, the launch is still expensive, but the Russians, Chinese, and French have all orbited Iridiums (without a single launch failure!) and the larger boosters can put up several at a time.
I knew the original Iridium CEO slightly, before Iridium was formed.