GPS satellite in orbit, image courtesy NASAThe Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio, allowing any GPS receiver (abbreviated to GPSr) to accurately determine its location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth.

GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes. GPS receivers can also gauge altitude and speed with a very high degree of accuracy.

The United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System), and launched the first experimental satellite in 1978. The satellite constellation is managed by the 50th Space Wing. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.

In late 2005, the first in a series of next-generation GPS satellites was added to the constellation, offering several new capabilities, including a second civilian GPS signal called L2C for enhanced accuracy and reliability. In the coming years, additional next-generation satellites will increase coverage of L2C and add a third and fourth civilian signal to the system, as well as advanced military capabilities.

The Wide Area Augmentation System (WAAS), available since August 2000, increases the accuracy of GPS signals to within 2 meters (6 ft) for compatible receivers. GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS).

While GPS is the natural choice for providing navigation in an outdoors environment, the urban environment places a significant challenge for positioning using GPS. The GPS signals can be significantly attenuated, and often completely blocked, inside buildings or in urban canyons. Personnel tracking inside buildings for improved command, control, and rescue rates high on the list of priority needs prepared by the U.S. Department of Homeland Security for first responders. Tracking firefighters will allow for better tactical decisions and faster fire suppression, resulting in decreased property losses.

A networked radionavigation approach that augments GPS signals with time-of-arrival observations using a software-defined radio can overcome attenuation, and often complete blockage, of GPS inside buildings or in urban canyons. The SDR can operate both as a GPS receiver and also as a 900 MHz transceiver operating within the ISM band. Applications for this technology include firefighters and other first responders, and military operations in urban terrain.

A software-defined radio (SDR) provides a flexible architecture that allows the same radio components to be reconfigured to perform different functions. The manufacturers have developed an SDR that includes the capability to operate both as a GPS receiver and as a 900 MHz transceiver operating within the industrial, scientific, and medical (ISM) band. Since both the GPS and communications functions reside within common radio hardware, this positioning and communications (POSCOMM) device can use the two functionalities to provide a positioning capability that leverages both the GPS-derived pseudorange and carrier-phase observations and the communications channel's time-of-arrival (TOA) observations.

The POSCOMM networked design allows this system to be rapidly set up around a building, providing indoor positioning capability without requiring fixed infrastructure. Military applications for this technology include improved military operations in urban terrain (MOUT). Commercial applications include firefighters as well as other first responders. This project will give firefighters, police officers and emergency officials an electronic vest and eyepieces that will provide their commanders with their location and their vital signs, as well as real-time video of their surroundings.

Since the POSCOMM technology has been developed based on a SDR architecture, we envision that future SDRs developed for first-responder communications will be able to be upgraded to include this essential positioning capability. The initial POSCOMM prototypes will be delivered to the U.S. Army in early 2007 for evaluation in an urban environment. The manufacturers are also working with commercial partners to embed the POSCOMM technology into first-responder networked communications products.

Thanks for the information, redbaron. It's amazing what they can do with technology nowadays.

Another writer and I were discussing the use of GPS devices inside of cell phones a few months ago and how a person/agent could be lost in the middle of nowhere is their phones were equipped with GPS. She discovered that if the phone is broken or turned off then the GPS device inside will not work. Thus, our wonderful beloved agents can be lost and not found. :unsure:

Same way with inside buildings. I wrote a fic where the main character was injured in an old warehouse and couldn't get help herself, couldn't give directions to where she was, etc. but had her phone. Using the cell towers they were able to figure out her general location but her GPS was useless inside the old building.

Working with technology every day in my job (phones, cellphones, radios, etc), it helps with some of the facts that I need in the fics that I write. :)