GPS (Global Positioning System) is truly one of the marvel innovations of the last century and certainly the best thing that emerged from the Cold War. Not only is the satellite infrastructure totally ubiquitous, almost all modern smart devices are GPS receivers.
GPS is based on a constellation of 24 satellites circling the planet in one of six orbits. GPS satellites broadcast radio signals providing their locations, status and precise time. A GPS-enabled device receives the signal and uses its exact time of arrival to calculate the distance from each satellite in view. Once a GPS receiver knows its distance from at least four satellites, it uses geometry to determine its location on earth.
Fast Fact: While the need for a global positioning navigation system was well-recognized world wide, the huge investment required would not have been possible if not for the nuclear threat during the Cold War. The first concrete steps toward GPS took place during a meeting in 1973 between the heads of the US military.
Due to the ubiquity of both the transmitters and the receivers, from an accessibility standpoint, GPS is a high performer. However, one factor led us to dock a point from GPS: power. As anyone who has ever used Google Maps on a road trip knows well, constantly searching for satellites makes it a huge battery drain and therefore a poor solution for "always-on" use. (-1)
GPS accuracy depends on a number of factors including atmospheric effects, sky blockage and receiver quality. Real-world data from the FAA shows the high-quality GPS receivers provide better than 3.5 meter horizontal accuracy. Higher accuracy is attainable using GPS in combination with augmentation systems. With these systems, accuracy is possible to within centimeters.
However, GPS accuracy breaks down indoors due to line of sight (e.g. no signal underground). Buildings block and refract the signal and multi-level buildings make it difficult to calculate altitude (and therefore which floor you are on). To use GPS for indoor location and positioning, it must be used in combination with nearby "anchors" or waypoints - most commonly, WiFi. Since we are comparing technology as standalone systems and we are focused on indoor, GPS receives a low score for accuracy. (-4)
GPS in and of itself is not invasive. It is simply four satellite signals sent that are recognized by a receiver and translated to location. The privacy and security risk is associated with the receivers (devices) and communication mechanism with the manufacturer's servers. That is to say, a GPS receiver is as secure as the manufacturer has made it using authentication and storage techniques. However, given the variability of receivers and well-known hacker issues related to this, we are giving GPS a medium score on privacy and security. (-2)
While, in reality, the infrastructure associated with GPS is an enormous cost, it is effectively free for the average consumer or business to tap into it.
Fast Fact: GPS is actually funded by American taxpayers. All GPS program funding worldwide is budgeted primarily through the U.S. Department of Defense with other government departments (e.g. Transportation, Agriculture) funding other civilian enhancements.
The main costs of GPS are the receivers - either personal devices or purpose-specific GPS trackers - and mobile applications required for business or use specific purposes. Purpose-specific GPS trackers, particularly ones with security features built in and improved accuracy, can be very expensive. Also, augmentation systems to achieve better accuracy can be a huge cost. (-1)
GPS continues to be the best location technology outdoors and especially over long distances. However, due to its battery appetite and lack of reliability indoors, GPS usually needs to be supplemented with other technologies for indoor location use.