C. GPS Explained
Global Positioning System (GPS) is United States space-based navigational system initially developed by the U.S. Department of Defense and later opened for civilian and commercial use in 1996. The GPS system currently utilizes a constellation of up to 31 satellites that orbit the earth every 12 hours and their ground stations, located in Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs, which together provide the user with longitude, latitude, and altitude information as well as the precise time.
1. How GPS Works
Each GPS satellite contains a high-frequency radio transmitter that sends information back to earth, where a GPS receiver locks onto the signals. The process by which the GPS receiver can accurately determine the satellite’s location is known as triangulation.
The term triangulation refers to the way a GPS receiver determines position by calculating the distance to three or more GPS satellites. The following is an example of how this process works:
Let’s assume that we find ourselves at a point in space 11,000 miles from a particular GPS satellite. This would mean that we could be located at any point on the surface of a sphere with a radius of 11,000 miles that is centered on the particular GPS satellite. Next, suppose that we determine the distance from our (as-yet-unknown) location to a second GPS satellite and find that this distance is 12,000 miles. This implies that we are also located on the surface of a second sphere with a radius of 12,000 miles centered on the second satellite. Now we can see that we must be located somewhere on the circle that is defined by where the two spheres intersect. If we calculate the distance between our location and a third satellite and find that this distance is, say 13,000 miles, then using the same logic as before we find that we must be located using the same logic as before we find that we must be located on either one of the two points defined by the intersection of the three spheres. Usually one of these two points can be eliminated as a possible location because it is unrealistically far from the earth . . . [h]owever, we can determine our true location without any guesswork by measuring the distance to a fourth GPS satellite. [Emphasis original.] Egbert & King, The GPS Handbook: A Guide for the outdoors 92003).
The satellites orbit the earth in such a way that there are always at least six satellites within the line of sight of almost every location on the earth’s surface. The GPS receiver determines the distance a particular satellite is from the earth by calculating the time it takes for a radio signal to reach it. The receiver takes a weighted average of the satellite is from the earth by calculating the time it takes for a radio signal to reach it. The receiver takes a weighted average of the satellite positions and translates that information into specific coordinates such as longitude and latitude. The satellites are equipped with atomic clocks, and the receivers decode the signals in order to synchronize the receiver with the satellite time.
2. How Cell Phones with GPS Work
While more and more cellular phones are equipped with GPS devices, those without the GPS receivers can still provide law enforcement with the location of a cell phone and its user. When the cellular phone is on, it automatically communicates with one or more cell sites or “cell towers.” A process known as “registration” begins when the cellular phone is powered on. The cellular phone constantly seeks out the site with the best reception, re-scanning for cell sites every 7 seconds or when the signal strength weakens, regardless of whether a call is made. The towers and phone communicate with each other identifying the phone, the phone’s owner, and the service provider, and the tower relays that information to switching office. AS the phone changes location, it automatically switches to the cell phone site that provides the best reception and continually re-registers as it moves along. Wireless service providers typically keep track of the identity of the cell towers serving a phone at any point in time and of each tower transmitting to the phone. When a phone is in touch with more than one tower, the service provider (or law enforcement, if given permission) can compare the signals and locate the phone through a process of triangulation.
3. GPS with Embedded devices that also serve as Wiretapping Devices
Cars equipped with telecommunication devices that provide the user with navigational assistance, location of commercial businesses, or recovery of a stolen car as well as request emergency or roadside assistance often combine GPS technology with cellular technology . Some systems are installed in the vehicles, allowing the driver to communicate with a designated call center has the ability to turn on the communication line, and, unbeknownst to the occupants of the vehicle, the conversations of the occupants of the vehicle can be heard and recorded. For example, GPS navigation systems such as General Motors’ OnStar offer car owners services including mapping capabilities, stolen vehicle tracking, remote door unlocking, directory information, and emergency roadside assistance. These systems determine the location with GPS and communicate with an assistance and services center through the cellular network, offering two-way transmission of both voice and data (including location data). Depending on how the system is managed, law enforcement officials may be able to obtain location information from the service provider, either in real-time or after-the-fact. An explanation of how this system functions can be found in In re U.S for an Order Authorizing Roving Interception of Oral Communications. (CA 9th 2003) 349 F3d 1132, 1134.
In analyzing cases where evidence is obtained through listening or recording communications with these type of systems, the federal and state wiretap laws control. For federal wiretapping law, see Title III. Section 2518 requires that the company be a provider of wire or electronic communication service,” and the interception must be accomplished “unobtrusively and with a minimum of interference with the services.” The court in In The Matter of application of the U.S. for an Order Authorizing the Roving Interception of Oral Communications (9th Cir 2003) 349 F3d 1132 reversed an order allowing interception of communications on the ground that the company involved could not “assist the FBI without disabling the System in the monitored car. Therefore, the “minimum interference” requirement could not be met.
The Morales Law firm would like to thank Scientific Evidence in California Criminal Cases for sharing this information with us.