Determination method for use with analog cellular system

DETAILED DESCRIPTION When the system desires to locate a particular mobile unit, the mobile unit is commanded to go to a predetermined and dedicated channel and transmit an audio tone over it's FM transmitter for a short interval, say one to ten milliseconds.
The audio tone's frequency should be above the speech spectrum, e.
g.
greater than 4 kHz.
At the end of the tone burst, the mobile unit returns to whatever it was doing previously, e.
g.
, continuing its call, idle mode, etc.
The channel frequency used for the position determination service would normally be dedicated to this purpose throughout the system and a system controller would ensure that only one mobile unit at a time transmits a positioning signal.
At the same time that the control message is sent to the mobile unit, the base stations are sent a control message indicating that a mobile unit is about to transmit a tone burst.
The base stations are equipped with GPS receivers allowing accurate time and frequency references to be available at each base station.
The base stations produce a tone reference signal at the same frequency and with synchronized phase based on the GPS receiver.
The base station measures the phase difference between the tone reference signal and the signal tone (if any) received from the mobile unit.
The measurement interval used is the same as the transmission time, normally about one to ten milliseconds.
The phase difference measurements are reported to the system controller along with a measurement of signal to noise ratio (S/N) of the measurement.
The mobile unit's position is calculated by computing phase differences between the tone burst phases reported by adjacent base stations.
For example, if two adjacent base stations report the same phase difference relative to the reference phase, then the mobile unit is known to be somewhere on the perpendicular bisector between the two stations.
If the phases are unequal, then the mobile unit is known to be on a hyperbola which is the locus of points having the same measured phase difference