Determination of position of mobile vehicles is usually accomplished by measurement of transit time of radio waves between mobiles and radio stations located at known points.
Direct determination of the transit time involves that times at which waves leave one station and arrive at the mobile receiver are accurately known.
This is theoretically the simplest way, but practical approaches to this elementary principle have only recently been made possible, by using atomic frequency standards (TORAN "0"). The accuracy is limited by a drift of a few meters per hour, due to the still imperfect stability of now available clocks.
Otherwise the only possibility of directly measuring distances between two points by radio is to transmit a signal at the first point, to receive it at the second point and to send it back, so that time elapsed between transmission and reception can be measured. Elapsed time is proportional to twice the distance between the considered points.
This involves that a transmitter be placed at both ends of the distance to be measured. This limits theoritically to one unit the number of mobile vehicles able to simultaneously determine their distance to a fixed known station.
This problem is set aside if difference of transit times of two signals radiated by two stations are received at the same points and are then compared. Constant time differences are then hyperbolae, the focal points of which are the two known transmitting stations, provided a means of synchronizing or knowing the transmission times has been secured. This way of operating has given birth to the hyperbolic system family. Mobile vehicles only carry receivers and their number is not limited.
But all these means of radio positioning have to be considered from another point of view: how to compare times of arrival of radio waves at a given point, for instance, on the mobile vehicle ?
Fundamentally, two ways of accomplishing this measurement are to be considered: pulse techniques and phase comparison. If the transmitters of the system radiate short pulses, leaving and arrival time of pulses can be compared provided that the steepness of pulse fronts is compatible with the required accuracy.
Phase comparison supposes that sinusoidal Waves are radiated to be phase compared in a phase meter. Phase measurement is very accurate-, but a phase can only be physically determined within one cycle of the wave, corresponding to one wavelength. This leads to the major drawback of phase comparison systems: the ambiguity. This ambiguity is the price payer for accuracy, for the shorter the wavelength, the higher the accuracy, but also the higher the ambiguity. Pulse systems are also subject to an ambiguity which is dependant of the repetition ratio pulses related to the measured distance. But the repetition rate is not directly bound to accuracy and can be chosen so that ambiguity is not a problem.