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Method of determining the footprint of a tire on the ground, using devices already installed in standard production tires. Since the frequency of the transmitter clock present in a wheel unit is perturbed by mechanical vibrations when the tire contacts the ground, the frequency variations of the reference clock are representative of the value of the footprint. The method therefore includes determining the footprint on the ground of a tire of a wheel by measuring continuously, as a function of time, a signal representative of the variations in frequency of the reference clock supplied at the output of the time control device; determining a time interval during which the frequency variations of the reference clock occur; and, on the basis of the rotation speed of the tire, deducing therefrom an angular range of contact (.THETA.) corresponding to the footprint between the ends representing the ground contact length of the tire.

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**Independent Claim 1.**A method for determining a footprint (.DELTA.L) of a tire (3), corresponding to a length of contact of said tire (3) with the ground, of a vehicle equipped with a monitoring system that includes one wheel unit (4) for each tire (3), each wheel unit (4) comprising a microcontroller (14), a speed sensor (12'), a transmitter (10), a reference clock (16) of the transmitter (10) configured to operate at a given frequency (F.sub.0) , and at least one time control device for the system, where an actual frequency (F) output by said reference clock (16) is subject to variations (.DELTA.F) with respect to the given frequency (F.sub.0) when the wheel unit is within the footprint of the tire, the method comprising: continuously measuring, by way of said time control device, for the variations (.DELTA.F) of the output frequency (F) of the reference clock (16) with respect to the given frequency (F.sub.0), as a function of timedetermining a time interval (T.sub..DELTA.L) during which said variations (.DELTA.F) of the frequency of the reference clock (16) occurand on the basis of the rotation speed of the tire (3), deducing therefrom an angular contact range (.THETA.) corresponding to the footprint (.DELTA.L) of the tire (3).**Independent Claim 6.**A method for determining a footprint (.DELTA.L) of a tire (3) of a wheel (1), corresponding to a length of contact of said tire (3) with the ground, of a vehicle equipped with a monitoring system that includes one wheel unit (4) for each tire (3), each wheel unit (4) comprising a microcontroller (14), a speed sensor (12'), a transmitter (10), a reference clock (16) of the transmitter (10) configured to operate at a given frequency (F.sub.0), and at least one time control device for the system, where an actual frequency (F) output by said reference clock (16) is subject to variations (.DELTA.F) with respect to the given frequency (F.sub.0) when the wheel unit is within the footprint of the tire, the method comprising the following successive steps: measuring continuously, and recording (30, 60, 60'), as a function of time, the frequency (F) at the output of the reference clock (16) and the variations (.DELTA.F) of the output frequency (F) with respect to the given frequency (F.sub.0)determining and recording the time interval (T.sub..DELTA.L) during which said variations (.DELTA.F) of the frequency of the reference clock (16) occurrecording a rotation speed of the wheel (1) as a function of time based on an output of the speed sensorusing the recorded time interval (T.sub..DELTA.L) of the variations of frequency (.DELTA.F) of the reference clock (16) and the recorded rotation speed of the wheel (1) to calculate and record an angular contact range (.theta.) of the tire (3)defining a detection threshold (F.sub.Min) below which said frequency variations (.DELTA.F) are disregardedif the frequency variations (.DELTA.F) are above the frequency threshold (F.sub.Min), calculating the footprint (.DELTA.L) on the basis of the contact angle (.THETA.), the time interval (T.sub..DELTA.L) and the speed of the wheel (1)defining boundaries (.DELTA.L.sub.min, .DELTA.L.sub.max) of the value of the footprint (.DELTA.L) beyond which the calculated value of the footprint (.DELTA.L) is not accepteddefining a frequency of calculation for determining the value of the footprint (.DELTA.L) of the tire (3)and restarting the calculation at the frequency thus defined.