Localisation
If there are multiple antennas or receivers, this permits localisation of the TPM such that the vehicle can tell from which wheel the pressure data has come. As an alternative to this method, the vehicle can be programmed at the time of manufacture with the position of the tire together with its TPM serial number. This allows the vehicle to display which tire has low pressure.
Also, some vehicles have low frequency radio transmitters mounted near to each wheel which can be used to force the individual TPMs to transmit at will. These typically use similar technology to 125 kHz RFID tags where the transmitted field is predominantly magnetic and can be easily detected by a small LF antenna located in the TPM. This method of localisation is often referred to as a high line system.
The LF antenna is also often used by the TPM for configuration and to force transmission so that localisation can be re-learned by the vehicle if a sensor is changed or the wheels rotated to even up tread wear.
A third method uses the UHF signal strength which is proportional to the distance of the TPM from the receiver. If the receiver is located towards the front of the vehicle, the signal from the front wheel TPM's will be stronger than that from the wheels at the rear.
The TPM also has a method of detecting the rotational direction of each wheel to identify which side of the vehicle the TPM is located and this information forms part of the message transmitted to the vehicle. This combination allows the correct wheel to be identified.
Read more about this topic: Direct TPMS