An access unit (5; 23) configured to communicate with a further access unit (5; 23) to control access to a vehicle, the access unit (5; 23) comprising: at least a first transmitter (11; 25) configured to transmit a polling signal; and at least a first receiver (11; 25) configured to receive an authentication challenge signal from the further access unit (5; 23) in response to said polling signal; wherein: the access unit (5; 23) is configured to transmit a response signal to be authenticated by the further access unit (5; 23) in response to the received authentication challenge signal; at least the first transmitter (11; 25) is a first ultra-wideband transmitter and at least the first receiver (11; 25) is a first ultra-wideband receiver; at least the first transmitter (11; 25) is configured to continually pulse the polling signal with a time interval between transmissions during an operating period in which the first transmitter is activated; the access unit (5; 23) is configured to determine from the communication between the access unit (5; 23) and the further access unit (5; 23) a distance between the access unit (5; 23) and the further access unit (5; 23); at least the first transmitter (11; 25) is configured: to increase the time interval between transmissions of the polling signal in response to an increase in the distance between the access unit (5; 23) and the further access unit (5; 23); and/or to reduce the time interval between transmissions of the polling signal in response to a reduction in the distance between the access unit (5; 23) and the further access unit (5; 23); and the first transmitter (11; 25) is configured to stop transmitting the polling signal if the access unit (5; 23) is stationary for a predetermined period of time so as to terminate the operating period. at least a first transmitter (11; 25) configured to transmit a polling signal; and at least a first receiver (11; 25) configured to receive an authentication challenge signal from the further access unit (5; 23) in response to said polling signal; wherein: the access unit (5; 23) is configured to transmit a response signal to be authenticated by the further access unit (5; 23) in response to the received authentication challenge signal; at least the first transmitter (11; 25) is a first ultra-wideband transmitter and at least the first receiver (11; 25) is a first ultra-wideband receiver; at least the first transmitter (11; 25) is configured to continually pulse the polling signal with a time interval between transmissions during an operating period in which the first transmitter is activated; the access unit (5; 23) is configured to determine from the communication between the access unit (5; 23) and the further access unit (5; 23) a distance between the access unit (5; 23) and the further access unit (5; 23); at least the first transmitter (11; 25) is configured: to increase the time interval between transmissions of the polling signal in response to an increase in the distance between the access unit (5; 23) and the further access unit (5; 23); and/or to reduce the time interval between transmissions of the polling signal in response to a reduction in the distance between the access unit (5; 23) and the further access unit (5; 23); and the first transmitter (11; 25) is configured to stop transmitting the polling signal if the access unit (5; 23) is stationary for a predetermined period of time so as to terminate the operating period. the access unit (5; 23) is configured to transmit a response signal to be authenticated by the further access unit (5; 23) in response to the received authentication challenge signal; at least the first transmitter (11; 25) is a first ultra-wideband transmitter and at least the first receiver (11; 25) is a first ultra-wideband receiver; at least the first transmitter (11; 25) is configured to continually pulse the polling signal with a time interval between transmissions during an operating period in which the first transmitter is activated; the access unit (5; 23) is configured to determine from the communication between the access unit (5; 23) and the further access unit (5; 23) a distance between the access unit (5; 23) and the further access unit (5; 23); at least the first transmitter (11; 25) is configured: to increase the time interval between transmissions of the polling signal in response to an increase in the distance between the access unit (5; 23) and the further access unit (5; 23); and/or to reduce the time interval between transmissions of the polling signal in response to a reduction in the distance between the access unit (5; 23) and the further access unit (5; 23); and the first transmitter (11; 25) is configured to stop transmitting the polling signal if the access unit (5; 23) is stationary for a predetermined period of time so as to terminate the operating period.