The invention relates to an access unit (5; 23) for communicating with a further access unit (5; 23) to control access to a vehicle (3). The access unit (5; 23) comprises at least a first transmitter (11; 25) for transmitting a polling signal; and at least a first receiver (11; 25) for receiving an authentication signal from the further access unit (5; 23) in response to the polling 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-wide-band receiver. The invention also relates to a vehicle access system (1) which is a combination of the access and further access units (5; 23). The invention also relates to methods of operating a vehicle access system (1) and a vehicle access unit (5; 23).