A processing module for a first lock device of a range determination system, the range determination system comprising a lock group comprising a plurality of lock devices (Anchor 1-4) of which the first lock device forms part, the plurality of lock devices physically spaced over a lockable element and configured to communicate with a key group (Tag1, Tag4) comprising a plurality of key devices, a shortest distance between any one of the key devices of the key group and any one of the lock devices of the lock group providing for access to the lockable element relative to a threshold distance, the processing module configured to: based on time synchronization between the first lock device and the other lock devices of the lock group, provide for transmission of a challenge message to the key group in time synchronization with the other lock devices, the message comprising a first common part comprising the same data transmitted by all of the lock devices at the same time and an identifier part, the identifier part including a unique identifier among the lock devices, the identifier part provided for transmission in a discrete predetermined time slot assigned to said first lock device relative to the common part and different to corresponding time slots assigned to the other lock devices of the lock group; and based on a received key response message from at least a subset of the key devices of the key group, the subset comprising each key device of the key group that received and provided a response to said challenge message, the received key response message including a unique identifier for each key device among the key group, begin a timer for timing future message exchanges between the first lock device and each lock device of the subset, the beginning of the timer based on a marker present in the key response message from that particular key device and the unique identifier of the particular key device; the distance between the first lock device and each of the subset of key devices based, at least in part, on a time measured by the timer associated with the lock device during exchange of at least one subsequent lock response message to the key group based on a marker therein and receipt of at least one subsequent, further key response message from the at least a subset of key devices of the key group in reply to the lock response message based on a marker therein and a time (T T1, T T1, T T4, T T4) measured by each of the key devices and transmitted to the first lock device comprising the time between a marker based on each key device transmitting the key response message, a marker based on receiving the lock response message and a marker based on transmitting the further key response message. based on time synchronization between the first lock device and the other lock devices of the lock group, provide for transmission of a challenge message to the key group in time synchronization with the other lock devices, the message comprising a first common part comprising the same data transmitted by all of the lock devices at the same time and an identifier part, the identifier part including a unique identifier among the lock devices, the identifier part provided for transmission in a discrete predetermined time slot assigned to said first lock device relative to the common part and different to corresponding time slots assigned to the other lock devices of the lock group; and based on a received key response message from at least a subset of the key devices of the key group, the subset comprising each key device of the key group that received and provided a response to said challenge message, the received key response message including a unique identifier for each key device among the key group, begin a timer for timing future message exchanges between the first lock device and each lock device of the subset, the beginning of the timer based on a marker present in the key response message from that particular key device and the unique identifier of the particular key device; the distance between the first lock device and each of the subset of key devices based, at least in part, on a time measured by the timer associated with the lock device during exchange of at least one subsequent lock response message to the key group based on a marker therein and receipt of at least one subsequent, further key response message from the at least a subset of key devices of the key group in reply to the lock response message based on a marker therein and a time (T T1, T T1, T T4, T T4) measured by each of the key devices and transmitted to the first lock device comprising the time between a marker based on each key device transmitting the key response message, a marker based on receiving the lock response message and a marker based on transmitting the further key response message.