Wireless localisation system

An apparatus for estimating the location of a remote node, the apparatus comprising:

• an antenna array comprising a plurality of elements in a fixed spatial arrangement, at least one element being a transmitting element configured to transmit a first wireless signal to the remote node, and at least two elements being receiving elements configured to receive a second wireless signal transmitted by the remote node in response to the first wireless signal, wherein the first wireless signal is used to measure a first time-of-arrival between the apparatus and the remote node and the second wireless signal is used to measure a second time-of-arrival between the remote node and the apparatus;
• and
• a signal processing unit connected to the antenna array, the signal processing unit being configured to:
• estimate a plurality of round trip distances using the first time-of-arrival and second time-of-arrival based on the first and second wireless signals, each round trip distance being from the transmitting element to the remote node and back to the receiving element;
• and
• estimate the location of the remote node using the round trip distance estimates;
• wherein each receiving element is configured to measure the phase of the second wireless signal relative to a common reference phase, and the signal processing unit is configured to estimate the location of the remote node using the phase measurements as well as the round trip distance estimates.