A system for proximity sensing on a marine vessel, the system comprising: a main inertial measurement unit positioned on the marine vessel at a main installation attitude and a main location; a first proximity sensor (72, 74, 76, 78) configured to measure proximity of objects from a first sensor location on the marine vessel; a first sensor IMU (62, 64, 66, 68) positioned on the marine vessel at the first sensor location and at a first installation attitude; a sensor processor configured to: receive main IMU data from the main IMU and first IMU data from the first sensor IMU (62, 64, 66, 68), wherein the main location of the main IMU on the marine vessel is known and at least one of the first sensor location and first installation attitude of the first sensor IMU (62, 64, 66, 68) on the marine vessel are initially unknown; characterised in that the sensor processor is configured to: determine a relative orientation transform between the main installation attitude and the first installation attitude by comparing the main IMU data and the first IMU data; and determine a relative position transform between the main location and the first sensor location based on the relative orientation transform and comparison of the main IMU data and the first IMU data. a main inertial measurement unit positioned on the marine vessel at a main installation attitude and a main location; a first proximity sensor (72, 74, 76, 78) configured to measure proximity of objects from a first sensor location on the marine vessel; a first sensor IMU (62, 64, 66, 68) positioned on the marine vessel at the first sensor location and at a first installation attitude; a sensor processor configured to: receive main IMU data from the main IMU and first IMU data from the first sensor IMU (62, 64, 66, 68), wherein the main location of the main IMU on the marine vessel is known and at least one of the first sensor location and first installation attitude of the first sensor IMU (62, 64, 66, 68) on the marine vessel are initially unknown; characterised in that the sensor processor is configured to: determine a relative orientation transform between the main installation attitude and the first installation attitude by comparing the main IMU data and the first IMU data; and determine a relative position transform between the main location and the first sensor location based on the relative orientation transform and comparison of the main IMU data and the first IMU data. receive main IMU data from the main IMU and first IMU data from the first sensor IMU (62, 64, 66, 68), wherein the main location of the main IMU on the marine vessel is known and at least one of the first sensor location and first installation attitude of the first sensor IMU (62, 64, 66, 68) on the marine vessel are initially unknown; characterised in that the sensor processor is configured to: determine a relative orientation transform between the main installation attitude and the first installation attitude by comparing the main IMU data and the first IMU data; and determine a relative position transform between the main location and the first sensor location based on the relative orientation transform and comparison of the main IMU data and the first IMU data. determine a relative orientation transform between the main installation attitude and the first installation attitude by comparing the main IMU data and the first IMU data; and determine a relative position transform between the main location and the first sensor location based on the relative orientation transform and comparison of the main IMU data and the first IMU data.