The design criteria for framing thermal imagers are well documented. However those for thermal linescanners are less easily available & are normally directed towards sensors optimised for fast, low flying manned aircraft. This paper aims to outline the considerations & characteristics of an infra-red linescanner specifically designed & optimised for UMA ( Un-Manned Aircraft ) or RPV (Remote Piloted Vehicle ) use. One of the major problems that has to be overcome is the lack of reliable real time interactive operator control so that the system must be capable of efficient autonomous operation under all operational conditions. The design of this system is described and in particular the control of the thermal gain of the scanner is considered. Here, as the scene is not multiply scanned, the use of the standard iterative algorithms derived for framing sensors could cause a serious performance degradation causing targets to be missed. To solve this problem the characteristics of various target scenes were analysed, (these included land, sea and coastal targets, both in the clear and in clutter). A method was derived for the linescanner to measure scene characteristics and to implement a predictive gain control to maximise target information. Also covered is the design of the main optical assembly to minimise manual alignment and simplify any field stripping that may be required and to maximise the reliability of a precise optical system in the highly hostile RPV environment. Finally the performance of the Barr And Stroud Infra-red Linescanner ( BASIL ), built from the above design work , is summarized together with the operational experience gained.