Over the Horizon Radars (OTHRs) operate by reflecting signals in the high frequency (HF, 2-30MHz) band off the ionosphere at altitudes around 250 km to provide a detect and track, beyond line of sight (BLOS) capability out to ranges between 500 km and 3000 km.
They are generally used to provide long-range air defence and intelligence capabilities. OTHR can also be used to monitor sea-state and sea ice. Whilst a conventional OTHR is formally a bistatic radar it can often be considered monostatic at typical detect and track ranges; consequently it can only measure the speed of a target along the look direction. This can mean the detection and track of ships is a problem for conventional OTHR. Furthermore, the detection and track of fast multiple targets are also a challenge.
To overcome the above limitations, we have proposed a relocatable and mobile, networked OTHR (NOTHR) system with a number of key features because it will employ a large transmitting antenna array and a small receiving antenna array which is contrary to the conventional configuration. Firstly, it will be able to detect and track low velocity targets against clutter; secondly it will be able to determine the target velocity (as opposed to the line-of-sight speed) within a wide search area and thirdly, the radar receiver will be able to stare at (not scan) multiple targets.
A crucial part of the development of this NOTHR system is a real-time ionospheric model, based on data from a variety of ionospheric sensors which may be part of, or independent from the NOTHR facilities. Optimally combining many different types of sensor data through a process of assimilation into a background model of the ionosphere will be necessary.