Beam-riding, also known as Line-Of-Sight Beam Riding (LOSBR) or beam guidance, is a technique of directing a missile to its target by means of radar or a laser beam. The name refers to the way the missile flies down the guidance beam, which is aimed at the target. It is one of the simplest guidance systems and was widely used on early missile systems, however it had a number of disadvantages and is now found typically only in short-range roles.
Beam riding is based on a signal that is pointed towards the target. The signal does not have to be powerful, as it is not necessary to use it for tracking as well. The main use of this kind of system is to destroy airplanes or tanks. First, an aiming station (possibly mounted on a vehicle) in the launching area directs a narrow radar or laser beam at the enemy aircraft or tank. Then, the missile is launched and at some point after launch is "gathered" by the radar or laser beam when it flies into it. From this stage onwards, the missile attempts to keep itself inside the beam, while the aiming station keeps the beam pointing at the target. The missile, controlled by a computer inside it, "rides" the beam to the target.
Beam riding is one of the simplest methods of missile guidance using a radar. It was widely used for surface-to-air missiles in the post-World War II era for this reason. An early example was the British Brakemine, first tested in 1944.
Early tracking radars generally use a beam a few degrees wide, which makes it easy to find the target as it moves about. These radars are normally used in conjunction with wide-scanning early warning radar systems, although in modern systems the two functions can be combined. When a target is detected, some radars had the ability to "lock on", and track the target automatically.
Beam riding systems can be easily adapted to work with such a system. By placing receiver antennas on the rear of the missile, the onboard electronics can compare the strength of the signal from different points on the missile body and use this to create a control signal to steer it back into the center of the beam. In practice, the systems used additional information from the signal, such as conical scanning, to more accurately calculate the centerline. With this simple system, the majority of the tracking problem is offloaded from the missile to the launching platform. In early examples, these were ground-based radars with all the equipment necessary for tracking.
The inherent disadvantage of the radar beam riding system is that the beam spreads as it travels outward from the broadcaster. As the missile flies towards the target, it therefore becomes increasingly inaccurate. This is not a problem at short ranges, but as many early surface-to-air missiles were designed to work at long ranges, this was a major issue. For example, earlier versions of the RIM-2 Terrier missile introduced in the 1950s were beam riders, but later variants employed semi-active radar homing to improve their effectiveness against high-performance and low-flying targets. In contrast to beam riding, semi-active guidance becomes more accurate as the missile approaches the target.
Another issue is the guidance path of the missile is essentially a straight line to the target. This is useful for missiles with a great speed advantage over their target, or where flight times are short, but for long-range engagements against high-performance targets the missile will need to "lead" the target in order to arrive with enough energy to do terminal manoeuvres. A possible solution for this problem was to use two radars, one for tracking the target and another for guiding the missile, but this drove up implementation costs. A more common solution for long-range missiles was to guide the missile entirely independently of the radar, using command guidance, as was the case for the Nike Hercules. Pure radar beam riding was rare by 1960.
Beam riding guidance became more popular again in the 1980s and 90s with the introduction of low-cost and highly portable laser designators. A laser beam can be made much narrower than a radar beam while not increasing the size of the broadcaster. Additionally, it is simple to encode additional information in the beam using digital means, which has a number of advantages. Missiles with small optical receivers on their tail can beam-ride on lasers with similar ease as earlier radar beam systems, but will be inherently more accurate.
Additionally, as the beam is very narrow by design, less power is needed than a semi-active design where the volume of space being "painted" is generally larger, in order to ensure the missile body does not block all of the signal. This makes it more difficult to be noticed by the target's warning receiver. Very low power signals can be used.