being carried out all around the target, allowing it to track in both altitude and azimuth. Systems that performed this automatically were known as "lock on" or "lock follow". 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. When used with conical scanning, the comparison can use several sets of paired antennas, typically two pairs, to keep itself centered in both axes. This system has the advantage of offloading the tracking to the ground radar; as long as the radar can keep itself accurately pointed at the target, the missile will keep itself along the same line using very simple electronics. The inherent disadvantage of the radar beam riding system is that the beam spreads as it travels outward from the broadcaster (see inverse square law). 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. Laser beam riding Beam riding guidance became more popular again in the 1980s