US Patent 7767945 Absolute time encoded semi-active laser designation

SAL designation uses absolute time coding of the pulse-stream to disambiguate the designator in a manner that reduces the number of pulses in a pulse-stream and reduces the total pulse energy on the target. This requires timing synchronization between the designator and receiver. For improved rejection of unintended returns, more precise time gating (narrower absolute time window) is required. This can be achieved by removing the path length and or firing time uncertainty errors. Absolute time coding reduces the number of pulses and total energy on target in two ways. First, the designator may only have to transmit the pulse-code once. The “spot” appears for the brief time associated with a pulse-stream and disappears; continuous lasing of the target is not required. Second, the designator can be disambiguated using a combination of pulse-code (relative spacing of pulses) and pulse-position (absolute timing of pulses) modulation. This allows the pulse-stream to be shortened considerably, perhaps to a single pulse.