A Radar Beacons is a small radar set consisting of a receiver, a separate transmitter and an antenna which is often omnidirectional. When another radar transmits a coded set of pulses at the beacon, i.e., interrogates it, the beacon responds by sending back its specific pulse code. The pulses from the beacon, or transponder as it is often called, may be at the same frequency as those from the interrogating radar, in which case they are received by the main station together with its echo pulses. They may alternatively be at a special beacon frequency, in which case a separate receiver is required by the interrogating radar. Note that the beacon does not transmit pulses continuously in the same way as a search or tracking radar but only responds to the correct interrogation.
One of the functions of a beacon may be to identify itself. The beacon may be installed on a target, such as an aircraft, and will transmit a specific pulse code when interrogated. These pulses then appear on the PPI of the interrogating radar and inform it of the identity of the target. The system is in use in airport traffic control and also for military purposes, where it is called identification, friend or foe (IFF).
Another use of radar beacons is rather similar to that of lighthouses, except that radar beacons can operate over much larger distances. An aircraft or ship, having interrogated a number of beacons of whose exact locations it may be unaware (on account of being slightly lost), can calculate its position from the coded replies accurately and automatically.
The presence of a beacon on a target increases enormously the distance over which a target may be tracked. Such active tracking gives much greater range than the passive tracking so far described, because the power transmitted by the beacon (modest though it normally is) is far in excess of the power that this target would have reflected had it not carried a beacon. This is best demonstrated quantitatively, as in the next section.
Beacon Range Equation:
Following the reasoning used to derive the general radar range equation, we may change Equation (16-18) slightly to show that the power intercepted by the beacon antenna is given by
where all symbols have their previously defined meanings, except that the subscript T is now used for quantities pertaining to the transmitter of the main radar, and B is used for the beacon functions. AOB is the capture area of the beacon’s antenna.
If Pmin,B is the minimum power receivable by the beacon, the maximum range for the interrogation link will be
Substituting into Equation (16-22) for the power gain of the transmitter antenna from Equation (16-11), and for the minimum power receivable by the beacon from Equation (16-15), and then cancelling, we obtain the final form of the maximum range for the interrogation link. This is
It has been assumed in Equation (16-24) that the bandwidth and antenna temperature of the beacon are the same as those of the main radar By an almost identical process of reasoning, the maximum range for the reply link is
To calculate the maximum (theoretical) range for active tracking, both Equations (16-24) and (16-25) are solved, and the lower of the two values obtained is used.