The Conical Scan
Figure 1: At the conical scan the antenna traces a cone pattern around its central axis
Some older target tracking radars use the conical scan to determine the exact target direction.
You can generate a conical scan pattern, as shown in figure 1, by using a rotating feed driven by a motor in the housing at the rear of the dish. The axis of the radar lobe is made to sweep out a cone in space; the apex of this cone is, of course, at the radar transmitter antenna or reflector.
At any given distance from the antenna, the path of the lobe axis is a circle. Within the useful range of the beam, the inner edge of the lobe always overlaps the boresight axis.
Let's assume that the conical scan is used for target tracking. If the target is exactly on the central axis, a relatively constant but very small echo is always received. But if the target is even a little away from this axis, then the levels change during an antenna beam rotation. For example, if the target is too far to the left, then the echo signal will show a maximum if the antennas radiator motor drive points to the left and if it points to the right, then a minimum is received. So if the target moves a little to the left from the minimum, then a computer evaluates this information and generates control signals for the servomechanism of the antenna, which now also moves a little to the left until a constant level of the echo signal is received again. In this way, the direction to the target is always determined automatically and very precisely in lateral and elevation angles.
Figure 2: An animation of the Conical Scan
It is also possible to perform a conical scan pattern only while the radar is in receive mode only (called COSRO, from Conical Scan on Receive Only). Different antennas are used for transmitting and receiving. Only the receiving antenna uses the conical scan. The antenna diagram of the transmitting antenna must be wide enough to cover all received directions of the receiving antenna.