Technical Background Currently, in radars searching and tracking the path of projectiles, successive rounds from the same gun or other trajectories will be detected and tracked with the radar using the same frequencies each time. Thus both the detection probability and the tracking errors will be correlated, since these depend on the frequency characteristics of the radar cross-section (RCS). RCS is the measure of a target's ability to reflect radar signals in the direction of the radar receiver.

Solutions are known in which the antenna orientation/position of the radar are changed. However, changing the antenna orientation/radar position will cause a period of time where the radar is not searching/tracking.

Another solution is to use two or more radars. Using several radars to get better search and tracking is expensive.

Brief summary of the invention It is an object of the present invention to provide a method in which a radar can obtain a better ability to detect and follow an object without having to relocate the radar or use more radar units.

This is obtained in a method as defined in the appended claims.

Briefly, the method consists of mechanically tilting the antenna and compensating for the tilting with beamsteering, said beamsteering being performed by changing the frequency of the emitted signal. For successive rounds from the same gun or other trajectories that are repeated, each round will be tracked using different frequencies. As a consequence, the detection probability will increase and the tracking error will be reduced, without the need for a break in operation time or the use more than one radar.

Brief description of the drawings The invention will now be described in detail in reference to the appended drawings, in which Fig. 1 shows an antenna that is being tilted in elevation, Fig. 2 is a schematic diagram of the hardware of a radar system, Fig. 3 is a sequence diagram showing the commands issued for changing the mechanical orientation of the antenna, Fig. 4 is a diagram showing the commands for adjusting the search horizon in view of the new antenna orientation, Fig. 5 is a diagram showing the sequence for predicting the next track point of a moving object, Fig. 6 is an example of how a tracking system can be implemented in a DPU.

Detailed description of the invention We want to steer the beam at the same place for search but with a different frequency to get a different RCS. This can be done by mechanically tilting the antenna and compensate the tilt by electrically steering, the beamsteering being performed by changing the frequency of the emitted signal.

The compensation must be done while the antenna is moving.

To be able to do this, we must measure the antenna orientation at a high rate. It is important that the compensation is done while the antenna is moving so we can search and track continuously.

Figure 1 shows the antenna at two different tilt angles, but we want to steer the beam at the same target. To do this, we can change the frequency so the electrical steering of the beam compensates for the mechanical tilt difference. In the figure the electrically steering must be IF and T'.

When the radar is tracking a target it shall predict the trajectory of the target to be able to steer the beam so it follows the target. In addition to this prediction, we must also take the antenna orientation into account and predict the antenna orientation the next time we measure the target position.

A schematic diagram of the hardware is given in figure 2.

The navigation system unit is located on the antenna and measures the antenna orientation. The turntable unit is responsible for physically/mechanically changing the antenna orientation. The Signal Processing Unit (SPU) and Data Processing Unit (DPU) can be located on the same or different hardware. These units can be constituted by PCs, workstations or real-time systems. The division of work tasks between SPU and DPU is a matter of convenience ; the SPU is often a fast processing front end unit realized in discrete electronics, while the DPU is a PC that handles the signal at a lower speed. As the processing rate of PCs increases, they will be able to handle faster signals and can then take on some of the tasks from the SPU.

Sequence diagrams Steering the antenna orientation We want the antenna to mechanically move continuously or with pauses between the movements. This is done by commanding the turntable unit to change the antenna orientation, as shown in figure 3.

Compensate for search The data processing unit tells the signal processing unit where the search beams shall be. Because the antenna is moving, these search beams must be adjusted in elevation.

Based on the antenna orientation, the DPU either gives the SPU new search beams or a message with elevation offsets for different elevation angles. This is shown in figure 4.

Compensate for tracking When the radar is tracking a target, we must steer the beam where it is most likely to see the target at the next measurement. When the antenna is moving, we must also take the antenna movement into consideration. To do this we can find the position of the target when we receive a track message from the SPU, and then predict where the target is at the next measurement (predicted track point), estimate the antenna orientation at the next measurement time, and finally, adjust the predicted track point based on the information on the antenna orientation. This is shown in figure 5. Figure 6 shows one possible solution in the DPU.

While the invention has been described with an antenna that is tilted in the elevation direction, the same principle can be used for adjusting the beam in any other direction.