This invention relates to a radar system which is defined for the purpose of this specification as apparatus for deriving information about the position or movement of a feature in a medium surrounding the apparatus by transmitting energy, through the medium, to a feature in said medium and observing the energy travelling to the said apparatus from the aforesaid feature. Whilst the invention is particularly applicable to apparatus which transmits and receives microwave radiation it is also applicable to acoustic or optical systems. Also, whilst the invention is particularly applicable to radar systems which sense the direction and time of arrival of the energy it is also applicable to systems which sense just the direction or just the time of arrival. It is also applicable to equipment which observes a Doppler shift between the transmitted and received signals. The term "radar" thus includes range finders, direction finders, and speed measuring equipment.
The invention particularly relates to radar systems employing pulse compression. In such a system a relatively long pulse is modulated before transmission, e.g., by sweeping its frequency from one value to another between the beginning and end of the pulse. On reception, the modulation of the pulse allows it to be compressed into a short time period. The use of pulse compression allows the transmission of a pulse which, because of its relatively long duration, has a relatively high energy whilst avoiding a loss of range resolution which would ordinarily be expected with an increase in pulse length.
When a pulse is received after reflection from two features at slightly different ranges, the received echoes of this pulse may overlap in time. The pulse compression system is normally able to separate such overlapping echo signals. However, if the aforementioned features are at close range the received power may be so strong as to saturate initial stages of the receiver, causing it to behave non-linearly. This prevents the pulse compression system from operating properly and thus a close range target adjacent to another larger target or a large source of clutter may not be detected. This problem was discussed by Glasgow in U.S. Pat. No. 3,945,011 at the time when pulse compression was first being considered for use in civil aviation radars. The Glasgow Patent proposed a solution to the problem which involved the transmission of a short unswept pulse after each long swept pulse. The long pulses were used in the receiver after reflection from long range targets and were subjected to pulse compression whilst the short pulses were used when reflected from short range targets and were not subjected to pulse compression. The aforementioned problem was thus overcome.
It is a principal object of this invention to provide an alternative solution to the same problem as that dealt with by Glasgow and described above.