TABLE OF CONTENTS If enough PRFs can be provided to resolve the growing number of range ambiguities that arise as the PRF is increased, pulse delay ranging can be employed successfully even at fairly high PRFs. However, a point is ultimately reached where the echoes return so "thick and fast" it is virtually impossible to resolve the ambiguities (Fig. 1). Range, if required, must then be measured indirectly, as in CW radars. The most common indirect method is linear frequency modulation, or FM, ranging. [1]
This chapter briefly describes the principle of FM ranging. It explains how doppler frequency shifts, which would otherwise introduce gross measurement errors, are taken into account and how a problem of ghosting similar to that encountered in PRF switching is handled. Finally, it briefly considers the accuracy which may be obtained with FM ranging.
[1]Another form of FM ranging which has advantages in some applications employs sinusoidal modulation.
With FM ranging, the time lag between transmission and reception is converted to a frequency difference. By measuring it, the time lag hence the range is determined.
In simplest form, the process is as follows. The radio frequency of the transmitter is increased at a constant rate. Each successive transmitted pulse thus has a slightly higher radio frequency. The linear modulation is continued for a period at least several times...
