754 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



nearby echoes on a frequency basis, much as pulse systems do on a time 

 basis. In FM-CW systems the shape of the signal versus range function 

 takes the form of the product of the Bessel function (of the same order as 

 the selected sideband), the sine function (of a period which is half of the 

 wavelength of the modulation frequency), and the normal range spread 

 signal reduction (usually the inverse square of the range). Hence, operation 

 to very low altitudes can be obtained if a sufficiently high modulation fre- 

 quency is used (several megacycles per second). However, the use of such 

 high modulation frequencies results in a relatively large number of "altitude 

 holes" (to be discussed below) within a given operational altitude range. 

 Due to the above-mentioned shape of the signal versus range function in 

 FM-CW systems, the velocity accuracy can decrease somewhat through the 

 undesirable weighting of signal returns from certain ranges, thereby affecting 

 the shape of the doppler spectrum. 



Both of the low-altitude phenomena of pulse and FM-CW systems 

 discussed above are similar and related to the so-called "altitude hole" 

 phenomenon in these systems. In fact, the low altitude region is often 

 referred to as the "first-order altitude hole." 



In pulse systems, the altitude hole phenomenon occurs at the periodic 

 altitudes at which the return pulse arrives near or at the time of the next 

 transmission pulse and can therefore be completely blanked out, if special 

 steps are not taken. The spectrum weighting error again occurs at and near 

 these altitudes, because of the gating-out process at certain ranges. Many 

 of the pulse systems employ a wobbled (frequency-modulated) pulse 

 repetition frequency (PRF) to reduce these altitude hole effects. In other 

 words, the pulse period is continuously changed (at a low rate) so that 

 some return signal is received at all altitudes. By this means, the signal 

 effect of the altitude hole phenomenon is largely eliminated and the 

 accuracy deterioration effect is greatly reduced. 



In some of the coherent pulse systems the pulse repetition rate is changed 

 in discrete steps at periodic altitudes, on the basis of barometric altimeter 

 information. For instance, the repetition rate is halved at every octave of 

 altitude (double the altitude) in order to yield a sufficiently long received 

 return signal at all times. The dependence of the operation of such a system 

 on barometric altitude information is a disadvantage, however, giving rise 

 to the possibility of operational difficulties over mountainous terrains. 



In FM-CW systems, altitude holes occur at the periodic altitudes 

 corresponding to multiples of the half-wavelength of the modulation 

 frequency as a result of the periodic signal versus range function discussed 

 earlier in this paragraph. Similar to the case of varying the pulse repe- 

 tition frequency in pulse systems, changes in the modulation frequency 

 are used in FM-CW systems in order to eliminate the altitude hole 

 problems. 



