Radar System Engineering

148 C-T RA DAR S YSTE.lf S [SEC. 59
We may note that l/-lA~ is analogous to the pulse time; it determines
the width of w-aveband transmitted and the range accuracy.
Next, one determines the modulation or repetition frequency j,.
As in pulse systems, this depends on the maximum range. But whereas
in pulse systems j, may actually be as much as c/2r, in the present systems
one must have f, markedly less than c/2r in order that not too large a
fraction of the time will be wasted while the sign of the beat frequency is
changing. Subject to this upper limit, the choice off, is determined by
considerations of bandwidth, response time, and apparatus convenience.
The smaller j,, the narrower the band and the slower the response.
Also, the beat frequency, which varies with ~,, should be kept in a range
suitable for the frequency meter to be used.
The maximum beat frequency is then determined as 4~.A~(r/c), and
an indicator working up to this frequency by steps of j, is designed. The
number of steps is thus about r ~~r.
This method of ranging, like the pulse method, makes no special
requirements on the carrier frequency, which is then chosen on the basis of
other considerations.
The power requirecl depends on the usual things, including the banclwidth,
which in this case is the bandwidth of one element of the frequency
meter—frhich should be about j..
This scheme, although it has not been highly developed, Trillundoubtedly
work much as describe(l. High range accuracy and small bandwidths
are possible-in fact one may ha~-e both at on~-e. Thus one
might ha~-e a repetition rate, ancl so a noise bandwidth, of, for example,
10 cps while lm~-ing a frequency swing of 40 31c sec., which \\-ouldgi~’e
range accuracy corresponding roughly to a *-psec pulse. And this
latter would be achie~-ed without a widebaml i-f or ~-ideo.
If, however, ~~e attempt to modify the system so as to allow the
presence of clutter, and therefore also a dopplei- shift of the signal coming
from the target, two difficulties arise.
First, although the clutter may, in principle, be eliminated by infinite
attenuation filters t~mmi to the repetition frequency and multiples
thm-eof, the nlmll>er of filters req~lired i< (of the ordrr of r ~r, and if this
number is lmgc the system may be impr~ctical. This limitation appeai-s
to be fundamental and arise.+in similar form in all other .s3-stems, But
cl-en if l\-e neglect the multiplicity of the filterx, there is considerable
doubt whethrr practical means of freq~leney modulation can be de~-ised
that will make succes.sire modulation cycles as nearly identical as is
necdecl for filtering out really serious clllttcr.
Second, as a re. ultof the dopplcr ~hift, e:ich target gives t~vo output
frequencies, and so two range indirati[ons The >eriom+nes (If this
depends on the ratio of the doppler frequenrl- to mean beat frequency,