Radar System Engineering

88 PROPERTIES OF R.41~.l R T.-lRGE~,~ [SEC. 311
denoting it by a function F(o). The aspect function F(e) {vill thus
include three effects: (1) the amount of power intercepted by the area .4
will depend upon the orientation of the surface with respect to the
incident wave; (2) only apart of the incident radiation will be reradiated
diffusely, the rest being absorbed (or, as in the case of sea return, reflected
specularly); (3) the scattered power may be reradiated preferentially
in certain directions, depending on the properties of the scatterers. If
we let F(6) include these three factors, the expression for the effective
cross section of an extended surface target becomes
u = (fik) . (+CTsec 0) F(6), (31)
Substitution of Eq. (31) into Eq. (30) will give the received signal S.
At medium and long ranges (compared to the altitude of the airplane),
the factor sec 8 is approximately unity and can be neglected. On the
other hand, for the computation of the altitude signal,l Eq. (31) is not
sufficiently precise, because the aspect angle o varies considerably over
the large area, directly below the airplane, which lies within the range
interval AR when R is approximately equal to h, the altitude. In that
case, the received signal must be obtained from an integration ovrr the
large area contributing to the instantaneous pow-er level in the radar
receiver. z
GROUND-PAINTING BY AIRBORNE RADAR
BY C. F. J. OVERH.~GE.w~ R. E, CI..\PP
Airborne radar equipment has been extmsively used in military
aircraft for navigation by pilotage under conditions of restricted visibility.
The performance of these radar sets in displaying topographic
features below the aircraft depends on point-to-point variations in the
radar-reflection properties of the earth’s surface. The information
contained in the received echo signals is generally presented to the
observer as a brightness pattern on an intensity-modulated persistentscreen
cathode-ray tube in which radial distance from the center corresponds
to slant range or ground range, and azimuth to relative or true
bearing. While the coordinates of this plan-position indicator (PPI)
presentation thus lend themselves tocornparison withmaps, the correlation
between the brightness pattern and the topographic features of
the ground is a matter of varying difficulty, depending on the nature
of the terrain, the experience and skill of the operator, ancl the particular
1The first signalto arrive is the reflectionfrom tllr ground directly I]elleaththe
aircraft; it is called the “altitu(ie signal” ljcctlll>(,itsr:illge isc,cltl:ll[otlle altitudeof
the aircraft.
ZSee the chapter on “The .MtitudeSignal,)’ R. E. (’IJPP, “.i Theoretical and
ExperiIllentalStudy of Radt~rGrotlr){iRet[lrll.'' RI, Ileport So. 1024,lYW