Radar System Engineering

SEC. 7.2] AIDS TO INDIVIDUAL NAVIGATION 217
marker. Once this index has been set on a radar echo by means of
adjusting knobs provided on the device, the index will move across the
face of the tube with the echo, provided that the wind setting has been
made properly, regardless of maneuvers of the aircraft.
A simplified schematic diagram of the GPI is shown in Fig. 7.3. To
the left of the box marked “ Rectangular to polar coordinate resolv w,”
the device is identical with the dead-reckoning computer just described.
Since the basic coordinates of the PPI are range and azimuth, it is necessary
to convert the cartesian position information to polar coordinates;
this is done in the resolver shown. The azimuth resulting from this
operation is compared with the instantaneous azimuth of the radar
True
air
speed
N.S fix knob
Compass
repeater — SpeedD
4 ON-S rate knob Altitude knob
1,
&ound position
integrators for
N-S and E-W
components
wind constant.speed speed
mechanism a E-Wrate knob
.* Azimuth from
radar scanner
E-Wfix knob
FIG. 7.3.—Simplifiedschematicdiagram of Ground Position Indicator.
scanner. When the two are equal, an azimuth mark is put on the radar
indicator electronically.
The range information provided by the resolver is measured horizontally
along the ground; it must be corrected for the altitude of the
aircraft if, as is usual, slant range is displayed by. the radar indicator.
This is done by means of the electrical triangle-solver labeled ‘‘ Slant range
circuit, “ into which the value of aircraft altitude is set by the operator.
The value of slant range computed by this device is used to put a range
marker on the PPI at the appropriate distance from the origin of the
sweep.
Taking a fix with the GPI involves turning the Ii-S and E-W fix knobs
until the intersection of the range and azimuth marks appears on top of
an identified target signal. The “fix” dials will then read the ground
range of the target from the aircraft, resolved into north-south and eastwest
components. As the aircraft moves, these dials will continue to
read the correct position of the target relative to the aircraft and the