Radar System Engineering

216 THE EMPLOYMENT OF RA DAR DATA [SEC.7.2
of dead-reckoning computers, or odographs, have been developed. The
control box of one such computer is shown in Fig. 72. This device,
developed in the Division 17 program of the NDRC, receives information
from a true-airspeed meter and a repeater compass. These data are
resolved into cartesian velocity components along the north-south and
east-west directions, and these velocity values are integrated to provide
continuous readings of air position with reference to any chosen point of
departure.
A mechanism isalsoprovided to indicate ground position by taking
account of the effect of wind. The direction and speed of the wind (as
determined from double drift readings, meteorological information, comparison
of earlier wind settings with the actual course made good, radar
fixes, or otherwise) can be set into the device by means of knobs shown
in Fig. 7.2. Wind velocity is resolved
into components along the
north-south and east-west directions,
integrated, and. the result
added to the air position to give
the ground position with respecb
to the point of departure.
Air position and actual ground
position both appear on the
counter dials shown at the lower
left. The output signals of the
unit can also be used to actuate
a plotting arm which moves over
1. ..; a map table not shown in Fig.
FIG. 7.2.—Control box of aircraft odograph
or dead-reckoningcomputer.
7.2. With this addition, the device
draws on a map the actual
ground track made good by the aircraft carrying the instrument. Various
map scales can be selected for the plotting table by means of a knob
on the control box. The control box can also be provided with a gear
train that actuates a mechanism showing latitude and longitude directly,
in addition to showing the departure from the last fix. In a carefully
made installation, the cumulative error of this instrument amounts to
only ~ per cent of the total distance traveled from the point of departure.
Ground-position Indicator, GPI.—Although the device just described
aids in performing dead reckoning, and to this extent assists in radar
navigation, it is convenient to present the results of the dead reckoning
directly on the radar scope. This has been done in a device called the
“ Ground-position Indicator,” or GPI.
The GPI provides an electronic index for the PPI tube of the radar
in the form of the intersection of a circle of constant range and an azimuth