82 SURVEY NAVIGATION, CORTEN
4. integration, of the speed (V,) over the time (t) so as to obtain the distance travelled
ft
$- | V, di;
0,
5. transformation of this information into usable form (by means of navigational com-
puters) and computation of the corrections which must be applied to gyros and to
platform.
Principle of operation.
The accelerations are measured with respect to a coordinate system which is re-
presented by the stable platform. The stability of this platform can be obtained by means
| LINE TO CENTRE
PS “OF EARTH
INTEGRATORS | Jee] à A-EARTH'S POLAR AXIS
NEC | | 0 AN
COMPUTER MULTIPLY E Y | Let
*| BY SEC a 1
W N |
p > EM
P d INTEGRATORS
ACCELERO-
METERS
[M : ii
ALL
LONGITUDE X
STABLE
PLATFORM
Fig. 22.
of a three-degrees-of-freedom gyro system in such a way that the platform is stable in
inertial space.
Figure 22 represents one of the possibilities of construction. Before the start of the
operation the primary axis of this system must be aligned with the earth’s polar axis;
this attitude is maintained throughout the operation. On the stable platform a horizontal
accelerometer platform is mounted; this can be horizontalized about two axes in such a
way that one of its accelerometers is oriented north-south and the other east-west.
If the vehicle does not move, the stable platform and the horizontal platform will
keep stationary with respect to space and consequently will rotate 15? per hour with
respect to the horizon according to the earth's rotation. In order to keep the accelero-
meter platform horizontal, a clock must correct the *polar axis" shaft continuously with
an angular speed equal to the negative earth's rotation speed.
As the accelerometer platform now remains stationary and exactly horizontal, no
accelerations are sensed by the accelerometers, not even the earth's gravity acceleration,
thus no signal is supplied to the integrators. The geographic position of the starting
point is now introduced into the navigational computer manually; its indication “latitude”
