− angular velocity of the platform,
i
a
− input acceleration, P − pendulosity of
the gyro floats about the output axis, H − gyro wheel angular momentum.
The platform angular velocity is usually read out by means of either an optical or
electromagnetic digital encoder. These devices produce a pluses train whose frequency
varies with the platform angular velocity.
If a pendulous gyro accelerometer is carried in an instrumented geographic
coordinate frame, there will exist a component of the earth's angular velocity about the
sensitive axis, the resulting specific force error will be in the range of a few mGal to a
fraction of an mGal depending on the scale factor of the particular instrument. For the
instrument described above, the error would be about 1 mGal at the poles. If the error
is significant, it can compensated by either introducing a compensating torque to the
gyro float through the torque generator, or by mounting the instrument on a table driven
about the vertical so as to null the vertical component of earth rate.
A PICA is currently being prepared for flight testing as a gravimeter by MIEIA
institute in Moscow.
Several sensors developed for land or sea use, such as the LaCoste-Romberg, the
Askania-Graf, and the Worden, has been modified for airborne use. These devices all
have been successfully tested in an airborne environment, but they do have some
disadvantages, primarily in the areas of data readout and dynamic range. There exists
a large class of specific force sensors developed for use as accelerometers in guidance
and navigation system. Several of these sensors seem particularly well suited to use in
AGS. One of the more promising devices, the pendulous integrating gyro
accelerometer or PIGA, is currently being readied for flight tests by the MIT
Experimental Astronomy Laboratory under Air Force Cambridge Research Laboratory
sponsorship. It is probably neither economically nor technically feasible to choose a
single navigation technique such as Doppler, inertial, etc. That can fully meet the
requirements of as AGS. Such system should be capable of indicating velocity to 0.5
knot or better and position to 0.5 mile or better for long duration flights at 500 knots.
An examination of the currently available sources of altitude data shows that a direct
and continuous determination of sea-level altitude to the accuracy required by an AGS
is not possible using any single source of information. Radar altimeter appears capable
of supplying data on sea-level altitude to a sufficient accuracy, but only when over
regular terrain or water of known elevation.
Combination of air-mass velocity measurements with ground velocity and
heading information from the navigation system can, through use of Henry's correction,
yield information on the slope of the isobaric surface being flown. Additional data on
the height of this isobaric surface can be provided by periodic radar measurements, and
by measurements made at surface weather stations.
- 1582 -