Download automatic astro-navigation

FLIGHT International, 22 November 1962
AUTOMATIC
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ASTRO-NAVIGATION
By A. A. B R O W N ,
ELESTIAL navigation has been brought into great prominence with the advent of modern high-speed jet aircraft,
and the increased accuracy now required, coupled with space
limitations and the greater work-load on the flight crew, have
drastically changed the requirements for navigational equipment.
Automatic astro-navigation equipment developed by Kollsman.
with inherent accuracy and simplified operation, provides a system
particularly adapted to requirements of modern aircraft.
Fundamentally, when used in aircraft, auto-astro provides two
primary outputs. Tt allows the navigator to correct his knowledge
of position and it indicates very accurately the true heading of the
aircraft. Both of these outputs are independent of errors cumulative with time, which are characteristic of dead-reckoning navigation such as Doppler or inertial navigation.
True heading output can also be used for other purposes such
as the initial and precise in-flight alignment of inertial platforms
and azimuth gyros and the subsequent monitoring of these platforms; and it can be obtained, without the use of inertial systems,
to accuracies at least one order better than that available from conventional magnetically referenced systems.
Celestial navigation has been practised for centuries in one form
or another, but rapid and positive position finding by the stars,
to the accuracy demanded today, has finally been accomplished
by the use of automatic astro-navigation equipment.
The inherent accuracy of celestial navigation methods is dependent on three factors, an accurate knowledge of time, an accurate
observation and an accurate reduction of the data. The cumulative errors with time of flight inherent in inertial systems do not
exist in celestial position-fixing.
The apparent motion of an astronomical body on a celestial
sphere is a combination of the real motion of the body in space and
rotation and revolution of the earth. The precise position of an
astronomical body on the celestial sphere at the instant of observation is obtainable in the Air Almanac to within 30sec of arc of
declination and GHA. These co-ordinates are the celestial sphere
counterparts of the_ terrestrial co-ordinates of latitude and longi-
C
Fig 2
BSc, AMIEE, A M I M e c h E , AFRAeS*
tude, used to express an observer's position on the earth's surface
(Fig 1). A sextant is used to measure the altitude of the celestial
body above the horizon, and this measurement, the time of observation and the ephemeris of the observed body are sufficient to
locate the observer on a locus which is a circle of equal altitude.
By the observation of two celestial bodies and the subsequent
narrowing of the loci to the intersection of two circles of equal
altitude, the possible positions of the observer can be reduced to
the two points of intersection. Since the two intersections are in
most cases widely separated, there is virtually no ambiguity and a
two-body fix is thus obtained. But if a third celestial body is observed there is no ambiguity of position at all (Fig 2).
Obtaining a three-body fix
Fig I
Celestial/terrestrial sphere
In addition to measuring the altitude of the celestial body, the
astro tracker unit also precisely measures the angle between the
centre-line of the aircraft and the line of sight of the star projected
to the horizontal plane (Fig 3). The azimuth angle, or relative
angle to north of the celestial body can be computed using the
basic knowledge of time, declination, latitude and local hour angle.
By combining these two angles it is possible to establish to a high
degree of accuracy the true heading of the aircraft.
Kollsman manufacture a wide variety of automatic astro-navigation systems, tailored to specific applications. At present,
B-52 and B-58, the GAM-77A missile and many other US and
British military aircraft, as well as orbiting vehicles and submarines,
are being fitted with such equipment. The KS-85 astro-navigation
system, standard installation on B-57s, is typical of the other
systems, and this equipment is described overleaf. The complete
system is composed of an astro tracker, a control computer and
indicator, and an amplifier unit (Fig 4).
* Managing Director, Kollsman Instrument
B
Ltd