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BACKGROUND OF THE INVENTION
This invention relates to improvements in aircraft flight safety control
systems and more specifically to a minimum safe altitude display and
warning system which automatically signals to the pilot on an ongoing
basis existing or imminent flight altitude requirements in order to avoid
hazard situations. The invention which may be applied to submarines, as
well as airplanes, for example, is herein illustratively described by
reference to its presently preferred forms as applied to aircraft and as
used in conjunction with currently available navigational computer
technology and related instrumentation. However, it will be recognized
that certain modifications and changes therein with respect to details may
be made and that the invention may be embodied in a variety of forms
without departing from the essential features involved.
A survey of prior art proposals potentially of interest as background to
this invention, revealed the following prior art U.S. Pat. Nos.:
2,568,568, 3,113,306, 2,585,605, 3,231,887, 2,606,317, 3,246,326,
2,809,340, 3,328,795, 3,060,426, 3,705,306, 2,582,588, 3,805,261,
3,680,046.
Of the foregoing patents, the following are believed to be representative
of the group in providing an indication of the state of the background art
of interest herein.
U.S. Pat. No. 3,113,306 discloses a fuel saving system involving a
precalculated trip altitude and vertical speed profile with deviation
monitoring to make appropriate enroute corrections in flight so as to
permit the aircraft to fly over a selected destination point at
preselected altitude.
U.S. Pat. No. 3,328,795 discloses a fix-taking or place-finding guidance
system using quantized topographic elevation data taken as a sequence of
discrete elevation readings at selected points during vehicle travel.
Position is determined for navigational purposes by comparison matching of
these sequential altitude values with prerecorded sequences also in
quantized form in the computer memory bank. Present position is determined
on the basis of "best fit" of the sequence patterns being compared. U.S.
Pat. No. 3,805,261 is generally of a similar nature.
U.S. Pat. No. 3,582,588 discloses a navigational system with a moving
recording medium representing aircraft motion associated with means
indicating altitude clearance and a means indicating oncoming elevation
hazards of which the pilot should be made aware in order to take
appropriate avoidance measures.
In U.S. Pat. No. 3,680,046 an assigned or command altitude corridor is set
by the crew using thumbwheel switches converting the altitude corridor
into digital form which is compared with a digital representation of
existing altitude using BCD logic to produce altitude deviation warnings.
A broad object of this invention is to provide an improved minimum safe
altitude indication and/or warning system that substantially avoids all
necessity associated with prior practices of relying upon the pilot's
observations, judgments and responses relative to ground hazards when
attempting to reduce altitude safely and to follow a safe course at low
relative altitude. A related object is to remove the added strain and
associated responsibility with attendant risk of human error placed upon
the pilot under those conditions regardless of whether or not radar
guidance or other hazard detection or course directing equipment or
personnel are available to assit. The risk, of course, becomes extreme
under poor visibility conditions, and yet, sometimes that risk is taken
because of operational mistakes or imperative demands to reduce altitude
in making final approaches.
In accordance with the present invention, by continuously indicating to the
pilot what is the minimum safe altitude and/or warning the pilot of
imminent descent below minimum safe altitude and by basing the
determinations thereof on data that is as reliable as the navigation
computer itself and other basic instrumentation, which data already
incorporates or is based upon "worst case" conditions, the necessity of
maintaining a collision "watch" and exercising low altitude hazard
avoidance judgments is minimized and the pilot can direct his principal
focus of attention upon other vital tasks.
In effect, the invention provides to the pilot ongoing information that
assures keeping the aircraft out of trouble from the standpoint of ground
hazards, while permitting the aircraft to be flown as low as safely
permissible within that specification. A related object hereof is to
provide an ongoing monitor and indication of minimum safe altitude that
combines the reliability of geographically based prerecorded present
position altitude data and projected position altitude data with the
reliability of navigation computer coordinate derivations.
A more specific object hereof is to provide a system which may be
implemented in a non-tactical or universally applicable form or
alternatively in a tactical form in which the prerecorded minimum safe
altitude data is utilized in conjunction with such variable factors as
ground speed and ground track as well as altitude change rate in order to
provide a higher degree of resolution in interpreting the prerecorded data
allowing the aircraft to fly safely at even lower indicated minimum safe
altitudes. In either design approach, the prerecorded minimum safe
altitude data is based on actual terrain features precisely located in
terms of geographic coordinates and related to other terrain features
similarly determined for location. These are accounted for in the
anticipatory or course projection determinations necessary to assure that
an aircraft in any given position can be safe only if above a certain
minimum altitude. Thus the determinations allow for terrain features
immediately ahead in whatever direction and at whatever velocity the
aircraft may be proceeding.
These and other objects and advantages of the invention will become more
fully evident from the description that follows
BRIEF DESCRIPTION OF THE INVENTION
This invention achieves its objectives by combining the basic capabilities
of conventional navigational computer means, operable to produce
coordinate signals ongoingly related to present position of the vehicle,
with data storage means in which is recorded by reference to geographic
coordinates predetermined minimum safe altitude values for all points or
grid sections that together make up or cover the geographic territory over
which the vehicle may navigate. In its rudimentary form the system is
completed by a data retrieval unit capable of utilizing the aforesaid
coordinate signals on an ongoing basis as the flight progresses
automatically to retrieve the currently applicable data from such storage
means and present it in an output. In a rudimentary form of the system the
prerecorded values of minimum safe altitude reflect not only man made or
terrain obstacle features, but also "worst case" flight assumptions for an
aircraft as it flies over each grid section. According to such "worst
case" assumptions, for example, the aircraft may be located at any point
in the grid section, it may be flying at any expectable velocity, it may
follow (or change direction to) any expectable course, and it may be
descending at any expectable rate. The predetermined, recorded minimum
safe altitude for the section, then includes not only allowance for the
terrain within the section, but also for the terrain in adjacent sections
within an arbitrarily selected range, regardless of direction of flight
out of the section.
As a further feature of the invention, the coordinate signals are further
related to aircraft ground speed and ground track. In making use of such
data, at least some of the "worst case" assumptions referred to
immediately above are eliminated from the prerecorded minimum safe
altitude data values for the respective sections and, instead, actual case
conditions are utilized with respect to those factors (i.e., ground speed
and track). As a result, the aircraft is given more tactical freedom by
the greater degree of ground hazard resolution capability allowed in the
selection of recorded minimum safe altitude values. As a result lower
minimum safe altitudes can often be presented as available to the pilot
that in the rudimentary system are foreclosed because of the necessity in
"worst case" planning to allow for any course heading and for any speed.
Similarly, additional resolution may be achieved by taking into account
present negative rate of change of altitude, if any, instead of assuming
the "worst case" possibility of the rudimentary system. The added
refinement provides still another basis in many situations to present a
lower minimum safe altitude as being "available" at different times along
a route.
By comparing minimum safe altitude determinations produced by the
monitoring system with actual altitude of the aircraft and detecting when
the difference becomes smaller than a predetermined value, visible,
audible or other sensory perception warning systems may be actuated,
drawing the pilot's attention to the situation. Likewise, the system may
include provisions by which these warning indications may be adjustably
advanced, for example, automatically in proportion to rate of descent of
the aircraft, to avoid risk of overshoot to unsafe levels.
These and other features of the invention, including combinations of
features thereof, will be evident from the more detailed description that
follows by reference to the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of the improved minimum safe altitude monitoring
system in a rudimentary form.
FIG. 2 is a map-like representation of a geographic mosaic of sections
defined by latitude and longitude grid lines that may be used along with
the terrain features or flight hazards therein for the purpose of
determining minimum safe altitude value for the respective sections or
areas.
FIG. 3 shows a somewhat more tactically oriented version of the system
shown in FIG. 1, and FIG. 4 shows a still more elaborate or refined
system, adding additional features.
DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS
The comparatively rudimentary system embodiment depicted in FIG. 1 is
intended for applications which, in effect, are nontactical, i.e., the
least demanding for high resolution and continuous updating of actual
minimum safe altitude determinations. Its readings and/or warnings are
universally applicable to all aircraft contemplated when computing the
minimum safe altitude values recorded, and to all flight conditions (i.e.,
velocity, track heading, etc.). For purposes of contrast, for example, a
tactically oriented system of this invention would apply to high-speed
low-level strategic military flight minimum safe altitude monitoring
wherein all possible measures must be taken to permit the aircraft to fly
safely as low as possible utilizing all available means to resolve and
compute the effect of oncoming ground hazard conditions in relation to
navigational position and predicted position of the aircraft.
In the case illustrated in FIG. 1, minimum safe altitude monitoring
requirements are met by recording, retrieving and utilizing minimum safe
altitude data on a gross area or low geographic resolution basis. In such
a system, as flight progresses along a chosen route, currently applicable
minimum safe altitude data recorded in memory device 12 for successive
discrete geographic areas being traversed is selectively retrieved from
the memory device by data retrieval unit 16 based on present position
coordinate signals received from navigation computer 10. This is performed
as an ongoing function providing either a continuous or intermittent
output that may be used in performing an indication, warning or control
function in the aircraft (i.e., such as automatic command to thrust and
attitude control to increase the aircraft's altitude). Memory device 12
may be of any suitable or conventional type capable of recording such
data. Examples of suitable memory devices for this purpose can be found in
core memory storage systems, tape memory storage systems or disc memory
storage systems. In the memory device 12 the specific values of minimum
safe altitude are systematically stored to be accessed selectively through
the appropriate channels of the device, such as by way of digital
registers which are organized pursuant to the system of geographic grid
lines employed to divide up the total navigational geographic area for
which the system is applicable. This may be accomplished in any suitable
manner using known technology or special technology to be developed for
the purpose, all as will be readily obvious to persons skilled in the
design and use of memory devices. The same applies with respect to the
nature and functioning of the data retrieval unit 16 or its equivalent as
a means responsive to present position coordinate signals derived from the
navigational computer 10 to selectively interrogate or access the memory
device 12 in order to retrieve the currently applicable values of minimum
safe altitude at different times during the progress of the vehicle on its
flight route.
No attempt is made in FIG. 1 to depict a particular form of response means
14 by which or through which the retrieved data value of present position
minimum safe altitude is put to any specific end use. As previously
stated, such end uses may vary. For example, the response means 14 may
comprise or activate a present position minimum safe altitude display that
the pilot and/or other crew members can read on the instrument panel of
the aircraft. The display may be either digital or galvanometric in form.
Alternatively, the response means 14 may comprise or activate a sensory
perceptible warning device that compares existing altitude with minimum
safe altitude and responds to an excessive difference so as to warn the
pilot of the approaching harzard condition.
With continued reference to the system of FIG. 1, more specifically the
total geographical navigational area of interest is effectively divided by
a grid system into discrete geographic areas or sections, each represented
in a corresponding memory "cell" or place in storage device 12. A single
data value representing minimum safe altitude for each area is determined
and prerecorded in the respective memory cells of device 12. Each such
data value thereby represents minimum safe altitude for any present
position of the aircraft within the effective boundaries of the related
geographic grid section or area.
The selection of these individual area minimum safe altitude values, as
well as the choice of grid line spacings defining the inidividual areas,
based essentially on an optimizing of the priorities given to three
interrelated, and to some extent, competitive design objectives:
1. The desire to have minimum safe altitude values recorded be as nearly
true safe minimums as feasible.
2. The practical consideration of limiting the total number of discrete
memory elements or cells required in an overall system so as to keep the
size and cost of the memory storage device within reasonable bounds. Here,
however, it should be noted that current progress in the art is making the
size and cost of "memory" capacity in apparatus of this nature rapidly
smaller, so that in time there may be no practical limitation of this
nature on the number of memory elements that can be used economically in
the system, and hence, no practical limitation on the degree of geographic
resolution applicable to the recording and retrieval of minimum safe
altitude data on a section by section basis.
3. The possible need, particularly when the system is designed for a coarse
grid mosaic (e.g., with area-defining grid lines widely spaced), to adjust
upwardly at least some of the area minimum safe altitude values so as to
avoid an excessive amount of increase in the recorded minimum safe
altitude value that can occur at the instant of crossing a grid section
line. If the monitor (system output) abruptly demands much higher minimum
safe altitude in the section being entered by the aircraft, the resultant
altitude maneuver required of the pilot in attempting to comply with the
change could be uncomfortable and perhaps in itself constitute a form of
danger, such as possible disregard to the system itself and a return to
dead reckoning methods. Avoidance of such a problem is readily achieved to
the extent necessary in a given design, however, simply by arbitrarily
increasing the recorded value of minimum safe altitude associated with
those grid section areas adjoining others for which the value is much
higher, thereby providing a moderating or dampening effect on the overall
pattern of section by section prerecorded minimum safe altitudes.
However, before reaching the question of making adjustments of the nature
discussed immediately above, basic determinations must be made of minimum
safe altitude values to be recorded for all the respective geographic
areas or grid sections represented. In so doing, these values are
determined in relation to a number of factors hereinafter discussed and
the observance of which forms a complete and omnibus basis for the pilot
of the aircraft in maintaining flight above minimum safe altitude. This
avoids the need to maintain a personal watch for, or to make personal
observations of instrument indications of coming specific hazards and the
like under conditions requiring rapid response to avoid impact. The pilot
now can safely assume under all expectable conditions that by heeding the
monitor the aircraft will be above safe altitude and still be permitted to
fly as low as it safely can at all times.
To achieve these ends in the rudimentary system described in FIG. 1, the
following factors and "worst case" considerations are taken into account
in determining the prerecorded values to be assigned as minimum safe
altitude values for each of the respective grid sections:
a. the aircraft's present position may be anywhere within the grid section.
b. The aircraft may be following any ground track (or may manuever to
change to any ground track) while within the grid section.
c. The aircraft may be flying at maximum expectable speed.
d. The aircraft may be descending at maximum expectable rate.
e. The time lag for the pilot and aircraft to execute a given change of
climb rate may be the maximum expectable.
The significance of the considerations and factors enumerated immediately
above is illustrated in the design and operation of the FIG. 1 system by
reference to FIG. 2. In this figure, longitude grid lines are designated
LO1, LO2, etc., whereas latitude grid lines are designated as LA1, LA2,
etc., (starting from territorial edges not shown). These latitude and
longitude grid lines make up a mosaic of geographic areas in the form of
square grid sections of selected size, such as one mile square. Other
geometric shapes could also be used for the sectioning if desired. In the
illustration, elevation-critical features include a high tower W and a
mountain peak A in section LO678, LO679-LA377, LA378. An airplane at point
P.sub.0 in section LO678, LO679-LA379, LA380 is shown flying on ground
track t. On this particular ground track, no notable elevation hazards are
imminent in the present position section or in the next two succeeding
sections in any direction of flight that would present an altitude hazard
above some preselected reference level. That reference level would be
typically the minimum required flight altitude level for the general
vicinity. Yet in determining what is to be the assigned minimum safe
flying altitude for the aircraft in its particular present position shown
in the figure for the rudimentary system design under consideration, the
presence of the town C (with possible high buildings and the like) and
peak B representing another potential hazard in next adjacent sections are
both considered, as is the presence of tower W and peak A, two sections
away. This is the case because minimum altitude selection assumes
arbitrarily that not only could the aircraft be at any location in the
section, but it could be on any ground track, and it is desired that the
determination be universal, that is, applicable to all flight conditions.
Stated otherwise, under the above enumerated worst-case assumptions
relating to the position of the aircraft, potential course changes,
maximum speed and response time required to execute corrective maneuvers,
the obstacles in establishing the minimum safe altitude for each grid
section include those obstacles lying within a region whose boundary is
the locus of a point which is a predetermined distance from the
latitudinal and longitudinal edges of that particular grid section. For
example, assuming a maximum expectable aircraft speed that allows
traversal of more than two grid section during a preselected time
interval, the minimum safe altitude that is stored in memory device 12 for
grid section LO678, LO679-LA379, LA380 of FIG. 2 is the altitude necessary
for safe overflight of the highest obstacle among tower W, peak A, peak B,
and town C. Thus, if an emergency develops or plans suddenly change and
the aircraft is required to change course and/or speed, the minimum safe
altitude reading still applies. Thus the pilot is assured he can safely
change course at any time without concern for new obstruction hazards to
consider.
Any of different available position tracking navigation devices can be
employed to provide aircraft position coordinate signals for purposes of
the system of FIG. 1. Preferably, of course, these signals are provided in
binary code for convenience in accessing the minimum safe altitude data
values stored by digital code indexing in memory device 12.
In the revised or more tactically refined embodiment shown in FIG. 3, it
becomes possible in the selection and prerecording of minimum safe
altitude values for the respective grid sections to omit "worst case"
assumptions b and c listed above in connection with the discussion of FIG.
1. Instead, utilizing the facilities of conventional navigation computer
means 10a, present ground track and present ground speed are determined
along with present position geographic coordinates to produce present
position coordinate signals. By simple projections of this data, predicted
position coordinate signals are produced based on a selected flight time
interval out of present position, such as one minute. These sets of
digitally coded coordinate signals are applied by the data retrieval means
16a to retrieve from the corresponding "slots" or cells of the memory
device the recorded values of minimum safe altitude for the respective
grid sections encompassing present position, predicted position within the
flight time interval selected together with those in between along the
present ground track. As an additional allowance or margin for error or
potential limited change of ground track direction within said selected
interval, computer 10a can also be programmed to provide the coordinate
signals of grid sections intersected by lateral zones on either side of
projected ground track t (i.e., between lines t.sub.1 and t.sub.2) and
thereby require correlator 16 to retrieve their respective recorded values
of minimum safe altitude as well. The highest one of those recorded values
of minimum safe altitude thus retrieved from memory 12a is then
automatically selected by the data retrievel means 16a, in order to
provide the controlling output that operates the response means 14. It
thus becomes possible at some points or in some areas along a route with
the system of FIG. 3 for tactical purposes, to fly lower at the indicated
value of minimum safe altitude than in the system of FIG. 1. This is true
because it is not necessary to take into account items b and c mentioned
above and, consequently, terrain features in grid sections off to the side
or behind the aircraft.
Thus, in terms of FIG. 2, the system of FIG. 3 predicting a position
P.sub.x for the aircraft within a selected flight time interval, such as
the next minute of flight from present position P.sub.0, operates through
data retrieval means 16a to retrieve from the memory device 12 the
recorded minimum altitude values only for those sections up to and
including position P.sub.x along the projected ground track. Preferably,
as indicated above, this ground track is translated as a widened or
divergent band of potential positions for the aircraft within the boundary
lines t.sub.1 and t.sub.2 centered on projected ground track t so as to
allow margin for error or change of course. In this example, therefore,
the minimum safe altitude values to be retrieved and compared for
selection of that which is greater includes those recorded for the
following grid sections.
LA379, LA380-LO678, LO679, LA377, LA378-LO677, LO678, LA379, LA380-LO677,
LO678, LA378, LA379-LO676 LO677, LA378, LA379-LO678, LO679, LA377,
LA378-LO676, LO677, LA378,LA379-LO677,LO678, LA376,LA377-LO677,LO678,
LA377,LA378-LO678,LO679.
In the system shown in FIG. 4, present altitude is provided by a suitable
means such as a conventional air data altitude computer 22 responsive to
pilot-static source inputs 24 and 26 representing total pressure and
static pressure, respectively. The present altitude signal produced by
computer 22 is in the form either of a digital value or an analog value
fed to conventional comparator 28. The latter also receives a present
position minimum safe altitude signal from data retrieval means 16a.
Either in the comparator or in the respective signal sources feeding the
comparator as mentioned, appropriate circuit arrangements assure that the
signals are in comparable terms or form enabling the comparator to
subtract one value from the other so as to produce a difference signal or
output that is compared in a warning circuit 30 against a reference value.
In the example, intended solely for purposes of illustrating desired
functions, the reference value is produced as the output of a selectively
variable reference potentiometer 36. When the difference between the
reference value signal and the difference signal produced by the
comparator drops to a predetermined level sensed in the warning device 30,
the warning device operates to trigger an audible alarm 32 and/or a
caution light indicator 34 on the instrument panel of the aircraft. This
alerts the pilot that minimum safe altitude is reached or is being
approached. By appropriate adjustment of the setting of the wiper arm of
reference potentiometer 36, any desired degree of advance warning may be
given to the pilot during descent of approach toward minimum safe
altitude.
As an additional feature shown in FIG. 4, negative altitude change rate is
utilized as another factor in determining the point at which the pilot is
warned or alerted in advance of the descending aircraft's approach to
minimum safe altitude. Again for purposes of illustrating function, to
utilize such negative change rate the system incorporates a suitable or
conventional altitude rate computer 38, the output of which is coupled to
the end of the winding of potentiometer 36 opposite the bias source E.
Thus, for a given setting of the potentiometer wiper and assuming no
change in the value E representing reference source energizing potential,
the amount of advance warning given to the pilot that minimum safe
altitude is being approached during aircraft descent goes up with
increasing descent rate. This enables the pilot more readily to respond in
time to avoid overshoot into an altitude lower than minimum safe altitude.
Additionally, an automatic pitch correction can be implemented in response
to sensing an approach to unsafe altitude level, this being indicated in
the diagram by the designated output arrow "To Pitch Controls." In a
production system the altitude rate computation and the advance warning
and/or control functions produced as minimum safe altitude is approached
may be performed in the air data computer 22 and/or in the warning
circuits 30.
If desired, potentiometer 36 may be calibrated to take into account pilot
reaction time and the responsiveness of the aircraft to change of its
angle of climb once the pilot makes corrective settings of the manually
operated flight control elements in the cockpit. Preferably, however,
potentiometer 36 is used solely for the latter purpose and the factors
associated with the response characteristics of the pilot and aircraft
itself are preferably set into the system by a separate potentiometer 50
providing an adjustable input to altitude rate computer 38. This setting,
for example, may be employed to vary the amplification factor of the
computer 38 to provide the effect described. Thus, for a given aircraft,
the setting of potentiometer 50 can be made once and for all as a factory
adjustment. Different settings would be used for other aircraft to reflect
their differing responses to change of thrust and attitude commands.
The system of FIG. 4 also illustrates the provision of a conventional radar
altimeter 20 as a check on terrain clearance beneath the aircraft,
particularly during low elevation flight conditions, and another indicator
for barometrically determined altitude. These and other conventional
instruments will, of course, be expected in the complete instrument panel
and navigation system of the aircraft.
These and other features and aspects of the invention, including variations
of the illustrated embodiments will be recognized by those skilled in the
art, and it is therefore intended that the claims set forth hereinafter
not be deemed restricted to the details of the illustrations as such.
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