Abstract: Disclosed is a collision avoidance device with reduced energy balance for aircraft, in particular for avoiding collision with the ground. This device comprises means for the geographical localization of the aircraft, a database containing a description of safety altitudes, means for the construction of an air floor about the vertical passing through the aircraft, the altitude of the floor being greater than or equal to the safety altitudes about this vertical, means for predicting the air position of the aircraft, means to compare this position with the air floor, means for the computation of avoidance paths and means to determine the energy balance of each of the computed paths. Application to equipment for aircraft, notably for civilian airliners.

Abstract: A ground proximity warning system (GPWS) for a rotary-wing aircraft provides a warning of excessive barometric altitude loss as a function of the product of time and the terrain clearance. The warning system is only enabled during a take-off mode of flight and is disabled during other modes of flight. The mode warning logic monitors the speed of the aircraft, the terrain clearance of the aircraft as well as the position of the landing gear to enable the altitude loss warning logic. When the barometric altitude loss as a function of the time product of the terrain clearance exceeds a predetermined value, the mode switching logic switches to an approach mode of flight to disable the excessive altitude loss warning system to minimize nuisance warnings during various maneuvers including ship-to-ship and oil platform-to-oil platform flight maneuvers.

Abstract: A mass memory stores a data base representing at least a substantial part of the terrestrial globe, in accordance with a grid configuration on several levels, said grid configuration being in particular more precise in the vicinity of an airport. Status indications are received representing the position of the aircraft with two horizontal components, and the altitude, and the velocity and acceleration vectors of the aircraft, as well as control indications coming from the flight deck. In accordance with the horizontal components of the position of the aircraft, a temporary local map is transferred into a fast access memory, on the basis of which map, an altitude envelope of the terrain is established in the zone where the aircraft is travelling. Anticollision processing make it possible to establish an alarm if the relation between a protection field and the altitude envelope meets a first condition which is defined at least partly by the control indications.

Abstract: The invention relates to an audible altimeter for use by parachutists. The altimeter comprises a sensor means 6 for sensing ambient air pressure, an electronic processor 5, a sounder device 4 and control buttons 2,3. The electronic processor 5 is programmed to determine the height of the altimeter above ground level and send said signals to said sounder device 4 during the parachutist's descent causing said sounder device 4 to emit a first audible signal at a first predetermined height above ground level and a second audible signal at a second predetermined height.

Abstract: A spatial disorientation detector system capable of warning a pilot of potentially disorienting flight conditions in response to Kalman filter modeling of human response characteristics. The Kalman filter models are representative of human semicircular canal and otolith responses and are capable of more accurate prediction of actual pilot disorientation conditions than are systems which respond with simple magnitude measurement of disorientation stress. Examples of disorienting environments are also disclosed.

Abstract: A warning system for aircraft equipped with an autopilot system having a radio altitude hold (R/A HOLD) function and/or a barometric altitude hold (ALTITUDE HOLD) function monitors the R/A HOLD and ALTITUDE HOLD function control switches, as well as the touch-controlled steering (TCS) control to determine when the functions have been engaged. When either of the R/A HOLD or ALTITUDE HOLD function control switches are selected while the autopilot system is engaged, the system samples the radio altitude or barometric altitude at the moment that the R/A HOLD or ALTITUDE HOLD functions are selected and stores the sample as a reference or datum. The system then compares the current radio altitude or barometric altitude with the datum. Should the actual radio altitude or barometric altitude exceed the datum by a predetermined amount, a warning is generated. In order to prevent spurious warnings, the system is disabled when the touch-control steering is active.

Abstract: A warning system provides a warning to the pilot of an aircraft during a take-off mode of flight of a relatively dangerous flight condition, such as improper configuration of the flaps during a take-off mode of flight prior to the aircraft becoming airborne. In order to avoid nuisance warnings, the warning system is disabled once the aircraft becomes airborne. The warning system monitors the flap position of the aircraft as well as the airspeed and provides a warning when the flaps are improperly configured during a take-off mode of flight. Different aural warnings are provided as a function of the airspeed of the aircraft.

Abstract: A mass memory stores a data base representing at least a substantial part of the terrestrial globe, in accordance with a grid configuration on several levels, said grid configuration being in particular more precise in the vicinity of an airport. Status indications are received representing the position of the aircraft with two horizontal components, and the altitude, and the velocity and acceleration vectors of the aircraft, as well as control indications coming from the flight deck. In accordance with the horizontal components of the position of the aircraft, a temporary local map is transferred into a fast access memory, on the basis of which map, an altitude envelope of the terrain is established in the zone where the aircraft is travelling. Anticollision processing make it possible to establish an alarm if the relation between a protection field and the altitude envelope meets a first condition which is defined at least partly by the control indications.

Abstract: An artificial horizon altitude warning system is provided for helping to prevent a controlled flight of an aircraft into the ground. The artificial horizon altitude warning system comprises an altimeter for gathering altitude information about the aircraft and generating an altitude signal; a laser assembly for producing a light in a cockpit of the aircraft; and a controller for receiving the altitude signal and for positioning the light in the cockpit to form an artificial line based on the altitude signal, the artificial line being positioned so as to be disposed along tin arcuate length corresponding to the location of an actual horizon as viewed by a pilot of the aircraft.

Abstract: Method for signalling insufficient maneuverability of an aircraft according to which:in a preparatory phase: with the aid of a plurality of N flight configurations, the weighting coefficients between three quantities G1, G2, G3 are determined, representative respectively of the acceleration, the slope angle of the trajectory and the risk of stalling, coefficients such that the weighted sum of said quantities is less than a threshold when maneuverability is insufficient;in an in-flight utilization phase: said weighted sum is calculated and a signal signifying insufficient maneuverability is generated if said sum is less than said threshold.Application to controlling aircraft.

Abstract: Warning systems for aircraft that provide a warning to the pilot of an excessive altitude loss or an excessive descent rate after take-off as long as he is flying below a predetermined altitude have the problem that they require a valid radio altitude signal for proper operation, and tend to generate nuisance warnings when used in low flying aircraft. This problem is solved by comparing the accumulated altitude loss after take-off (12, 14, 62, 34) with the product of a barometrically derived altitude and the length of time the aircraft has been flying (12, 14, 30, 32, 34). A warning is generated if the altitude loss is excessive for the accumulated time-altitude product after take-off (38, 40). Thus, the system is more sensitive immediately after take-off and becomes less sensitive as flight time is accumulated to permit low level maneuvering without generating false or nuisance warnings, and does not require a valid radio altitude signal to generate a warning.

Abstract: An aircraft landing information system is disclosed which provides to the aircraft pilot information regarding the actual and preferred sink rate of the aircraft, the distance between the landing gear wheels and the runway, and deviation of the actual sink rate from the preferred sink rate. An altitude determining sensor is provided on the aircraft and the information transmitted therefrom is fed into a microprocessor or like system which then presents the relevant data to the pilot in an audio and/or visual format to allow the pilot to touch down at the preferred sink rate.

Abstract: A low airspeed alerting system for alerting the pilot of an aircraft of insufficient airspeed during a landing monitors various readily available signals representative of flight parameters of an aircraft such as pitch angle, vertical speed and true airspeed and advises the pilot when the airspeed of the aircraft is approaching an airspeed (kinetic energy) that is too low with respect to the ground to maintain the aircraft flying along the desired flight path without stalling, or without action by the pilot in time to recover.

Abstract: Performance of an aircraft ground proximity warning system can be improved, especially where the performance of the aircraft itself has been degraded by a factor such as wind shear, by extending Mode 1 and Mode 3 warning envelopes down to within five feet of the ground. Additional improvements in warnings can be made by monitoring flight path angle, stall margin, angle of attack and deceleration profile, when the aircraft is close to the ground.

Abstract: A system and method for extending the useful life of anti-collision lights comprising inert gas flash tubes and which are included on commercial, military, and business jet aircraft. When the aircraft is cruising in high altitude tightly controlled airspace (where instrument flight rules are controlling, i.e., at altitudes above 18,000 feet), and when the aircraft is not undergoing a substantial change in altitude, the power input to the anti-collision lights is reduced. The reduction in the power provides a significant increase in the useful life of the flash tube while causing only slight reduction in the intensity of the light output of the flash tube.

Abstract: A ground proximity warning system monitors the flight path angle or barometric or inertially derived descent rate of the aircraft and modulates the glide slope alerting function in accordance with flight path angle or descent rate when the aircraft is not descending rapidly even though it is below the glide slope in order to reduce nuisance warnings without compromising system sensitivity. Alternatively, the repetition rate of the alert may be varied as a function of the flight path angle or descent rate to provide less frequent alerts when the aircraft is not descending rapidly.

Abstract: A hand-held instrument adapted selectively to provide a sportsman, such as a mountain climber, with readily-readable indications of the existing altitude or direction. The instrument includes a magnetometer which yields an analog signal as a function of direction and a pressure sensor which yields an analog signal as a function of atmospheric pressure. These analog signals are converted into corresponding digital signals and applied to a microprocessor which acts to control an LCD display having two display modes--one providing a numerical readout of altitude, the other a 360 degree readout of direction. When the microprocessor is switched to the altitude mode, it then processes the signal derived from the pressure sensor to yield a digital output for operating the display to indicate the existing altitude. When the microprocessor is switched to the direction mode, it then processes the signal derived from the magnetometer to yield a digital output for operating the display to indicate the existing direction.

Abstract: Warning systems that monitor the altitude above ground and descent rate of an aircraft and generates a warning if the descent rate of the aircraft is excessive for the altitude in which the aircraft is flying are well suited to aircraft whose flight and operational characteristics are readily predictable but not as suitable for aircraft, such as tactical aircraft whose operating conditions are not so predictable. To overcome this problem, the criteria for providing the warnings are altered (30, 32) as a function of flight configuration (16, 18) to optimize the warnings for different flight conditions.

Abstract: A ground proximity warning system monitors the descent rate of an aircraft and generates a warning if the descent rate is excessive for the current altitude above ground of the aircraft according to a predetermined relationship. The predetermined relationship is altered as a function of the deviation of the aircraft from the glidescope to generate the warning earlier if the aircraft is below the glidescope and later if the aircraft is above the glidescope. The warning is given as a specific aural message indicating the specific flight parameters of the aircraft.

Abstract: A ground proximity warning system for aircraft having flight path angle based warning criteria generates a warning in the event of an excessively steep flight path angle during an approach to a landing. The warning criteria are altered to provide an earlier warning upon the occurrence of a low intensity wind shear or a below glideslope condition. A below glideslope warning system that is biased as a function of decreasing performance shear is also provided. A descent rate based warning system usable above a predetermined altitude may also be provided to supplement the flight path angle warning system.

Abstract: A method and system for avoiding ground collision through pilot warning messages which is implemented in aircraft having a plurality of sensors, onboard inertial navigation units, and altimeters for measuring aircraft performance or flight characteristics. Data is collected through a digital interface unit from data buses connected to these sources, transferred to one or more storage locations and a data validation unit for determining the validity of current altitude data. A series of data processing elements use the aircraft data to extrapolate an aircraft altitude path forward during a predetermined period of time as a function of the aircraft flight data. The extrapolated altitutde allows extrapolation of an aircraft flight path as a function of predicted terrain variations and changes in altitude in response to recovery from various aircraft maneuvers such as rolling, banking, diving, climbing, or accelerating.

Abstract: A system for warning the pilot of a tactical aircraft in a tactical maneuver such as a dive of the point at which recovery must be immediately initiated to prevent the aircraft from descending below a minimum desired altitude monitors and the altitude, descent rate and pitch angle of the aircraft, and issues a voice warning when recovery must be initiated. The system also provides warnings of dangerous flight conditions during non-tactical or common route segments of a flight such as the take-off, landing and cruise segments of a flight. Circuitry responsive to the arming of the weapons of the aircraft enables the tactical warning portion of the system and disables certain functions of the non-tactical portion of the sysem that may cause false warnings to be genrated during tactical manuevers.

Abstract: A low altitude radio altimeter of the FM/CW type which transmits a sawtooth frequency wave and has a transmitting and a receiving aerial. A beat frequency signal Fb.sub.t between the transmitted and received waves is produced at the output of a mixer. The altimeter includes means for digitizing Fb.sub.t at the rate of n samples per sawtooth, a first memory (M1) to store n.times.k samples S.sub.j.sup.i (where i varies from 1 to k and j varies from 1 to n), a second memory (M2) to store n sums ##EQU1## first computing means (PR) for determining ##EQU2## representative of Fb.sub.4, for storage in a third memory (M3), and additional computing and storage means (PR, M4) producing difference signals Fb.sub.t -Fb.sub.f.

Abstract: Warning systems that monitor the altitude above ground and descent rate of an aircraft and generate a warning if the descent rate of the aircraft is excessive for the altitude in which the aircraft is flying are well suited to aircraft whose flight and operational characteristics are readily predictable but not as suitable for aircraft, such as tactical aircraft whose operating conditions are not so predictable. To overcome this problem, the criteria for providing the warnings are altered (30, 32) as a function of flight configuration (16, 18) to optimize the warnings for different flight conditions.

Abstract: A system for warning the pilot of a tactical aircraft in a tactical maneuver such as a dive of the point at which recovery must be immediately initiated to prevent the aircraft from descending below a minimum desired altitude monitors the altitude, descent rate and pitch angle of the aircraft, and issues a voice warning when recovery must be initiated. The system also provides warnings of dangerous flight conditions during non-tactical or common route segments of a flight such as the take-off, landing and cruise segments of a flight. Circuitry responsive to the arming of the weapons of the aircraft enables the tactical warning portion of the system and disables certain functions of the non-tactical portion of the system that may cause false warnings to be generated during tactical maneuvers.

Abstract: A proximity to terrain warning system for aircraft utilizes a first set of warning criteria to generate a warning in the event of insufficient terrain clearance after take off. A different set of criteria are employed to generate warnings when the aircraft in in a cruise or approach phase of flight or on a tactical mission. Mode switching is provided to select the appropriate criteria and to select an excessive descent after take-off set of warning criteria in addition to the first set of criteria when appropriate.

Abstract: A warning system for rotary wing aircraft that monitors the altitude above ground of the aircraft and issues a warning when there is insufficient terrain clearance. The warning criteria are optimized for rotary wing aircraft and are altered as a function of landing gear position and airspeed. Two distinct warnings are given. One indicates insufficient terrain clearance and the other warns the pilot if he attempts to land with the landing gear up. Logic circuitry is provided to modify the criteria necessary to generate a warning, and to inhibit certain warnings, as required by the mode of operation of the aircraft.

Abstract: A warning system for aircraft compares the rate of descent of the aircraft with its altitude above ground after take-off, and generates warning if the aircraft is experiencing an excessive descent condition for the radio altitude at which the aircraft is flying. The position of the landing gear, the speed of the aircraft and the engine power are mointored to enable the system only during the take-off or missed approach phases of operation in order to minimize false warnings during other phases. The relationship between radio altitude and descent rate required to generate a warning is optimized for small, high performance aircraft such as fighter or attack aircraft.

Abstract: A wind shear warning system monitors the flight path of an aircraft and wind shear in the vicinity of the aircraft and generates an advisory or cautionary message for the pilot based both on the magnitude of the wind shear and the flight path of the aircraft. When the aircraft is flying a normal flight path, a wind shear warning is generated only when relatively high negative wind shears are present. If the flight path of the aircraft is not normal, for example, if the aircraft is below the glide slope or descending too rapidly, a cautionary alert is given at lower values of wind shear. The alert or warning is selected to provide the pilot with information defining the nature of the hazard or potential hazard being encountered.

Abstract: A system that warns the pilot of an aircraft performing low level maneuvers of a dangerous flight profile monitors the altitude of the aircraft above ground, and provides a first specific warning to the pilot if the altitude of the aircraft drops below a predetermined minimum altitude above ground. The system further monitors the roll angle and descent rate of the aircraft to provide a second specific warning if the descent rate of the aircraft exceeds a predetermined rate determined by the roll angle of the aircraft if the aircraft is below a second predetermined altitude above ground.

Abstract: To provide a predictive emergency warning to the pilot of flight and terrain conditions which will result in a collision with the ground unless the pilot takes immediate action, without issuing excessive nuisance warnings, the system uses input parameters from other aircraft systems such as the radar altimeter. Inertial Navigation System, and Central Air Data Computer which are processed in an on-board computer to determine when a warning is required. A software program which is part of the warning system provides a logic link between the on-board aircraft parameters and the on-board voice command ("pull-up, pull-up"). The software program is readily adaptable to all aircraft applications with varying amounts of modification depending on specific mission requirements for which this protection is required. A feature is the use of a continuously computing predictive warning algorithm (based on classical flight dynamics equations) in combination with unique inhibit logic equations.

Abstract: A system for providing a voice warning to the pilot of a fighter/attack aircraft when the aircraft is landing with the landing gear up monitors the radio altitude, airspeed and engine power of the aircraft and provides a predetermined voice warning if the landing gear is not down when the airspeed is less than a predetermined speed, for example, approximately 200 knots, the radio altitude is less than a predetermined altitude, for example, approximately 100 feet, and the engine is not developing take-off power.

Abstract: A ground proximity warning system is disclosed which can recognize when an aircraft is on a final approach to an airport without utilizing a landing flap signal input. Airports together with the surrounding terrain topography are modeled by a simple geometric shape, such as, an inverted truncated cone, and stored on-board the aircraft. The system uses navigational data to determine the distance of the aircraft from the geometric model. Once the aircraft is determined to be within the area defined by the geometric model, the system provides an enabling envelope indicative that the aircraft is on a final approach for enabling various ground proximity warning systems. Also disclosed is a system for altering the enabling envelope as a function of the aircraft's alignment with a particular runway.

Abstract: An alarm means including a digital microcomputer having a memory programmed for storing a look-up table defining an alarm envlope which, above a predetermined altitude, is based on a relationship between altitude and ground-closure rate which is independent of ground velocity, and below said predetermined altitude, is based on a relationship between altitude and ground-closure rate which is dependent on ground velocity.

Abstract: An aircraft ground proximity warning system having an excessive descent rate warning mode which generates a warning signal when the descent rate of the aircraft exceeds a predetermined limit for the particular altitude of the aircraft above ground. The warning system provides for three separate warnings. A SINK RATE advisory warning is generated for combinations of lesser descent rates and greater altitudes. A PULL UP warning is generated for higher descent rates at lower radio altitudes. After a predetermined time after the SINK RATE warning is generated a TERRAIN warning is provided to indicate to the pilot of the aircraft that an excessive sink rate condition exists and the aircraft is flying over undulating or mountainous terrain. Also disclosed is logic circuitry for detecting that the aircraft is flying over undulating terrain.

Abstract: An aircraft ground proximity warning system is disclosed having an inadequate terrain clearance warning mode, wherein the criteria required to generate the warning is modified when the aircraft is flying over undulating or mountainous terrain. The system is responsive to signals representative of the radio altitude, the barometric altitude, the speed of the aircraft, the flap position and the landing gear position to provide warnings when the aircraft is operating below a predetermined altitude with either or both of the landing gear of the flaps not in a landing configuration. The warning system also provides a warning when the aircraft is traveling above a predetermined speed and below a predetermined altitude irrespective of the configuration of the landing gear or the flaps. Also disclosed is logic circuitry for detecting that the aircraft is flying over undulating or mountainous terrain and modifying the warning envelope during such condition to provide longer warning times.

Abstract: A warning system for rotary wing aircraft compares the accumulated altitude loss after take-off of the aircraft with its altitude above ground, and generates a warning if the altitude loss is excessive for the altitude above ground at which the aircraft is flying. The position of the landing gear, the speed of the aircraft and its altitude enable the system only during the take-off and missed approach phases of operation in order to minimize nuisance warnings during other phases. The relationship between radio altitude and altitude loss required to generate a warning is optimized for rotary wing aircaft such as helicopters.

Abstract: A ground proximity warning system especially adapted for a tactical aircraft provides the pilot with a warning in the event that the closure rate is excessive for the altitude above ground at which the aircraft is flying. The descent rate of the aircraft is monitored and used to bias the closure rate warning signal to reduce nuisance warnings when the excessive closure rate is part of an intentional maneuver such as a dive during a tactical mission.

Abstract: A system for warning the pilot of a rotary wing aircraft such as a helicopter of a flight condition that, if left uncorrected, would result in the tail of the aircraft striking the ground monitors the altitude above ground, descent rate, pitch and pitch rate of the aircraft and provides a warning if the relationship between the aforementioned parameters is such that a tail strike is imminent.

Abstract: A warning device, for a helicopter with a tail rotor and a mechanical protection device therefor, for giving during emergency power-loss landings a flare signal to the pilot to initiate flare followed by a tail/ground contact warning signal when the helicopter tail approaches too closely the ground. The warning device comprises in one embodiment a height-finder with two switchable transmitting/receiving antennas mounted at the helicopter tail to produce respectively, when switched into use, a height-finding beam backwards and obliquely downwards or forwards and obliquely downwards. Height signals obtained initially using antenna 40 are processed by computing means and initiate at a predetermined initial flare altitude a flare warning signal whereafter height signals from antenna 26 are used to determine the sink rate of the tail and in dependence upon this sink rate and the instantaneous height, a warning signal is initiated if this corresponds to a predetermined relative relationship.

Abstract: An aircraft ground proximity warning system having an excessive terrain closure warning mode and a below glide slope warning mode monitors the glide slope signal and modifies the terrain closure warning envelope to accept a lesser terrain clearance when the aircraft is within the glide slope beam.

Abstract: A ground proximity warning system utilizes geographic position data to alter the warning criteria as is required by the terrain features of particular geographic areas. To determine whether the aircraft is within a particular geographic area, the world is subdivided into latitude bands that are in turn subdivided into non-overlapping zones by predetermined lines of longitude, with the position of each zone being defined by the latitude and longitude coordinates of one of its corners. The zones are further defined by regions which may overlap other zones. The regions are limited in size, and their positions are also defined by the locations of one of their corners. The regions are further subdivided into one or more geographic areas that are defined by their shapes and locations within the region.

Abstract: A system for warning the pilot of an aircraft flying at low level, particularly over rising terrain, of an excessive closure condition that could result in controlled flight into terrain if left uncorrected. Specifically a warning system is disclosed that monitors the terrain closure rate of an aircraft under certain conditions. These conditions relate to the weapons not being armed, the aircraft flying between 100 and 1800 feet and the air speed exceeding 200 knots. When these conditions are met a warning is provided to the pilot that the terrain closure rate exceeds a predetermined safe limit for the radio altitude at which the aircraft is flying.

Abstract: In order to reduce nuisance warnings and to increase the effectiveness of real warnings produced by a ground proximity warning system, the criteria for generating a warning is varied for certain airport areas to take into account the terrain in those areas. Along with position data, certain flight parameters such as heading, altitude, glideslope and localizer deviation are checked by the system in order to ensure that the aircraft is located in the desired warning modification area. In addition, the ground proximity warning system includes a mechanism for ensuring that the corrected barometric altitude of the aircraft is within acceptable limits.

Abstract: A warning system for providing an aural warning to the pilot of a rotary wing aircraft monitors the altitude above ground and barometric descent rate of the aircraft and generates the warning if the descent rate of the aircraft is excessive for the altitude at which the aircraft is flying. The system provides two distinct warnings, one being an early warning advising the pilot of a hazardous condition, and the other being a warning alerting the pilot of imminent danger. The system also includes tactical and non-tactical modes of operation, wherein the warning criteria are adjusted to meet tactical and non-tactical flight conditions.

Abstract: To increase the effectivity of warnings and to decrease nuisance warnings in a ground proximity warning system having several modes of operation, the switching from one mode to another is done as a function of time. In addition, in a ground proximity warning system where a warning signal is generated in accordance with a predetermined relationship between flight parameters, one or more of these parameters can be varied as a function of time in order to, for example, increase the altitude above ground as a function of time from take-off that a terrain clearance warning may be generated or to decrease as a function of time, the radio altitude below which a negative take-off after a climb warning may be generated.

Abstract: An apparatus to continuously monitor a preset altitude for a pilot of an aircraft. It senses a change in altitude and an audible as well as a visible alarm tells the pilot that he has deviated from his preset altitude by a given height. Increase in altitude is signaled by a high pitch tone and by a yellow light while a decrease in altitude is signaled by a low pitch tone and a red light. The invention can also be used to warn a small boat operator of a pressure drop.

Abstract: To increase the effectivity of warnings and to decrease nuisance warnings in a ground proximity warning system having several modes of operation, the switching from one mode to another is done as a function of radio altitude and time. In addition, in a ground proximity warning system where a warning signal is generated in accordance with a predetermined relationship between flight parameters, one or more of these parameters can be varied as a function of radio altitude and time in order to, for example, increase the altitude above ground as a function of radio altitude and time from take-off that a terrain clearance warning is generated or to decrease as a function of radio altitude and time, the radio altitude below which a negative climb after take-off warning is generated.

Abstract: An altitude sensing arrangement which combines the advantages of a barometric altimeter with a radar altimeter. The apparatus monitors the radar altimeter's associated radar validity signal and selects the radar altimeter's reading when the validity signal indicates a valid condition. Alternately, when the validity signal does not indicate a valid condition, the invention computes the difference in barometric altitude since the last valid radar altimeter reading and sums this difference with the last valid reading from the radar altimeter to produce a combined altitude reading.