Abstract: A map display system for displaying a linear object on a map in accordance with stored map data is disclosed. The coordinate components of at least two specified points on the linear object and other points thereon crossing straight lines drawn in spaced relationship and in parallel with the straight line connecting the specified points are stored as map data in a memory. The coordinate components thus stored are converted into a display coordinate by a display control to generate a display signal. In accordance with the display signal the linear object is displayed on a display within a predetermined display section.

Abstract: A navigation system for a road vehicle comprises a route-determining unit and position-locating means. The route-determining unit works together with a mass memory which stores geographic data of a road network. The route-determining unit determines a driving route between a starting point and a destination supplied by the driver of the vehicle. The position-locating devices determine the vehicle position coordinates recurrently from the speed and acceleration of the moving vehicle provided by a measuring unit. A correction unit derives a correction vector for correcting the generated vehicle position coordinates. The correction vector is determined by performing a transposition of the generated vehicle position coordinates to an actual road position as repeated by coordinates of the road network stored in the mass memory. An acceleration operator is derived from the measured speed and lateral acceleration and from the generated vehicle position coordinates.

Abstract: A navigation system for a vehicle generates a guide display in the form of a diagrammatic roadmap during travel on surfaces roads and in the form of a textual road-name display exclusively during travel on freeways. In addition, the navigation system has a manual reset switch which triggers the change of the display image back to the diagrammatic roadmap after freeway travel and replacement of a deduced vehicle position with the known position of a road feature at which a driver is prompted to push the reset switch.

Abstract: An artificial horizon device producing a projected light bar perceivable with peripheral vision. The device includes a scanner assembly with a pair of rapidly movable scanning mirrors each selectively pivotal about respective mutually perpendicular axes. A beam of light is directed upon a first of the pair of scanning mirrors for reflection upon the second of the mirrors, and thence to illuminate a surface for perception as a relatively movable horizontal reference line. A single adjustment mirror is interposed between the source of the light beam and the first of the scanning mirrors. The adjustment mirror is selectively movable about two mutually perpendicular axes each substantially intersecting the reflective plane of the adjustment mirror in order to set a selected point of incidence for the light beam upon the first of the scanning mirrors.

Abstract: Apparatus and method for use on a measurement vehicle to measure road surface profiles while correcting for surface slope (cross slope and/or grade). Transducers are mounted on the vehicle suspended mass for measuring distance to the road surface and acceleration normal to the road surface as the vehicle is propelled over the road surface. A third transducer is responsive to fixed increments of vehicle travel over the road surface. Surface profile is determined in the spatial domain over each successive incremental distance. Total acceleration of the vehicle in the direction of the slope is measured, from which acceleration due to vehicle motion is subtracted. Elevation is determined as a trigonometric function of slope, and corrections are implemented to the profile signals.

Abstract: An improved system and technique is disclosed which enables an aircraft control system to bring an aircraft to hover with no direct measurement of ground speed. The system includes a device for numerically calculating a value for ground speed based on a differentiation of aircraft position as measured from conventional ground reference navigational aids. The computed value of ground speed is compared with values representing heading and true air speed to generate a computed value of wind. The computed value of wind is then resolved into vector components of headwind and crosswind and the headwind component and air speed are summed to provide a signal used to control aircraft speed until zero ground speed is achieved.

Abstract: A map display apparatus displays a road map of a desired display district in accordance with stored map data. The apparatus includes a map data storing medium storing, in correspondence to a plurality of basic sections forming the whole area of the road map, a plurality of map information representing the road configurations in the basic sections and a plurality of name information representing the names of various parts in the basic sections, a display selecting device for selecting the desired display district from the whole area of the road map, a display control unit for generating a display signal in accordance with the map information group and the name information group of the basic sections contained in the selected display district, and a display responsive to the display signal to display the selected display district.

Abstract: A novel electronic map display system is disclosed for displaying a map on the basis of a map data stored. Various discontinuous points in each of predetermined areas included a map to be displayed are stored as structured data in the form of vectors. The discontinuous geographic data in each of the areas is stored as image pattern data. A selection signal is produced from a display selection unit for selecting an area to be displayed from the map. In accordance with the selection signal, a display signal is generated from the pattern data and the structured data corresponding to the area to be displayed. A map of the particular area is displayed on the basis of the display signal of a display control unit.

Abstract: An aircraft instrumentation system includes a plurality of air-driven instruments and a primary air pump for operatively driving the instruments. An air pressure regulator, coupled to the primary pump, selectively exhausts compressed air discharged by the primary pump so as to maintain a constant desired pressure at the instruments, and a backup pump is provided for operating the instruments in the event the primary pump fails. To avoid excessive cooling of the backup pump when the backup pump is not being operated and the aircraft is at altitude, a shroud is mounted so as to form an air space around the backup pump. Warm air, heated through compression by the primary pump and exhausted through the pressure regulator, is collected by a housing surrounding the pressure regulator and is conveyed, through a conduit, to the air space to warm the backup pump when the primary pump is operating.

Abstract: A crossflow vorticity sensor is provided for the detection of crossflow vorticity characteristics. The crossflow vorticity sensor is comprised of crossflow sensors (10) which are non-invasively adhered to swept wing laminar surface (12) either singularly, in multi-element strips (11), in polar patterns (40), or orthogonal patterns (41). These crossflow sensors (10) are comprised of hot-film sensor elements (18) which operate as a constant temperature anemometer circuit to detect heat transfer rate changes. Accordingly, crossflow vorticity characteristics are determined via cross-correlation. In addition, the crossflow sensors (10) have a thickness which does not exceed a maximum value h in order to avoid contamination of downstream crossflow sensors (10).

Abstract: A test apparatus for measuring side-slip and gravity slip of a vehicle moving along a curved road or travelling across downhill. The test apparatus is adapted to be towed by a vehicle. The test apparatus is connected to a vehicle by a bar which has the other end connected to a vertically extending bushing which houses a pivot member extending perpendicularly to a riding surface. The lower end of the pivot member is connected pivotably to a wheel fork horizontally extending and supporting a test wheel in contact with a riding surface, while the other end of the pivot member extends out of the upper portion of the bushing and is connected by a horizontally extending level member which has the other end connected to a counterweight. An angle-measuring sensor is disposed over the upper end of the pivot member and is supported on a bracket secured to the upper end of the bushing.

Abstract: In a navigator for vehicles, a distance sensor detects a distance travelled by a vehicle and a heading sensor detects a direction of travel of the vehicle. In response to the detection signals from the sensors a microcomputer computes the present position of the vehicle and generates a position signal. In accordance with the position signal from the microcomputer and the selection of display selection switches a display tube gives a graphic display of a road map or a character display of a road map data.

Abstract: An inertial measurement unit which provides three axis acceleration and angular turning rate detection with a cubical magnetically suspended sensor mass disposed within a cubical outer assembly. The sensor mass is free to move mechanically independent from the outer assembly. The sensor mass has a plurality of sensing and suspension elements of particular orientation on a selected plane for each axis of detection which face a corresponding set of sensing and suspension elements respectively on interior surfaces of said outer assembly. The sensing elements are advantageously cross-coupled to minimize cross-axis sensitivity. The invention utilizes several servo-control loops of conventional design to process outputs from the pickoffs and maintain the sensor mass in proper orientation.

Abstract: A retroreflective attitude marker includes at least two intersecting reflecting planes oriented at 90 degrees with respect to one another, and having the property that incident laser beams which are perpendicular to the line of intersection of the two planes are reflected directly back to the laser source. Each retroreflective attitude marker therefore has a single retroreflecting plane perpendicular to line of intersection of the two mutually perpendicular planes. Using three layer illuminator/receivers, directed toward a spinning object, on which a retroreflective attitude marker is located, the rate of spin and the orientation of the spin axis of the object on which the retroreflective marker is located, may be determined mathematically.

Abstract: Vacuum systems deriving their trustworthiness by means of simplified apparatus, wherein, the vacuum source will be derived from simple venturis common to the state of the art, whereby, ice prevention will be achieved by use of bleed air from the turbine section of the jet engine used to power the aircraft. Venturi ice prevention will be selectable by the pilot and will fail safe to the on position should the aircraft encounter a complete electrical failure. The remaining units comprising of the vacuum system and connective tubing will all be common to the state of the art. One of the vacuum systems will supply regulated vacuum pressure to the earth reference gyros by means of a venturi being located within the inlet mouth of a jet engine, said pressure then routed to a pressure regulator and then routed to the gyros, therefore, regulated pressure will be available immediately following engine start and will remain available in flight even should its respective engine fail.

Abstract: A pressure sensing device for a body movable in a fluid in a direction transverse to the longitudinal axis of the body. A chamber is provided within the body and a plurality of bores extend radially from the chamber to outer ends opening to ambient fluid so that, as the body moves, any pressure differential between the outer end and inner end causes fluid flow in the bore. Each bore is a flow restricting device such as a valve or fluidic diode, and operable to restrict outward flow from the inner end to the outward end more than inward flow thus causing the pressure in the chamber to be higher than the average pressure at the outer ends. A fluid pressure measuring device is provided in the chamber.

Abstract: A gyrocompass navigation system for land vehicles is disclosed which includes self-contained heading and navigation means. The system determines the orientation of the vehicle with respect to grid north; provides land navigation information in universal transverse mercator (UTM) coordinates (northings and eastings); and provides range and bearing information for destination location. The system features a single degree of freedom gyroscope for both north seeking and navigational modes and further includes simplified means for tilt measurement and temperature compensation.

Abstract: A navigational apparatus for automotive vehicles comprises manually operated command entry keys for specifying a desired page of a road map and specifying one of first and second display scales. Road map data is stored in a memory, the data representing a plurality of nodes along travelling routes that appear on each page of the map and comprising first node data having a first level of significance and second node data having a second level of significance which are addressable in accordance with the specified scale. A data processor reads the stored data from the memory according to the specified page and the specified scale so that the first node data is read when either of the first or second scale is specified and both of the first and second node data are read when the second scale is specified. The processor converts the storage location of the read data to a coordinate location of the memory to cause a display to provide a road map.

Abstract: A navigator for use aboard a vehicle includes means for detecting the running distance of the vehicle, means for detecting the angle of the advancing direction of the vehicle with reference to terrestrial magnetism, arithmetic operational means for calculating the coordinates of the position of the vehicle in response to the detected running distance and angle of direction, a memory for storing data regarding the coordinates of the points of correction set at intersections of roads on a map corresponding to an area in which the vehicle is traveling, the distances between the points of correction, and the angles of roads branching from the points of correction with reference to terrestrial magnetism, a locating means for judging whether the vehicle is within a predetermined distance from a point of correction by comparing the coordinates of the present position of the vehicle with the coordinates of the points of correction stored in the memory, a means for judging whether the vehicle has made a turn, and a se

Abstract: A turn-and-bank indicator can integrate with regard to time the displacement of a pointer of the turn-and-bank indicator, the pointer indicating the turning rate of the airplane. It includes a non-contact converter which outputs an electrical signal in proportion to the deflection of the pointer; an amplifier which amplifies the output of the non-contact converter; an integrator which integrates the output of the amplifier with respect to time; and a digital display unit which displays the output from the integrator at a close position with the pointer.

Abstract: A navigation system for displaying, on a display screen, road map information and running traces of a car superimposed thereon comprises a cathode ray tube for displaying the road map information and information regarding the car running traces and the like, bearing sensors for detecting a running direction of the car, a detector for detecting a running distance of the car by detecting a revolution of the car wheel, and a controller receiving outputs from the bearing sensors and the running distance detector, for computing a current position of the car and selecting road map information corresponding to the current position for display thereof on the cathode ray tube.

Abstract: An automotive navigation system wherein a memory included therein has not stored therein a picture information of a map itself but instead has stored therein geographical names and positional information thereof. When the geographical names of a departure point and a destination point are entered through an input unit, a control unit reads out from the memory the geographical names and the positions of the points. A CRT displays marks respectively indicating the departure point, the destination point, and the current position of a vehicle on an adequately reduced scale on the display scope. The arrangement of this automotive navigation system can perform a fully practical navigation function even with a small sized and cheap memory and arithmetic unit.

Abstract: An automotive navigation system wherein a storage means included in the system has not stored therein picture information of an actual map, but instead has stored therein the geographical names and the geographical positions of points as well as the declination of the earth magnetism of the points. When the geographical names of a departure point and a destination point are entered through an input unit, a control circuit reads out from the storage means the respective positions of the points entered. The control circuit controls a display unit to display on a display screen marks respectively indicating the departure point, the destination point, and the current position of a vehicle on an adequately reduced scale determined by the positions of these points. A point nearest to the current position of the vehicle while the vehicle is being driven is determined and retrieved by the control circuit from the storage means.

Abstract: An automotive navigation system wherein a memory included therein has not stored therein picture information of an actual map, but instead has stored therein geographical names and the geographical positions thereof. When the geographical names of a departure point, a destination point and one or more passing points are entered through an input unit, a control circuit reads out the respective positions of the points from the memory. The control circuit controls a display unit to display on a display screen marks respectively indicating two or more of the points and the current position of a vehicle.

Abstract: An automotive navigation system wherein a memory included therein has not stored therein picture information of an actual map but instead has stored therein geographical names and the geographical positions thereof. When the geographical names of the departure point and the destination point are entered through an input unit, a control unit reads out from the memory the respective graphical names and the positional information thereof. A display device displays marks indicating the departure point, the destination point, and the current position of a vehicle on an adequately reduced scale on the display screen. Furthermore, a display control is carried out such that the mark indicating the current position of the vehicle does not extend beyond the display screen during the running of the vehicle.

Abstract: Phase I (25) and Phase II (20) and III (24) AFRS (Automatic Fault Reporting System) signal processing implementations (FIG. 3) of fault related data permits utilization in steps, viz. Phase I and Phases II and III on board operational aircraft of AFRS, thereby permitting gradual phase in and substitution of AFRS for present state of the art flight crew FRM (Fault Reporting Manual) and ground personnel FIM (Fault Isolation Manual).

Abstract: In a highway survey vehicle, grade and cross fall measurements are made in relation to references established by a displacement type pitch and roll gyroscope with a gravity referenced erection system. Lateral acceleration of the vehicle is calculated independently and used to provide a correction for errors caused by spurious erection induced by such lateral acceleration, provided that such acceleration falls within certain limits such as to permit effective correction. The lateral acceleration may be calculated from measured rates of displacement and change of the direction of the vehicle.

Abstract: An inertial sensor assembly (ISA) includes a cluster of three ring laser gyros, each gyro producing an output signal having a pulse repetition rate representative of the rate of angular deviation of the ISA about one of three coordinate axes X, Y, and Z. The ring laser gyros are synchronously dithered at a relatively constant rate. The ISA also includes a triad of three accelerometers, with each accelerometer producing an output signal representative of the rate of velocity deviation of the ISA along one of the X, Y, and Z coordinate axes. A first processor, P1, accumulates the pulses produced by each ring laser gyro over its dither period. The resultant counts are stored in registers for subsequent sampling by the P1 processor at a periodic sampling rate which is greater than the dither rate. The P1 processor then synchronizes each sampled pulse count to a common sampling interval, thereby eliminating errors otherwise caused by using positional data values taken at different times.

Abstract: An evolutionary development of my prior inventions into an improved display form, more compatible to the desires of the overwhelming majority of pilots. This new invention is a natural derivative of my (co-pending application for patent, Ser. No. 629,850, file date July 11, 1984 for Bank and Pitch Indicator) and (co-pending application for patent, file number pending, file date Mar. 25, 1985 for Improved Pitch Indicator for Bank And Pitch Indicator).

Abstract: Continuous display of pitch attitude beyond normal limits of plus or minus 20 degrees is provided by an extended pitch indicator whose operation is coordinated with a primary pitch indicator which provides pitch information through primary ascent and descent ranges (plus or minus 20 degrees). Operation of the extended pitch indicator is coordinated with operation of the primary pitch indicator in such a manner that the extended pitch indicator is hidden from view during flight maneuvers within the primary climb and dive ranges, but which is automatically exposed during flight maneuvers outside of the primary climb and dive ranges. In the preferred embodiment, the extended pitch indicator element is attached to the pitch gimbal and a horizon mask 15 interposed between the extended pitch indicator and the primary pitch indicator scale.

Abstract: An aircraft control position indicator is provided that displays the degree of deflection of the primary flight control surfaces and the manner in which the aircraft will respond. The display includes a vertical elevator dot/bar graph meter display for indicating whether the aircraft will pitch up or down, a horizontal aileron dot/bar graph meter display for indicating whether the aircraft will roll to the left or the right, and a horizontal rudder dot/bar graph meter display for indicating whether the aircraft will turn left or right. The vertical and horizontal display or displays intersect to form an up-down-left-right type display. Internal electronic display driver means receive signals from transducers measuring the control surface deflections and determine the position of the meter indicators one each dot/bar graph meter display.

Abstract: A solid-state heading sensor is comprised by a three-axis Hall effect magnetometer and a three-axis accelerometer. The magnetometer is comprised by a cube-like structure (10) of non-magnetic material on each of three orthogonally related faces of which is located a respective Hall effect device (11, 12, 13), for example of GaAs. The three-axis accelerometer may be comprised by a three-axis module (15) disposed in a recess in the cube-like structure, or by three separate accelerometer structures (23, 24, 25), for example planar etched silicon accelerometers, each disposed on a respective one of the orthogonally related faces adjacent the Hall effect device (20, 21, 22) thereon.

Abstract: The rotor suspension electronics of an electrostatic gyro suspension system is required to perform flawlessly to sustain the life of the gyro. Transient abnormalities as short as one millisecond duration will cause the spinning rotor to come in contact with the surrounding electrodes and result in destruction of the gyro. Thus, not infrequently after a catastrophic dropped rotor incident no trace of the fault is evident upon ensuing check of the system. To remedy this problem a built in test (BIT) system is used to provide fault isolation. The test circuitry is built into the navigation system electronics and integrated with the system program. The fault isolation electronics comprise sensor circuits to monitor functional subdivisions of the electronic suspension system for signal abnormality and a processing circuit which receives the sensor circuit outputs and identifies the primary source of the fault. The faults are detected and the source is flagged at the time of occurance.

Abstract: An indicator instrument for boats with outboard drive units which can be adjusted to various angles in relation to the transom and which has an indicator, preferably an LCD which shows the angle of the drive unit within the entire adjustment range, and an indicator, preferably an LED stack, which provides a rough indication of the drive unit setting within the trim angle range.

Abstract: In a travel guidance system for vehicles, a map data storage medium preliminarily stores administrative division name data and/or landmark data, and a control unit has the function of retrieving the name of an administrative division to which the present position of a vehicle belongs and/or a landmark in the vicinity of the present position in accordance with present position data and displaying the retrieved administrative division name and/or the retrieved landmark on a display.

Abstract: A basic flight instrument to provide the pilot the means, by which, he can easily remain upright during instrument conditions. This flight instrument will be highly trustworthy, in that it will be absolutely tumble free due to its basic input derivatives. The instrument will be provided with two separate erection systems, whereby, the roll erection system will consist of a gimbal cage, spring tension erected and the pitch erection system will consist of a drum, the rotation of which will be properly erected by means of a gravitational weight and rotational reversing system. The pilot will be provided with simultaneous display of roll and pitch of reliable derivative. The roll rate of the gyro may be speed and altitude compensated if desired.

Abstract: Method and apparatus for use in a floating vessel subjected to wave action, used to determine the proper vessel heading so that the longitudinal axis of the vessel will be positioned parallel to the direction of oncoming waves, thereby reducing the risk of capsizing of the vessel. The apparatus comprises a mass elastically suspended from a portion of the vessel structure. Observation of the direction of rotation of the mass allows the helmsman to determine what direction to steer the vessel so that the vessel remains headed into the waves.

Abstract: A vehicle turn angle detecting device calculates an average value .DELTA..phi. of steering wheel rotation angle at every unit time together with a travelled distance .DELTA.L. And this device calculates a vehicle turn angle .DELTA..theta. at every unit time by using the equation shown below. By adding the calculated turn angles .DELTA..theta., the vehicle running direction to the standard direction can be detected..DELTA..phi.=K.multidot.tan.DELTA..phi..multidot..DELTA.

Abstract: A simple accurate arrangement for determining the instantaneous angular position of a moving object comprises rotation acceleration sensors located in reference axes of the article. Because of the double integration, the low-frequency static angular signal obtained is not reliable so that a device for measuring the static angle is used which supplies these values accurately, it is true, but which is not reliable in high-frequency respects. A correct angular signal is obtained by filter circuits and mixing. A piezoelectric element is used in the rotation acceleration sensor, while the device for measuring the static angle is an electrolytic leveling instrument.

Abstract: An aircraft attitude display device comprising: a circular array of radially extending selectively activatable display elements (5), the radially inner ends of which constitute a boundary defining a central space; within the said central space, a fixed aircraft symbol (7); and means for selectively activating the said display elements in accordance with aircraft attitude so that the selected display elements form a visible pattern, e.g. a sector of the array, bearing a geometrical relationship with respect to the aircraft symbol indicative of the aircraft attitude.The device is suitable for implementation using liquid crystal display technology to give a stand-by display of aircraft pitch and roll which is readily understandable by pilots since it is similar to the display given by conventional rolling ball action aircraft attitude instruments.

Abstract: An artificial horizon device producing a projected light bar perceivable with peripheral vision.

Abstract: A method of visually reproducing actual and criterion value signals without giving any numerical values includes signal processing of the measuring values forming the basis of the actual value signal, actual and criterion values being fed to a measuring instrument, there to operate at least one indicator (8, 9), the sweep of which defines a surface (S) constituting a measure of the discrepancy between the actual and criterion value signals. The indicator may be of the analogue or quasi-analogue type. Coincidence between actual and criterion signals is indicated by a line, i.e. the surface is reduced to zero.

Abstract: An aircraft operating in a gravitational field and having conventional sensors for measuring true air speed (sensors 10, 11), angles of incidence and yaw (sensors 13, 12), rotation about x, y and z axes and acceleration therealong (sensors 14, 15, 16) is provided with means for calculating the inertial component of the acceleration from data concerning the true air speed, heading and rotation of the aircraft obtained from the sensors, and means for comparing the inertial component with the total acceleration sensed, thereby to deduce the orientation of the gravitational component and hence obtain an estimate of the pitch and bank angles of the aircraft.

Abstract: A gyro flight instrument utilizing variable rate inputs, such variable being proportional to the speed of the craft, the result of which will be a lateral display nearly corresponding to the true lateral reference. Such lateral display will be in combination with a pitch display, such display will be variable in proportion to the angle of attack of the craft in relation to the relative airstream. The resultant display will nearly match the display of a gyro horizon with the exception of extreme excursions of pitch and bank. This will result in a highly trustworthy instrument the indication of which is totally derived from basic, non-tumbling reference, totally immune of earth reference requirement.

Abstract: A self-contained, standby flight instrument includes sensors, processing circuitry, rechargeable power supply, voltage regulation circuitry, and display hardware arranged to provide a continuous display of aircraft airspeed, altitude, roll, pitch and heading regardless of any partial or complete failure in the primary instrumentation, an interrelated subsystem, or the electric power supply.

Abstract: A solid state attitude and director indicator (ADI) is disclosed utilizing liquid crystal display technology to represent attitude of a vehicle in two axes. A visual representation of attitude changes is provided on the liquid crystal display as well as numeric data, in a preferred embodiment. Additionally, glideslope, localizer, and approach speed may be displayed as well as heading and command bar information.

Abstract: In a map display system for vehicles, spot coordinates are preliminarily stored which represent map forming spots and are provided with a coordinate origin of a specified spot, and display coordinates indicating map changing frames are also preliminarily stored. The display coordinate of the present position of the vehicle is compared with the display coordinate of a map changing frame. When the present position reaches the map changing frame, the spot coordinate of the present position is compared with the preliminarily stored spot coordinates so as to extract spots forming a new map centering the present position of the vehicle.

Abstract: An instrument and method for continuously measuring by differential air pressure the surplus of kinetic energy of an aircraft mass and the resulting potential of wing lift (POWL) above an empirical datum which coincides with the incremental onset of mushing sink, and for coincidentally and continuously displaying the margin of measured airspeed above the calibrated datum within the airspeed range of transitional flight. The instrument includes a sensing probe having a pair of fixed plane surfaces normal to a common vertical plane and adjustably mounted in a region of induced, stable, incident airflow under the wing or other part of an aircraft to sense differential impact pressure on the fixed plane surfaces, and an instrument to measure and display the sensed differential pressure.

Abstract: Apparatus has a closed, vented, damping chamber. A gas duct terminates in the damping chamber and air is bubbled through the gas duct. The pressure in the gas duct is read using a manometer, and indicates the distance below the water surface of the exit opening of the gas duct. The damping chamber is connected to the surrounding water by a water duct whose effective cross-sectional area is much less than that of the damping chamber.

Abstract: A system to control the effect of northerly turning error which may occur in any direction indicating navigation instrument which includes a component responsive to the earth's magnetic field. In one form, an aircraft magnetic compass is compensated by electric current in an electromagnet which is made responsive to the rate of turn as sensed by a turn and bank indicator. In another form, an aircraft type slaved gyro directional system is protected from the effect of northerly turning error which occurs in its remotely located magnetic azimuth sensor by disconnecting its control circuit from the gyro when the turn rate reaches a predetermined level, thus allowing the gyro to act solely as a directional gyro during the turn.