Electromagnetic waves and barometric sensors system for the measurement of distances and heights. The system at issue is of great use in the field of topographical, cadastral and any other kind of ground survey, including the measurement and mapping of interiors. Such system allows the measurement both of distances between points and of their heights, even when the points to be surveyed are not visible from the survey stations. It employs at least two portable stationary units (9) and one portable mobile unit (1). The above-mentioned units are built as rods, each made of a tube (12) of watertight and insulating material provided with both a lower pointed end (13) and a spheric level (2). The accumulators (6), the transceiver instruments (4), the aerial (15), the barometric sensor (3), together with the electronic components (5) or (10) are housed inside each rod. As far as the electronic components are concerned, they are differently structured according to the portable unit they are in, be it the first stationary unit (9), the second stationary unit (9) or the mobile unit (1). Each portable mobile unit (1) also includes a portable control keyboard (8).

A system and method for providing highly accurate measurements of the altitude above ground level (AGL) of an aircraft flying over local terrain. A current AGL altitude of the aircraft over local terrain is obtained by activating a radar altimeter on the aircraft for a single short duration or pulse. A mean sea level (MSL) elevation of the local terrain is determined by identifying the terrain from the then-current aircraft geographical position coordinates and utilizing known terrain topography data. The actual MSL altitude of the aircraft can then be determined. An uncorrected MSL altitude of the aircraft is then determined from conventional static air pressure measurements and the difference between the actual MSL altitude and the uncorrected MSL altitude of the aircraft yields a local barometric correction factor for use in determining MSL altitude measurements of the aircraft as the aircraft flies over and continues its flight away from the local terrain.

An ambiguous dial indicator intuitively communicates the relative value of setpoints to an operator. The ambiguous dial indicator includes an ambiguous scale and a pointer or needle. The setpoints are represented by "bugs" which are adjacent to the ambiguous scale. The bugs are limited to a predetermined range about the pointer such that when a setpoint is outside of the predetermined range the associated bug is limited to an edge of the range. When several setpoints are outside of the predetermined range the several associated bugs saturate or park at the edges of the range. The invention is particularly useful in head-up displays and helmet mounted displays.

A combined altitude and height-above-ground indicator for an aircraft enables the altitude and the height of an aircraft to be displayed simultaneously. The combined indicator includes a first sensor and a second sensor which respectively sense altitude and height above ground, means for processing the information supplied by the first and second sensors, and means for displaying the sensed altitude and height-above-ground information by superimposing the altitude and height-above-ground information on the same scale against a pointer. The scale includes a transparent first part which represents a pressure-altitude display, and a second part which is a mask positioned under the first part so that it masks the background of the indicator for altitude levels which are below the measured height above ground. An advantage of the indicator is the ability to incorporate into an altimetric indicator adjusted by a pilot a device capable of warning the pilot of an abnormal situation such as unintentional closeness to the ground as a result of poor altimeter adjustment.

An altimeter has four display panels each being a matrix array of light-emitting diodes. An energizing unit energizes selected diodes to produce a representation of a number in each panel indicative of the value of the input. A knob on the altimeter selects input signals from a barometric source or a radar source. When barometric signals are selected, the altitude is represented in the panels by bright numbers against a dark background. When radar signals are selected, the energizing unit changes the format of energization of the display to represent altitude by dark numbers against a bright background. Other formats are also possible.