UNIT 16 DEAD RECKONING

AIM OF THE UNIT: - to describe dead-reckoning and navigation

systems

- to understand the grammar point the category of

comparison of the adjective

TASKS

1 Do your best to answer the brainstorming questions. 2 Read the text for general understanding.

3 Make up questions to the text. 4 Find the sentences with the new words in the text. Give the Kazakh or Russian

equivalents of the words.

5 Write sentences with the new vocabulary. 6 Do the given exercisesfor better remembering the topic.7 Study grammar point ‘The category of comparison of the adjective` do the tasks

and find the sentences with the category of comparison of the adjective in the

text.

8 Speak on the topic. Given schemes and questions will help you to remember and

understand the topic.

9 Find more information about the text and prepare a presentation or a project

work.

Brainstorming questions

 

1 What are the simplest dead-reckoning systems?

2 What is the oldest heading sensor?

3 What is the usual speed-sensor on an aircraft or helicopter?

The simplest dead-reckoningsystems measure aircraft heading and speed, resolve speed into the navigation coordinates, then integrate to obtain position. The oldest heading sensor is the magnetic compass: a magnetized needle or an electrically excited toroidal coil (called a flux gate), or an electronic magnetometer. It measures the direction of the Earth’s magnetic field to an accuracy of 2 degrees at a steady speed below 60 degrees magnetic latitude. The horizontal component of the magnetic field points toward magnetic north. The angle from true to magnetic north is called magnetic variation and is stored in the computers of modern vehicles as a function of position over the region of anticipated travel. Magnetic deviations caused by iron and motors in the vehicle can exceed 30 degrees and must be compensated in the navigation computer.

A more complex heading sensor is the gyrocompass, consisting of a spinning wheel whose axle isconstrained to the horizontal plane by a pendulous weight. The aircraft version (more properly called a directional gyroscope) holds any preset heading relative to Earth and drifts at more than 50 deg/hr.Inexpensive gyroscopes (some built on silicon chips as vibrating beams with on-chip signal conditioning) are often coupled to magnetic compasses to reduce maneuver-induced errors and long-term drift.The usual speed-sensor on an aircraft or helicopter is a pitot tube that measures the dynamic pressure of the air stream from which airspeed is derived in an air-data computer. To compute ground speed, the velocity of the wind must be vectorially added to that of the aircraft. Hence, unpredicted wind will introduce an error into the dead-reckoning computation. Most pitot tubes are insensitive to the component of airspeed normal to their axis, called drift. Another speed sensor is Doppler radar that measures the frequency shift in radar returns from the ground or water below the aircraft, from which ground-speed is inferred directly. Multibeam Doppler radars can measure all three components of the vehicle’s velocity relative to the Earth. Doppler radars are widely used on military helicopters. The most accurate dead-reckoning system is an inertial navigator in which accelerometers measure the vehicle’s acceleration while gyroscopes measure the orientation of the accelerometers. An on-board computer resolves the accelerations into navigation coordinates and integrates them to obtain velocity and position. The gyroscopes and accelerometers are mounted either directly to the airframe or on a servo-stabilized platform. When fastened directly to the airframe (“strap-down”), the sensors are exposed to the angular rates and angular accelerations of the vehicle. In the 2000s, virtually all inertial navigators were strap-down. Attitude is computed by a quaternion algorithm that integrates measured angular increments in three dimensions at a faster rate than the navigation coordinates are calculated.When accelerometers and gyros are mounted on a servo-stabilized platform, gimbals angularly isolate them from rotations of the vehicle. The earliest inertial navigators used gimbals. A single-axis sensor chip and a 2-axis sensor chip are mounted orthogonally at the end opposite the connector. The sensor chips are magneto-resistive bridges with analog outputs that are digitized on the board. GPS-Inertial Navigator. The inertial instruments are mounted at the rear with two laser gyroscopes and electrical connectors visible. The input-output board is next from the rear; it excites and reads the inertial sensors. The computer board is next closest to the observer and includes MIL-STD-1553 and RS-422 external interfaces. The power supply is in front. Between them is the single-board shielded GPS receiver. Round connectors on the front are for signals and electric power. A battery is in the case behind the handle. Weight 10 kg, power consumption 40 watts. This navigation set is used in the F-22 and many other military aircraft and helicopters. Photo courtesy of Northrop-Grumman Corporation.

Navigators were only used on specialized high-accuracy military aircraft. The gimbal angles measure attitude directly, without computation. The instruments are in a benign angular environment and held at a constant orientation relative to gravity, which simplifies the computations and reduces the error in mechanical instruments. Inertial systems measure vehicle orientation within 0.1 degree for steering and pointing. Most accelerometers consist of a gram-sized proof-mass that is mounted on a flexure pivot. The least expensive ones measure the deflection of the proof mass; the most precise accelerometers restore the proof mass to null, thus not relying on the structural properties of the flexure. The newest accelerometers are etched into silicon chips. The oldest gyroscopes contained metal wheels rotating in ball bearings or gas bearings that measured angular velocity or angular-increments relative to inertial space; more recently, gyroscopes contained rotating, vibrating rings whose frequency of oscillation measured the instrument’s angular rates. The newest gyroscopes are evacuated cavities or optical fibers in which counter-rotating laser beams are compared in phase to measure the sensor’s angular velocity relative to inertial space about an axis normal to the plane of the beams. Vibrating hemispheres and rotating, vibrating bars are the basis of some navigation-quality gyroscopes (drift rates less than 0.1 deg/hr). Fault-tolerant configurations of cleverly-oriented redundant gyroscopes and accelerometers (typically four to six of each) detect and correct sensor failures. Inertial navigators are used in long-range airliners, business jets, most military fixed-wing aircraft, space boosters, entry vehicles, and manned spacecraft.