Methods for determining the coordinates of the location of aircraft. Structure of a typical navigation system
To determine the location of the aircraft can be used the following methods of navigation: • dead reckoning; • position; • surveillance - comparative. Dead reckoning method is based on calculating the distance through the known initial position of the aircraft. The input to this task are the velocity or acceleration of the aircraft and its course. Determine the coordinates of the location of the aircraft in navigation systems is performed using expressions (for orthodromic coordinate system):
;
.
Depending
on the information of the sensors used are several types of dead
reckoning: Doppler, air (aerometrichesky), inertial and combinations
thereof.
The principle of the Doppler ground speed and
drift angle (DISS) is based on measuring the frequency shift that
occurs in the reflected signal from the earth's surface during
irradiation it from a moving aircraft. The geometrical shape of the
multipath pattern Diss provides a measurement of the velocity of
three aircraft in the coordinate system attached to the antenna. To
recalculate the components of ground speed of the antenna system in
the horizontal coordinates are special calculators that take into
account the roll and pitch angles of the aircraft. Based on the
values of the vector components of the ground speed due to its
single integration estimated coordinates of the location of
aircraft.
The advantages of navigation systems that are
based on Diss are: the independence of their work on the visibility
conditions, the time of day and year, altitude and flight speed.
However, Doppler navigation systems are not sufficiently accurate
location control (error is 1 ... 2% of distance traveled) by a noise
immunity and are sensitive to the form of the underlying
surface.
Navigation system, built on the basis of the
system of air signals (SAF), have simple technical performance and
high reliability. However, due to rather large errors in the
measurement of air speed, these systems have a low accuracy of the
positioning of aircraft. The error is 3 ... 6% of distance
traveled.
The basis for the construction of the complex
navigation has now become the inertial navigation system (INS). ANN
built on the principle of dead reckoning by double integration of
acceleration measured in the defined coordinate system using
accelerometers. The advantages of ANN are:
- autonomy;
-
regardless of weather conditions, time of day, year;
- noise
immunity;
- continuity of information.
However, the
main disadvantage is the increase in the INS error to determine the
position of the aircraft flight time. ANN modern middle class have
standard errors location about 5 ... 10 miles per hour of flight.
Positional method is based on determining the location of the aircraft relative to pre-linked to the card surface transmitting stations.
The main advantage of the positional method compared with the method of dead reckoning is the independence of the accuracy of the coordinates of the aircraft of the distance to the point of measurement, ie, errors do not accumulate over time. Among the positional navigation systems most widely used radio near (RSBN) and distant (RSDN) navigation and landing systems. The mean square error value positioning of aircraft using the system RSBN-6C is 0.25 km, and the system RSDN 3C - 1.5 ... 2 km. The disadvantages of radio navigation systems are the ability to determine the coordinates of the aircraft only in certain areas of the outer stations (non-autonomous), and low immunity. Promising direction of development are RSDN satellite radio system (SRNS). SRNS are global and have a much higher accuracy (maximum error of positioning can be 50 ... 150 m).
Overview and comparative methods are based on the collation of the measured fields of the Earth (magnetic, gravity, topography) with their standards, pre-stored in their special on-board devices. Navigation system, based on measurements of the surface topography fields followed by comparing them with the standards, called correlation - extreme navigation systems (КЭНС). The disadvantages of these systems is the difficulty of creating the necessary information management, as well as inefficiency in areas on Earth where there is no abnormality used geophysical field. Rms error locating aircraft with КЭНС TERCOM is about 100m. In the future, this value can be reduced by up to 10 m.
Analysis of the possibilities of specific methods and means of navigation can be concluded that no single system can provide navigation and piloting of modern aircraft. Therefore, the main way to improve aircraft navigation equipment is to create an integrated navigation system (SNS). The essence of aggregation is to use information and structural redundancy to improve the accuracy, reliability and immunity measuring navigational parameters. Information redundancy provided obtain a uniform information from multiple sensors of different physical nature, followed by a joint processing of this information in a specialized calculator. Redundancy structure of the complex provides its performance if one of the navigation sensors. In this case, there may be some deterioration in accuracy. One of the major challenges in creating a rational choice CND of navigation systems and optimum combining them into a single set. Structure of a typical modern aircraft navigation system is shown in Fig. 3. The basic method of determining the coordinates of the aircraft in such a complex is an autonomous dead reckoning, which is based on ANN. This complex has a functional redundancy techniques dead reckoning. This is ensured by the presence of Diss, SAF, and vertical rate of SLE.
Coordinate correction resulting notation, using data from various systems of compensation, are used to determine where the aircraft positional method. When correcting dead reckoning coordinates performs two operations: - the coordinates of the aircraft by using selected as the corrector (RSBN, radar, optical sight, etc.); - replacement of dead reckoning coordinates to the new measured values. - Inclusion in the SNS several means to provide the necessary correction allows accurate navigation in any situation.