Measuring magnetic field parameters used to determine the orientation and direction of motion. Historically, this is the first navigational instruments for targeting. The principle of operation is based on the use of Earth's magnetic field.
North Magnetic Pole is currently located at coordinates 70 ° north latitude and 95 ° west longitude and south magnetic pole - at coordinates 72.5 ° south latitude and 154 ° east longitude (Fig. 63). In addition, the magnetic axis of the Earth is not stable and moving, respectively, and the magnetic poles are moving too.
Fig.63 Earth's magnetic field.
Earth's magnetic field is characterized by its intensity T. Vector T tension is the result of compiling vector of individual fields: which are caused by: То Uniform magnetic field magnetization of the body of the Earth; Тм – inhomogeneous magnetic field of the inner layers of the Earth; Та – magnetic field of magnetization earth crust, called anomalous field; ΔТа – external source field on the Earth, called geomagnetic field variations.
Vector sum called the intensity vector of the normal geomagnetic field. The vector is decomposed into components: H horizontal, vertical Y and longitudinal Z (Fig. 64). Angle between vector Т and horizontal plane are called inclination angle.In the magnetic equator angle of inclination is zero. During the movement of the magnetic equator to the poles of the magnetic angle increases and approaches 90 ° near the magnetic poles.
Fig.64 Elements of the intensity vector of a magnetic field of the Earth
If you take a magnetized arrow and set it to the middle dot resistance, it will be installed in the direction of the horizontal component of H along the line north-south, while in the northern hemisphere vertical component Z will incline north end of the arrow tip down. Accordingly, in the southern hemisphere vertical component Z will incline down the south side.
The horizontal component of the tension vector N does not coincide with the direction geographic meridian. Direction component of H is called the magnetic meridian place.Angle between magnetic and geographic meridians are angle of declination.Declination Considered positive if the magnetic needle deviates northern end to the east of the geographic meridian and negative when the arrow is rejected to the west.
The values of the declination plotted on maps of the magnetic declination special and captured using a magnetic compass. Since the magnetic declination different in size and sign in different parts of the earth's surface, for their consideration in the determination of the need to know the whereabouts of the aircraft.
Heading of the aircraft called the angle between the plane of the meridian and geographical projection aircraft axis on a horizontal plane. The course is measured clockwise from the North. The course is measured in degrees and may range from 0 to 360 °.
Instruments designed to measure the course aircraft, called Compass.
There are true, magnetic and compass courses.
True course is called the angle measured from the geographical meridian.
Magnetic course called the angle measured from the magnetic meridian. If the measure compass course, such a magnetic, indications will differ from the true and magnetic courses through the action of error inherent in the device. That is why it is called a compass course.
The principle of the magnetic compass is based on the properties of the needle set towards the magnetic field lines of the Earth (Fig. 65) .
A fundamentally different method of measuring magnetic induction course is to use the method. By this method the magnetic field of the Earth operates on electromagnetic field generated in the system electrical coils. As a result of changing inductance parameters that are fixed with special equipment and acquire values proportional to the magnetic course.
Fig. 65. Magnite reserve compass (S.I.R.S.)
At present there are a large number of sensors that measure all components of the vector magnetic field and have a very small size. Examples of sensors Rockwell Collins and Honeywell, to measure all components of the vector magnetic field shown in Fig. 66 and 67.
Sensor HMR2300 built by magnetoresistance measurements using components of the magnetic field , which is based on property magnetization direction changes in the internal structure of NiFe alloy under magnetic field (Fig. 68). As a result, depending on the angle α between the direction of magnetization current and changing resistance R alloy NiFe.
The change of resistance recorded using bridge circuit connection of resistors.
Fig.68. The principle of building a magnetic field sensor by using magnetoresistive method
To determine the direction of the magnetic field lines can be used sensors based on the Hall effect .
Magnetic field sensors with gyro instrument forming Attitude and heading reference system (Magnetic Heading Reference System - MHRS or Attitude Heading Reference System - AHRS). AHRS - a system that gives the user the value of the angles between geographic coordinate system and coupled with the aircraft coordinate system. Magnetic field sensors indicate the aircraft magnetic course , and a group of three gyro sensors placed on the respective axes - angular coordinates of aircraft. Typically, such systems complect as acceleration sensors. The main function AHRS - ensuring angle measurement rate of roll and aircraft pitch to display them on display EFIS, used in the flight control system, FMS and other systems of the aircraft.
The main producers AHRS are Rockwell Collins (AHS 1000, AHS 3000)  and Honeywell. It should be noted that the aircraft magnetic course can be computed using some mathematical transformations based on known angles of deviation from the geographic coordinate system. Accordingly, some AHRS provide relatively accurate value of the aircraft magnetic course, but in its structure does not contain magnetometers (Rockwell Collins: AHS 4000 and Honeywell: AN-2100). So, AN-2100 (Fig. 68) measure angular coordinates with accuracy 0,15°, cousre – with accuracy 1°, and the deviation course is only 0,1° per hour.