INS-Inertial Navigation System
INS avionic system is a long range navigational aids and it stands for Inertial Navigation System. It provides aircraft velocity (3D) and position fix (3D). In fact, INS is based on DR, i.e. we first need to obtain the aircraft vectorial acceleration, then integrate once to obtain the velocity, and finally integrate a second time to get the position. Also, what makes this technology quite amazing is that it is a self contained system. Hence no ground stations are required for operation.
In the Aircraft there are two types of INS as mentioned below.
1. Stable-Platform INS or Gimballed INS:
The stable platform isolates the gyroscopes and accelerometers from the aircraft angular motion, and hence remains in-synch with the earth co-ordinate system. The system contains 3 Gyroscopes(G) viz. GROLL, GPITCH and GYAW.
It ontains 3 movable Accelerometers (A) viz. ALAT (AN-S), ALON (AW-E) and AALT.
2. Strap Down INS:
This type of INS has no moving parts; and therefore, the accelerometers are solidly connected to the airframe and gyroscopes are well aligned with the aircraft X-Y-Z coordinate system.
It contains three laser gyroscopes viz. GROLL (Gx), GPITCH (Gy), GYAW (Gz).
Laser gyroscope: The two laser light beams are sent one clockwise, and the other counter clockwise.
It contains 3 non movable accelerometers viz. Ax, Ay and Az.
3. In addition to velocity and position, INS provides following parameters:
• Track to fly
• Off-track distance
• Distance between 2 points
• Stores alternate destination positions
• Determines true North direction
• Recalculates ETAs
1.) Following are advantages of INS avoinic system.
• INS is a self-contained airborne system that does not need any outside NAV source.
• It displays in real-time the Aircraft velocity and position.
• It operates at all the altitudes.
• Sometimes GPS is used as an aid to INS in order to correct or attenuate errors.
2) Stable-Platform INS:
• It gets aligned with the earth coordinate system despite aircraft angular motion.
• ccelerometers and gyros are protected from malfunctioning due to severe maneuvers since they are not directly connected to the airframe.
3) Strap Down INS:
• Mechanically simple to realize.
• Laser gyros are more robust than traditional ones.
1.) In General:
• Drift Error is approx. about +/- 0.5 kts (+/-k 1 Km/hr)
• Errors are generated by non orthogonality of accelerometers.
• Vibration and thermal variation may cause flaws in information data.
• INS is an expensive technology.
2) Stable-Platform INS:
• Mechanically more complicated to realize.
• Errors generated by non-orthogonality of gyros.
• Gyros may suffer from EMI.
3) Strap-Down INS:
• NAV accuracy is highly dependant on the A/C maneuver, given that the accelerometers and gyros are directly connected to the airframe.
• Demands more computations as first need to convert accelaration from Aircraft coordinates to the earth coordinates Then perform DR to get the velocity and position.
Combination of INS and GPS is used as one completes the other. GPS can calibrate INS drift error, while INS attitude data can aid the GPS.
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