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| LN-3 Inertial Navigation System | |
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| LN3-2A Inertial Platform at RNlAF Electronics Museum, Rhenen, The Netherlands. |
The LN-3 Inertial Navigation System is an Inertial Navigation System that was developed in the 1960s by Litton Industries. The Lockheed F-104 Starfighter was the first supersonic fighter aircraft to be equipped with this Inertial Navigation System (INS). An inertial navigation system is a system which continually determines the position of a vehicle from measurements made entirely within the vehicle using sensitive instruments. These instruments are accelerometers which detect and measure vehicle accelerations, and gyroscopes which act to hold the accelerometers in proper orientation.
Contents
Background
The Cold War missile race spurred the development of smaller, lighter and more accurate inertial systems. Independent of its environment, the inertial system provides velocity and position information accurately and instantaneously for all manoeuvres, as well as being an accurate attitude and heading reference. The LN3-2A was the first inertial navigation system small and light and accurate enough to be fitted in a high performance fighter.The early F-104's, model A through F, did not have an Inertial Navigator. It was the development of the F-104G, around 1959, for the European Air Forces with tactical bomber/strike capabilities, that brought the LN-3 into the aircraft.[1] The LN-3 gave the F-104G the capability to navigate at low level in adverse weather and to drop a nuclear weapon at a range of 1,000 km with the best possible precision; this being vital to the F-104G program.
The LN-3 is a full 3-degrees-of-freedom, 4-gimbal inertial navigator, covering the flight performance envelope of the F-104G which ranged from 0 to 70,000 feet altitude; 0 to Mach 2+ speed, and accelerations from -5 to +9 g.
Introduction
The functional description of the LN3-2A requires some knowledge of some basic principles of inertial navigation to understand their application to the LN3-2A. The principal component of the system is the stable platform to which are mounted three accelerometers and two gyros. This stable platform is mounted in a system of platform gimbals. The acceleration of the airplane in any plane or direction is measured by the accelerometers and integrated in the computer to obtain velocity. Velocities in turn are integrated to obtain distance. With a known reference point representing initial position of the airplane with respect to earth, this data can be converted to distance and heading traveled, and distance and bearing to destination.The following characteristics of the platform are described:[2]
a. Three accelerometers in orthogonal directions provide the basic sensing elements. They measure acceleration along the two grid coordinate axes and the vertical (Z) axis. The Z accelerometer is not used by the LN3-2A itself but provides vertical acceleration data for the automatic flight control system. The east-west and north-south X and Y axes are used for the LN3-2A. The accelerometer outputs torque the gyro's in their sensitive axes, while the airplane is in flight, to maintain earth and grid north orientation of the stable platform through the platform gimbals.
c. Platform gimbals are the assemblies which actually keep the platform accelerometers stable and enable the airplane to maneuver about the gyro-stabilized earth-oriented platform. The LN3-2A platform is a four-gimbal system (outerroll, pitch, innerroll and azimuth) allowing the airplane 360 degrees of rotation in all directions. The azimuth, pitch and outerroll gimbals use sliprings and brushes for electrical contacts to allow unlimited freedom. The innerroll gimbal provides a built-in redundancy to prevent a gimbal lock situation when the azimuth and outerroll gimbal axes become aligned at 90 degrees of pitch.
The LN3-2A Computer controls the platform, computes navigational information and provides special ac and dc voltages required for equipment operation. The functions of the computer are:
a. to position the azimuth, pitch and roll gimbals of the platform. The basic sequence is that the gyro precession error due to airplane maneuvering is sensed and fed to the platform azimuth synchro resolver. The gyro signals are resolved into pitch and roll error voltages which are amplified in the computer. The computer drives the platform roll and pitch gimbal servomotors. The lower gyro is torqued to precess in azimuthh to drive the azimuth gimbal motors. The upper gyro is caged to the lower gyro in azimuth. The gimbal servomotors position the gimbals to compensate for the original deviation.
b. to provide the voltages for the starting and running of the gyro spin motors. During the start of the system the gyro's are brought up to spin speed by airplane 115VAC, 400 Hz power. After the 1 minute coarse align phase the frequency source for the gyro's is an electric tuning fork which provides a 3 kHz reference frequency which is divided by 8 to provide an operating frequency of 375 Hz and a running voltage of 90 Volts.
c. to control the heating of the component oven, the platform, the gyro's and the accelerometers. Some circuits within the computer, like amplifiers, require a very stable amplificationfactor which can only be maintained if certain components are kept under precisely held temperature. These components are placed within the Component Oven at 71 °C. Also the gyro's and accelerometers are kept at 71 °C ± 1.1 °C. The ambient atmospheric temperature inside the platform is maintained at 51.7 °C by a set of heaters and a circulating fan, and a motordriven cooling air valve controlling the flow of pressurized air through the doublewalled platformcover.
d. to compute velocity and distance information from acceleration. These navigational computations are performed with carefully designed electronic circuits in harmony with precision electromechanical components. The electronic parts are the Accelerometer Restoring Amplifier which give a voltage that is proportional to the acceleration. Ranging from micro-G's to units of G they span a very impressive dynamic range. Also the Servo Amplifiers, picking off the tiny Gyro signals and amplifying this to control the platform gimbalmotors, have tight specifications. The actual integration of the accelerometersignal to a velocitysignal is performed by an electronic amplifier which controls a velocity-motor which drives a capacitance-tachometer. This cap-tach feedback provides the basic integrator signal since the speed of the cap-tach is proportional to acceleration input. The feedback nulls the acceleration input to stop the motor. The motor positions the velocityshaft to pick-off the appropriate potentiometer signal which represents velocity. A dead zone network drives the velocitymotor in steps which are smoothed to provide the integrated acceleration (= velocity) signal. The velocity integrators operate in a similar manner to the acceleration integrators, except that the output signal is not smoothed because the so-called M-Transmitters are step-function devices. The M-transmitters send the integrated velocity (= distance) signal to the Position and Homing System PHI-4.
e. to sequence and control the coarse- and fine-align phases in conjunction with platform temperature.
f. to sense malfunctions to actuate the go, no-go circuitry of the inertial navigator.
g. Since the LN-3/PHI-4 navigation system is to be used around the globe of the earth, some systematic corrections for use on this rotating sferoid are implemented in the LN-3 : Earth rate-, Transport rate-, and Corioliscorrection. And to suppress inherent errors the system is Schuler-tuned.
Operation of the LN-3[3]
Before starting the Inertial navigator, the pilot has to enter the coordinates of the starting point in the "Align Control" panel in the right-hand console of the F-104G. The first selection in the starting sequence is to rotate the mode selector switch of the "Inertial Navigation Control" Panel from Off to Standby.In this mode the platform and component oven are brought up to operating temperature; indicated by the "heat" light on the IN Control Panel, which takes several minutes depending on outside and system temperatures.
All at operating temperature the system may be switched to "Align", allowing the machine to commence operation. The computer is powered up and nulls its velocity shafts; the gyro's are powered by 115V and 400 Hz and revving up; the platform is leveled in pitch, inner and outer roll relative to the aircraft using the gimbal synchrotransmitters; and the azimuth axis is driven to the grid north direction using the magnetic heading sensor. This phase of Alignment takes 1 minute and is called coarse align.
After this 1 minute the system switches to the fine align phase, during which the gyrospinmotor power is brought down to 95V and 375 Hz to avoid any magnetic interference with any other aircraft system using 400 Hz. The leveling of the platform is taken over by the X and Y accelerometers sensing even the smallest component of gravity which is an indication of not being precisely level. The leveling of the stable element is achieved by torqueing the respective gyro torquers which makes the gimbal motors to follow up and level the stable element. The distance shafts are set to zero; the gyro's are at operational speed and the computer is continuesly feeding the gyro's, and thereby the stable element, with corrections for local earth rotation. This is called the leveling phase of fine align.
Leveling ends automatically when the computer decides that the platform stable element is exactly locally level, which may take a few minutes. If level, the final phase of alignment is switched on; gyrocompassing. The stable element is exactly level and Schuler-tuned but the gyro's are not yet aligned with the earth rotation axis. Therefore the stable element tends to turn off-level, which is sensed by the Y accelerometer which signal is fed to the gyrotorquer to rotate the azimuth axis of the stable element. This process continues for a few minutes until the correction signal is getting smaller and can be kept almost zero for 50 seconds, which gives confidence that the system is level and aligned. This is visible for the pilot because the green Nav light flashes.
The system is now ready for use and the pilot selects "Nav" on the IN Control Panel, and all circuitry that was involved in the various alignment phases is switched to the navigate mode.
Other possible modes are Compass only which may be selected after a LN3 in-flight failure, and Alert Align to shorten the alignment phase. After the last flight but before shutting down aircraft power the precise heading of the running LN3 is stored and can be used at starting up the next time, if the aircraft is not moved.
- Performance
During manufacturer's development flying at Palmdale, some 1167 flights were made up to October 1961, and the c.e.p. of the LN-3 and PHI-4 combined was a mile or so outside specification. From October 1961 to January 1962 a further 123 flights at Palmdale were assessed, following incorporation of the -9 modifications, and the c.e.p. came almost up to specification.
At Edwards AFB, during Category 2 testing, and at Palmdale during the "avionics marriage" period, mean time between failures of pre-9 systems was considerably below the 200 hr specified, but the target has been exceeded since then.[4]
Genealogy
Litton Systems Inc., or Litton Industries, the Guidance and Control Systems Division at Beverly Hills CA, were one of the major producers of inertial systems in the USA in the 50's and 60's, and have made a series of systems for a number of American aircraft.[5]The Genesis of inertial navigation systems is explained in the following reference.[6]
- The LN-1 was a development attitude reference for the XB-70 Valkyrie.[7]
- The LN-2A (military designation AN/ASN-31 or -36) was a Doppler-inertial system for the A-6A Intruder
- The LN-2B was the system for the E-2A Hawkeye,
- The LN-3-2A (or LN3-2A) was the Inertial Navigation System used in the F-104G Super Starfighter. (development 195?-195?, production 1960-196?) Improved versions of the LN3-2A were -9, -11 and -13.[12]
- The LN-3-2B is the Inertial Navigation System used in the Canadian CF-104.[13]
- The LN-3-13 is fitted to the Italian F-104S/CI and F-104S/CB; enhanced variants of the F-104G from 1969 and onward.[14] In the early 80's a further upgrade led to the F-104S ASA version which kept the original LN-3; but the ASA-M version of the '90s was equipped with the LN-30A2 inertial navigation system.[15]
- The LN-4 is a miniature inertial system for "a manned orbital vehicle" [16]
- The LN-5 is a (1963)"state of the art experimentation astro-inertial system installed in a Convair 340 R4Y ".[17]
- The LN-7 is an astro-inertial-Doppler system for a classified application.[18]
- The LN-12A/B series are an evolution of the LN-3 and are used in F-4C (AN/ASN-48), the F-4D and F-4E (AN/ASN-63), the RF-4C (AN/ASN-56), all with slight differences.[19]
- LN3-2A notation.
Other U.S. Inertial Systems of the early 1960s.
The Litton LN-3 was one of the first inertial navigators on a production aircraft, but other systems, either Inertial Navigators or Inertial Measurement Units, of other brands and for various applications with comparable technology existed.The Autonetics Radar Enhanced Inertial Navigation System (REINS) of the North American A-5 Vigilante was more or less comparable to the LN-3/PHI-4. This system was derived from the XN-6 system developed for the SM-64 Navaho, the N5G system for the AGM-28 Hound Dog and the N2C/N2J/N3A/N3B system for the XB-70, and was related to the N6A-1 navigation system used in the USS Nautilus (SSN-571) and the N10 inertial guidance system for the LGM-30 Minuteman.[22] Note that the Boeing history claims the REINS to be the first inertial navigation in a production airplane.
Nortronics had developed and produced Astro-Inertial guidance/navigation systems for the SM-62 Snark. The system developed for the GAM-87 Skybolt was later adapted for use in the Lockheed SR-71 Blackbird and mostly referred to as NAS-14 and/or NAS-21.
The UGM-27 Polaris missile was equipped with a MIT-developed inertial system, which later evolved to the Delco produced IMU of the Apollo PGNCS.
The Saturn V was equipped with a MSFC-developed ST-124-M3 inertial platform which was a further development of the PGM-19 Jupiter's ST-90.
LN-3 Maintenance and Test equipment
The LN-3 system was designed to constantly monitor critical parameters, and warn the pilot in case of a malfunction. Depending on the problem the pilot could switch-off the system, or continue in a dead reckoning mode. In case of serious self-diagnosed problems the system would auto shut-down.- Flight line maintenance of the LN-3, like systemchecks and fault isolation, was performed using specific test equipment
- MATS (Mobile Automated Test System)
- Line Test Analyzer
- Gyro Bias Test Set
- At base (nav)shop level the Platform, Computer and Adapter units were tested and repaired using the following test equipment
- System Test Console (STC).
- Bench Test Console (BTC).
- For repairs beyond the capabilities of base level, the RNlAF Electronics Depot (and possibly others) were equipped with specific testequipment and tooling to handle the (higher) depot level repairs of the LN-3 system. The main teststations in use were
- Platform Functional Test Console (PFTC).
- Module Test Console.
- Industry support
LN-3 units on display
- Germany
- The Wehr Technische Studiensamlung (WTS) at Koblenz.
- Netherlands
- Museumcollection of Navigation and Communication systems at Royal Netherlands Air Force Logistic Centre Woensdrecht, location Rhenen, former Air Force Electronics maintenance depot (DELM).
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