Historical Document · 1953

Description of Ground Position Indicator Type A-1

This document describes the Ford Instrument Company Ground Position Indicator (GPI) Type A-1, a four-unit airborne system prepared for the U.S. Air Force that continuously computes and displays present latitude and longitude of an aircraft in flight. The system weighs approximately 45 pounds (exclusive of cables), operates from 115 V 400-cycle and 28 V DC power, accepts true airspeed, compass heading, wind direction, wind force, and magnetic variation inputs, and achieves a maximum total air-position error of about 1.5% of computed nautical miles traveled. The document covers operating principles, component descriptions (Preset Control Box, Computer Unit, Amplifier Unit, Ground Position Indicator panel), dimensional drawings, and the trigonometric geometry underlying the latitude/longitude integration.

Manufacturer: Ford Instrument Company
System: Ground Position Indicator Type A-1
Year: 1953
Type: Historical Document
Language: English
Learning track: specific applications
Pages: 9
Credit: Prepared for U.S. Air Force by Ford Instrument Company, Division of The Sperry Corporation, Long Island City, New York, 1953.

Ground Position Indicator Type A-1
Ford Instrument Company
ground position indicator
airborne navigation
dead reckoning
electro-mechanical computing

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DESCRIPTION OF GROUND POSITION INDICATOR TYPE A-1 Prepared for U. 5. Air Force by FORD INSTRUMENT COMPANY Division of The Sperry Corporation Long Island City, New York 1953 DESCRIPTION oF FORD GROUND POSITION INDICATOR TYPE A-l The Ford Instrument Company Ground Position Indicator System Type A=-1 consists of four airborne instrue ments that continuously compute and display present latitude and longitude of an aircraft in flight. GEN ERAL_ DESCRIPTION Input data are obtained from instruments solely within the aircraft. For greater output accuracy and auto- matism, G.P.J. Type A=-1 can be operated with radar inputs. Components of the G.P.I. system ares: The Indicator Unit, Preset Control Box, Computer Unit, and Amplifier Unit. The Indicator Unit (Figure 1) is mounted on the instrument panel; it displays on counters the present lati- tude and longitude of the aircraft; Lt also holds two slewing switches for setting in the latitude and longitude of the point of departure. The Preset Control Box (Figure 2) is console mounted within convenient reach of the pilot; wind direction, wind force, and variation are manually set in this box. When radar is used, wind force is zeroed. The Computer Unit (Figure 3) and the Amplifier Unit (Figure 4) may be placed at any accessible location. The Computer Unit receives true air speed and compass heading from other instruments, =Be The weight of the system, exclusive of cables, is about 45 pounds. Range is unlimited, but system opera- tion is limited to latitudes less than 70°, Electrical requirements: 115 v., 400~%; 28 v. D.C. PRINCIPLES OF OPERATION The variables entering into the ground position determination are shown in Figure 5. To determine the aircraft's ground speed, and from this speed the ground position, we must know the vectors of true airspeed Vt and of wind force Vw. Wind direction Hw and force Vw are as- sumed to be available. True air speed Vt is also available; we can find its true heading Ht by algebraically adding to the magnetic heading He the values of magnetic deviation De and magnetic variation Va. The latitudinal and longitudinal components of true air speed and of wind force are found by reference to Figure 6. The latitudinal component of true air speed --- Vt cos Ht, is added to the latitudinal component of wind force --- Vw coos Hw. The addition yields the latitudinal speed of the aircraft; Vt cos Ht + Vw cos Hwee. To find the longitudinal speed of the aircraft, we must allow for meridian convergence. This allowance is made by multiplying the east-west component of the aircraftts ground speed by the secant of present latitude. The east~ west component is the sum of the east-west components of true air speed and wind force: Vt sin Ht + Vw sin Hw. Accordingly, the longitudinal speed of the aircraft is (Vt sin Ht + Vw sin Hw) sec Lap, where Lap is the present value of latitude. Present Position The changes in latitudinal and longitudinal positions are generated by the time integrals of the latitudinal and longitudinal speeds. These changes are added to the values of initial latitude and longitude, which are put into the system at take-off, to yield the aircraft's present ground position. In equation form, present latitude and present longitude (Lop) may be expressed ass t Lap -/ {Vt cos Ht + Vw cos Hw)dt + Lai, ) t Lop -/ (Vt sin Ht + Vw sin Hw} sec Lap dt + Lol, o where Lai and Loi denote initial latitude and longitude, respectively; and t denotes the "time variable of integration. The Ford Ground Position Indicator Type A=-1 uses the preceding equations in the following form: t Lap / (cos Ht + (Vw/Vt)oos Hw) d(Vtet) + Lai, o Lop (sin Ht + (Vw/Vt)sin Hw) d(Vt sec Lapet) +Lole Radar Operation When the Ford Ground Position Indicator Type A=1 is used with associated radar the wind force knob is zeroed. Automatics inputs are true ground speed, drift angle, and magnetic heading; manual input is variation. DESCRIPTION OF COMPONENTS The packaging of the Ford Ground Position Indicator Type A-l features miniaturized units, readily producible, extremely flexible, compact and light in weight. The system consists of four self-contained separate units: Preset Control Box, Computer Unit, Amplifier Unit, and Indicator Unit. Prasat Control Box The Preset Control Box contains components neces- sary to set remotely wind direction, wind force, and variation data into the system. The input control knobs are connected to coarse and vernier dials, and to transmitters for con= veyance of the signals to the Computer Unit. Computer Unit The Computer Unit includes the majority of mechanical and electro-mechanical computing elements. The unit receives wind force, wind direction, and variation signals from the Preset Control Box. True air speed is received from a Type A-l, A-2, Cel, or C-2 True Airspeed Computer, or equal. Magnetic heading is received from a Type J-2, J-4, or Nel Slaved Gyro Magnetic Compasse Magnetic heading is corrected by a cam=-type compensator for residual deviations. The deviation corrections and magnetic heading are then added to magnetic variation to generate true heading. Amplifier Unit The Amplifier Unit includes electronics and electrical components for operation of the other units. ‘The power supply section delivers 26-volt 400-cycole power to synchros, 24=«volt 400-cycle power to resolvers. Power is obtained from the 115-volt 400-cycle single-phase aircraft supply, and from a 28~volt D.C. power sources. Ground Position Indisator The Ground Position Indicator is a panel-mounted unit that receives changes in latitude and longitude from the Computer Unit, and continuously displays present latitude and longitude on veeder=type counters. The counters are re- settable during operation of the system. ACCURACY The maximum total air-position error is expressed as a percent of the computed nautical miles traveled, If the maximum component errors and maximum combinations of these errors are assumed, then the actual air~position error should be about 1.5%. For radar operation, the figure should approximate 0.7%. Since it is improbable that all errors will reach their maximums at the same time, it is expected that the overall errors may be less. -o0c08 5.365 3.375 +9 1688 Sl is] fs { coles DEVIATION CAM ADJUSTMENT QOae? i @o ote © © ©! ll @e 2 2 Se2 : 6.813 Lomas HY ! 8 === L_ i$ $5.19 ———_ 76 -— h#—— 5.86 ——»} Figure 4. 94: 37 Vw sIN Hw Hw i | Vt sIN Ht Vw cos Hw VT cos Ht + Vw cos Hw Figure 5. ~ PRUE NORTH Rh, De DIRECTION OF NORTH INDicaTeD ON AIRCRAFT COMPASS GEOMETRY FOR THE GROUND POSITION INDICATOR Figure 6,