Development of Aerial Camera Stabilizationand Its Effect on Photogrammetryand Photo Interpretation*

TIMOTHY TROTT, Electrical Dept. Head,Aeroflex Laboratories Inc.,

34-06 Skillman Avenue,Long Island City 1, N. Y.

ABSTRACT: From the inception of aerial photography, the photographersearched for methods of holding his camera steady and vertical despite thevehicle's motions. Early attempts were generally based on the pendulousmass principle. As vehicle speeds increased, the motions became toosevere for crude methods and the use of the gyroscope was introduced.Finally, it was recognized that center of gravity mounting was necessary,and improved gyroscope technology began to make possible the goal ofcomplete isolation of the camera from the vehicle. This goal has been closelyapproached by the use of modern gyroscope and servo techniques.

T HE object of stabilization is two-fold.First and most important is the re­

moval of the effects of aircraft motions,enabling the photographer to get labora­tory resolution from his camera and film.The second object is to keep the camera soclose to true vertical that subsequentrectification (except, possibly, for a small­scale change) is not necessary.

This paper shows to what extent thesegoals have been achieved. It also sketchesthe story of how this has come about andindicates what the future holds in store.It might be noted here that while perfec­tion is theoretically the goal, the law ofdiminishing returns operates to force amore practical view. Thus, in steadiness,the attempt is to get less than 5 or 6seconds of arc of motion in any exposureperiod for a 36 inch lens. This givesnegligible resolution degradation. For ver­ticality, 3 minutes of arc error is desired,this being the limit of accuracy of rectifi­cation. Both of these requirements havebeen met by present equipment. The con­ditions necessary for this will appearlater.

Before plunging more deeply into thematerial, some of the reasons for stabiliz­ing will be examined briefly. These may

Timothy Trott

be classified as civil and military. In civilphotography the advantages of stabilizingmust be evaluated almost entirely on aneconomic basis. They may be brieflyou tIined as follows:

(1) The automatic attainment of stead­iness and of even a low degree ofverticality eliminates the necessityfor a skilled photographer to operate

* Presented at the 22nd Annual Meeting of the Society, Hotel Shoreham, Washington, D.C.,March 21-23, 1956.

122

AERIAL CAMERA STABILIZATION 123

the camera, and reduces the load onthe pilot.

(2) Improved steadiness makes prac­tical the use of longer exposures.This has the advantage of extendingthe photographic day, or providinggreater freedom in the choice ofaperture and film.

(3) The greater imperviousness to air­craft motions makes possible photog­raphy in clear air turbulence, thusgiving many more photographicdays in the year.

(4) The enormously increased prob­ability of good pictures reduces thenumber of necessary repeat flights.

(5) With sufficiently good verticalitythe necessity of rectification couldbe eliminated, and in some caseseven ground control could be elimi­nated or simplified.

Before proceeding to survey the presentstate of the art, the history of the art willbe described very briefly.

The first self-conscious attempts tostabilize an object in a moving vehicleoccurred in the Age of Discovery. Asships ventured further from land the im­portance of accurate navigational in­struments became apparent. The firstinstrument to receive this attention wasthe compass; this was suspended in gimbalrings to isolate it from the ship's motion.The first gimballed compass appeared in1537.

The word "gimbal" is derived from theLatin "gemelus" meaning twin. From thiscame the Old French gemel, a term for aring formed of two linked hoops used asbetrothal or friendship rings in the 16thCentury. The earliest mention of such adevice occurs in the work of the 13th

FIG. 1. Stabilized navigator of the 16th century.

In military photography the argumentis even more forceful. There are two allimportant areas of usefulness.

(1) Guaranteeing a high degree ofverticality is mandatory in mappingareas where no ground control ispossible.

(2) Obtaining the highest probability ofhigh resolution is essential. A mis­sion is flown at great danger topersonnel and equipment. It is notdesirable to repeat a flight. It maynot even be possible since themilitary picture may be in a stateof flux. A mission must be fruitful.

Century architect Wilars de Honecort. Hedescribed a hand warmer, a charcoalbrazier supported in six gimbal rings tohold it top-side up. The six rings are ap­parently an erroneous attempt to eliminategimbal lock. De Honecort added ..."youmay turn it about in any way and thecinders will never fall out. It is excellentfor a bishop, for he may boldly assist athigh mass, and as long as he holds it inhis hands they will be kept warm.... "

After the use of the device on the com­pass it came to be used quite extensivelyfor shipboard instrumentation. In thesame century a suggestion was even made

124 PHOTOGRAMMETRIC ENGINEERING

FIG. 2. Stabilized chronometer by Huygens.

to gimbal the navigating officer himself fortaking Star Sights (Figure 1).

In 1659 Huygen's Marine Chronometerwas gimballed and heavily weighted withlead so as to maintain verticality for theclock pendulu m. Some success was ob­tained but time sufficiently accurate forLongitude measurements had to awaitHarrison's Chronometer using a torsionalpendulum independent of vertical. It maybe noted that almost all of the earlyattempts were based on the pendulousgimbal principle (Figure 2).

The gimbal was first applied to aerialphotography in 1858. In this year, only 19years after the invention of photographyby Daguerre, a French balloonist, F.Tourachon, engaged in aerial photography,obtained a patent on a device to hold acamera in a vertical and steady position,by means of a pendulous gimbal sus­pension. One would think that a balloonwould be steady enough for photographybut there must be remembered the slowspeed of the wet plates then in use. Figure3 shows a view of Boston taken from aballoon in 1858. Messrs. King and Blackexposed eight wet collodion plates andsecured two good pictures, of which thisis one.

The use of a pendulous suspension isundesirable because the unbalance neces-

sary to get verticality causes the mountto respond to lateral accelerations. Someof the defects of this type of suspensioncan be alleviated by using a gyroscope.This instrument, however, was not at thistime a practical device.

Gyroscopic principles had been knownsince early times and Newton formalizedsome of these motions, but the subject wasstill a learned discussion of an interestingtoy. The word gyroscope was coined in1852 by the French physicist Foucault.The etymological meaning is "rotationviewer" from the Greek, and was chosenon account of a method Foucault used todemonstrate the rotation of the earth. Hisapparatus is portrayed in Figure 4. Sincethis pre-dated electric motors, the wheelwas demountable. It was put into thedevice in the center of the figure whichgeared it to a hand crank. It was broughtup to speed and then put back into thegimbal structure. The microscope wasused to examine the motion of a pointerattached to the gimbal.

Gyroscopic instruments had to awaittechnology capable of getting power tothe wheel without coercing the suspension.In 1878 an electric motor was introducedand Foucault's experiment was success­fully run by Hopkins. Subsequently awhole series of interesting applicationsappeared su'ch as the early gyro compass

FIG. 3. Aerial view of Boston, 1858.

AERIAL CAMERA STABILIZATION

FIG. 4. Foucault's gyroscope, the first such instrument.

125

shown in Figure 5. This was one of thefirst instru ments using an electric motor.

The first suggestion for the use of agyroscope to stabilize an aerial camerawas made by Stolze in the British Journalof Photography in 1881. The suggested

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FIG. 5. Electric gyro-compass.

device was to attach a gyroscope rigidlyto a gimballed camera.

With the introduction of the airplanethe problem of vibration had to be faced.In 1912 Gasser in Germany introducedthe use of springs and a pneumatic cushionto the mounting of aerial cameras. Withthe airplane the other motions also be­came more severe, and a burst of activitywas produced in the stabilization field.

Some of the first work was done in theUnited States. In 1915 A. Brock receiveda patent on a gyroscopically controlledcamera mount with the gyroscope beingrigidly attached to the gimballed camera.The wheel is in the center of Figure 6.

In 1922 a patent was granted to Cookefor a gyroscopic mount. In this device wesee the first attention given to balance.To avoid the gyroscopic precession causedby unbalance, it was necessary to take thefilm off the gimbals through the bellowsarrangment in Figure 7.

Figure 8 ill ustrates the first Servo ap­proach. S. Fairchild received a patent in1923 for an arrangment which used a gyrofor reference only. Some means of datatake off was provided, and a system oflevers and motors used to keep the cameraslaved to the gyro. The gyroscope used inthis mount is illustrated in Figure 9.

This scheme failed of success only be-

126 PHOTOGRAMMETRIC ENGINEERING

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cause at that time there were no gyros ofsufficient accuracy and no servo deviceswhich would ensure low enough reactiontorques on the gyro. A typical verticalgyro of that period is shown in Figure 10.*On account of these difficulties the stabi­lized mount disappeared from view andno further developments occurred untilthe start of World War II.

Early in that war the principal prob­lems were those of vibration. A series ofsponge rubber mounts were developed byRobinson and manufactured by Aeroflex

FIG. 7. Gyroscopically stabilized cameramount with balance control in 1922.

FIG. 8. Camera mount servo controlled togyroscopic reference in 1923.

* Compare this crude device to a modernvertical gyro manufactured by Aeroflex Labora­tories specifically for aerial camera stabilization;see-Figure 11.

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FIG. 10. Typical vertical gyro of the early twenties. FIG. 11. Modern vertical gyro by Aeroflex Laboratories Incorporated.

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128 PHOTOGRAMMETRIC ENGINEERING

FIG. 12. Vibration isolated mount ofWorld War II.

Laboratories. Typical of these mounts arethe A-17 illustrated in Figure 12 and theA-8, A-27, A-17 and the NR-l, 2 for theNavy. Depicted in Figure 13 are a coupleof typical installations.

Among the first to feel the necessity forimprovement were the British. In late1941 the British completed development ofa gyroscopically stabilized mount. Thiswas probably the best of its type and was areapplication of the original ideas in thefield. A gyroscope was rigidly attached to abalanced gimbal suspension carrying thecamera. Great care was taken to balanceit accurately. The greatest cisadvant ageof this type of mount is the enormous sizeof the gyroscope required. This not onlymakes manufacture of a sufficiently ac­curate gyro difficult, but actually con­stitutes a physical danger to the ship. Thismount also incorporated provision forrotating the mount to compensate forforward motion of the aircraft.

When work was undertaken in theUnited States it was early realized that theprinciple of primary importance for stead­iness is the accurate location of the centerof gravity at the intersection of the axesof rotation. The early work was done bythe Harvard Optical Research Lab. whichmounted 2 K-40 cameras on a t inchsteel ball, located at the common center ofgravity. The results indicated excellentelimination of the influence of vibration,but no important improvement of the longperiod motions of the aircraft.

After the war much of the work doneby the Germans came to light. This workculminated in the Steinheil mount whichwas perhaps 75 per cent complete when thewar's end stopped work. The men and themount were brought to the United Stateswhere the work was completed and themount became a prototype for the AirForce A-28 mount. The principle of thiswas the use of gyros as reference unitsonly. A servo system using an opticaldata pick off was used to slave the mountto the gyros. The mount is shown inFigure 14.

This mount was redesigned to useAmerican components and to take a seriesof American cameras. The resulting mountis the A-28 production mount (Figure 15)manufactured by Aeroflex Laboratories.

The accuracy of verticality is perhaps! to 1 degree. The isolation from longperiod motions is excellent but fromvibrations is not very satisfactory. It de­pends upon excellent low frequency isola-

FIG. 13. Two typical installations of vibration mounts, World War II.

AERIAL CAMERA STABILIZATION 129

FIG. 14. Air force prototype A-28 (Stein­heil) mount.

tors for this purpose. Really satisfactoryisolation from fast disturbances cannot beobtained with geared servos since themotor and gearing constitute a restrainton the gimbal. Where a skilled operator isavailable without too high a cost, it isbetter to eliminate the servo and to returnto the idea of a balance point mount. Sucha mount was designed for the GeologicalSurvey by Aeroflex Laboratories and is de­picted in Figure 16. This mounts two T-11cameras on a small Hooke's joint ofextremely low friction. The photographerkeeps the mount level and the cg suspen­sion keeps resolution high.

If automatic operation is required thesame or better results may be obtainedby the use of torquers to slave the mountto a vertical gyro. Such a mount is shownin Figure 17. The design has progressed

FIG. 15. Production A-28 mount.

to the point of making the camera essen­tially independent of aircraft motions. Themount was designed for night operationswith exposures of 0.1 second. Motion dur­ing this exposure is kept to less than 6seconds of arc in any exposure periodgiving essentially laboratory resolution forthe K-36 camera. The verticality is onlyslightly different from that obtained withthe A-28.

Pictured in Figure 18 is a mOunt re­cently designed to give extreme precisionin verticality coupled with the high levelof steadiness. This utilizes torquer servosand rate gyros to keep the mount motions

FIG. 16. Balance point mount for two T-ll cameras.

130 PHOTOGRAMMETRIC ENGINEERING

FIG. 17. Torquer driven mount with verti­cal gyro reference.

down. Verticality is principally a functionof obtaining a good reference, which ispresent in the Aircraft for which thismount was designed. The verticality errorof this mount is less than 3 minutes of arcbut the size of the equipment for thevertical reference is enormous.

Future work in this field must and willbe directed toward reducing the size,complexity and cost of the accurate ver­tical reference and of the mounts in gen­eral. Several development programs arenow in progress toward that end. Theauthor feels that the problem of steadinesshas been adequately solved, and theprogram is now directed toward simplifica­tion and increasing reliability. A highaccuracy vertical reference really suitablefor photographic work has not yet beenperfected however. Most of the long termdevelopment must be directed toward thisend. The advantage of eliminating rectifi­cation and simplifying ground control areapparent.

FIG. 18. Torquer driven mount with external high accuracy reference.