Survey navigation and determination of camera orientation elements

Commission 1

ll~vited p a p e r

251

Survey Navigation and Determination of Camera Orientation Elements by F. L. C O R T E N I.T.C., Delft. D e d t c a t e d to the centenary olc a e r i a l ~urrcy

Contents.

Introduction 1. Purely navigational methods 1.1. Dead reckoning navigation 1.2. Contact navigation or visual navigation according to line-of-sight (incl. the use of some special flight instruments) 1.3. Overlap regulation and bloc photography 2. Navigation and orientation methods deter~nining planimetric position 2.1. Shoran and H i r a n r a d a r 2.2. Decca n a v i g a t o r 2.3. Doppler r a d a r 2.4. Aerodist 3. Navigation and o~'ientation methods determining flying height, ~'elative flying heights and terrain elevations 3.1. Barometric a l t i m e t r y 3.2. Statoscope a l t i m e t r y 3.3. R a d a r a l t i m e t r y 4. Navigation and orientation methods determining the angular orientation 4.1. Fore a n d a f t photography 4.2. Solar periscope 4.3. Horizon photography 4.4. Vertical gyro 5. Navigation and orientation methods determining attitude 5.1. S t a r t r a c k i n g 5.2. Inertial navigation, incl. inertial vertical

complete spatial "position a~td

6. Conclusions 7. Reference literature list Introduction.

The development of aerial survey navigation over its 100 year period can be visualizcd by two pictures: the f i r s t aerial photograph taken of Paris by Tournachon in 185~" and the first spatial photograph taken by Lunik III in 1959 (figures I and 2). Tournachon did not apply any navigation method, the Lunik photography was guided by automatic self-contained navigation, exposm'e control, scanning and radio transmission of the photograph. This report deals with the situation of survey navigation to-day with special attention 1 7

252

Fig. 1. F i r s t aerial photograph, made over P a r i s by Tournachon ( N a d a r ) in 1859.

Fig. 2. F i r s t spatial photograph, moon's back side photographed by Lunik III in 1959.

Photog~ammetria, XVI, No. $

Commission I

Survey Navigation etc.,

Corten

253

a. to new methods which may be of importance to p h o t o g r a m m e t r y in the n e a r f u t u r e ; b. to conventional methods t h a t need some special remarks. The two aspects of this report are airborne navigation and airborne determination of camera oq'ientation elements. Until recently, these two were completely separate fields, the former being not nearly as accurate as the latter. However, the demands to supersonic speed navigation, to spatial navigation and to automatic guidance have become so extremely exacting t h a t the methods and instruments developed for these purposes must, in future, perform vcith the same accuracy in flight as the photogrammetrist has always required for his orientation elements. It is not possible to determine all elements in flight with sufficient accuracy to be used for photogrammetric orientation without reference to ground control. Some of the older and some of the newest methods, however, are capable of a. producing near-perfect photo coverage; b. supplying camera's orientation data with sufficient accuracy for greatly economizing photogrammetric control. In establishing these elements, the performance of the methods is such t h a t the accuracy of each individual measurement is not as high as the accuracy of the photogrammetric orientation. The influence of their errors, however, is mainly of a constant nature: the error propagation is negligible as compared with the propagation of the photogrammetric orientation errors. Their application is still limited mainly to their use as "auxiliary data for aerial triangulation" but with the advent of some highly accurate systems this situation may change in the n e a r future. We refer to Brandenberger in Comm. III [4a] and to Schermerhorn in Comm. IV [4b].

1.

Purely navigational methods.

1.1. Dead reckoning navigation. Principle. Dead reckoning navigation (DR) is based on the knowledge of a s t a r t i n g point position; if the vectorial components of the airplane's movement (i.e. speed and direction) are known, these can be integrated over the time to give the displacement from the known point and, consequently, the present position. A number of corrections must be applied such as corrections for compass deviations, d r i f t vectors, etc. Because the DR errors can be large~ and are always propagating, the final result is of insufficient accuracy to be used in survey navigation. In addition, the time consuming computations and the determination of fixes necessary to correct for the DR error propagation make its application to survey navigation of very doubtful value. Remarks. Some of the newest navigational methods, however, are capable of determining the displacement values with extremely high accuracy; examples are inertial navigation and Doppler navigation. In addition, they can be coupled onto computers to present the results in~nediately in any coordinate system or in the form of a command to the automatic pilot. These methods (coupled with the proper computers) can be considered as modern versions of the dead reckoning navigation. They become of g r e a t value for survey navigation and are dealt with in p a r a g r a p h s 2.3 and 5.2. 1.2. Contact navigation. Visual navigation according to line-of-sight. Most aerial photography missions are applying visual methods of survey navigation.

Photogrammet~a, XV1, No. -4

254

The quality of this navigation is one of the determinants of the economy of photogrammetric mapping. P r o p e r survey navigation shall result in the smallest possible number of photographs. Requirements to be fulfilled are: s t r a i g h t flight lines, constant overlap as small as feasible, constant side lap as small as feasible, constant flying height. Notwithstanding the f a c t t h a t the above-mentioned principles are generally recognized, t h e r e is still too much survey photography showing poor results. In m a n y cases this is caused by the fact t h a t r a t h e r a r b i t r a r y trial and e r r o r methods are applied even by experienced survey navigators. Optimum results can be obtained only if the survey is carried out by an extremely competent crew applying sound navigational principles. Various possibilities do exist, some of them described in literature. The best method i n a s f a r as is known by the author is the method designed by the Groupe des Escadrilles Photographiques of the Institut G~ographique National, France. This method is s t a n d a r d procedure applied by the crews of 12 B-17 and 8 HD 34 survey airplanes; their results are excellent and constant over many years. Principle

of the

I.G.N. visual

navigation

for

constant

side lap.

F o r the positioning of the flight lines no DR procedures are used because these depend on compass heading references which cannot be sufficiently accurate for aerial survey; the same applies to drift, to integration of air speed over the time and to other data. For this reason, the method is based on the use of visual landmarks, applying them as if they were beacons in line. A!

Mean datum plane Z o is deterd~

•

mined according to

I Z 0 ~

Zma x --

q.

where Zmax = highest •point of the area q = mean side lap s = photo scale.

Fig. 3.

Flying height is then determined; it is established by means of the barometric altimeter and kept constant within twenty meters by means of statoscope. The flying height differences are statoseope recorded.

Flight line parallelity is obtained in flight line R n by visually picking o u t land marks on the next flight axis Rn+ 1 in such a way t h a t tan fl

a(1--q) f

where a = photo size.

A pelorus visor can he used but a gyro stabilized d r i f t sight is more appropriate. In mountainous terrain, if the elevations differ from mean datum plane, by a value s . sin 2fl dZo, corrections dfl must be applied to anglefl i.e. dfl = f . dZ 0 (figure 3). Experienced navigators estimate the t e r r a i n elevations with sufficient accuracy to determine the new flight line's axis within 1% tolerance of side lap. This method is called "la m6thode en parall~le" 1). 1) In comparison with the I.G.N. methods, it may be noted that the U.S.S.R. navigation uses a solar compass as heading reference in s t a n d a r d procedure; figure 4.

Commi.qsion 1

S~rve~/ Navigation etc.,

In case no s u f f i c i e n t n u m ber of good l a n d m a r k s is a v a i l able, t h e heading n e c e s s a r y to m a k e t h e d e s i r e d t r a c k c a n be d e t e r m i n e d u s i n g p o i n t s of t h e p r e v i o u s f l i g h t line. T h e s l a n t a n g l e ~ to t h e s e p o i n t s c a n be e x p r e s s e d , m e a s u r e d a n d used to d e t e r m i n e t h e h e a d i n g a n g l e a ( f i g u r e 5). Precision t u r n s a r e exec u t e d a t t h e end of each f l i g h t line in o r d e r to a r r i v e a t t h e exact starting point for the n e x t f l i g h t line. T h i s c a n be achieved

Corte~t

~5

I.

Fig. 4. S o l a r c o m p a s s w i t h photo-electric s e n s o r a n d a. by m a n u a l p i l o t i n g to a p o i n t with servo to pilot's h e a d i n g display. S t a n d a r d equipchosen v i s u a l l y by m e a n s of m e n t in U.S.S.R. s u r v e y a i r c r a f t . pelorous v i s o r a c c o r d i n g to a n g l e fl; b. a c c o r d i n g to I.G.N. ( f i g u r e 6) by i n t r o d u c i n g the line s p a c i n g L, d r i f t (i a n d speed V

~

n

Fig. 5.

into t h e c o m p u t a t i o n to find t h e r a t e of t u r n a n d t i m e of t u r n to be executed m a n u a l l y . A special slide rule is c o n s t r u c t e d . F i g u r e 7. c.

a c c o r d i n g to B r u c k l a c h e r , u s i n g t h e s a m e principles h u t f l y i n g c o m p o u n d t u r n s cons i s t i n g of s e g m e n t s a t t u r n rate~ 1 °, 2" or 3 ° p e r second by a u t o pilot;

Photogrammet~'ia, XVI, No.

256 d. according to Fleming, by u s i n g auto pilots with a fixed rate of t u r n of 3 ° per second and executing S-turns; e. according to U.S.S.R. practice, f l y i n g compound t u r n s on t h e auto pilot or f l y i n g v a r i a b l e r a t e of t u r n s m a n u a l l y by m e a n s of a visual b a n k i n g indicator (Ser~en, f i g u r e

i Rv

,-v

t P v

V

~

--

-~

!

."

o

8). Fig. 6.

II II /I'lllllll

1

II

I

I II

I

Fig. 7. Survey navigation slide rule for c o m p u t i n g r a t e of t u r n and time of turn. S t a n d a r d procedure I.G.N. Performance

of

the

methods.

Visual s u r v e y navigation, if executed by properly c o m p e t e n t a n d experienced crews, can supply a photographic coverage which is perfectly suited for topographic m a p p i n g of f l a t or m o u n t a i n o u s areas. Over a n u m b e r of y e a r s the following r e s u l t s have shown to be routine p e r f o r m a n c e with I.G.N.: a. Air p h o t o g r a p h y 1 : 50.000 i f topographic m a p s of the a r e a a r e available: orientation of t h e f l i g h t lines is parallel within a m e a n e r r o r of ± 0.1°; lateral overlap is kept within a variation of -+ 5% overlap. b. A i r p h o t o g r a p h y 1 : 6 5 . 0 0 0 if no m a p s are available: the m e a n deviation f r o m the flight lines (length approx. 120 km for one-degree blocs) = + and - - 2 % side lap, max. deviation - + and - - 5 % side lap; f i g u r e 9. Fig. 8. B a n k i n g indicator for p2~cision t u l ~ s in U.S.S.R. s u r v e y equipment.

c~ r~ c~

°°

o o r~

~o

7 o

o ~0

o

258

Photogrammetria, XVI, No. 4

On account of these close tolerances, the lateral overlap (side lap) can be kept at its smallest value, thereby saving a large percentage of the n u m b e r o f photographs. This a d v a n t a g e is valid for all topographic surveys in all types of terrain. 1.3. Overlap, longitudinal coverage and bloc photography. F o r most photogrammetric surveys, the m i n i m u m longitudinal overlap shall be = = 55% ; The largest value of overlap should be kept as low as possible. A reduction of t h e average overlap percentage can have considerable economical dp a d v a n t a g e s because a saving dp in overlap p saves a n u m b e r o f photographs = 100 ~ %. F o r example, reduction of average overlap p f r o m 65% to 57% (dp = 8% of total image size; 1 - - p = 35%) saves 23% of the original n u m b e r s of photographs. I t is therefore necessary to keep the overlap constant and on its m i n i m u m desired value. Principles of overlap control. F o r a given aerial camera, tl~e overlap is determined by the flying height H , airp l a n e ground speed V a, and exposure interval t. The required exposure pulse interval is

Vg

"

directly proportional to the ratio ~

being the a n g u l a r speed of the ground with respect

to the airplane's coordinate system. This velocity/height ratio must be determined and introduced into the camera's exposure command mechanism. Practical possibilities. Overlap can be controlled in t h r e e different w a y s : 1. U s i n g a drift sight, stop watch and intervalometer. T h e interval in seconds of time, necessary to obtain the required overlap, is measured a t the d r i f t sight. This interval is introduced by the operator into a t i m e r (erroneously called "intervalometer") which supplies the exposure pulses to the camera a t the set rate. The method is simple b u t has the disadvantage that~ in practical operation, it does not take into consideration the con~tinuously v a r y i n g flying height, t e r r a i n elevation, wind speed, wind direction, a i r p l a n e speed, etc. In this way, the overlaps obtained a r e always l a r g e r t h a n strictly necessary and this method is therefore uneconomical. 2. Using a ~visual interval regulator coupled with the camera. The interval regulator Vg measures the ~ ratio by means of a continuously variable and moving reference g r i d ; this is incorporated either in a telescope viewer (Wild and others) or in a projection ground glass (Zeiss). In addition, either the c a m e r a data plus the desired overlap, B

or the desired ~

ratio is introduced into the i n s t r u m e n t as input.

The o u t p u t is a correct minimum overlap if the operator keeps the i n s t r u m e n t t u n e d V~ to t h e v a r y i n g ~ rate. Experience has shown t h a t this is possible within a very few % of overlap, even in mountainous t e r r a i n . 3. U s i n g an automatic photo-electric scanning intervalometer. The airplane's a n g u l a r ground speed can be measured by projecting the moving earth's tonal brightness differences onto a series of photo-electric cells in a scanner which must be gyro stabilized. A f t e r a n u m b e r of electrical operations the resulting signal is proportional to the a n g u l a r ground speed. This D.C. voltage, now proportional to ~ ,

is used to drive

Commission I

Su~'vey Navigation etc.,

Co,'ten

25¢~

a variable overlap control which regulates the pulse repetition rate of the aerial camera. Experience with the P A P I (Precision automatic photogrammetric intervalometer) has shown t h a t this can be achieved - even in mountainous t e r r a i n - with mean deviations = 1½% overlap in the nadir point; as the instrument cannot anticipate t e r r a i n elevations ahead of the nadir point, the overlap variations at the image edges, however, will be larger. Bloc

photography.

Until to-day, air photography is nearly always carried out as "strip photography". The flight axes are positioned so as to give constant side lap, but the photo positions are regularly distributed in the flight line direction only; in ~ --A-~ the perpendicular direction their mutual positions are com~-~-pletely a r b i t r a r y as is illustra- ~ --c~ ted in figure 10. ~o--With the advent of bloc adjustment methods and beFig. 10. Fig. l l . cause of the f a c t t h a t a better Strip photography. Bloc photography. photo distribution will allow for g r e a t savings in minor control economy, it would be appropriate to arrange the individual photo positions in a completely regular bloc p a t t e r n as is illustrated in figure 11. F o r this a r r a y we propose the designation "bloc photography". Each control point is now situated a t the edge of the three-fold overlap and is common to six photographs. The saving is appreciable: in each area covered by m flight lines of n photographs, the saving in number of control points is no less than = n (m + 1). In practice, this amounts to a factor ½ approximately. Bloc photography is possible in various ways: a. Individually aiming and pin-pointing each photo nadir point separately in large scale surveys carried out by experienced crews. Regular practice at the I.G.N. b. Brucklacher (1957) f i r s t suggested the use of 90% longitudinal overlap thereby obtaining photographs situated within 5% o f the ideal position. A f t e r the selection of ~A of the photograps, the other ~ are not used. With this method the film consumption is four-fold, but bloc photography is obtained within a spread of + and - - 5 % photo size; the reduction of minor control greately outweighs this higher film consumption. c. With the use of instrument navigation, such as shoran, Doppler, aerodist, inertial, etc., it becomes possible to couple this navigation not only to the a i r c r a f t guidance but also to the camera exposure command. The economy of bloc photography as compared with strip photography will soon j u s t i f y the e x t r a trouble involved in the design and construction of this additional function. With Doppler, for instance, it should not be complicated to design a control t h a t performs within ÷ and - - 5 % . 2,

]Navigation and arientation methods determining planimetric position.

2.1. Shoran and Hiran radar. These methods have found wide application in aerial survey. They are used for various purposes: a. Measurement of long geodetic lines; p r i m a r y and secondary geodetic points are obtained by trilateration. The range of this method is obtained by the use of "line cros-

260

b. e. d.

e.

Photogrammetria, XVI, No.

sing" aircraft. Accuracy of distances ( a f t e r a d j u s t m e n t of trilateration net) = 1 p a r t in 60.000. Determination of horizontal control by means of Shoran controlled aerial photography ("Shoran photo"). Shoran controlled photography in flight lines used for horizontal bridging. Shoran controlled cross flight pattern for determination of secondary control points. The accuracy obtained is of the same order as t h a t of the average point obtained by field astronomy. Shoran used as a navigation aid. Flight lines can be predetermined, plotted on a Shoran map and displayed during flight on a Shoran straight line computer.

Remazks.

The application of Shoran and H i r a n in aerial survey, to-day, is well-established; for f u r t h e r information we refer to the extensive literature concerned. 2.2. Decca navigator. Decca navigator is a phase comparison radio system, measuring a station's position with respect to three or more ground based " m a s t e r " and "slave" stations. Its air station is light but its ground "'chain" is still relatively bulky. Decca can be used for: a. t e r r e s t r i a l positioning (of terrestrial survey parties). F o r this purpose it is accurate if the t e r r a i n i~ homogeneous or if a monitor station can be set up; b. aerial positioning of vertical photographs. It can be used for the same purposes as shoran in this respect; c. measurement of long lines. Its errors are too large, however, to use it for trilateration; d. navigation of airplanes and helicopters. F o r aerial survey it can be used in combination with the Decca flight log, flying predetermined flight lines and plotting them on a map. Its accuracy v a r i e s according to the disposition of the a i r c r a f t ' s station with respect to the ground stations; it can be expressed as a fraction of the "lane" width, the mean e r r o r of the single coordinate being = ± 0 . 0 1 . . . 0.005 lane, the mean e r r o r of a " f i x " being larger according to the e r r o r ellipsis. The use of this system is well known and very well described; we r e f e r to the photog r a m m e t r i c literature concerned.

2.3. Doppler ~ada~. The application of Doppler r a d a r has proven its value in aerial survey. The equipment is being improved continually: lightweight precision systems are developed and we may expect t h a t Doppler r a d a r is going to play an increasingly i m p o r t a n t rSle in photogrammetry. Definition. Doppler r a d a r is a self-contained airborne velocity measuring system and is characterized by 1. beams of electromagnetic energy in microwave form which are t r a n s m i t t e d to the e a r t h ' s surface; 2. diffuse reflection of this energy a t the e a r t h and reception of a fraction of this energy a t the airplane; 3. frequency s h i f t (according to the Doppler principle) of the signal received, with respect to the original signal t r a n s m i t t e d ; 4. m e a s u r e m e n t of this Doppler frequency shift;

Commission I

Survey Navigation etc.,

Corten

261

5. processing of this frequency shift information into t e r m s of velocity components of the vehicle (airplane) with respect to the diffusing surface (earth). Main components. A complete Doppler system consists of 1. a Doppler r a d a r supplying the components of a i r c r a f t motion with respect t(~ the earth; 2. a heading reference supplying the direction of the coordinate frame, e.g. magnetic or geographic coordinates; 3. a computer processing the a i r c r a f t velocity vectors into t e r m s of horizontal ground speed, drift, wind speed, and capable of integrating the velocity information over the time: in such a way the "distance travelled" is presented as output; if the computer has a dead reckoning capacity, the "present position" can be supplied as output.

DISTANCE DISPLAY POSITION DISPLAY [ TO COMPU?ER~

t__REOUENYo,

D/SC,^',NAOR p a 4

°uRc'- I L_

SOURCE

f~ _ f

t

,

\

°

I'

~f

\ \ \

\ N\

\

\

\

EARTH Fig. 12. Principle of operation. The principle of Doppler radar measurements is shown in figure 12. Microwaves.of frequency f and wavelength ), generated by a radio frequency source, are transmitted by the antenna A under angle 7 to the e a r t h ' s surface. The return signal will arrive at A with a I)oppler shifted frequency f~. The frequency shift zJf-~ f n - - f is a measure for the airplane ground speed V according to the relation 2V. cos 7 2-

A f = fn - - f . . . .

(13).

This frequency shift is measured, t r a n s f o r m e d into speed units and displayed o1' fed into a computer. If the antenna is rotated about a vel~ical axis, the maximum frequency shift will be found in the flight direction and the angle between this flight direction and the airplane's longitudinal axis is the drift angle. In order that the three airplane velocity com-

262

Photog.rammetria, XVI, No. 4

ponents can be determined, we need at least three non-coplanar beams. Most Doppler systems used in aerial survey have four-beam radar. Figure 14. In the design and operation various theoretical and practical requirements must be fulfilled. Some of these considerations are: 1. Diffuse reflection must take place; complete specular reflection such as takes place on water surfaces will spoil the return of a signal unless the prevailing wind is at least 2 Beaufort. In that case the velocity is measured with respect to the moving wave pattern; this can be accounted for by introducing this wave propagation (ap-

// // // //

I

/i ii ii

I

/ l

1 /

/I \

\

\ -

\\ \

\ \ \

I I I

Fig. 14. prox. 20% of the Doppler-measured wind vector) as a correction value into the computer. 2. Depression angle ~ of the beam must be known. This means that the antenna must be gyroscopically stabilized, or that the airplane's rotational components must be measured and introduced into the computer in order to correct for these pitch and roll movements. 3. Equation (13) holds for one ray. In reality the beams have a half power width of approximately 3.5°; this means that the return frequencies form a spectrum which has a certain hand width, a certain energy distribution and certain signal-to-noise ratios (figure 15). These quantities are not constant either but they vary with the reflecting characteristics (and hence with the topography of terrain or water), with the strength of the return signal (and hence with the flying height and photo scale according to the inverse square law), w i t h the accuracy of antenna stabilization or correction, etc.

Commission 1

S~rvey Navigation etc.,

Corten

263

M a n y o t h e r c o n f l i c t i n g r e q u i r e m e n t s m u s t be fulfilled, w h i c h g a v e rise to t h e develo p m e n t of a l a r g e n u m b e r of m o d e l s ( a t l e a s t f i f t e e n c o m m e r c i a l models to d a t e ) s o m e of w h i c h a r e p a r t i c u l a r l y v a l u a b l e f o r aerial s u r v e y .

~ ~

.~

airplanespeedvector

\\

\

\\

\

\\

\ \

\\\

I

max. signal strength-

._' . . . . . ~_~l~,, i I

I .1~1

I

i.~

r

[% ,

f

I

~ "lhl

~___ $___.~ho,f

: ,:,iF

background no~e

J I

I

l°mp/itude

..~_'

"

I

~',~J

............... [

max.

I powerfrequency

l

powoc sp,ctr.~ width I

_ ~ . ,~,~,.

~return frequencyfR

Fig. 15. Performance

of

present-day

T h e basic m e a s u r e m e n t s can be g r o u n d speed V¢ a n d of 0.1" of drift c a n be v e r y s m a l l a n d i n a s m u c h as e r r o r s w h i c h a r e s m a l l a s well, we II

7

systems. c a r r i e d o u t with m e a n e r r o r s of 1 : 1 0 0 0 of a i r p l a n e angle. I n a s m u c h a s dead r e c k o n i n g c o m p u t e r e r r o r s constantbearing survey flying introduces additional m a y e x p e c t m e a n e r r o r s in distance t r a v e l l e d s =

Vg dt of 1 . . . 2o/o o of flight line length

o

T h e d i r e c t i o n a l e r r o r , h o w e v e r , is d e t e r m i n e d m a i n l y by t h e h e a d i n g r e f e r c n c c w h i c h is h a r d l y m u c h b e t t e r t h e n 1/:~° m e a n e r r o r if a c o n v e n t i o n a l t y p e of a c c u r a t e c o m p a s s ( g y r o c o m p a s s o r f l u x g a t e ) is used. I m p r o v e m e n t s in t h i s r e s p e c t c a n be expected only if a n i n e r t i a l h e a d i n g r e f e r e n c e s y s t e m is u s e d in c o m b i n a t i o n w i t h D o p p l e r r a d a r . I n e r tial s y s t e m s c a n also s u p p l y e x a c t vertical r e f e r e n c e s a s will be described in p a r a g r a p h 5.2; f o r t h i s r e a s o n t h e y m a y become o f p r a c t i c a l v a l u e for p h o t o g r a m m e t r y in" t h e n e a r f u t u r c a n d t h e y could t h e n be used for s u p p l y i n g t h e D o p p l e r h e a d i n g r e f e r e n c e as well. T h e D o p p l e r s y s t e m can, in r e t u r n , be used to keep t h e e r r o r p r o p a g a t i o n { i n s t r u m e n t d r i f t ) of a n i n e r t i a l s y s t e m w i t h i n close limits. T h i s is a t y p i c a l e x a m p l e of w h a t we m a y e x p e c t in f u t u r e p h o t o g r a m m e t r i c n a v i g a t i o n : c o m b i n a t i o n s o f precision n a v i g a t i o n s y s t e m s , l i m i t i n g each o t h e r ' s e r r o r s to e x t r e m e l y close values.

Photogrammetria, X VI, No. 4

264

ER 1c . ~ o Computers. The Doppler infor""'-~JPosl~,ON I "°""O">IDIGr,"NCE ~ o~-p&~ l mation, to be used for navigation, can be com~NT~__~.~VELOCITY I I ...... J bined with the computer as illustrated in figure 16. HEADING ' . CAMERAEXPOSURE For most navigaIHEA' DESIIRED COMMAND tional purposes, the DESTINATION computer must transHEADING form the Doppler inREFERENCE formation and the Fig. 16. heading information into polar coordinates (course t distance vector) but for survey navigation the output should be supplied in rectangular or grid coordinates in order to enable the airplaine to cover predetermined parallel flight lines; this can bc achieved by having' the computer command the autopiIot, or by manual flying on the display which 6-:~-"-~)2t -¢h-v-"-~' g-k-:'-L - J ] , r .............. -I Oopplerrodoc may be a map with planned survey ' , Abtrift flight lines. Examples are the Anadac, the Automatic navigation plot] steue~k~,~s[ ~o~pa~ I ting board, Marconi's Ground Position Indicator, and the Automatische K°Pp"r"chn'" L_ E,g,,- ] E,9,-] Koppelkarte. The functioning of such a system may be as follows (fig. 17). i # # ~ 1 Kur$ LJber G r u n d Input into the computer: Doppler X y Wind' velocity, d l i f t angle, magnetic bearing, airplane speed IAS, (these data W ' 14,' ~ ' d, ,h"~ ' inputs by means of instruments), and gtJ,s u b ~ r Gr~nd the grivation (by means of manual input). The computer delivers as outKur| put : course over ground (ma~metic), zorn Z*¢l course over ground (in grid coordinates) airplane ground speed, ground • -*YSL distance travelled, present position zum~lel (in grid co(~rdinates), drift angle, wind vector. This output is displayed at a map, supplied with a transl l a parant flight line grid disk. Predetermined survey flight lines can 'Kartengeraf thus be flown within close tolerances. Wirld Korr, WinO [-W I~-$ The compute)" takes ()vet' as a Fig. 1 7 . dead reckoning computer (using the air speed and wind vector) in case Abtrift - - Drift the I)oppler signal fails, e.g. due to Eigengeschwindigkeit - - Indicated air speed (IAS) the presence of water surfaces (see G r u n d g e s c h w i n d i g k e i t - Ground speed p a r a g r a p h 1.1). The U S A F AutoKartenger~it - - Map display matic navigation plotting board Koppelrechner - - DR Computer consists of a compute)' handling a Kreiselstabilisierter numb':r of input data and plotting Kompass - - Gyro compass continuously the airplane's present Kurs fiber Grund - - Track position on standard A.F. charts. Kurz zum Ziel - - Course to destination.

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Commission I

Survey Navigation etc.,

Corten

265

Fig. 18. Marconi Doppler transceiver, tracking unit and display unit as is used in Hunting's survey airplanes.

Pre-determined photo positions. Bloc photography. In order to be of optimum economical value for photogrammetric mapping, a Doppler survey system equiped with a computer as described before, can also be equiped with a camera exposure command in order to position the exposure stations in the regular rectangular pattern of bloc photography. Par. 1.3. Practical date.

l)l~l"t A Y

experiences

to

Doppler systems in use for aerial survey are : Radan PC-201 of General Precision Laboratory Inc.; Canadian Marconi's Airborne Doppler; AD 2000 of Marconi's Wireless Teleg r a p h Co., Figure 18, and others. Large areas have been covered by aerial photography and airborne geophysical surveys, using Doppler. Experiences are : Drift determination, abortive flying, gap runs, searching for the s t a r t i n g point and other ineffective f l y i n g was reduced practically to zero. In APR and in geophysical surveys the "positioning" of the data is greatly facilitated, in many cases the previous flying and construction of mosaics was not necessary; savings of 25% and 40% flight time are reported. Drift info~maatlon is continuously available, thereby improving the isobaric surface's slope for APR. Precision turns for exact mutual positioningof flight line axes. Doppler is not yet sufficiently accurate for determining the camera's planimetric coordinates; in some cases,

Photogrammetria, XVI, No. 4

266

however, a m e a n error of I : 1000 of distance (e.g. m e a n s c a l e o f f l i g h t line axis) is sufficient f o r construction of semi-controlled mosaics. On f u r t h e r improvement, particularly in combination w i t h inertial n a v i g a t i o n , it m a y eventually develop to a s y s t e m suitable for trilateration. 2.4. Aerodist positioning. The Tellurometer s y s t e m of distance m e a s u r e m e n t , based on radio microwave r a n g ing, h a s proven to be m o s t useful for the e s t a b l i s h m e n t of geodetic lines. A n a c c u r a c y of 1 : 300.000 + 5 cm can b e obtained by this method of p h a s e comparison between the cont i n u o u s wave t r a v e l l i n g f r o m M a s t e r to Remote station a n d a pulsed r e t u r n w a v e f r o m Remote to Master. T h i s system, designed for g r o u n d distance m e a s u r e m e n t s , h a s now been modified for use in a i r c r a f t to m e a s u r e g r o u n d - t o - a i r distances a n d is called Aerodist. I t is a n e w method, u n d e r g o i n g its f l i g h t t r i a l s in U.S.A. Principle

of the

Method.

M a s t e r s (M) are placed in the a i r p l a n e a n d Remotes (R) are positioned a t t h e ground. I n f o r m a t i o n about distance M . . . R is obtained by p h a s e comparison between two continuous waves of approx. 1 Kc f r e q u e n c y (in t h e s t a n d a r d Tellurometer, p h a s e differences of one continuous wave and one pulsed r e t u r n w a v e are m e a s u r e d ) . These p h a s e differences increase and decrease w i t h distance a n d are m e a s u r e d a t the M a s t e r s t a t i o n s in t h e airplane. Their t r a n s m i t t e r c h a r a c t e r i s t i c s - including t r a n s m i s s i o n power a n d a n t e n n a b e a m width - h a v e been modified to a d a p t to a i r c r a f t operation. The g r o u n d based o p e r a t o r s m u s t keep the a i r c r a f t t r a p p e d within the Remote's a n t e n n a beam. A Duplex s y s t e m consists of two, a Triplex s y s t e m consists of three sets Master-plus-Remote. Principles

of operation.

To solve the distance triangles, the p l a n i m e t r i c position of two g r o u n d s t a t i o n s R 1

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Fig. 19. a n d R 2 and t h e h e i g h t s of these stations a n d of the a i r p l a n e M 1 and M 2 m u s t be known. T h e a c c u r a c y of these elevations n e c e s s a r y to exploit the Aerodist method to the full extent, m a y well p r e s e n t a practical problem to the surveyor.

Commission I

Survey Navigation etc.,

Corten

267

Ground-to-air d i s t a n c e s of about 100 miles can be m e a s u r e d . T h i s can be done: 1. by m e a n s of line c r o s s i n g recording the s u m s of distances to two g r o u n d s t a t i o n s a n d e s t a b l i s h i n g the m i n i m u m s u m f o r r e a s o n s of t r i l a t e r a t i o n ; or 2. by m e a n s of a i r c r a f t position d e t e r m i n a t i o n d e t e r m i n i n g the n a d i r position of each photo e x p o s u r e s t a t i o n ; or 3. by m e a n s of d e t e r m i n a t i o n of a t h i r d point's position if two g r o u n d stations are known, f i g u r e 19. It m a y be done w h e n the u n k n o w n point is s e p a r a t e d f r o m the known points by distances up to 150 miles; t h i s method is called: position f i x i n g by continuous t r i l a t e r a t i o n . A s continuously c h a n g i n g distances are m e a s u r e d in the moving airplane, all measu r e m e n t s m u s t be recorded. It is n e c e s s a r y to h a v e good radio line-of-sight conditions between a i r c r a f t and g r o u n d stations. The necessity of k n o w i n g the Aerodist g r o u n d s t a t i o n s ' a n d a i r c r a f t s t a t i o n s ' h e i g h t s s u g g e s t s the u s e of barometric h e i g h t determinations. However, t h e application in combination with r a d a r a l t i m e t e r and with statoscope will probably allow for t h e m o s t economical operation. Performance. The over-all a c c u r a c y is stated to be = ± 1 m e t e r s t a n d a r d i n s t r u m e n t error + s t a n d a r d e r r o r of 1 p a r t in 100.000 of the m e a s u r e d distance. The limitation of accuracy is set by the knowledge of the v a r i a t i o n s of r e f r a c t i v e index of the atmosphere. If the p r e s e n t t e s t s would fulfill t h i s expectation in actual s u r v e y operation, this method would become an i m p o r t a n t m e a n s of airborne t r i l a t e r a t i o n a n d of d e t e r m i n i n g the c a m e r a ' s p l a n i m e t r i c position a t each exposure station.

3.

Navigation and orientation methods determining flying height, relative flying l~eights and terrain elevations.

3.1. Baro'metric altimetry. There does n o t exist a physical phenomenon t h a t c a n be used f o r correctly m e a s u r i n g the absolute altitude of t h e c a m e r a station over d a t u m . One of the s u b s t i t u t e s is the relation between static a i r p r e s s u r e a n d elevation. The a i r p r e s s u r e is not only dependent on the a i r p l a n e ' s h e i g h t b u t also on a large n u m b e r of other v a r i a b l e s ; consequently, t h e r e variables should be known a n d introduced into t h e m e a s u r e m e n t a s corrections in order t h a t static a i r p r e s s u r e correctly e x p r e s s e s the f l y i n g h e i g h t over d a t u m . In practice, m a n y of these correcting d a t a are known only partially, t h u s s u b j e c t i n g the altitude (scale) d e t e r m i n a t i o n to l a r g e errors. It is reported t h a t the U.S.A.F. Central air d a t a c o m p u t e r system, h a n d l i n g all i m p o r t a n t d a t a in flight, applies as m a n y corrections as possible to the altimeters. One of these is the t u r b u l e n c e a r o u n d the pitot tube or p r e s s u r e probe; t h e problem h a s been solved to a g r e a t e x t e n t by u s i n g a n angle of a t t a c k s e n s o r a n d a compensation f o r static p r e s s u r e a n d angle of attack. The a l t i m e t e r indication is, u n d e r n o r m a l s u r v e y conditions, corrected to a value w i t h only 1.2% H deviation f r o m t r u e altitude. 3.2. Statoscope altimetry. Principle. Small differences in f l y i n g h e i g h t can be m e a s u r e d b a r o m e t r i c a l l y by m e a n s of a m i c r o b a r o m e t e r h a v i n g a n e x t r e m e l y h i g h sensitivity and, consequently, a v e r y limited range. These m e a s u r e m e n t s refer to a n isobaric s u r f a c e which, in general, will not be horizontal n o r flat.

18

268 Sources

Pho$og~r~mme~ia, XVI, No. 4 of

errors.

a.

B y s u i t a b l e d e s i g n a n d c o n s t r u c t i o n t h e i n s t r u m e n t a l e r r o r of t h e s t a t o s c o p e is l i m i t e d to m = 1/30 mb o n l y ( e q u i v a l e n t to 30 c m a p p r o x . ) . b. T h e i n f l u e n c e of t h e isobaric s u r f a c e ' s slope c a n be n e a r l y e l i m i n a t e d i f t h e r e is control a t t h e b e g i n n i n g a n d a t t h e e n d of t h e s t r i p . L i n e a r i n t e r p o l a t i o n is t h e n possible. c. T h e r e s i d u a l e r r o r s a r e m a i n l y due to t h e f a c t t h a t a l i n e a r i n t e r p o l a t i o n does n o t a c c o u n t f o r local a n d t e m p o r a r y v a r i a t i o n s in a l t i t u d e o f t h e isobaric s u r f a c e . T h e s e v a r i a t i o n s are, u n d e r n o r m a l s u r v e y c o n d i t i o n s , of t h e o r d e r of -- 1 m. T h e above f a c t s c o n f i r m t h e g e n e r a l e x p e r i e n c e in p h o t o g r a m m e t r y t h a t s t a t o s c o p e h e i g h t d i f f e r e n c e s c a n be reliable to a s t a n d a r d e r r o r m = +- 1 , 5 . . . 2,0 m.

18

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F i g . 20.

Instrument

s.

M o d e r n s t a t o s c o p e s record t h e m e a s u r e m e n t s on t h e m a i n c a m e r a ' s n e g a t i v e film. T h e Z e i s s s t a t o s c o p e w i t h electrical r e c o r d i n g is w e l l - k n o w n , w h e r e a s W i l d h a s cons t r u c t e d a n e w version. T h e U.S.S.R. s t a t o s c o p e s u p p l i e s a c o n t i n u o u s record, p r o d u c e d b y t h e i l l u m i n a t e d m e n i s c u s w h i c h is p r o j e c t e d onto a c o n t i n u o u s l y m o v i n g film. T h e d i f f i c u l t y of r a n g e l i m i t a t i o n is o v e r c o m e b y u s i n g t w o s t a t o s c o p e s : n u m b e r 1 is in a c t u a l o p e r a t i o n a n d n u m b e r 2 t a k e s over a t t h e m o m e n t t h a t n u m b e r 1 r e a c h e s i t s l i m i t ; t h i s a c t i o n is i n i t i a t e d by electrolytic c o n t a c t a t t h e l i q u i d ' s m e n i s c u s a n d is c a r r i e d o u t b y a s e r v o m e c h a n i s m , f i g u r e 20. I.G.N.'s s t a t o s c o p e will be d e s c r i b e d a t t h e C o n g r e s s .

Commission I

S u r v e y N a v i g a t i o n etc.,

Corten

259

3.3. R a d a r altimeter. A i r b o r n e profile recorder. Principle. a. A self-contained radar, its narrow beam vertically stabilized by gyro, measures the airplane's ground clearance. b. A microbarometor measures the airplane's flying height variations with respect to isobaric surface. If these two measurements are combined, the reflecting surface's profile with respect to a datum parallel with the isobaric surface is found. In order to convert this into useful information, corrections must be applied such as: the isobaric surface's slope with respect to survey datum plane, t e r r a i n reflectance characteristics, instrumental errors, etc. As a result, a profile of the t e r r a i n is obtained. Instruments

available.

RVTD Radiovysotomer, constructed by the Central'nyj NauSno-Issledovatel'skij Institut Geodezii Aeros-emki i K a r t o g r a f i i (CNNGAiK). It is used in combination with the statoscope and with photographic recording. Airborue Profile Recorder Mk 5, Canadian Applied Research Ltd. N a r r o w Beam Altimeter EA-NBA-2, Electronic Associates Ltd. The U.S.S.R. r a d a r altimeter, consisting of an oscillator unit, transmitter-receiver unit and indicator unit, displays its indication on a circular scope screen which is synchrophotographically recorded. The APR, in its improved version Mk 5 contains a hypsometer as a sensitive pressure altimeter. This contains toluene, the boiling t e m p e r a t u r e of which is measured by means of a thermistor. Its sensitivity, particularly at the g r e a t e r flight altitudes, is equivalent to approx. 30 cm flying height difference. The indicator supplies a continuous record of the ground clearance, the airplane's height v~riations and the terrain profile. Remarks. In Canada, operational research was carried out on the A P R ' s proper ground location, on the proper choice of elevation points, and on the determination of the isobaric surface. The inner accuracy of the instrument was found to be bettor t h a n 1 m, but external influences such as the precise location error, the elevation point choice within the illuminated area, etc., a u g m e n t this error to m z = ± 1,5 m a t shorter distances and from lower altitudes, and to m z = ± 3 m in aerial triangulation strips with flying heights of 9.000 m. In U.S.S.R., the r a d a r altimeter is used: - - for small scale topographic contour mapping with the topographic stereometer; --as auxiliary in radial triangulation establishing planimetric control (increase of ground control point efficiency by a factor 1 , 5 . . . 2 through reduction of scale error accumulation) ; for elevation control extension as auxiliary data in aerial triangulation; - - for compilation of semi-controlled mosaics; q for evaluat{on of position determination in flight by means of r a d a r r a n g i n g at each camera station as is applied for 1 : 100.000 mapping and is proposed for use in topographic mapping 1 : 25.000 and 1 : 10.000.

]

Photogr. XVI, 4

Photogrammetria, XV1, No. 4

270 4.

Navigation and orientation methods determining the angular orientation.

4.1. Fore and aft photography. Principle. Oblique p h o t o g r a p h s can be t a k e n s y n c h r o n o u s l y w i t h the m a i n vertical photograph. On t h e s e p h o t o g r a p h s s t r a i g h t lines in f l i g h t direction can be determined a n d t r a n s f e r r e d onto t h e m a i n vertical p h o t o g r a p h s by m e a n s of a " S t r a i g h t line plotter". W h e n b r i d g i n g the strip, t h i s f l i g h t line l i n e a r i t y is used to control t h e a z i m u t h and to keep the e r r o r prop a g a t i o n of y b e n d i n g within close li~nits. Performance. E x a c t i n f o r m a t i o n about t h i s method and i t s i n h e r e n t a c c u r a c y is not yet available. It is felt t h a t more statistical m a t e r i a l is needed for p r o p e r evaluation of its m e r i t s and performance. 4.2. Solar periscope. S a n t o n i ' s solar periscope is a n i n s t r u m e n t f o r use with aerial t r i a n g u l a t i o n . It determines t h e inclination of the c a m e r a ' s optical a x i s a t the exposure station by m e a s u r i n g two a n g l e s : one direction to the s u n a n d the o t h e r d~ a z i m u t h . The position of the aerial c a m e r a w i t h respect to t h e s u n ' s direction is synchro-recorded by m e a n s of a "'solar c a m e r a " ; its i m a g e is m e a s u r e d in the "solar c o m p u t e r " which - a f t e r introduction of the m a i n p h o t o g r a p h ' s d~ - delivers as o u t p u t t h e v a l u e s of ~ and o~ to be used in the b r i d g i n g procedure. Performance. C o m p a r a t i v e e x p e r i m e n t s of O.E.E.P.E. h a v e shown t h a t the method can p e r f o r m w i t h r e m a r k a b l e a c c u r a c y ; m e a n e r r o r s of 3 e were obtained. F o r f u r t h e r i n f o r m a t i o n we r e f e r to the l i t e r a t u r e concerned. 4.3. Horizon photography. B y m e a n s of p h o t o g r a p h y of the horizon synchronized w i t h the vertical photography, the inclination of t h e aerial c a m e r a ' s a x i s w i t h respect to t h e vertical can be recorded. T h i s method is known a n d applied since 1930, a n d improved to a remarkable degree of accuracy, b u t its use in p h o t o g r a m m e t r i c r o u t i n e has, until now, been limited to F i n land. T h e m e t h o d can be carried out w i t h a special p h o t o g r a m m e t r i c c a m e r a photograp h i n g t h e horizons - via p r i s m s - on the main vertical c a m e r a film, or it can be applied with s e p a r a t e s y n c h r o n o u s p h o t o g r a p h y by m e a n s of additional horizon recording cameras. Principle

of

the

method.

One or more p h o t o g r a p h s of t h e horizon a r e t a k e n in exact synchronization w i t h the m a i n vertical exposure. A n y inclination of the c a m e r a s y s t e m will become a p p a r e n t in t h e horizon p h o t o g r a p h y in two components: one will show u p as a n inclination of the horizon image, the other one a s a n elevation of t h e horizon image. Both a n g u l a r inclination components (a and fl) of t h e vertical c a m e r a can be m e a s u r e d for each exposure with respect to t h e f o r e g o i n g exposure's and to t h e following exposure's inclination d a t a ; the r e s u l t i n g v a l u e s are inclination differences between consecutive photographs. The two c a m e r a inclination components a a n d fl can be related to the orientation elements a n d w if the c a m e r a ' s orientation differences du about t h e vertical axis a n d drift angle ~ a r e known.

Commission 1

Survey Navigation etc.,

Corten

271

Performance. T h e m e a s u r e m e n t s t a k e o n l y a f e w m i n u t e s p e r p h o t o g r a p h , t h e y c a n be c a r r i e d o u t in e a c h single p h o t o g r a p h or stereoscopically. I f t h e h o r i z o n a s s u c h i s n o t c l e a r l y visible, t h e y a r e c a r r i e d o u t a t a cloud line o r a t a n i n v e r s i o n l a y e r . T h e relative v a l u e s o f t h e i n c l i n a t i o n s o f c o n s e c u t i v e p h o t o g r a p h s , i f stereoscopic m e n s u r a t i o n is applied, a r e obt a i n e d w i t h n o t m o r e t h a n ma, # = - 2 c. . . 3 c m e a n e r r o r . I n o r d e r to o b t a i n t h e absolute v a l u e s of t h e c a m e r a ' s i n c l i n a t i o n s , it is n e c e s s a r y to h a v e g r o u n d control, p r e f e r a b l y a t t h e b e g i n n i n g a n d a t t h e e n d of e a c h f l i g h t line. A n a l t e r n a t i v e to t h e u s e o f g r o u n d control d a t a is t h e u s e o f t h e a v e r a g e v a l u e of s p i r i t level i n d i c a t i o n s . T h e objection a g a i n s t t h e horizon m e t h o d t h a t , in m a n y cases, a u s a b l e horizon i m a g e c a n n o t be o b t a i n e d , is denied b y t h o s e h a v i n g e x p e r i e n c e w i t h t h i s m e t h o d . T h e r e m a r k a b l e f a c t t h a t t h i s s i m p l e anzl a c c u r a t e m e t h o d h a s h a r d l y f o u n d a n y application o u t s i d e o f F i n l a n d can, p e r h a p s , be e x p l a i n e d b y t h e e x p e c t a t i o n s t h a t we h a d of t h e f u t u r e of t h e v e r t i c a l g y r o in p h o t o g r a m m e r r y . It is n o w k n o w n t h a t it is b a s i c a l l y impossible to h a v e g y r o s c o p i c s t a b i l i z a t i o n w i t h t h i s s a m e a c c u r a c y ( p a r a g r a p h 4.4); t h e only v e r t i c a l r e f e r e n c e s y s t e m s c a p a ble of p e r f o r m i n g w i t h s t a n d a r d e r r o r s of 2 c . . . 3 c or 1 ' . . . 1,5' o f a r c a r e : t h e h o r i z o n camera method, the inertial vertical system a n d t h e s o l a r periscope. A modern horizon camera, recording f o u r horizon i m a g e s s y n c h r o n i z e d w i t h t h e m a i n c a m e r a , is c o n s t r u c t e d b y W i l d a n d is n o w u n d e r t r i a l ( f i g u r e 21). A s t h e e x a c t r e c o r d i n g a n d t h e photogrammetric use of the vertical deviation is t h e a l t e r n a t i v e to c o m p l e t e l y a u t o m a t i c -- and consequently more complicated -i n e r t i a l v e r t i c a l g u i d a n c e , t h e r e s u l t s of t h e s e trials are awaited with interest.

Fig. 21. W i l d h o r i z o n c a m e r a coupled w i t h RC8.

4.4. Gyroscopic ve~'tical recording and se,'vo stabilization. I n a m o v i n g a i r c r a f t t h e e x i s t e n c e of m a n y a c c e l e r a t i o n f o r c e s p r e v e n t t h e e s t a b l i s h m e n t of " t h e v e r t i c a l " , i.e. t h e p e r p e n d i c u l a r to t h e geoid a t s e a level. T h i s f a c t c h a l l e n g e d t h e p h o t o g r a m m e t r i s t to a p p l y t h e p r o p e r t i e s of t h e g y r o s c o p e f o r e s t a b l i s h i n g a v e r t i c a l r e f e r e n c e in f l i g h t ; i n a s m u c h as it is p h y s i c a l l y i m p o s s i b l e to e t a b l i s h " t h e " v e r t i c a l , s u c h a r e f e r e n c e s h o u l d a t l e a s t be a s t a b l e one. R e s e a r c h a n d d e v e l o p m e n t h a v e been c o n c e n t r a t e d on t h r e e m a j o r a i m s : 1. to e s t a b l i s h a s t a b l e r e f e r e n c e d i r e c t i o n f o r r e c o r d i n g t h e deviation o f each a e r i a l photograph's axis from a vertical position; 2. to e s t a b l i s h a s t a b l e r e f e r e n c e w h i c h c o n t r o l s a n d c o m m a n d s t h e c a m e r a m o u n t i n g to a n e x a c t l y vertical optical axis; 3. to e s t a b l i s h a stabilized c a m e r a p l a t f o r m in o r d e r to prevent image deterioration ( u n s h a r p n e s s ) d u e to a n g u l a r m o v e m e n t s of t h e c a m e r a a t t h e m o m e n t o f e x p o s u r e . Principles

of

the

gyro

reference.

A g y r o t e n d s to r e m a i n f i x e d in o r i e n t a t i o n relative to i n e r t i a l s p a c e (the s t a r s ) . If

272

Photogrammet~'ia, XV1, No. 4

its axis is initially oriented along the vertical, the e a r t h ' s rotation will c a r r y the local vertical away f r o m the gyro's axis which remains fixed in space (earth's rotation effect). I f the gyro is placed in a moving vehicle, the displacement of this vehicle around the e a r t h will also c a r r y the local vertical away from the gyro's axis (earth's profile effect). A n y imperfections in the gyro cause it to move slowly in inertial space (gyro d r i f t ) . E x t e r i o r torque forces applied to the gyro will cause it to rotate perpendicularly to these torque forces (gym-dynamic) effect. These effect make it necessary to provide the gyro with a vertical pendulous reference to which it is slaved. I f this slaving (erection) is made strong, the gyro will be forced to follow nearly all deviations of the pendulum from the vertical, including the short-period oscillations; if the slaving is made weak, the gyro represents a reference which only follows the long-period deviations (such as long-term accelerations of aircraft, increase of d r a g or speed, r a t e s of t u r n below the detection level of the autopilot, coriolis acceleration during the displacement, change of the local vertical direction e.g. 5' pe r minute of time, etc.). Thus a gyro is basically subject to p a r t l y gyroscopic errors and partly acceleration e r r o r s ; the acceleration errors are independent of the gyro quality. The errors of the purely gyroscopic vertical reference or of a vertically slaved gyro servo system will, in survey flight, always be of the order of 20' mean e r r o r from local vertical. If, by means of special measures, some of the above-mentioned errors are compensated for, the remaining errors in the most advanced purely gyroscopic systems will still be of the order of 10 sexagesimal minutes of arc. This is illustrated by the performance of a number of gyro stabilized references constructed for photogrammetry during the last 20 y e a r s : Nistri's nadir recording camera showed mean errors ms. B = -+ 2' on the laboratory stand; in O.E.E.P.E. flight ma. B = ± 10' of arc. The same values are s t a n d a r d errors with the U.S.S.R. gyro stabilized A F A - T E camera. ,The Steinheil and the A-27 gyro stabilized mountings, the Dubuisson gyro recording and the B a r r and Straud type 67 m o u n t i n g show mean deviations between 10' and 20' in slow nominally s t r a i g h t and level flight. A Sperry vertical gyro within the a u t h o r ' s experience made to perform 1' on the s t a n d showed 20' during in-flight operations. The Aeroflex ARX-3 gyro, however, corrected by long-period erection and acceleration-sensors, performs in-flight mean errors = 5 ' . . . 10'. Such performance can be particularly useful: it enables 3rd order plotting instruments to be used also in mountainous terrain. All gyro stabilized vertical references are equivalent to physical penduli and are subject not only to the e a r t h ' s gravity but also to all other accelerations; the "earth pendulum" is, according to Schnier, the only g r a v i t y reference t h a t is basically independent of all horizontal accelerations. The applications of this principle are discussed in p a r a g r a p h 5.2 dealing with inertia systems. The bottleneck of vertical accuracy is not formed by the mounting's stabilization servo but by the gyro and its accelerations; consequently, using the gyro not as a stabilization device but solely as a vertical reference will not appreciably improve the situation. In conclusion, it can be stated t h a t gyro references in photogrammetry can be useful for rectification (or its elimination), for reconnaissance photography, as n a d i r indication for APR, Shoran, magnetometer, etc. Highly corrected stabilized systems become available and can be useful in combination with 3rd o r d e r plotting instruments. A gyro stabilized mounting shall have such erection rates with such torquer couplings t h a t photo image quality is improved through improved steadiness. It is difficult to obtain sufficient gimbal freedom and no peak erection r a t e s nor gearing vibration. This can be obtained by means of electro-magnetic coupling (e.g. T-28 mounting). The construction of stabilized servo mountings shall be specially designed so as to meet the camera's and the airplane's characteristics; universal mountings cannot deliver optimum performance.

Commission I 5.

Survey Navigation etc., Corten

2"]3

Navigation and orientation methods determining camplete spatial position and attitude.

5.1. Star tracking. Automatic star tracking is a relatively new development. In space navigation, celestial bodies provide the only rigid reference frame. Principle. A telescope, mounted on an inertial platform, is supplied with a photo-electric sensor; this sensor, once it is locked onto a star, feeds a servo system so as to continuously point the telescope to the star. I f the p l a t f o r m is truly horizontal, and if two angles - e.g. altitude and true azimuth of the celestial body - are measured, the standpoint's position can be computed. This is done in an anolog computer. The output can be displayed or be introduced into other systems. Automatic astro compass. Applications of these principles are the automatic astro compasses e.g. the MD-1 of Kollsman I n s t r u m e n t Corp. The MD-1 tracks a celestial body photo-electrically and supplies 1o a true heading output continuously with mean error not more t h a n ~ . Some of the difficulties inherent to this principle are the signal-to-noise ratio in daylight and the necessity of a highest-accuracy inertial platform. Applications. In aerial survey, such extremely accurate heading information can be useful a. for survey navigation as such: conservative precision compasses do not supply the heading closer than 1/3° ; b. for coupling this accurate heading to a Doppler DR computer (the Doppler not being more accurate than its attitude information); c. for c6upling this angular reference onto an inertial system in order to keep its drift within close limits. Applications b. and c. are examples of the useful combination of various systems. Similar system integrations of extremely high accuracy may become of value to photog r a m m e t r y in the n e a r future. 5.2. Inertial navigation. Great e f f o r t s are being expended in the development of inertial navigational systems. These systems provide help in flight line navigation, particularly if provided with a navigational computer, and they supply verticality infoq~mation f o r the control of the camera's optical axis. I t may be expected that inertial navigation can provide basic improvements to future aerial photography. Definition. Inertial navigation systems can be characterized by: 1. an inertial or stable platform which is gyro stabilized with respect to space or with respect to the e a r t h ' s horizon; 2. accelerometers mounted on this inertial platform, sensing the vehicle's accelerations and transducing these accelerations into electrical signals; 3. integration of these accelerations (a) over the time (t) so as to obtain the vehicle's speed V g = o f t a ' d t ;

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Photogrammetria, XVI, No. 4

4. integration, of the speed (Vg) over the time (t) so as to obtain the distance travelled s -----

Vg. dr;

0 5. t r a n s f o r m a t i o n of this information into usable form (by m e a n s of navigational computers) and computation of the corrections which must be applied to gyros and to platform. Principle of operation. The accelerations are measured with respect to a coordinate system which is represented by the stable platform. The stability of this platform can be obtained by means LINE TO CENTRE ~'O,: EAR., /

COMPUTER

I MULTIPLY I

STABLE

~

*~;"../ ~

iiiI~

/

/~"-. ~

/

I /

I

r Fig. 22.

of a three-degrees-of-freedom gyro system in such a way t h a t the platform is stable in inertial space. F i g u r e 22 represents one of the possibilities of construction. Before the s t a r t of the operation the p r i m a r y axis of this system must be aligned with the earth's polar axis; this attitude is maintained throughout the operation. On the stable platform a horizontal accelerometer platform is mounted; this can be horizontalized about two axes in such a way t h a t one of its accelerometers is oriented north-south and the other east-west. If the vehicle does not move, the stable p l a t f o r m and the horizontal platform will keep stationary with respect to space and consequently will rotate 15 ° per hour with respect to the horizon according to the e a r t h ' s rotation. In order to keep the accelerom e t e r p l a t f o r m horizontal, a clock must correct the "polar axis" s h a f t continuously with an a n g u l a r speed equal to the negative earth's rotation speed. As the accelerometer platform now remains stationary and exactly horizontal, no accelerations are sensed by the accelerometers, not even the e a r t h ' s gravity acceleration, thus no signal is supplied to the integrators. The geographic position of the s t a r t i n g point is now introduced into the navigational computer manually; its indication "latitude"

Commission 1

Survey Navigation etc., Corten

'975

and "longitude" will not change as long as there is no other movement than the earth's rotation and its exact compensation a t the horizontal platform. A f t e r the vehicle s t a r t e d moving, say, in north-south direction, accelerations will be supplied to the aecelerometers, the resulting electrical signal (aN) supplied to the int e g r a t o r s ; a f i r s t integration will t r a n s f o r m the acceleration (aN) into the north-south speed V~

of'

a N . dt and a second integration will t r a n s f o r m this speed (V~) into the

/,

north-south distance travelled s N =

Vs¢. dt; the final

output of the integrators is

0

proportional to the distance travelled s N = f f

a~-. dt ~. The navigational computer adds

g2"

HORIZONTAL PLATFORM

it

I

[

\

Fig. 23. this distance, expressed in latitude coordinate distance, to the latitude of the s t a r t i n g point which was introduced into the system before the start. The final indication will continuously show the latitude (qJ) of position. The horizontal platform, however, needs a continuous correction about the E-axis in order to keep its horizontal attitude (figure 23). This compensation for the vehicle's north-south displacement is applied by the same latitude displacement information: the distance travelled is not only fed into the navigational computer but also into the correction computer which rotates the E-axis over angle ~o in order to keep the accelerometer platform horizontal. If the movement takes place in an a r b i t r a r y direction, the acceleration a E caused by the east-west movement vector is measured and integrated in the same way as a N. In addition, however, the dis1 placement must be multiplied by - in order to compensate for the varying lengths of c o s qo the parallels. The information about the longitude of position is used by the navigational computer to add 2 to the original longitude of the s t a r t i n g point, and by the correcting computer to rotate the aeeelerometer platfmTn about the polar axis in order to keep the platform horizontal during the east-west displacement 2. Corrections for a-sphereicity of the earth and for eoriolis accelerations must also be applied. Performance

of

inertial

systems.

There are two main causes of errors in inertial navigation systems:

Photogvamme~ria, XVI, No.

276

1. the fact t h a t the systems are self-contained without references to outside sources gives rise to a serious propagation of errors, and 2. the fact t h a t the systems are complicated servo loops gives rise to unwanted oscillations. The f i r s t source of errors (build-up of errors) must be met by designing and constructing component p a r t s with incredibly high accuracy, e.g. gyros, a c c e l e r o m e t e r s l ) , servo meters, computers and integrators. In addition, the propagation of errors is greatly reduced by slaving the inertial system to another reference such as Doppler radar, astrocompass, etc. The second source of errors - the oscillations of t h e pendulous servo systems - is greatly reduced by slowing the oscillations down to a 84 minutes oscillation period. An oscillating s y s t e m h a v i n g t h i s oscillation period T = 8 4 m i n = 2 ~ V R i s

equivalent to

a physical pendulum of an earth's radius (R) length. Such an earth pendulum or Schuler pendulum will be influenced by horizontal accelerations in such a way t h a t it oscillates with a radius = R, consequently its attitude will follow the earth's curvature, consequently it will "remain horizontal" when it oscillates due to accellerational disturbances. The f a c t t h a t cumulative errors can also be restricted, is an important additional advantage of Schuler-tuned servo systems. Instruments

available.

A complete inertial navigational system may consist of an inertial platform (containing gyros, accelerometers, gimbals, preamplifiers, heaters, synchros and torques), electronic unit, navigational computer and display or output. Such a system can provide outputs of heading, present position, speed, acceleration, in other words complete attitude and behavior on e a r t h or in space. These systems are extremely complicated. For photogrammetry, Aeroflex designed an inertial system, containing only w h a t is needed for a photogrammetric vertical reference in nominally s t r a i g h t and level, low-speed survey flights. F i g u r e s 24 and 25. The functioning of this P V R U (photogrammetric vertical reference unit) is described in a paper by T. Trott. It is under flight test and is expected to command a t r u l y vertical camera axis with mean errors m~,~ = - 2 . . . 3 e. I f this system should come up to these expectations, it would provide an imp o r t a n t contribution to photogrammetric operations.

1) For instance, the errors of acceterometers for precision systems should not exceed 1 p a r t in 100.000 of acceleration response.

Fig. 24. Aeroflex inertial p l a t f o r m PVRU.

Commission I

Suxvey Navigation etc., ~

i /~

i

Corten

277

¸

Fig. 25. Aeroflex inertial vertical; power supply a n d computer of the PVRU.

Acknowledgement. In compiling this report, the a u t h o r was assisted by members of the subcommission 13, p a r t i c u l a r l y by M r Eldon D. Sewell.

278

Photogrammetria, X V I , No. 4

CONCLUSIONS. General.

Instrumental navigation methods are in rapid development to extremely high accuracies; many of them Can provide for near-perfect survey navigation; some of them reach a degree of accuracy which enables the determination of camera's orientation elements at each exposure station. Survey navigation. Of the various possibilities, a line-of-sight navigation method executed by a completely competent crew 1) according to sound principles has shown to supply near-perfect results (for topographic surveys IGN's variations in side lap are not more than 5%). Overlaps shall be kept to a minimum, with only a few percent tolerances, in favour of appreciable reduction in the number of photographs. Longitudinal overlap is reduced by using overlap regulators, either of the visible type or of the photo-electric automatic type. The use of an intervalometertimer is uneconomical. Bloc photography, having all n a d i r points within 5% of the ideal position, should, in many cases, be preferred over strip photography. It provides for points common to six photographs which is optimum economy of minor control. I t can be obtained in three ways : either by taking 90% overlap and rejecting s/~ of the photographs, or it is obtained by individual pin-pointing, or it shall be made possible by coupling the exposure command to an automatic guidance such as a Doppler (or an aerodist) navigational computer. Doppler navigation and 40% savings) and conventional operation. In the n e a r future, of Doppler navigation aerodist.

is reported to provide appreciable savings in flight time (25% g r e a t improvement in flight line positioning as compared with f u r t h e r improvements may be expected from the combined use with inertial systems and star tracking, and from the use of

Camera orientation. For elevation control, statoscope (flying height differences, m z = ± 1 . 5 . . . 2 m) and the r a d a r altimeter (ground clearance m z = ± 1 m plus one or more meters t e r r a i n influence) have proven their value. The combination of these, A P R (terrain elevations and profiles, spot heights m z -- ± 1 . 5 . . . 3 m at well-defined hard points) is recommended. Planimetric control can be obtained by means of shoran and decca; a promising new development is aerodist (expected mx, y - - ± 1 m + 1/100.000 distance). Verticality information can be obtained by means of the solar periscope (ma,~ ~ ± 3 c is possible) ; the horizon camera in its new version may prove to be of g r e a t importance (verticality indication expected ma, fl ~ ± 3c) ; another development of g r e a t potential interest is the new automatic inertial vertical now under trial (vertical photographs, expected ma, ~ - + 3~). Gyro verticals can and will not be sufficiently accurate for photogrammetric stereoscopic orientation; they can be used for planimetric mapping purposes (verticality information errors ma, ~ = 1 0 ' . . . 2 0 ' ; one highly corrected gyro system performs ma. fi ~ 5 ' . . . 1 0 ' ) and for the use of 3rd order plotting instruments. In many cases single methods alone are incapable of performing with highest accuracy (compass, gyroscope, inertial instruments, etc.); there is a trend towards combining various methods - each one with t h e i r particular merits and deficiencies - into integrated precision navigation systems such as: Doppler plus dead reckoning, inertial plus Doppler, inertial plus s t a r tracking, Doppler plus s t a r tracker on inertial platform, etc. In this way, they can be made to limit each other's error propagation and i n s t r u m e n t 1) navigators of this class are of a pilot's educational and experience level.

Commission I

Survey Navigation etc.,

Corten

279

d r i f t to within extremely close limits. In addition, the output of these systems is processed in computers - analog or digital - to bring them in a form usable either by the automatic pilot or by the camera orientation elementg recorder, or by both. Whether these complicated systems will become of value for civil photogrammetry must become a p p a r e n t in the future. R E F E R E N C E LITERATURE.

This list is limited nearly exclusively to articles published in the period 1956... 1960. General. [1]

B 1 a c h u t, T. J., Airborne controlled method of aerial triangulation. Photogrammetria, XII, 4, 1955-1956. [2] K o n s b i n , M. D., Primenjaemye metody Opredelenija v. poletje elementov vnesnego orientirovanija (Methods for determination of the elements of outer orientation of the camera in flight). Moskva 1959. [3] ~ e r l e n , A. I., Aerofotos'emka, (Aerial photography). Geodezizdat, Moskva 1958. [4a] B r a n d e n b e r g e r, A., Aerial triangulation with auxiliary data. Internat. Archives, Comm. III, London 1960. [4b] S c h e r m e r h o r n, W., P l a n n i n g in modern aerial survey. Internat. Archives, Comm. IV, London 1960. [5] R a m s a y e r, K., Neue MSgliehkeiten der Luftbildnavigation. Bildmess. u. Luftbildwes., 1960. [6] C o r t e n, F. L., Photo quality economizes aerial survey, U.N. Cartographic Conference, Bankok 1959.

Navigational methods. [7] [8] [9] [10] [11] [12J [13]

[I4] [15] [16] [17] [18] [19]

U.S. Navy Hydrographic Office, Air navigation, H.O. Pub. No 216, Washington 1955. S o k o I o v, V. I., SpravoSnik aviacionnogo iturmana. Reference book for the aviation navigator. Moscow 1957. Wild H e e r b r u g g A. G., Description of survey cameras and overlap regulators. Zeiss Aerotopograph, Description of survey cameras and IRU overlap regulator. W o l v i n, J., Precision automatic photogrammetric intervalometer, Photogramm. Engng., Dec. 1955, p. 773. G 1 e i z e, P., a.o., Institut G~ographique National, Les m~thodes de navigation en photographie a~rienne. Internat. Archives, Comm. I, London 1960. Brucklacher, W., A contribution to the navigation of photoflights covering large areas, English version of publication in German. Bildmess. u. Luftbildwes., Dec. 1952. F 1 e m i n g, J., Survey navigation, precision turns, private report ITC 1957. B r u c k I a c h e r, W., Beitrag zur Planung, Vorbereitung und Darchfiihrung photogrammetrischer Bildfliige. Deutsche geodiit. Komm., Reihe C, No 25, 1957. G1 e i z e , P., a.o., Institut Gdographique National. Aerial survey navigation methods in use at the I.G.N. Internat. Archives, Comm. I, London 1960. P o w e 1 l, The Decca navigator system as an aid to survey. A series of publications edited by the Decca Navigator Comp, Ltd., London. R o s s, J. E. R., Geodetic problems in Shoran. Geodetic Survey of Canada, Publ. No 76, Ottawa. Aslakson, C. I., U.S. Coast and Geodetic Survey (Washington), and Aero Service Corp. (Philadelphia). A number of publications in various journals describing shoran and hiran operations and measurements.

280 [20]

[21] [22] [23] [24] [25] [26]

[27] [28] [29] [30] [31] [32] [33]

[34]

Photogram~ne~'ia,, XVI, No. $ Z a r z y c k i, J. M., Application of shoran and establishment of horizontal control for photogrammetric mapping of inaccessible areas. Canadian Aero Service Ltd., Ottawa. Geodetic Survey of Canada, Geodetic application of shoran. Publ. No 78, Ottawa. Wilson, A. N., U.S. Air Force, H i r a n aerial electronic surveying. Proc. of Commonwealth survey officers' conference ]959. R o s s, J. E. R., Use of shoran in photogrammetric operation, U.N. Seminar on topographic mapping. Teheran 1957. B erger, F. B., The design of airborne Doppler velocity measuring system, I.R.E. Transactions, ANE-4, No 4, 1957, 157-175. F r i e d, W. R., Principles and performance analysis of Doppler navigation systems. I.R.E. Transactions, ANE-4, No 4, 1957, p. 176-196. S m i t h, R. D., Position determination by the Tellurometer system of line measurement using aircraft. Proc. of Commonwealth Survey officers' conference 1959. W a d l e y, T. L., The Tellurometer system of distance measurement. Emp. Survey Review, XIV, No 105 and 106, 1957. B e r g e r, F. B., The nature of Doppler velocity measurement. I.R.E. Trans. on aeronautical and navigational electronics, ANE-4, p. 103-112. Aero Service Corp., Exploration over featureless t e r r a i n with r a d a r navigation, Washington, 1959. W a d l e y, T. L., Electronic principles of the Tellurometer. Trans. S.A.I.E.E., 1958, 49, 5. Hunting Surveys Ltd., Application of Marconi AD 2000 Doppler navigation in aerial survey. London 1959. B o r n, C. J., Use of shoran controlled aerial photography in medium scale mapping. U.S.A.-A.M.S. Report. V e t s t e l l e, J. Th., Use of the Decca navigator survey system in New Guinea for hydrography and as a geodetic framework. Intern. Union of Geodesy and Geophysics, The Hague, Toronto 1957. Canadian Applied Research Ltd., Anadac, Automatic navigation display and computer system, Toronto 1959.

Altitude. [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]

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Survey Navigation etc.,

Corten

281

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Vertical. [48] [49]

[50] [51] [52]

[53] [54] [55] [56]

[57] [58] [59]

[6o] [61] [62]

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Spatial attitude. [63] [64] [65] [66] [67] [68] [69] [70]

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