632 PHOTOGRAMMETRIC ENGINEERING

avoided if another project were undertakenwhile some others could not. One time-savingdevice highly desirable would be a printingcounter for machine coordinates, now stand­ard on the newer first-order instruments. Ex­perience gained by personnel would permitgreater efficiency in a similar undertaking.As was pointed out at first, it will probably bedifficult to find another area with so manyfactors favorable to photogrammetry. Theexpense of going into the area twice and set­ting up a camp is a disadvantage of thismethod. A complete survey would permit a

T he Use of Doppler Radarin Present and FutureMapping Operations*

comparison more favorable to photogram­metry than the skeleton survey describedhere. One conclusion seems certain, themethod is not adapted to a survey of a smallarea. The extent to which it will be used forother original surveys is a matter for furtherstudy.

This project is an example of one of a num­ber of techniques under consideration by theBureau of Land Management as an aid inspeeding up surveys in Alaska and the un­surveyed lands in the other States of theWest.

HANS MEIER,

Aero Service Corporation,Philadelphia, Pa.

ABSTRACT- The removal of the military security restnctwns on the DopplerRadar Navigation System has made possible adding a new tool to the commer­cially available electronic distance-measuring devices.

The purpose of this paper is to explore the possibilities which this new instru­ment offers to the surveying and mapping profession. Since only a very limitedamount of practical experience exists in this new application of Doppler, theideas presented herewith should be considered as suggestions for studies to ob­tain the necessary statistical data to verify and prove our fondest hopes.

AT PRESENT, our position is the same asthat of enthusiastic and optimistic

pioneers after the Second World War, whenShoran was a navigation instrument and itsdevelopment as a geodetic measuring in­strument was proposed. The theory and in­strumentation of the Doppler system whichmeasures ground speed and drift angle bymeans of radar signals emitted from the air­craft and reflected from the terrain is as­sumed to be known. However, a short de­scription of the Doppler principle seems to bein order.

Figure 1 shows an aircraft emitting twopencils of radiation downward and which arereflected back to the aircraft. The difference

between the emitted signal-frequency and thereceived signal-frequency from a forward andbackward-looking pair of radiation beams ismeasured. This difference is proportional tothe aircraft's ground speed. If we use fourbeams radiating symmetrically to the air­craft axis, covering the ground in an x-shapedpattern, the drift-angle of the aircraft canbe measured. The Doppler echo frequency­shifts from the two diagonal pairs of pencilsare compared. If they differ, the antenna isnot aligned with the actual path being flown,and the aircraft is drifting. The frequencydifference actuates a servo mechanism, whichrotates the antenna, aligning the beams withthe actual path flown. The angle through

* Presented at the 25th Society's Annual Meeting, Hotel Shoreham, Washington, D. C. March 8 to11, 1959.

USE OF DOPPLER RADAR IN MAPPING OPERATIONS 633

FIG. 2

The Doppler computer determines con­tinuously the true air-speed, the sine of the

is a constant for a certain flight-lineis the true air-speedis the sine of the drift-angle, andis the distance between exposurestations.

!!.h = 0.0351 X sin", X s X V A X sin 0

where

sin cf>VA

sin 0s

formation is available in digital form to thepilot and the data compiler. Since it is quiteeviden t that these data are exactly what isneeded for photogrammetric purposes, theonly question which must be answered satis­factorily is: "\Vhat is the accuracy?" As ofnow, we have only the word of the manufac­turer and the en thusiastic reports from pilots,Lacking are statistics substantiating theseastonishing claims.

It appears that distance accuracies of 0.2%or better can be obtained. This error magni­tude is within the realm of photogrammetricapplications.

A test flight was made with Aero Service'sRadan along a straight railroad track wherethe 9" X9" mapping camera was triggered bythe Doppler computer at equidistant in­tervals. The actual exposure intervals werethen checked by photogrammetrically meas­uring the ground distance between the nadirpoints of the photos. The maximum error was9 feet for a 1,500-foot base.

Let us look into the future and see how wewould use the Doppler information.

1. The system can be used immediately inconnection with the airborne profile recorder(APR) to determine the slope of the isobaricsurface, which is usually given by the for­mula:

HANS MEIER

FIG. 1

which the antenna was rotated is the drift­angle.

Figure 2 shows the four-beam principle.The important advantage of Doppler is thefact that all measurements are made in theaircraft without any assistance from groundstations. This short explanation of the Dop­pler principle will be considered sufficientsince we are concerned here only with th~application of the system to photogrammetricand surveying problems.

Aero Service Corporation owns and oper­ates Radan, a commercial version of themilitary model, which is installed in a DC-3for magnetometer survey navigation. ThisDoppler system consists of transmitter, an­tenna, receiver, gyrocompass and a specialcomputer which determines continuously thedistance traveled as well as the lateral devia­tion from the pre-selected track. All this in-

634 PHOTOGRAMMETRIC ENGINEERING

drift-angle and the distance along the track.I t is, therefore, possible to determine auto­matically the elevation difference of succes­sive exposure stations. It might be pointedout that only the height corrector of the APRor a statoscope is necessary for this procedure.

Since the drift angle is the most criticalquantity in the above equation, its continuousand more accurate determination by Dopplerwill increase the slope accuracy considerablyover the present method of measuring thedrift angle with a driftmeter at relativelylarge distance intervals.

2. A photo mission flown with a Doppler­equipped aircraft allows keeping the sidelapto a minimum, reducing the number of stereo­models. Gaps in the photography due topoor navigation can be completely eli minated,reducing the amount of costly reflights.

3. If further tests prove sufficiently ac­curate for determining also the distance be­tween exposure stations, aerial bridging willbe greatly facilitated. Combined with theheigh t-difference information explained be-

height conKlor at"

FIG. 3

aircraft path

isobaric sur10Ct

fore, we would have an excellent system forphotogrammetric control-extension. The testflight along the railroad track, which we havementioned earlier, indicates that this proce­dure is well within the capability of the Dop­pler system.

Going a step further, we visualize the useof Doppler in connection with Shoran orHiran surveys. The tri-Iateration network willfurnish the basic horizontal control to whichthe Doppler controlled photos are adj usted.The selection of Shoran stations is then di­vorced from the photo aspect, and they canbe placed on easily accessible points withoutthe present restrictions imposed on them bystation angle limitation. The occupationperiod of the ground station will be muchshorter, since they are not needed for theaerial photo work, thus reducing the prob­lems of logistics considerably.

Cumbersome reconnaissance trips to thehighest points in the survey area, to deter­mine the horizon profiles, necessary forpresent Shoran controlled photography work,are reduced to trips to more accessible loca­tions, since complete horizon profiles are notrequired for tri-Iateration flying.

Shoran equipment maintenance will beeasier, since it can be returned to base after arelatively short station occupation, thus re­ducing also the length of time between equip­ment calibrations.

Selected Doppler flight lines can be checkedwith Shoran, providing an excellent means forcalibration of the Doppler system.

In order to strengthen the Doppler con­trolled photography, more Shoran tri-Iatera

FIG. 4. Antenna of self-contained Doppler navigation system mounted flush in belly of Aero's DC-3.

USE OF DOPPLER RADAR IN MAPPING OPERATIONS 635

tion stations will be needed than in regularShoran controlled surveys. This, however,need not be a disadvantage, because the re­strictions imposed on present Shoran stationlocations are greatly reduced. The tri-latera­tion net"'ork obtained in this manner will beused in all future survey projects which willcertainly follow a small-scale Doppler con­trolled mapping program.

Pure Doppler flights can be used to deter­mine the geographic positions of flight-lineintersections within an accuracy obtainableby usual astro field methods. This procedurewas used by Aero Service on a recent mag­netometer survey over the Libyan Desert.The flight path was photographed on stripfilm from 1,500 feet above ground. At 4 km.intervals, a fiducial mark was exposed on thefilm triggered by the Doppler computer.Flight-line intersections were then deter­mined from the strip film interpolating theinterval for distance. The strip film intersec­tions were then transferred on to existing9" X 9" photographs and a radial phototemplet-laydown was made, using the astropositions as basic control. The Doppler dis­tances were compared with the astro con­trolled templet distances resulting in an aver­age distance error of 0.22%. However, thistest cannot be considered as conclusive forthe attainable accuracy. The distance com­parison is based on astros in an area which isknown to have large deflections of the ver­tical and is lacking ideal identifiable terrainfeatures. This leaves too much to unac­coun table facts.

I t would be highly desirable to perform atest over well-mapped territory with theDoppler lines flown in such a manner thatenough redundant measurements are avail­able, and where errors due to the insufficientreference data are eliminated.

Since the azimuth of the flight line is de­termined by reference to a compass aboardthe aircraft, the accuracy and dependabilityof this compass become matters of great im­portance. Vile use a Kearfott J-4 compass,either as a high-precision magnetic compass,or in free mode, as a low-drift inertial ref­erence. Its operation has been satisfactory,but the magnetic compensation of the air­craft has not yet been carried out to the limitwe desire.

We realize that the feasibility of the ideaspresented here depend on further investiga­tion of the capability of the Doppler systemfor photogrammetric purposes. I t has provenits value as a navigation device, and it seemsto us, based on the short experience gainedwith the instrument, that its use for small­scale mapping is well within its range. Atpresent there are at least four different Dop­pler systems commercially available: theDecca, the Marconi, the Laboratory forElectronics and the Radan. Each of thesemakes has different characteristics, advan­tages and disadvantages.

Aero Service will welcome the opportuni tyof keeping the Society advised of future ex­perience gained by the Company with itsRadan equipment.

ACIC Objectives for PhotographicQuality Control*

ARNOLD F. TRACHSEL, Cartographer,Aeronautical Chart and Information Center,

St. Louis, Mo.

(Abstract is on next page)

A RECENT magazine article stated that dustry would rate under such a system is notscience and technology have been ex- definitely known, but from a user standpoint

panding at an annual rate of 5 to 7 per cent. the author would be inclined to place it at orSuch a rate of expansion means a doubling in near the top.about 15 years. 1 How the photographic in- A part of the mission of the Aeronautical

* Presented at the Society's 25th Annual Meeting, Hotel Shoreham, Washington, D. c., March 8 to11,1950.

1 Foote, P. D., Industrial and Engineering Chemistry, Vol. 51, No.2, p. 91A (1959).