The Use of Doppler Radar in Present and Future Mapping ...
Transcript of The Use of Doppler Radar in Present and Future Mapping ...
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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-
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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.
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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).