FRONTISPIECEReproduction of a portion of a semi-eontrolled SLAR mosaic.

JANW. VAN ROESSEL Earth Satellite Corp. Berkeley, Cali$ 94704 ROGERIOC. DE GODOY Ministerio das Minus e Energia Rio AJaneiro GB, SLAR Mosaics for Project RADAM

Semi-controlled mosaics made with side-looking radar imagery comprise one of the mapping products of the Amazon and the Brazilian Northeast. (Abstract on next page)

INTRODUCTION a standard map product with a certain NTIL THE organization of Project ADA AM (for geometric accuracy. U~adar~mazon) in October 1970, the To date, the minor o1,jective has almost Brazilian Amazoil Basin was one of the been acco~nplished:160 semi-controlled SLAR largest poorly mapped areas in the world. At mosaic sheets have been released for pul~lic that time, however, the Brazilian use. Each inosaic sheet covers a rectangular decided to undertake a reconnaissance sur- area of 1 degree latitudinally by 1.5 degrees vey of the Amazon and the adjacent Brazilian longitudinally. Preliminary checks have in- Northeast in a most unconventional manner, dicated that the geometric accuracy of each namely, by executing the largest commercial sheet and of the overall nlosaic can be ex- side-looking radar (SLAR)remote-sensing proj- pected to be well within the contractual re- ect ever undertaken. quiren~ents,which will be described pres- The major objective was to collect informa- ently. tion on mineral resources, soils, vegetation, Mapping such an extensive area in such a and land use; a minor objective was to obtain short period required an uncoilventional PHOTOGRAMMETRIC ENGINEERING, 1974

mapping tool, for which only SLAR qualified reliable cartographic delineations, a certain because of its capability to penetrate clouds. minimum data quality had to be achieved. Indeed, the pay-off has been proportional to This minimum level was at least maintained the high risk taken in the use of a commer- by observing technical specifications in- cially unproven mapping device, to the effect cluded in the data acquisition contract in the that a good set of reconnaissance maps is now form of a Technical Annex. Many of the initial available for all of Brazil north of the 8"s technical specifications were provided by parallel. Dr. S. B. Levin of Earth Satellite Corpora- In this paper we shall briefly discuss the tion. background of Project RADAM (de Azevedo, A special team (ApoioTbcnico) was formed 1971) as well as the contractual requirements to work jointly with the contractors to and the project instrumentation, and we shall evaluate image quality and geometric fidelity then consider in somewhat more detail the of the radar imagery, and to implement SLAR specific SLAR geometry, ground control con- mosaic compilation methods. siderations, semi-controlled mosaic compila- Some highlights of the SLAR quality re- tion and accuracy evaluation. quirements are the following:

IMAGE QUALITY PROJECTRADAM Dynamic range 20 dB; resolution 16 M. The project is supported mainly by the * Brazilian Ministry of Mines and Energy. The GEOMETRIC FIDELITY OF IMAGE STRIPS initial plan covered an area of about * Along-track and across-track scales uniform 1,500,000 km2, but the project was gradually within 1 percent.

ABSTRACT:Semi-controlled SLAR mosaics covering most of Northern Brazil (more than 4,500,000 km2) are being compiled as a part of Project RADAM of the Brazilian Ministry of Mines andEnergy. These mosaics represent the first large-scale commercial effort to manufac- ture maps from SLAR imagery. A special combination of SLAR in- strumentation and airborne and ground navigation equipment is used for the project, creating special metric problems with respect to ground control, flight configuration mosaic compilation and accu- racy estimation. extended to cover approximately 4,600,000 * Along-track scales of adjacent strips consis- km2. This extension was made possible by tent within 1 percent. the successful performance of the system. * Angular distortion not to exceed 10 mrads in The project area is shown in Figure 1. any one swath. * Image sidelap average 25 percent but not to For acquisition and processing of the radar be less than 10 percent at any point. imagery and related remotely sensed data, a contract was signed with two associated en- GEOMETRIC FIDELITY OF SEMI-CONTROLLED MOSAICS terprises: LASA Engenharia e ProspeF~oes * Cumulative scale discrepancy in any direc- S. A. and Aero Service Corporation. Aero tion not to exceed 1 km. Service's role was mainly to obtain the radar Angular distortion not to exceed 10 mrads imagery, whereas LASA provided logistical within any one mosaic. planning, executed the ground survey and * Comer positions of each mosaic to be accu- assembled the radar images into mosaics. rate to within 1 km with probability of 95 Earth Satellite Corporation was contracted percent and to within 0.5 km with probabil- by the Brazilian Government to select and ity of 50 percent. evaluate proposals made by qualified con- * Tic mark grid orientation to be correct within 10 mrads. tractors, to provide the contractual and tech- nical specifications for the project, and to ad- vise Project RADAM on matters of quality con- trol and imagery interpretation. RADAMflights were spaced 15 min of arc apart and were generally flown North-South. The flight spacing provided 25 percent To permit a homogeneous interpretation of sidelap for the radar strips and 8 percent the area's natural resources and to provide sidelap for concurrently obtained infrared SLAR MOSAICS FOR PROJECT RADAM

LEGENDA

ARLI COY COBEIITUR~ TOTAL SHORAN

FIG.1. The project area superimposed on the . The cross- hachured area was controlled through E-W tie lines. Triangles indicate TRANSIT points. photographs which were taken with 66 per- Inertial guidance platforms-2 Litton cent forward overlap. For the initial area all LTN-51 systems, indicating present posl- flights were tracked with SHORAN For the tion, attitude, heading, drift angle. and add-on areas, a new and less expensive ground velocity. Radar altimeter-Stewart Warner APNl195. ground configuration was adopted for which Accuracy +. 50 m. only East-West tie lines were tracked with Barometric differential altimeter SHORAN Figure 1 shows the extent of the ini- -Rosemount 803C. tial and the add-on areas, as well as the con- SHORAN master station-RCA APN-84. figuration of ground control points. Digital data handling equipment-Lancer The remote-sensing platform was a twin- digital data system. Kenedy tape recorder. jet Caravelle flying at an altitude of 12 km Monroe datalog printer. with a speed of approximately 690 kmthour. The digital-data system integrates the iner- On board were the following remote sensing tial velocity signal and triggers the cameras and navigation devices: and the fiducial marks on the SLAR imagery for Side-looking radar-Goodyear Mapping every 10 kilometers. On digital tape are re- System 1000 (GEMS).Optically correlated, corded the SHORAN ranges, the LTN-51 out- coherent, and focused beam. Ground puts, the radar altimeter outputs and the time, range presentation. Operational one record per km. parameters-scale 1:400,000, ground range To provide ground support for the SHORAN delay 11 km, ground sweep 37 km, swath system, accurate point positions for 45 overlap 25 percent. ground points were determined using TRANSIT Aerial cameras-Zeiss RMKA 23/85 satellite locating equipment. The Magnavox super-wlde angle, used with color-infrared film and W15 filter. 12S Mark I camera used MX-702 provides coordinates accurate to with B&W infrared film and47,57,25A, and within 15 meters. A few observations were 89B filters. on points of known coordinates adjusted to Video tape system-3 cameras Javelin the Brazilian Corrego Allegre datum (HIRAN SC-950 with Sony videotape recorders. points). The discrepancies due to the differ- PHOTOCRAMMETRIC ENGINEERING, 1974 ences between the Brazilian and satellite el- At the same time the slant range r is lipsoids were alinost constant, namely, l sec- situated in the X'Z1-plane for which we can ond for latitude and 3 seconds for longitude write the following equation: (da Rocha, 1971). The SH~RANsystem was not used in the usual manner, namely for the determination of base distances between ground stations, where but instead the recorded ranges to two ground stations with known coordinates A' = sin K cos 4 + sin 4 cos K sin w, were used to intersect the air-station posi- B' = COS K COS w tions. C' = -sin 4 sin K + cos K sin w cos 4 Logistics plans called for the operation of four SHORAN transponders at a time so that one with K, 4 and o representing the yaw*, roll spare transponder was always available. and pitch angles, respectively, ofthe antenna Considering that a great number of ground pod. stations were located in uninhabited regions, If we assume that Zc is the flying height and that flight operations were almost con- above the plane in which P is situated, then Z tinuous, the sinall amount of untracked flight = 0, so that Equations 1and 2 reduce to two lines (16 percent) indicates a high perfor- equations with two unknowns, from which mance level of the SHORAN system and its we can solve for X and Y, namely operating crew. In all, 300 flight missions were made, ac- X =A1C'Zc? B'. counting for a total of 1,500 hours of flight [(r2 - Z2,) (ArZ+ Brz) - ~'~22~1'+ X,; time. Af2+ Bt2 SLAR GEOMETRY and by permutingAf and B' in Equation 3 we It was the task of the Apoio Tbcnico team to obtain the expression for Y. Considering the monitor the quality and geometric fidelity of look direction, there is no difficulty in iden- the incoming iinages and also to advise on the tifying the proper solution. implementation of mosaic colnpilation Formula 3 and its Y equivalent can be used methods. To accomplish this task, the mem- to handle any orientation of the antenna pod. bers and advisors of the team had to gain However, recorded antenna pod orientation some insight in the relevant SLAR geometry. parameters were not available for Project For the purpose of controlled mosaic con- RADAM,even though the orientation param- struction two types of control points were specified in the contract. The primary geopoint (G1 point) was nlainly a point de- termined by the ~~~~s~~equipment.The sec- ondary geopoint (G2 point) was specified as any radargrammetrically or photogrammetri- cally derived point. For the semi-controlled mosaic (henceforth referred to as MSC)radar- grammetric G2 points proved to be of prime importance as the image strips were matched at the G2 points with the plotted positions of these points. Analytically, G2-point coordinates can be computed from the known slant range and the air station coordinates. Air station coordi- nates can in turn be computed froin the re- corded SHORAN ranges. In Figure 2 the antenna axis system X', Y', Z' is at the air station C, whose coordinates X,, Y", Z, are expressed in the XYZ-system. The slant range from C to P is r. As many authors FIG.2. SLAR geometry under flat terrain assump- (Leberl, 1970; Rosenfield, 1968; Konecny, tion. 1970) have indicated before, the slant range can be considered as the radius of a sphere *Yaw as measured from the north-pointingY-axis with the equation: and therefore equivalent to the tn~eaz~muth ofthe flight direction at any one point. SLAR MOSAICS FOR PROJECT RADAM

eters of the aircrafi- were recorded on magnetic tions, so that their along-track scale could tape. Unfortunately, these parameters could readily be computed and used to evaluate the not be used as the antenna pod had an inde- across-track scale of the N-S lines. pendent motion conlpensation systeln (drift Under the flat-ground assumption, how- angles of as much as 6" could be accounted for ever, we discovered thstthere exists a simple bv orienting the antenna in the direction of method to conlpute the across-track scale di- zero Doppler shift). Thus, for Project RADAM, rectly from the recorded radar altimeter read- Equation 3 and its Y equivalent could be ings. Across-track scale variation nlainly re- simplified for general flight directions by as- sults from the difference between the as- suming zero pitch and roll, so thatA1 = sin K, sumed mean terrain clearance used in the B' = cos K, andC' = 0, yielding1 = G cos K + slant range to ground range conversion func- X, and Y = -G sin K + Y, where (: renresents tion and the actual terrain clearance. the ground range (G = (r2-Z2,)h).A hrther Slant range mark spacing as a function of simplification could even be made iol north- the assumed mean terrain clearance can be south flights for which K can be assumed expressed as follows: equal to zero, so that X = G + X, and Y - Y, Incidentally, this saine solution is obtained for K = 0°, o = 0°, but 4 # 0°, as can be seen from the expression for the orientation matrix where d, 1s th_e distance between range nlarks elements and Equation 3 and its Y i and i+l, h is the assunled mean terrain equivalent. Thus, for sinall yaw and pitch clearance, r, is the slant range of inark i, andE angles. roll can be neglected and this is logi- is the representative fraction of the nolninal cal as the position in the Y plane is deter- scale (400,000). For the same distance on the mined by the radar system in the time do- ground a similar expression can be wdtten in main. which the term 1IE is omitted and h is re- For the MSC compilation, the X = G + X, placed by h, the actual terrain clearnnce. Di- equation was simply ilnpIenlented by offset- vision of the two expression5 then give5 the ting the ground range from the plotted air representative fraction of the actual average station perpendicular to the flight path. scale between the two marks: Afier correlation, seven slant range marks are visible at every 10-km fiducial on the radar film. The time delay for each range mark can be calculated from the fact that there is a 30.88 microsecond increment for each pulse. Then, knowing the speed of By rearranging Equation 5 and using trun- light and the atnlospheric refractive index, cated series developments, an expression can accurate slant ranges can be determined from be developed in which r, +I has been replaced the time delays. The slant ranges in turn can by ri + d,. Then by obtaining the limit of this be converted to ground range5 reflecting the expression ford, + 0, we can derive the fol- height ofthe aircraft. The seventh slant rallge lowing continuous expression for the across- mark in the far range channel was selected for track scale: the computation of G2 points. They were generally spaced 50 km apart both in the along-track and across-track direct~on. Throughout the course ot the image acquis- Equation 6 presented in graphical form ition phase there was a concern for the con5is- shows a series ofhyperboliccurves with Illax- tency of along-track and across-track scale. In imum scale distortion closest to the aircraft the inltial conbact a clause was included call- ground track and minimum distortion in the ing for daily overflight of a geometric test far range. These graphs proved to be ex- area. As work began, however, it soon be- tremely useful in assessing transversal scale came apparent that it would be more feaslble variation by examining radar altimeter rec- to fly a series of east-west (E-W) tie lines (not ords. For instance, it was found for one ex- to be confused w~ththe SHORAN controlled tie tre~nelylong tie line flight that the error lines for the add-on areas). Such lines were around the mean terrain clearance was 128 111, flown and serveda two-fold purpose, namely, and that a maximum deviation of 422 111 oc- to provide insurance wgninst a total collapse curred. Using Equation 6 we determined that of the ground-control system and to facilitate this corresponded to a 1-percent scale varia- transversal scale checklng of the nolth-south tion in the near part of the near range and to (N-S) lines. The E-W tie llnes in the original less than 0.25-percent scale variation in the area were flown over identifiable ground sta- f'arthest part of the far range. PHOTOGRAMMETRIC ENGINEERING, 1.974

CLOSURE ERROR IN LATITUDE (MIN OF ARC)

CLOSURE ERROR IN LONGITUDE (MIN OF ARC) FIG.3. Histograms of inertial guidance platform closure errors.

-z- J L., - I-I'C-lIIIIlIIIIIIIIlIIIII I1 I I I ~l.o~~~a~"O~~~~"o~~"0~~~- 8,- /119 ,_ r8DUII.l UY" FIG.4. Schuler periodic error curves of flight 33.

support system with partial SHORAN control. On mapping missions, the aircraft was al- After the LTN-51 performance data had been ways guided by an inertial guidance plat- considered, the latter course of action was form. Two Litton LTN-51 systems were on- taken and a series of East-West board; one situated in the cockpit, the other SHORAN-CO~~~O~~~~tie lines was flown. system located in the middle of the plane. We could evaluate the LTN-51 behavior in The two systems were interchangeable. two ways. When the aircraft returned to its In the initial stages of the project, inertial point of departure, the inertial positions were platform behavior was of concern because of read from the displays and the differences its effect on scale variation in the image between them and the starting positions strips. At a later stage, knowledge of this be- were noted. These closure errors provided havior became crucial in considering ground insight into the secular drift of the platforms. support methods for an add-on area known as Histograms of latitude and longitude closure RADAM North. In this area SHORAN support of errors are presented in Figure 3, which were every strip became impractical due to the compiled from 80 flight missions of an aver- scarcity of qualifying airports in the remote age duration of 5 hours. Absolute values of northern jungle areas. Thus, a decision had to the closure errors were used to make the his- be made as to whetherto continue solely with tograms. Root Mean Squared Errors (RMSE~) LTN-51 position data or to develop a ground were computed from these data, which SLAR MOSAICS FOR PROJECT RADAM proved to be 1.10 minutes of arc (1.9 km) in curves of Figure 4. (The Foucault period is longitude and 2.56 minutes of arc (3.4 km) in much larger than 5 hrs.) latitude. The second platform was less accu- Brown has suggested the following error rate with RMSES of 2.35 and 4.60 minutes of model to describe the error curves: arc (4.3and 8.3 km) in longitude and latitude, respectively. Perhaps due to the predominant N-S flight directions with mostly changes in latitude, where s(t) represents the latitudinal or lon- LTN-51 indicated latitudes were more sub- gitudinal error as a function of flight time. In ject to error than longitudes. However, on the Table 1the least squares fitted coefficients of relatively predominant E-W flights larger Equation 7 are listed as computed from the closure errors for latitude than for longitude data of Figure 4. were also observed. Time t = 0 was assumed at the beginning of The second method to evaluate the the curves of Figure 4 at fiducial 110 and the LTN-51 behavior was to compare the plat- units of the fitted data were minutes of arc. form indicated latitude and longitude with A significant spin-off of the evaluation of the position data computed from the re- corded SHORAN ranges. This method provided us with a continuous evaluation ofthe inertial TABLE1. LEASTSQUARES FITTED COEFFICIENTS OF platform error over some very long flight EQUATION(7) lines. The latitude and longitude differences for Coefficients Latitude Longitude an E-W tie line are presented in Figure 4. For both graphs the abscissa indicates the posi- ao -.451150 .027302 tion difference in minutes of arc, whereas the ordinate indicates the fiducial number. The fiducials were spaced approxi~nately0.8 km apart(not 1km, due to system error). The line was about 1100 km long and accounted for about 93 minutes of fli~ht- time. It was flown in the southeastern part of the original RADAM area with the plane taking off from Belem and the Schuler-periodic error curves was that we landing in . Inspection ofthe curves could estimate the accuracy ofthe SHORAN sys- shows the latitude error to be larger than the tem. The random variation exhibited in the longitude error most of the time, consistent error curves of Figure 5 is due to SHORAN with the closure errors of this flight- and the operator error, atmospheric conditions and data of the closure error histograms. ground station geometry. Theoretically, it An important feature of the error curves of would be difficult to propagate all the error Figure 4 is the periodicity of the position sources through the complicated procedure errors. This phenomenon is sometimes refer- used to derive the air station coordinates. However, the knowledge that the error red to as the Schuler period. The periodicity is caused by initial misalignment of the plat- curves should be smooth, as there are no high form and is further a result of the complex- frequency oscillations in the platform, pro- vided us with the unique opportunity to es- ities of unscrambling the acceleration in spherical coordinates on a rotating earth timate the combined SHORAN errors. The esti- mated standard deviations of latitude and while accounting for the earth's non- longitude computed from the fitting of Equa- sphericity and gravitational variations. tion 7 were 236 and 138 meters, respectively. In its oscillating state the platform seems These numbers are very good estimates for to behave much like a Foucault pendulum the accuracy of the computed air station (Broxmeier, 1964). The period of rotation in coordinates. the plane of oscillation is F = 2.rr (a sin A), The Schuler-period error curves became where fl is the rate ofrotation of the earth axis further important in the course of the project, and h is the latitude. If the pendulum is ob- as the Apoio TBcnico team could use them as sewed with respect to a geographic reference a device to trace errors in the SHORAN system, the projection of its oscillations on computations due to faulty programn~ing,er- the x and y axes will have the appearance of rors in ground station coordinates, tracking, beat waves with the period S = 2mI(Rlg)" 5 etc. Three different ground station combina- 84 min, where R is the earth radius and g is tions were used for the flight of Figure 4, as the gravitational constant. The same period is indicated by the Roman numerals, yet no also the predominant period in the error jumps in the error curves can be detected. PHOTOGRAMMETRIC ENGINEERING, 1974

indicating good computational results (a part cially true considering that 25 percent of the

ofthe curves is missing due to s~o~~~failure).near range.-, where across-track scale distor- The model of Equation 7 might even be used tions are most significant, was not used for to smooth the computed air station coordi- the mosaic compilation. nates. Such a procedure and its ~otentialben- The prepared copies of the radar strips are efits are outlined in "Geometric Evaluation assembled on a piece of Masonite hardboard. of Radar Mosaics" (van Roessel, 1972). The strips are glued down and when the glue has dried they are inspected to see that linear features such as roads and rivers are continu- Initially, mosaic compilation was begun ous and that the G2 points coincide with their with the manufacture of uncontrolled plotted positions on the overlay. The Mason- mosaics. At a later stage MSC compilation was ite boards cover 1.5 degrees in longitude started and at that time it became an impor- (one mosaic width) and up to 8 to 10 degrees tant responsibility of the Apoio Tbcnico team in latitude. The breakaway method is used to to monitor and advise on compilation provide continuity from column to column. methods. After the mosaic column has been inspected for image quality and general assemblage, MOSAIC COMPILATION WITH FULL SHORAN CONTROL individual mosaics are cut from the column. These are annotated with place names and tic The first step in the MSC production is the establishment of a geometric base by com- marks and are then copied onto a negative at a puting the coordinates of the G2 points to be scale of 1:250,000. This negative is joined plotted on a stable base overlay. The proce- with a standard border to form the final nega- dure begins with the processing of a paper tive from which the final mosaic copies are made. tape, produced by the TRANSIT equipment, As an example of the final product, a por- containing satellite range inforn~ationfrom tion of semi-controlled mosaic in the vicinity approximately 20 satellite passes. Computer processing of this tape provides the coordi- of the city of , is shown in the Fron- tispiece. nates of the SHORAN ground stations. Also, the magnetic tape with the data logged in the MOSAIC COMPILATION WITH PARTIAL SHORAN CON- aircraft is cleaned up and standardized. The TROL standardized tape is used as input to the Outside the original area, only s~o~~~programtogether with the ground sta- SHORAN-controlledtie lines were available to tion coordinates; then, geographic and UTM produce MSCS In this instance, the tie lines coordinates are computed for air stations 10 are glued down on the masonite board with km apart along the flight line. the help of the tie line G2 points which are The air stations are plotted on a stable base similarly plotted on an overlay. A study of the overlay at a scale slightly larger than the final tie-line intersections with a selected number mosaic (1:200,000) with the help of an invar of N-S lines is then made and the along-track grid plate and a small coordinatograph. The scale of these N-S lines is then adjusted piece G2 points are then plotted by off-setting their by piece, such that the E-W and N-S images ground range from the plotted air stations, will coincide at the tie lines. The N-S lines perpendicular to the flight track, as judged are then glued down and their scale is trans- from adjacent air stations. The ground range ferred to other N-S lines. After all lines have to the G2 points used for off-setting is pre- been put into position, the tie line has be- computed as a function of the mean terrain come co~npletelycovered. clearance. The actual mosaic compilation is then begun. Based on the overlay, overlapping Another task of the Apoio Tbcnico team was pieces of the same radar strip negative are to evaluate the accuracy of the mosaics. For enlarged through an anamorphic lens. This this task three potential methods were consid- lens is capable of differential enlargement in ered. two orthogonal directions. However, only the First, one could think of performing a com- along-track scale is adjusted whereas the plete theoretical error propagation, taking across-track scale is left unchanged. An into consideration errors in ground station analysis of across-track scale variation based positions, errors in SHORAN range measure- on radar altimeter data and using Equation 6 ments and reduction, terrain distributions, convinced us that this practice (at least in the flying altitude and attitude distributions, eastern part of the original area) would not plotting and mosaicking errors, etc. However, cause across-track scale errors outside of the there is no need to study these aspects contractual specifications. This was espe- thoroughly before deciding whether this SLAR MOSAICS FOR PROJECT RADAM

Ill -500 -400 -300 -200 -100 0 100 200 LL ERROR IN UTM NORTHING (Meters) 0

ERROR IN UTM EASTING (MeterJ FIG 5. Histograph of coordinate differences of points obtained from Brazilian Navy (Diretoria de Hydrografia e Navega~o). method would be prohibitively cornplex and One has to keep in mind that these results tedious. are obtained from a small set of points clus- The second method is one in which the tered in a small area and are therefore sul~ject coordinates of measured mosaic points are to small-sample variation. A third method of directly compared with the coordinates of accuracy evaluation is available, however, ground points established by astronomic ob- part practical, part theoretical, that is applic- servation, TRANSIT determination, HIRAN sur- able to larger areas. vey, etc. With this method, the compilation proce- The objection against this method is that dure is considered step by step, with an ap- these points are often difficult to locate on the propriate practical or theoretical error esti- radar imagery, as they are often not marked mation for the coordinates at the end of each on any kind of image. However, Project step, given the coordinates of the previous RADAM was lucky to have availal~lea set of 16 step. For the ~sccompilation process the astronomically derived points furnished by step? are as follows: the Brazilian Navy (Diretorio de Hyclrografia e Navega~Xo),which were well marked on 1. TRANSIT ground station coordinates. Some TRANSIT determined points were alro part of aerial photographs. a HIRAN triangulat~onnet The HIRAN The points were identified on the radar coordinates could be compared w~ththe mosaics and their coordinates were then de- TRANSIT coordinates and the differences termined by the multi-laterative method could be used to provide error estimates. (Wolf, 1967), for which only an accurate scale 2. SH~RANair-\tation coordinates. Random is needed. Differences in Easting and North- vanation around the Schuler-penodlc error ing with the known UTM coordinates were curves of the LTN-51 platform5 could be computed. Histograms of the differences are used to prov~deextremely good est~matesof shown in Figure 5. In all, 16 points were the accuracies of the SHORAN air stations, glven a set of ground station coordinates. used, situated on the North-East coast of 3. C2 point coord~nate\.Here we could use Brazil. A positive bias of 150 meters was pres- theoretical error propagation methods ent in Easting, whereas a negative bias of uslng the partial derivatives of Equatlon 3 133 meters was present in Northing. RMSES and ~tsY equivalent. The error sources used were 190 and 306 meters in Northing and as input to the derived error equations were Easting, respectively. (a)the variation in fly~nghe~ght (from radar PHOTOGRAMMETRIC ENGINEERING, 1974

Column 60" - 61'30' long.

Intermediary ERMSE ERMSE Error Estimation Coordinates E Bias N Bias Method

TRANSIT 76 74 30 29 Comparison with coords. triangulation net.

Air station - Use of random coords. variation around computed from fitted Schuler SHORAN period error ranges. curve.

Analytically - Theoretical error determined propagation. Stdv G2 point flying height = coords. 128 m; stdv azimuth = 0.4"; stdv pitch = 0.1".

Coords. of -97 Comparison of plotted G2 analytic with points plotted coords.

G2 point Comparison of coords. plotted with mosaicked coords. Estimated 274 74 510 -68 Combining the accuracy at above error the estimates with mosaicked (8) G2 points

Estimates from Navy points for comparison

altimeter records), (b),azimuthal error in mosaics in the Eastern part of the original off-setting the ground range (statistics com- RADAM area is presented in Table 2. piled from computed air station coordinates In Table 2, ERMSE stands forE stimated Root of several flight lines), (c)pitch error (this Mean Squared Error. The ERMSE is equal to error is not too important and a reasonable the variance plus the bias squared, so that the value was assumed). 4. Plotted G2 point coordinates. Coordinates errors for two error sources are combined as of plotted points on the overlay could be follows: determined with the multi-laterative method and the results could be compared with the analytically computed coordinates. 5. Coordinates of G2 points on 1:200,000 mosaics. Similarly as above, G2 points on Of course, the implicit assumption of inde- the mosaics could be measured with the pendent error sources is present in the above multi-laterative method, and the results can process, and also errors in Easting and North- be compared with the coordinates of the plotted G2 points or the analytically deter- ing are assumed uncorrelated. These as- mined coordinates, if Step 4 is omitted. sumptions are quite reasonable because the five steps mentioned are independent and An error budget resulting from such a most operations are either performed in East- stepwise analysis made for a column of ing or Northing. The bias product term of SLAR MOSAICS FOR PROJECT RADAM

5z*''h\ I x: FLIGHT TIME \>

FIG.5. Linear scale and position adjustment of N-S lines to E-W lines; estimation of error component due to Schuler periodic variation. I Equation 8 contributed only to the overall This adjustment is in fact what happens when ERMSE when all error sources have a bias term, the scale of the N-S line is changed by a which is not the case in Table 2. An estimate constant and its position is changed by a sim- - of the overall bias term is obtained simply by ple rotation to make the strip coincide with adding individual bias terms. the E-W strips at the crossing points (pointsA The estimates of Table 2 apply only to the and B in Figure 9). G2 points. A further analysis is necessary to Let the error curve be s(t) of Equation 7. gain insight into the variation at other mosaic Alsos(t,) = si ands(t,) = s,, where tj = ti+k,and I points, but at least the variation at those k is the flight duration from one tie line cross- points is bounded by the expected values of ing to the next crossing. The distance be- b the estimates at the G2 points. tween A and B on the straight line X is desig- It is interesting to compare the results of nated r and 1 = s, -s . Table 2 with the analysis of the Navy points, We are interested3in the deviations of s(t) the results ofwhich are included in Table 2 in fromX as given by the function which we will parentheses. The ERMSES for Easting are in call q(t,t,) for various positions oft, and vari- good agreement but the values for Northing ous values of k. differ by a factor of2, as do the bias terms. The The phase of the curve s(t) is randomly L signs ofthe bias terms are in correspondence, determined and different for every flight, so I however. that ti can be considered a random variable with a uniform distribution. The time t is also MOSAICACCURACY FOR AREASWITH REDUCED considered a random variable with uniform I SH~RANCONTROL distribution as we are interested in the error along the N-S strips at randomly selected The above estimates are probably also points with no preference for certain loca- valid for the G2 points on the tie lines of the tions. Under this assumption, probabilities area with reduced SH~RANcontrol. However, fort and ti will be 11s and Ilk, respectively. for the N-S lines in these areas, additional Then we can express the contribution to inaccuracies will be present. I the RMSE of the position of the points in the We debated whether the Schuler-periodic I N-S strip due to the Schuler-periodic varia- error would be a significant error source. This tion as follows: periodic variation causes the film scale to vary periodically and also causes a wander- ing terrain effect in the across-track direc- tion. The following analysis gave us insight I into the significance of the Schuler period in terms of point inaccuracy, and also provided us with criteria for E-W tie-line spacing. A periodic error curve, to which a straight The expression q(t,t,) can easily be found line is fitted such that it coincides with the by rotating from the X'Y' system to the XY error curve at two points, is shown in Figure 6. system, namely: PHOTOGRAMMETRIC ENGINEERING, 1974

TABLE3. EXPECTEDCONTRIBUTION TO ERMSES FOR POINTSON N-S STRIP FITTEDBETWEEN E-W TIELINES DUE TO SCHULERVARIATION (Flight 33)

Contribution to ERMSE

Distance Between Tie Lines Flight Time Distance (min. ofarc) (meters} (see) (km) Lat. Long. Lat. Long.

compilation methods. This team had to un- derstand the project instrumentation, the The integration was perfornled numeri- contractual requirements and the relevant cally using the fitted coefficients of Table 1 SLAR geometry, as well as the navigational for the coefficients of s(t). The results are methods used. presented in Table 3. Special problems arose with respect to From Table 3 we can see that the contribu- scale estimation, lnosaic compilation tech- tion to the ~~~s~increasesexponentially with niques and mosaic accuracy evaluation. Solu- tions to these problems were discussed and the tie line spacing. For Project RADAM the tie line spacing varied from 200-300 km. Com- methods used were outlined. In particular, bining the 1,500-sec flight time estimates an evaluation of the behavior of the inertial guidance platform LTN-51 was made and the (211 and 362 m) with the ERMSE values from Table 2 (upon interchanging latitude and results were used to specify a modified longitude as the values apply to tie lines) we ground control system, relying on SHORAN obtain values of 450 m and 550 m for latitude controlled E-W tie lines. Accuracy estimates and longitude, respectively, as preliminary for both the full and partial control systems ERMSE estimates for areas with reduced SHORAN were made (a200-300 m, and + 500-600 m, control. To obtain more realistic estimates, respectively). several other minor error sources would have Our preliminary conclusion is that, cer- to be considered but it seems that the 1 km tainly for the fully controlled area, the mosaic requirement would not be violated. accuracy will be well within specifications. To test the accuracy of the tie-line control- For the tie line controlled areas this is proba- led areas, the Brazilian author of this paper bly true also, but additional study is required conducted an additional test in which 46 to produce estimates based on more exten- points were measured, some of which were sive samples. established with the TRANSIT satellite locating In general, however, we can conclude that equipment. The results of the test are RMSES of the initial requirement, namely, to produce a 310 and 393 meters in Easting and Northing, set of reconnaissance maps with some respectively, a slightly better result than ex- geometric consistency, has been more than pected from the above considerations. satisfied.

1. de Azevedo, L. H. A., "Radar in the Amazon," Project RADAM is a project of the Brazilian proceedings of the seventh ~~~~~~~~i~~~l Ministry of Mines and Energy. It was estab- Symposium on Remote Sensing of Environ- lished with the objective to map the Amazon ment, Vol. 111, pp. 2303-2306, May 1971. and the Brazilian North-East with side- 2. da Rocha, G. A., A Cadografia Brasileira e a looking radar. Geodesia por Satellite&,Services Aerofotog- A technical support team was formed to rammetricos Cruzeiro do Sul S. A., Rio de monitor image quality and to advise on the Janeiro GB, July 1971. implementation of semi-controlled mosaic 3. Leberl, F., "Metric Properties ofImagery Pro- SLAR MOSAICS FOR PROJECT RADAM

duced by SLAR and IRLS," Proceedings of the the Netherlands. September 1970. ISP Commission IV Symposium, Publications 6. Broxmeier, C., Inertiul Nuvigution Systems, + of the ITC, Delft, the Netherlands, September McGraw-Hill Electronic Sciences Series, 1970. McGraw-Hill. 1964. 4. Rosenfield, G. H., "Stereo Radar Tech- 7. van Roessel, J. W., "The Geometric Evalua- niques," Photogrummetric Engineering, 34:6, tion of Radar Mosaic$," unpul~lishedmanu- 587-594. 1968. script, 1972. 5. Konecny, "Metric Problems in Remote Sens- 8. Wolf, P. R., "Trilaterated Photo Coordinates," ing," Proceedings of the ISP Commission IV Photogrammetric Engineering, 35:7,543-547. Symposium, Publications of the ITC, Delft, 1969.

New Sustaining Members

Aero-Metric Engineering, Inc.

ERO-METRICENGINEERING, INC.,was founded 1130 System, and Auto-Trol Digitizer. A in 1969 to provide complete photogram- Photo laboratory production utilizes the metric services to the engineering profes- Durst V-184 Color Enlarger and Brown Copy sion. In the first two years, production was Camera. Aero-Metric performs all mapping mainly concentrated on producing large- utilizing color photography. The LogEtronic scale topographic maps. The corporation has Printer and Kodak Color Enlarger, in addi- grown to its present staff of 38 professional tion the LogEtronic Strip Printer, round out and technical photogrammetric personnel. the photo lab equipment. Aero-Metric Engineering currently has its Aero-Metric Engineering also performs operations and production in Sheboygan, land surveying and sub-division planning in Wisconsin in a completely new and modern addition to photogralnnietric control utilizing plant and office building. The Photogram- the latest in survey instruments and E.D.M. metric Engineering Department is equipped equipment. with first-order Stereometrographs, second- The area of operations has increased to order Topocarts and Jena Orthophot Sys- cover most parts of the with a tems. In addition, we use the Wild PUG point heavy emphasis 011 State and Federal Map- transfer device. Data processing and compu- ping Programs. tations are performed with an in-house IBM

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ICOMED CORPORATIONmanufactures a com- verting fill11 images to computer mag-tape Dplete line of digita 1 equipment for use in and also for converting mag-tape to color or computer-based image processing systems. black-&-white film images. The product line includes digitizers for con- In addition to its standard product line, verting photographic film images to DICOMEDdevelops and supplies specialized computer-compatible images, and digital digital image peripherals to original equip- film recorders for converting digital image ment manufacturers of various systems. data to high-quality color (and DIC~MEDmaintains a well-staffed Cus- black-&-white) film images. Other products tomer Service Department for close support include digital tape units, computer- in field nlaintenance and, in addition, offers interfaces and channel expanders to facilitate training courses in the operation and routine the interconnection of DIC~MEDperipherals maintenance ofits standard equipment. Dem- with a wide variety of computers and to pro- onstrations of standard peripherals are vide modular flexibility in system configura- conducted by appointmetlt at the Corporate tions. DICOMEDalso maintains a well- headquarters in Minneapolis, Minnesota. equipped Data Services operation for con-

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