SECOND INTERNATIONAL SYMPOSIUM ON OPERATIONALIZATION OF REMOTE SENSING, AUGUST 1999, ITC ENSCHEDE, NETHERLANDS

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1 SECOND INTERNATIONAL SYMPOSIUM ON OPERATIONALIZATION OF REMOTE SENSING, AUGUST 1999, ITC ENSCHEDE, NETHERLANDS STABILIZED AND DIRECTLY GEOREFERENCED IMAGERY BASED ON INS/(D)GPS Prof. Dr. Manfred Bäumker Dipl.-Ing. Rainer Brechtken Prof. Dr. Franz-Josef Heimes University of Applied Sciences Lennershofstraße 140 D Bochum Dipl.-Ing. Torsten Richter UC Umweltconsulting Werksstraße 5 D Hattingen Abstract A high precision stabilized platform has been developed at the University of Applied Sciences, Bochum. The platform is designed to carry different types of remote sensing devices - with the exception of large format mapping cameras. The system is able to provide precisely stabilized imagery even for low flying light aircrafts under turbulent air conditions. Residual image inclinations and yaw deviations will be recorded. Together with the data of the (D)GPS augmented inertial navigation system (INS) the complete exterior orientation data are delivered. The GPS receivers used are the single frequency receivers LEICA MX 9400 providing an accuracy of about 0.3 m. GPS positioning is based on very precise code phase measurement with the advantage that the ambiguity resolution problem does not exist and thus operation is possible up to several hundred kilometers far from the GPS ground reference station. The inertial navigation system is the low cost strapdown INS LCR 88 from LITEF, Germany. In the GPS augmented mode the inertial navigation system (INS) delivers the phi and omega angles with an accuracy of 0.02 and the kappa angle with an accuracy of 0.1. The perfect and directly georeferenced (i.e. without ground control points) images are very well suited for aerial monitoring in the fields of bodiversity, forest-clear-cut, afforestation, coastal zones, soil-erosion or for planning and control of pipelines, and for investigations and help in case of natural- and man-induced hazards. Digital cameras like the Kodak DCS 420 and DCS 460 have been applied up to now. Tests with a digital video camera from Sony are in preparation. On the basis of this imagery rapid generation of DEM s and production of digital orthophotos is possible as well as vector data capturing in stereo mode for GIS. Also high resolution digital mosaics of georeferenced imagery may be directly superimposed with satellite imagery for ground truthing purposes. At the same time the system is very well suited for local photogrammetric surveys with medium or small format cameras. At present adapters for the Rollei 6006 metric camera or the Contax RTS III (with film fattening and fiducial marks) are available.

2 1.0 INTRODUCTION Since the Global Positioning System (GPS) is fully operational GPS has been established as a standard guidance and navigation system for numerous applications on aircrafts, land vehicles and ships. Considerable improvements with respect to the standard positioning service of the GPS based upon C/A code receivers can be achieved by applying the differential GPS technique (DGPS) or the latest milestone: the ambiguity resolution on the fly (OTF). All GPS methods are suffering under the loss of the satellite signals due to obstacles and manoeuvres of the vehicle, or other disturbances are influencing considerably especially the successful application of the OTF method. Furtheron high precision navigation applications, especially for remote sensing applications, require accurate lever arm corrections to transfer the position of the antenna phase centre to the desired site. But lever arm corrections depend on the availability of attitude (roll, pitch) and heading of the vehicle which cannot be delivered by the GPS. If the velocity vector have to be transformed additionally rates are needed. For remote sensing applications without ground control the angles phi, omega and kappa are additionally required. Furtheron, to get perfect images a stabilized platform for the sensor is needed. All of these requirements can be ideally solved with a (D)GPS augmented inertial navigation system delivering all the data to control a stabilized platform with camera and a flight management system for the shutter release. 2.0 THE SYSTEM CONCEPT Since several years a lot of effort has been spent at the University of Applied Sciences Bochum (in cooperation with the UC - Umweltconsulting) to develop an aerial survey system on the basis of small format aerial photography (Heimes et al. 1992a, 1992b, 1993, 1994, 1996, 1997, 1999). The basic configuration of a first prototype system consisted of a GPS receiver, stabilized mount and a notebook computer. The aircraft's attitude data for camera stabilization were derived from standard gyro instruments (vertical artificial horizon gyro and directional gyro) with synchro outputs. The converted digital signals were used as inputs for the control loops. Practical trials have shown that the stabilization was too slow due to the signal processing and that the performance of the cardanic type of mount was not satisfactory. Therefore, a completely new system was developed.; the mechanical components (sensor mount) were constructed and built in cooperation with the University of Applied Sciences Cologne (Prof. Dr.-Ing. H. Gartung) and the company IBF (Ingenieur-büro Freudenberg). The new high performance system is especially designed for light aircrafts. Different medium- and small-format cameras or other remote sensing sensors can be used. At present adapters are available for the following cameras: Rollei 6006 metric, Contax RTS III (with film flattening and fiducial marks), the digital cameras Kodak DCS 420 or DCS 460 and the Sony digital video camera. The new system (see figure 1) consists of following components:

3 Fig. 1: Stabilized camera platform: precise three-point suspension, three highly dynamic servo motors Figure 2: Components of the hybrid system a standard strapdown attitude and heading reference system according to ARINC 705 (LCR 88 from LITEF, Germany) a 12 channel single frequency C/A-Code GPS receiver LEICA 9400 used as reference station a 12 channel single frequency C/A-Code GPS receiver LEICA 9400 used as rover a data link (telemetry, 433 Mhz, at least 1200 Bd). This is optional. a processing unit (486 PC) with a smart ARINC 429 interface board and a multiple serial and digital IO-card a mount control unit a stabilized sensor platform with camera a flight management system (FMS)

4 The strapdown INS LCR 88 incorporates two mechanical dry tuned gyros (DTG) and three pendulum accelerometers. Their typical accuracies are listed in table 1. The original 50 Hz inertial measurements (rates and accelerations) are acquired via a smart ARINC 429 board by the processing unit and then combined in a Kalman Filter with the measurements of a (D)GPS using two single frequency C/A code receivers LEICA 9400 as reference and rover. To use the raw data of the reference receiver in the optional DGPS mode additionally a standard data link is required. The synchronization of the inertial and GPS data is managed with the help of the PPS-signal of the rover receiver and the internal clock of the ARINC 429 board. The system is able to perform a self alignment within two minutes and can bridge GPS data gaps during severals minutes. In the GPS augmented mode position accuracies of 0.3 m can be achieved. Roll and pitch angles can be measured with an accuracy of 0.02 and heading with 0.1. Details of the system, its mechanization and the applied Kalman Filter are given in (Bäumker 1995a, b). Gyro accelerometer Drift / bias 0.3 /h 0.5 mg Scale factor error 1000 ppm 1000 ppm noise 0.05 /Öh 10 mg Table 1: Accuracies of the inertial sensors The processing unit (PC) calculates and outputs with a frequency of 50 Hz all the data (position, velocity, attitude angles, heading, track angle, body rates, body accelerations) which are required to drive the flight management system and to control the stabilized sensor platform by the mount control unit. The flight management system is responsible for the shutter release of the camera and delivers the flight line information for the display of the pilot. The mount control unit controls the stabilized sensor platform and determines the control residuals (residual inclinations and yaw deviations) which are fed back to the processing unit. In case of the shutter release the instant residuals are stored togehter with the actual position, velocity, rates, heading, roll and pitch angles on the hard disk of the processing unit. The stabilized platform consits of a stable three-point suspension and three highly dynamic servo motors. Angular motions of the aircraft measured by the inertial platform and the calculated attitude and heading angles are processed by the mount control unit to stabilize the platform on which the camera or other types of sensing devices can be mounted. This stabilization ensures perfect images for numerous applications even for low flying light aircrafts under turbulent air conditions. Due to the continous feedback of the control residuals and of the shutter release signal of the camera to the processor unit the complete exterior orientation data are immediately present at the time of exposure without any ground control. 3.0 SYSTEM INSTALLATIONS Several flight missions have been carried out up to now. The system has been installed in a Cessna 172 Rocket, for which disadvantagely no camera hatch is available. Figure 3 shows how the stabilized mount was fixed outside the luggage compartment. The authors wish to thank LSV Freudenberg for the support given. - Due to low dimensions and low weight the complete system can be installed in nearly every light aircraft.

5 Figure 3: System installed in a Cessna 172 Rocket Figure 4 demonstrates how the system fits into the pod of the new light surveillance aircraft S15 under construction by Stemme aviation, Strausberg near Berlin, Germany. Figure 4: Installation of the system in a pod of the light surveillance aircraft Stemme S15 Another example (Figure 5) shows the high performance two-seater Pulsar XP, an experimental aircraft being under construction since four years by one of the authors - ready to fly within a couple of weeks. This plane is equipped with an autopilot S-TEC 50. The system integration will be realized in coorperation with the manufacturer in such a way that a survey flight line pattern can be flown completely automatic. Figure 5: Pulsar XP, a high performance two-seater experimental plane - under modification for system installation. On the right: two floor hatches, two seat back hatches, autopilot roll- and pitch servos 4.0 SYSTEM TESTS AND TEST RESULTS

6 4.1 Testflight University Bochum Figure 6 shows the photo index of the test flight University Bochum. For this area about 60 precisely determined ground control points are available. Five runs, seven photographs each, 60% forward overlap, 30% sidelap, photoscale 1:25000, were planned and flown. The camera used was the (metric) Contax RTS III with f = 28 mm resulting in a flight height of 700 m. The flight was carried out under rough air conditions, no clouds. The photo index and the actual flight track plotted together with the actual photo positions (s. Figure 7) demonstrate the perfect execution of this survey flight. Test Area University Bochum (2,2 Km x 2,2 Km) Camera: RTS III with Filmflattening and Fiducial marks Format: 24mm x 36mm Focal Length: 28mm Flight Height: 700m Photo Scale: 1 : Overlap: 60% longitudinal, 30% lateral Number of Flight Lines: 5 Number of Photos: 5 x 7 = 35 Number of Control Points: 60 Figure 6: Photoindex of Test Flight University Bochum Figure 7: Flight Track with Photo Positions, Plotted According to (D)GPS Augmented INS-Coordinates Coordinates of perspective centres and control residuals (residual inclinations and yaw deviations) are calculated for each camera station (s. Table 2). Control Residuals Photo Latitude Longitude Height Roll Pitch Yaw Table 2: Sample Data: Coordinates of Perspective Centres and Control Residuals In figure 8 are presented the roll-, pitch- and yaw data as well as the angular rates - as an example for run number one. All data are calculated and recorded with a frequency of 50 Hz.

7 The data demonstrate the high performance requirements which have to be fulfilled by a stabilization system to be applied in a light aircraft under turbulent air conditions. Results show that yaw is compensated better than roll and pitch in spite of the fact that the accuracy of the roll and pitch angles delivered by the inertial system are better than heading (yaw). This is due to the regular flight dynamics of the aircraft because yaw rates are much slower than those for roll and pitch. The control residuals recorded by the system are below 0.15 (yaw), 0.20 (pitch), and 0.5 (roll) resulting in perfect vertical images wtih common scaling. Figure 8: Heading- and Attitude Data as well as Angular Rates for Line Number one. ( Flight Time appr. one minute, 50 Hz-Data) Evaluation of photos was carried out within a diploma thesis (Annette Hohaus). Different bundle adjustments were calculated for different control point configurations. One bundle adjustment was calculated without any ground control point. For the camera feedback signal a systematic delay-error of 130 ms was detected. The (D)GPS/INS coordinates were corrected for this effect. σ X (cm) σ Y (cm) σ Z (cm) Number of Control Points Adjustment with Coordinates of Perspective Centres no yes no control points yes Table 3: Standard Deviations of Tie Points for Bundle Adjustments with Different Control Point Configurations (Small Format 1: ) The results for bundle adjustment without any control point are better than expected. The angular values kappa, phi and omega which were also determined in-flight could not be introduced into the bundle adjustment because of the mentioned synchronization error (delay of camera s feedback signal). 4.2 Project Herborn

8 Figure 9 shows the photoindex of project Herborn. In total the area was flown four times (twice with 50 mm and twice with 80 mm lens, photo scale in all cases 1:5.000). Flight heights were very low: 250 m and 400 m resp. In spite of the turbulent air conditions all photo coverages are perfect. Project Herborn (600m x 600m) Aerial Photography of the Historical City: Data Capturing for 3D-Modelling, 3D-Visualization (Computeranimation) Camera: Rollei 6006 metric Format: 55mm x 55mm Focal Length: 80mm Flight Height: 400m Photo Scale: 1 : Overlap: 60% longitudinal, 30% lateral Number of Flight Lines: 3 Number of Photos: 3 x 5 = 15 Figure 9: Photoindex Project Herborn For the area Herborn 18 control points have been determined (by means of GPS). Lothar Vagedes investigated within his diploma work the performance of the INS/(D)GPS system. The elements of exterior orientation determined in-flight could be compared with those derived from a bundle block adjustment based on 18 control points (for angular conversions see chapter 4.3). The following accuracies were calculated for the elements of exterior orientation determined in-flight by means of INS/(D)GPS: σ X,Y,Z = 0,5 m; σ κ,ϕ,ω = 0,1 o In the following table standard deviations for tie point coordinates as calculated from bundle adjustments with different control point configurations are listed: σ X (cm) σ Y (cm) σ Z (cm) Number of Control Points Adjustment with Coordinates of Perspective Centres No Yes no control points Yes Table 4: Standard Deviations of Tie Point Coordinates after Bundle Block Adjustments with Different Control Point Configurations, Rollei 6006, f = 80 mm, M b = 1 :5.000 The standard deviations as calculated from the bundle block adjustment without any control point are nearly the same as for the test flight University Bochum (Contax RTS III, photo scale 1:25.000). This means that the accuracy of the coordinates of perspective centres as measured in-flight is determining here the final accuracy of the object points. - The angular values kappa, phi and omega as measured in-flight were not taken into account in the bundle adjustments. Unfortunately accuracy is lost because of the mechanics of the stabilized mount. Meanwhile the system is in the stage to be modified in such a way that the INS will be fixed with the camera. Thus the INS is used for electronic control of stabilization as well as for direct measurement of the control residuals.

9 4.3 Mosaic University Bochum The mosaic below was calculated from 25 digital images taken from a flight height of 1125 m with the Kodak DCS-420 (appr x 1500 Pixels). This kind of mosaic can be directly geo-referenced. - A major US governmental organization is going to apply the system described in this paper to produce directly georefenced mosaics (orthophotos) for superimposition on satellite imagery. The purpose is ground truthing for biological monitoring. The mosaic is calculated from a coverage with 60 % longitudinal and 30 % lateral overlap. - Besides this the imagery and the elements of exterior orientation as determined in-flight also have been investigated. This was done within diploma work at the Institute of Photogrammetry, Uni-versity Hannover, supervised by Dr. K. Jacobson. As reference first of all a bundle block adjustment was carried out based on 29 horizontal and 27 vertical ground control points, resulting in σ 0 = 7.7 μm and mean square corrections of object points σ X,Y = 0.65 m and σ Z = 3.7 m the accuracy of Z-coordinates being so much worse because of the low base to height ratio of 1 : 7. For determination of the installation off-sets (difference between INS reference frame and image coordinate system) the photogrammetric angles phi, omega and kappa from the bundle adjustment were converted into the INS system related to the aircraft body with the angles pitch, roll and yaw software Dr. Jacobson, integrated in his bundle adjustment system BLUH. The determination of the installation off-sets appeared to be a problem because due to the small camera angle (f = 18 mm, chip size = appr. 9 mm x 14 mm) there exist very strong

10 correlations between the image inclinations phi, omega and the camera stations. As a solution the off-sets were determined in two steps. The yaw off-set was calculated on the basis of the reference bundle adjustment - low correlations for kappa. The pitch and roll off-sets were derived from longitudinal and lateral deviations at ground control points (coordinates of ground points from bundle adjustment and coordinates calculated by photogrammetric intersection from image coordinates and elements of exterior orientation determined in-flight - kappa corrected for installation off-set, first step). The angles pitch, roll and yaw as determined in-flight by the INS now could be corrected for installation off-sets. After this the conversion to the photogrammetric system with the angles phi, omega and kappa was performed and ground coordinates for appr. 80 points were calculated from image coordinates and corrected elements of exterior orientation determined in-flight. These ground coordinates could be compared with the ground coordinates as determined from bundle adjustment. The square mean of differences is appr. σ x,y = 4 m, σ Z = 14 m (base to height ratio = 1 : 7). From this result a conclusion may be drawn as compared to the project Herborn (s. 4.2). Very much accuracy is lost because of the present set-up of the system: the INS fixed to the aircraft`s frame providing the reference for the electronic control of the camera mount (stabilization), control residuals being derived from encoders attached to the axis of the servo motors. Very soon the INS will be fixed to the camera to be used as reference for stabilization control and for direct measurement of the control residuals. The new INS with FOG (fibre optical gyros) and micro accelerometers from Litef is very suitable for this and will be available very soon. 4.4 Project Road Data Base In September 1998 the system was installed in a survey aircraft of Hansa Luftbild - German Air Survey. In a pilot project for setting up a road data base aerial image data were collected for 80 large road intersections. The camera used was the digital camera Kodak DCS-460 (appr x 3000 pixels). At a flight height of 840 m and with a focal length of 24 mm this resulted in a photo scale of 1: Many intersections could be covered by only one stereo modell. For most of the intersections three up to six images were to be taken. But with the Kodak DCS-460 only the first two images of a series can be taken with a short time interval, after that the time interval between two images is appr. 8 seconds. Because of this fact most of the short flight lines had to be flown twice. Figure 11 shows the complete flight track. Of course the survey navigation and camera release were performed by means of computer supported flight management. Figure 10: System Installed in a Survey Aircraft of Hansa Luftbild German Air Survey

11 Figure10: Images Taken with the Digital Camera KODAK DCS-460 (appr x 3000 Pixels), Flight Height = 840 m, Focal Length = 24 mm, Image Scale = 1: Two up to six images were to be taken per road intersection. Figure 11: Complete Flight Track to Take Digital Images of 80 Road Intersections. Most of the Intersections had to be Flown twice because of the Problem of Time Intervall between two Images.

12 For this project it was specified to produce digital orthophotos per road intersection as well as vector data (axis of roads and road lanes). The required accuracy was σ X,Y,Z = 1m. Figure 12: Digital Orthophoto of one Road Intersection with Superimposed Vektor Data (Road Axis and Axis of Road Lanes). Within this pilot project there are existing topographic maps in scale 1: From these maps ground control points can be obtained for the purpose of orientation of images. For the future it is intended to carry out the photogrammetric work with a minimum number of ground control points, most desirable without any control point: Direct Georeferencing without Ground Control! With the image- and flight-data available now testing has started to find out how much the number of control points can be reduced if the elements of exterior orientation determined inflight are used in the photogrammetric orientation process. First results demonstrate, that an absolute accuracy of σ X,Y,Z = 2 m is obtained without ground control. With only one ground control point the accuracy specifications (σ X,Y,Z = 1 m) most probably can be fulfilled. Very important of course is a well calibrated camera (camera constant, principal point, radialsymmetrical and decentering lens distortion, orthogonality and affinity), s. Shortis, Robson, Beyer, 1998.

13 6.0 CONCLUSIONS The lighter the aircraft (and the lower the flight height) the higher the requirements on the stabilization of the photogrammetric camera or other remote sensing devices. To fulfil these requirements a stabilized platform has been developed and improved in the last years at the University of Applied Sciences Bochum. The system consists of a stable three-point servo-driven mount controlled by the data of a (D)GPS augmented Inertial Navigation System (INS, LCR 88 from LITEF). The GPS receivers used as reference and rover are the single frequency receivers LEICA 9400 providing a position accuracy of appr. 0.3 m. Besides the position the system delivers the phi, omega and kappa angles which are derived from the measurements of the INS and the control residuals (inclinations and yaw deviations). Thus the system provides stabilized and fully georefenced imagery - elements of exterior orientation are determined in-flight. The perfect images allow aerial monitoring (biodiversity-, forest-clear-cut-, afforestation-, coastal-, soilerosion-, natural-desaster-, and other monitoring applications) without ground control. Furtheron the system is well suited for local surveys with medium format cameras and digital cameras (e.g. Kodak DCS-460). A PC based digital photogrammetric workstation with stereo capability will especially be adapted for GIS data capture (cooperation with GEODELTA, Delft, Netherlands). The adaptation of a digital video camera for monitoring purposes (e.g. oil pipe lines, gas pipe lines, shore lines etc.) is in preparation. Also high resolution digital mosaics of georefenced imagery may be directly superimposed with satellite imagery for ground truthing purposes. 6.0 REFERENCES M. Bäumker, "DGPS-gestütztes Fahrzeugnavigationssystem mit faseroptischen Kreiseln", Journal for Satellite-Based Positioning, Navigation and Communication, Vol. 4/95, pp , December 1995a. M. Bäumker, "Basiswissen Inertial- und Sensortechnik", Journal for Satellite-Based Positioning, Navigation and Communication, Vol. 4/95, pp , December 1995b. Heimes, Bretchken, Puruckherr, "Computer Controlled Survey Flight Based on a Low Cost GPS C/A Code Receiver", Photogrammetric Record, Vol. 14 (80), pp , 1992a. Heimes, Balve, Brandenburg, Hülsebusch, Ludwig, Lorenz, "Hochauflösende Photographie mit der Contax RTS III - Meßkammer mit Filmansaugung", Proceedings ISPRS Congress Washington, 1992b. Heimes, Poole, Brechtken, Puruckherr, "LEO - Local Earth Observation", Proceedings International Symposium Operationalization of Remote Sensing", Heimes, Bäumker, Brechtken, Richter, "Photogrammetric Data Acquisition from Light Aircraft on the Basis of the High performance Camera Contax RTS III", GIM, 8/94 pp , August Graham, Read, Warner, "Small Format Aerial Photography", Whittles Publishing, Caithness, Graham, Digital Imaging", Whittless Publishing, Caithness, 1997

14 Bäumker, Brechtken, Heimes, Richter, High-Precision Stabilized Sensor-Platform Based on INS/(D)GPS, International Airborne Remote Sensing Conference and Exhibition, Copenhagen, 1997 Bäumker, Brechtken, Heimes, Richter, Practical Experiences with a High-Precision Stabilized Camera Platform Based on INS/(D)GPS, The First North American Symposium on Small Format Aerial Photography, 1997 Shortis, Robson, Beyer, Principal Point Behaviour and Calibration Parameter Models for Kodak DCS Cameras, Photogrammetric Record, 15(92),1998 Bäumker, Brechtken, Heimes, Richter, Hochgenaue Stabilisierung einer Sensorplattform mit den Daten eines (D)GPS-gestützten Inertialsystems, ZPF Zeitschrift für Photogrammetrie und Fernerkundung, 1/1998 Bäumker, Brechtken, Heimes, Richter, Direkte Georeferenzierung mit dem Luftaufnahmesystem LEO, X. Internationale Geodätische Woche Obergurgl (Universität Innsbruck), 1999