1. Civil and Environmental Engineering and Geodetic Science Airborne Integrated Mapping System (AIMS  ) and Land-based Applications of GPS/INS/CCD Sensor Assembly Dorota A. Grejner-Brzezinska Civil and Environmental Engineering and Geodetic Science The Ohio State University 470 Hitchcock Hall Columbus, OH 43210 Tel. (614) 292-8787 E-mail: dorota@cfm.ohio-state.edu OSU

2. Civil and Environmental Engineering and Geodetic Science Presentation outline   AIMS: the concept   GPS/INS integration in AIMS INS component GPS component Imaging component Performance analysis   Land-based applications

3. Civil and Environmental Engineering and Geodetic Science Airborne Integrated Mapping System (AIMS™)   Fully digital data acquisition system for large-scale mapping and other precise positioning applications   Designed for airborne applications   Incorporates state-of-the-art GPS/INS positioning and digital imaging technologies.

4. Civil and Environmental Engineering and Geodetic Science   Theoretical foundation and expertise build-up at The Center for Mapping – –GPSVan (first Mobile Mapping System) – –Real-time cm-level positioning (Construction industry)   Technology component from leading manufactures – –GPS (Trimble) – –INS (Litton) – –CCD (Lockheed Martin Fairchild)   Commercialization through industry partners AIMS ™ Resources

5. Civil and Environmental Engineering and Geodetic Science AIMS™ Positioning Module Objectives   Direct Platform Orientation (DPO) by GPS/INS integration   Tightly coupled GPS/INS integration (single Kalman filter optimally estimates errors in position, velocity, attitude, as well as errors in GPS and INS)   Post-processing mode   4-7 cm position, ~10 arcsec orientation over long baselines   Real-time operation capability (future)

6. Civil and Environmental Engineering and Geodetic Science   1 Hz GPS data supports estimation of the INS errors used to calibrate the INS on a continuous basis   Enables precise positioning when number of satellites drops below four   High accuracy of the INS system over short periods of time enables the correction of cycle-slips and losses of lock affecting GPS phases   Makes it possible to perform fast and robust OTF ambiguity resolution based on the precise positioning obtained from the integrated filter Why Tight Integration?

7. Civil and Environmental Engineering and Geodetic Science Why Tight Integration?   Limitations: Complex and hard to build Tough timing requirements (precise, high-rate data exchange and exact synchronization of subsystems)   Advantages: Superior performance as compared to loose integration More robust and jam-resistant Optimal Kalman filtering GPS tracking loop aiding Can use low cost INS/IMU

8. Civil and Environmental Engineering and Geodetic Science   System design and implementation   GPS/INS component   Imaging component   GPS ambiguity resolution for GPS/INS integration   Kalman filter design AIMS™ Architecture

9. Civil and Environmental Engineering and Geodetic Science AIMS Positioning Component   Tightly coupled GPS/INS   Target accuracy: 4-7 cm position, ~10 arcsec orientation over long baselines   Trimble 4000SSI dual-frequency GPS receivers   Litton LN-100 Strapdown Inertial Navigation System Zero-lock TM Laser Gyro (ZLG TM ) A-4 accelerometer triad 0.8 nmi/h CEP, gyro bias – 0.003  /h, accelerometer bias – 25  g

10. Civil and Environmental Engineering and Geodetic Science GPS Component   Trimble 4000SSI dual-frequency GPS receivers   Double-differenced L1 phases (optionally L2)   Initial ambiguity resolution On-The-Fly (OTF)   INS-aided OTF ambiguity resolution   Stochastic modeling of the DD ionospheric effect (random walk)   1Hz data acquisition rate   Timing provided by GPS time and   1pps signal supports GPS and INS clocks synchronization

11. Civil and Environmental Engineering and Geodetic Science   Litton LN-100 Inertial Navigation System – –Modified firmware for AIMS – –IMU data are collected   Data Collection – –IMU output data collected at 400 Hz – –INS internal timer is synchronized to GPS time – –Time-tagging INS data using both GPS and INS time – –Navigation solution provided at 256 Hz INS Component

12. Civil and Environmental Engineering and Geodetic Science AIMS Imaging Component   Digital frame camera – –based on Lockheed Martin Failrchild 4K  4K CCD sensor – – 60mm by 60mm imaging area (15-micron pixel size) – –Hasselblad 553 ELX camera body with Zeiss lens – –6 second image acquisition rate   Fully Digital Airborne Data Acquisition System – –direct connection to softcopy systems – –corridor and other large-scale mapping applications   Long-term objective – –Create a 9K by 9K advanced AIMS digital camera

13. Civil and Environmental Engineering and Geodetic Science BigShot /Hasselblad Camera

14. Civil and Environmental Engineering and Geodetic Science Digital Camera/INS Mount

15. Civil and Environmental Engineering and Geodetic Science Image Acquisition Control and Storage Image Acquisition Control and Storage Strapdown Navigation Solution Tightly Coupled GPS/INS Kalman Filter Strapdown Navigation Solution Tightly Coupled GPS/INS Kalman Filter User Interface, Control & Display Unit User Interface, Control & Display Unit 4K by 4K BigShot  Digital Camera 4K by 4K BigShot  Digital Camera Rover GPS Station Rover GPS Station LN-100 Base GPS Station Base GPS Station L1 and L2 phase observable Delta V Delta  L1 and L2 Image Data Exposure Control Time Tag/Sync EO Data Host/Slave Communication Control Signal Optimal Position, Velocity, Attitude Estimates AIMS™ Prototype Architecture

16. Civil and Environmental Engineering and Geodetic Science GPS AntennaImaging PC GPS Base Station BigShot™ Hasselblad Camera Trimble 4000SSI LN-100 INS/GPS PC AIMS™ Hardware Configuration

17. Civil and Environmental Engineering and Geodetic Science Tight GPS/INS Integration Processing Flowchart Processing Flowchart Stop Cycle-Slip Detection Compute INS DD KF Measurement Update time > t_end Any Cycle-Slips? Integer Ambiguity Search Use INS Position to Fix Cycle-Slip Is INS Good Enough to Fix It? Start Get Processing Control Parameters KF Time-Update INS Computation GPS DD Available? Next GPS Measurement Time? Yes No Yes No Yes No (no valid DD) Initialize the Kalman Filter Initialize INS Computation Module Compute GPS DD No Is the Search Successful? No Yes No GPS data (1 Hz) IMU data (256/64 Hz)

18. Civil and Environmental Engineering and Geodetic Science   Short baseline between the rover and the base assumed   Simultaneous estimation of widelane and geometry-free ambiguities using combinations of four independent GPS code and phase observations for four primary satellites   Implementation of the least-squares ambiguity search, based on the estimated widelane (N 1 - N 2 ) and geometry free (N 1 - 77/60 N 2 ) ambiguities to resolve N 1 and N 2   Positions computed based on fixed ambiguities are used to initialize the GPS/INS filter Initial Ambiguity Resolution

19. Civil and Environmental Engineering and Geodetic Science 99% level, 80 samples, 1 sec = 1epoch Long baseline ~16 km Short baseline < 1.5 km Speed of Ambiguity Resolution

20. Civil and Environmental Engineering and Geodetic Science OTF Ambiguity Resolution   Tight GPS/INS integration enables fast and reliable correction of cycle-slips and losses of lock OTF for long baselines.   As long as the level of DD residuals is below 0.3 1, no cycle- slips are assumed.   Cycle-slips are fixed based on the known rover position provided by the filter updated by valid DD.   In case of total loss of lock the search loop is activated for 4 highest satellites.   The standard deviation of the ambiguity candidate is obtained from the covariance matrix of the predicted positions and ionospheric estimates. It is subsequently used to build the ambiguity search interval.

21. Civil and Environmental Engineering and Geodetic Science Compute DD using satellite coordinates and INS-predicted position and ionosphere,  Compute DD using satellite coordinates and INS-predicted position and ionosphere,  Form DD using measured phase  Form DD using measured phase  Check geometry-free and widelane conditions Check geometry-free and widelane conditions cycle slip ? Compute  =  p -  o Compute  =  p -  o Search First Guess   tol. Yes No Kalman Filer Update Kalman Filer Update success ? Yes Use only satellites with fixed ambiguities Use only satellites with fixed ambiguities No INS-aided OTF Ambiguity Resolution

22. Civil and Environmental Engineering and Geodetic Science   Error states chosen (as opposed to whole-value filter states)   INS psi-angle error model adopted AIMS GPS/INS Kalman Filter Design where  v,  r, and  are the velocity, position, and attitude error vectors respectively;  is the accelerometer error vector;  g is the error in the computed gravity vector; and finally,  is the gyro drift vector.   Observations: double differenced L1/L2 phase measurements

23. Civil and Environmental Engineering and Geodetic Science   The complexity of the INS error model depends on the error models for IMU sensor measurement errors, as well as the gravity uncertainty. For example, a 24-state model may include the following variables: Subscript b stands for bias Subscript f stands for scaling factor AIMS GPS/INS Kalman Filter Design

24. Civil and Environmental Engineering and Geodetic Science Where X Nav, X Acc, X Gyro, X Grav, X Ant and X GPS are, respectively, the error vectors of the inertial navigation solution (position, velocity, attitude), the accelerometer measurement errors (bias, scaling factor), the gyro measurement error (drift; may include scaling factor and bias), the gravity anomaly and deflections errors, the antenna lever arm errors, and the GPS ionospheric errors; w Nav, w Acc, w Gyro, w Grav, w GPS and are all zero-mean Gaussian white noise vectors.  State equations based on the chosen error model AIMS GPS/INS Kalman Filter Design

25. Civil and Environmental Engineering and Geodetic Science AIMS GPS/INS Kalman Filter States

26. Civil and Environmental Engineering and Geodetic Science AIMS  Performance Analysis   Level of double-difference residuals   Covariance matrix analysis   Comparison with traditional aerotriangulation   Ground truth test

27. Civil and Environmental Engineering and Geodetic Science Time [sec] 0100200300400500 600 700 -0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02 DD Residuals [m] Double Difference Residuals ~ 10 km baseline

28. Civil and Environmental Engineering and Geodetic Science Double Difference Residuals ~ 180 km baseline 050010001500200025003000 -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 Time [sec] DD Residuals [m] ~180 km Back to base

29. Civil and Environmental Engineering and Geodetic Science L1 Double Difference Residuals ~ 350 km baseline Back to base

30. Civil and Environmental Engineering and Geodetic Science Position Estimation Accuracy 0100200300400500600700 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 GPS Week Time (416455 - 417176 seconds) position errors (1 sigma, meters)

31. Civil and Environmental Engineering and Geodetic Science Velocity Estimation Accuracy time (seconds) 0100200300400500600700 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 velocity errors(1 sigma, m/s)

32. Civil and Environmental Engineering and Geodetic Science Orientation Estimation Accuracy time (seconds) 0100200300400500600700 0 10 20 30 40 50 60 orientation errors (1 sigma, arc seconds)

33. Civil and Environmental Engineering and Geodetic Science Differential Ionospheric Delay

34. Civil and Environmental Engineering and Geodetic Science Differential Ionospheric Delay

35. Civil and Environmental Engineering and Geodetic Science INS Positioning Errors when GPS Observations are not Used GPS Week Time (418798 - 418848 seconds) 400410420430440450 0 0.05 0.1 0.15 0.2 0.25 Positioning Errors (m)

36. Civil and Environmental Engineering and Geodetic Science Horizontal Error Growth During GPS Outage Introduced at Epoch 60 with Gravity Compensation (known deflections of the vertical)

37. Civil and Environmental Engineering and Geodetic Science Effect of (partially) Known Deflections of the Vertical (DOV)   Estimability of the attitude components improves by adding partially compensated (1 arcsec RMS) gravity information   Kalman filter converges faster and the standard deviation spectrum is less sensitive to the lack of maneuvers   Free inertial navigation during the GPS outages with DOV compensation shows slower error accumulation, especially in the horizontal components

38. Civil and Environmental Engineering and Geodetic Science Gyro Magnitude Spectra 020406080100120140 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 frequency (Hz) magnitude spectrum of angular signals Magnitude Spectrum for x-gyro data Magnitude Spectrum for y-gyro data Magnitude Spectrum for z-gyro data

39. Civil and Environmental Engineering and Geodetic Science Difference between GPS/INS and GPS Antenna Array Solution   The differences containing only the random component (after boresight removal) practically match the 3DF accuracy for the 10-meter baseline separation

40. Civil and Environmental Engineering and Geodetic Science AIMS Estimated Standard Deviations as Function of Base- Rover Separation

41. Civil and Environmental Engineering and Geodetic Science Callaghan FL Test Flight   Scale 1:6,000 (altitude ~ 300 m)   GSD ~ 10cm   Ground control good to 2 cm (1 sigma)   Maximum base-rover separation ~ 350 km   Several passes with phased image capture, to resolve the conflict between the required high image acquisition rate (1.5sec) and the limited camera cycling time (6 sec)

42. Civil and Environmental Engineering and Geodetic Science Callaghan FL Test Area -81.9-81.88-81.86-81.84-81.82-81.8 30.52 30.54 30.56 30.58 30.6 longitude [deg] latitude [deg] - control points *

43. Civil and Environmental Engineering and Geodetic Science Aerotriangulation Results   SoftPlotter data reduction   SoftPlotter and OSU adjustment packages   2 cm accuracy assumed for control points   7  for image coordinate observations

44. Civil and Environmental Engineering and Geodetic Science The Purpose of Aerotriangulation   Camera/INS boresight calibration   based on the set of imagery containing a set of control points)   depends strongly on the AT quality   Direct orientation performance testing   by comparison with AT results   uses images different from those used for boresight calibration

45. Civil and Environmental Engineering and Geodetic Science GPS/INS Positions vs. AT (projection center coordinates) GPS/INS Positions vs. AT (projection center coordinates) 051015 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 control points coordinate difference [m] x difference y difference z difference

46. Civil and Environmental Engineering and Geodetic Science GPS/INS Attitude vs. AT GPS/INS Attitude vs. AT heading pitch roll

47. Civil and Environmental Engineering and Geodetic Science Boresight Estimation Quality Performed on ground control points (natural objects) The standard deviations for the boresight components · · linear displacements : 0.22, 0.08 and 0.06 m · · rotation angles , ,  : 0.01, 0.03 and 0.04 deg Possible reasons: · · Photogrammertic processing accuracy – – affected by poor signalization of control points · · Mechanical problems with the camera body/mount · · Image time tagging (rather unlikely)

48. Civil and Environmental Engineering and Geodetic Science Average differences between nominal and manual measurements of control point coordinates 3D coordinates were measured manually in four stereo models oriented by DPO, each one containing two to five control points

49. Civil and Environmental Engineering and Geodetic Science AIMS  Summary - Outlook   Promising test results   Automation and cost reduction offered by multi- sensor systems   Further refinements needed:   camera body rigidity   camera/mount/INS assembly needs improvements   boresighting should be performed at a special calibration range prior to the survey   high-resolution (9K  9K) digital camera   real-time operation capability

50. Civil and Environmental Engineering and Geodetic Science Land-based Tests   Single side-looking camera, tilted downwards by 5°   50-mm focal length   Imagery collected along the surveyed road (edge/center line location)   Stereo-pairs formed by subsequent images   7-8 m object distance   8-20 m object distance for ground control points

51. Civil and Environmental Engineering and Geodetic Science Direct Orientation Land-based System Digital camera GPS antenna INS

52. Civil and Environmental Engineering and Geodetic Science Ground coordinate difference for check points measured from 44 different stereo-pairs Ground coordinate difference for 15 check points measured on stereo-pairs from different passes

53. Civil and Environmental Engineering and Geodetic Science Test Against Ground Truth Control points were determined by differential GPS with estimated accuracy of 1-2 cm per coordinate