Rainer Sandau
Camera Head
/ IMU
Camera
Head
Electronics N
POS Computer Camera Computer
External "
Interf *| Recording , Stabilized
RTCM nterface Mount
Interface
System [| 1
Data pm " - |
GPS
mn ron as
as L^
Antenna
Figure 8. Main components of tight integration of IMU/GPS and a
three line sensor camera
High rate incremental velocity and angle data from the IMU are integrated in a strapdown navigator to produce a full
six degrees of freedom navigation solution of the camera (position, velocity and orientation). Position and velocity
information from the GPS receiver are then used to observe and correct the low-frequency errors in the navigation
solution. The net result is a high-rate, high-bandwidth position and orientation solution that has the dynamic fidelity of a
pure inertial based solution and the absolute accuracy of GPS.
The IMU is mounted inside the camera head. Unlike existing frame cameras, the ADS40 has been designed for the IMU
to be mounted directly on its focal plane. This eliminates the problem of relative motion between the camera and the
IMU that can sometimes arise when the IMU is mounted externally.
The POS computer system and the GPS receiver are embedded in the camera computer system. Data from the IMU are
transmitted to the POS computer via a fibre optic link. Other inputs include RTCM 104 differential GPS corrections and
gimbal encoder input from the stabilised platform.
The ADS40 POS system generates both a real-time and post-processed position and orientation solution. The real-time
solution is used for input to the flight management system and for control of the stabilised mount. By providing both
position and orientation, POS has the ability to control automatically the stabilised platform yaw to remove crab and
drift. The post-processed solution is generated using Applanix’s POSPac software, which processes carrier phase GPS
measurements and raw IMU data in an optimum Kalman filter/smoother to produce the smoothed best estimate of
trajectory (SBET) navigation solution. The SBET navigation solution is then applied to the image data to build up a
sequence of rectified line images for each flight line. The rectified images are then triangulated using the SBET and
automatic tie point matching/bundle adjustment techniques to produce a georeferenced photogrammetric block.
10 SUMMARY AND CONCLUSIONS
LH Systems has chosen the three-line scanner approach for photogrammetric imaging. The very short development time
for the ADS40 of less than three years was possible owing to the transfer of knowledge from DLR’s space sensor
development program within a joint development project. To achieve a high ground resolution with large swath width,
the staggered array principle is applied, resulting in the equivalent of up to 24000 pixels. Additional spectral imaging
lines (RGB, NIRI and optional NIR2) with 12000 pixel arrays are used for multispectral imaging. The spectral channels
are chosen to allow remote sensing applications also. Both photogrammetric and multispectral imaging occur
simultaneously during a single flight. The ADS40 is optimised for image acquisition with a dynamic range up to 12-bit
and a radiometric resolution of 28-bit (including the effects of the Poisson distribution of the incoming light). The
compressed data are stored in an on-board mass memory with a storage capacity up to over a half a terabyte. The
imaging process (sensor control and flight guidance) is controlled by an appropriate ADS40 software system
implemented in the digital computer system connected to the camera head system via optical fibre links.
264 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000.
