ISPRS Commission III, Vol.34, Part 3A ,,Photogrammetric Computer Vision“, Graz, 2002 
  
INDIRECT GEOREFERENCING OF AIRBORNE MULTI-LINE ARRAY SENSORS: 
A SIMULATED CASE STUDY 
Daniela Poli 
Institute of Geodesy and Photogrammetry 
Swiss Federal Institute of Technology, Zurich, Switzerland 
daniela@geod.baug.ethz.ch 
KEY WORDS: Orientation, Modelling, Triangulation, GPS/INS, Three Line, Simulation 
ABSTRACT: 
Multi-line array sensors, carried on airborne or satellite, acquire images with along or across track stereo viewing and are used for 
photogrammetric mapping at different scales. The main characteristic of the imagery provided by this kind of sensors is that each 
image line is independently acquired with a different sensor external orientation (position, attitude). If positioning instruments 
(GPS/INS) carried on board provide the sensor external orientation of each line, the ground coordinates of the observed points can 
be estimated with direct georeferencing. Anyway the positional and angular displacements of the GPS/INS instruments with respect 
to the image frame with origin in the sensor perspective centre must be estimated, together with additional measurement errors 
contained in the observations. Therefore a triangulation integrated with the sensor external orientation modelling (indirect 
georeferencing) has been implemented. The algorithms have been tested on a simulated testfield, supposing an airborne three-line 
sensor with optical system consisting of one lens. After simulating the sensor trajectory and the coordinates of 40 object points, the 
image coordinates of each point in the three images were calculated with back projection. In order to test the indirect georeferencing 
model, some perturbations and constant offsets in the correct sensor external orientation were introduced and afterwards estimated 
with the proposed integrated triangulation. The RMS obtained on the checkpoints using different Ground Control Points (GCPs) and 
  
Tie Points (TPs) distributions are presented. 
1. INTRODUCTION 
Today a wide class of linear CCD array sensors that acquire 
images in a pushbroom mode exists. Some of them are carried 
on aircraft (e.g. ADS40, DPA and WAAC from DLR, AirMISR 
from NASA) or helicopter (e.g. TLS from STARLABO), others 
on spacecraft (e.g. SPOT from CNES, IRS from ISRO, MISR 
and ASTER from NASA, IKONOS from SpaceImage, WAOSS 
from DLR) and can be used for photogrammetric mapping at 
different resolutions. 
The stereoscopy of the images is achieved across- or along- 
track with respect to the flight direction. Sensors with across- 
track stereo capability, usually carried on spacecraft (SPOT, 
IRS), combine one linear CCD array with a rotating mirror. 
Stereopairs are acquired from different orbits, with a time delay 
in the order of days or months. On the other hand, sensors with 
along-track stereo capability scan the terrain surface with linear 
CCD arrays placed parallel to each other and with different 
viewing angles along the flight direction. The advantage of this 
geometry is to enable the acquisition of a larger number of 
images with a small time delay. A very common along-track 
stereo system used both on airborne (ADS40, TLS, DPA, 
WAAC) and satellite (MOMS-02, WAOSS) consists of three 
CCD lines looking forward, nadir and backward the flight 
direction. Within pushbroom sensors with along-track stereo 
viewing, the number of lenses is variable: some sensors 
(ADS40, TLS, DPA, WAAC, WAOSS) use one lens common 
for all the CCD arrays, others (AirMISR, MISR) have one lens 
for each group of CCD lines looking in the same direction. 
The images provided by linear CCD array sensors consist of 
lines independently acquired with a different exterior 
orientation. Therefore a classic bundle adjustment is not 
realistic for the georeferencing of this kind of imagery, because 
the number of unknowns would be huge. 
In case of sensors carried on satellite, due to the smooth 
trajectory, the exterior orientation can be modelled as a 
polynomial function depending on time, including the physical 
properties of the satellite orbit as constraints (Kratky, 1989: 
Ebner, 1992,). In this case a sufficient number of well- 
distributed Ground Control Points (GCPs) is required. 
For airborne applications, where the trajectory is not 
predictable, the direct measurement of the external orientation 
is indispensable. Thanks to the successful improvement and 
rapid expanding of positioning systems, the exterior orientation 
can be directly measured with high precision with GPS and INS 
systems carried on board (Cramer et al., 2000), allowing direct 
georeferencing and rectification of the images (Schwarz, 1996; 
Haala et al., 1998, Tempelmann et al., 2000). 
Anyway the data provided by GPS/INS do not refer to the 
perspective centre of the lens, but to additional reference 
systems centred in the instruments themselves. The required 
offset vectors and misalignment angles between the systems are 
measured before the flight with surveying methods. Anyway 
these data are not always available and must be estimated with 
post-flight calibration procedures. Moreover the GPS/INS 
observations can be affected by additional errors. Therefore for 
high precision applications the errors contained in the GPS/INS 
data have to be modelled and integrated in the bundle 
adjustment of the imagery, resulting in an indirect 
georeferencing (Lee et al., 2000; Chen, 2001). 
In this paper a model for indirect georeferencing of multi-line 
CCD array sensors is proposed. After an overview on direct 
georeferencing of multi-line sensors (Section 2), the proposed 
model for indirect georeferencing is presented in Sections 3. 
Then, after the description of the generation of a simulated 
testfield for an airborne three-line sensor (Section 4), the results 
obtained from the indirect georeferencing are presented and 
discussed in Section 5. Conclusions and future work about the 
extension of the model to multi-lens linear CCD array sensors 
will close the paper. 
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