tanbul 2004 
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Quality and 
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ication in: 
e, May 23- 
tacy. JPL. 
Geometric 
llites, The 
ry, Remote 
erdam, Vol. 
MODULATION TRANSFER FUNCTION MEASUREMENT METHOD AND RESULTS 
FOR THE ORBVIEW-3 HIGH RESOLUTION IMAGING SATELLITE 
K. Kohm 
ORBIMAGE, 1835 Lackland Hill Parkway, St. Louis, MO 63146, USA — kohm.kevin@orbimage.com 
KEY WORDS: Remote Sensing, High Resolution, Sensor, Quality, Metric, Analysis 
ABSTRACT: 
The Modulation Transfer Function (MTF) is a fundamental imaging system design specification and system quality metric often 
used in remote sensing. MTF is defined as the normalized magnitude of the Fourier Transform of the imaging system's point spread 
function. Alternatively, the MTF describes the attenuation of sinusoidal waveforms as a function of spatial frequency. Practically, 
MTF is a metric quantifying the sharpness of the reconstructed image. On-orbit measurement techniques are discussed to quantify 
the along scan and cross scan MTF profiles. While many measurement techniques exist, the technique utilized is designed to provide 
accurate measurements for high resolution imaging systems. Additionally, a confidence interval is assigned to the measurement as a 
statement of the quality of the measured value. 
The classical slant-edge measurement technique for discrete sampled systems is employed. Fixed high-contrast targets are used to 
obtain MTF measurements in the center of the array. As access to such targets is limited, suitable edges for analysis are identified in 
nominal operational imagery. The measurement results from the specialized targets are used to confirm the large number of 
measurements from the operational imagery. 
The data sets used in the analysis are from the OrbView-3 (OV-3) High Resolution Imaging Satellite, launched June 26, 2003. 
Results are presented for the 1 meter ground sample distance (GSD) panchromatic band of the OV-3 system. 
1. INTRODUCTION 
1.1 Motivation 
On-orbit quantification of MTF for remote sensing systems is 
desirable from multiple perspectives. During sensor 
commissioning, the system MTF is compared against the design 
requirements to verify expected performance is achieved. For 
the end user, system MTF can be used to compare the intrinsic 
quality of imagery from various sources as well as analytically 
equalize the sharpness of multiple images from different sensors 
in a combined product. 
This work was initiated to characterize the MTF of the 
OrbView-3 system during commissioning and continuing 
throughout the life of the program. Two specific goals for the 
measurement technique include determining MTF without the 
use of dedicated targets and providing an intrinsic quality 
metric of the measurement. 
While dedicated targets have been shown to produce quality 
results, access to such targets is limited. Using features found in 
nominal scenes allows for many more measurement 
opportunities and increasing the sample size decreases the error 
estimate of the results. Additionally, features in nominal 
imagery will be positioned at various locations across the linear 
array. This enables MTF characterization across the extent of 
the linear array without the extensive specialized tasking that is 
required with fixed targets. MTF results from fixed targets 
complement the measurements from nominal scenes. 
1.2 Objective 
The objective of this work is to develop a robust on-orbit MTF 
measurement technique for remote sensing systems with GSDs 
on the order of 1 meter or less. One dimensional components of 
MTF in the along scan and cross scan directions are required. 
The technique must operate on both nominal image scene 
well as specialized, fixed targets. Error estimates are to be 
provided for cach measurement. 
un 
a 
N 
1.3 MTF Estimation Methods 
Multiple methods have been proposed for determining the MTF 
of remote sensing systems on-orbit. These include imaging lines 
or points and potentially using imagery from a system with 
known MTF during an underflight (Schowengerdt, 1985). In 
general, these measurement techniques require a particular size 
and orientation of targets based on the GSD and scan direction 
of the sensor to achieve good performance. 
Another approach is to use edges to determine MTF. The edge 
spread function (ESF) is the system response to a high contrast 
edge. The derivative of the ESF produces the line spread 
function (LSF), which 1s the system response to a high contrast 
line. The normalized magnitude of the Fourier Transform of the 
LSF produces a one-dimensional slice through the two- 
dimensional MTF surface. Other methods exist for computing 
the system MTF directly from the ESF that remove the need for 
differentiation (Tatian, 1965). 
A requirement for determining MTF from edges is to have a 
high fidelity. representation. of the ESF. The slanted edge 
algorithm uses the change in phase of the edge across the 
sampling grid to create a "super-resolved" ESF (Reichenbach, 
1991). For sensors with GSDs on the order of 1 meter or less, 
high contrast, slanted edge targets are preferred for MTF 
analysis.