Rigorous Sensor Modeling and Triangulation for OrbView-3 - asprs

Replacing the rigorous sensor model with an RPC approximation is popular mainly for its simplicity. The
January 2003 issue of Photogrammetric Engineering & Remote Sensing contained four papers discussing RPC and
RPC triangulation. An RPC model, especially when embedded in standard imagery formats such as NITF, allows
software developers to implement the RPC model once for any sensor using the standardized format. Unfortunately
RPC modeling does not lend itself to long image segments where high frequency corrections cannot be accounted
for in the standalone RPC model. The Replacement Sensor Model (McGlone, 2004) is a promising extension to the
RPC model. But, until the RSM is fully accepted by the end-user, data provider, and software developer
communities, rigorous models will continue to be used for complex modeling of airborne and satellite sensors.
ORBVIEW-3 SENSOR MODELING AND ASSOCIATED SUPPORT DATA
The OrbView-3 satellite carries a linear array pushbroom sensor, providing one meter panchromatic and four
meter multispectral imagery. GeoEye sells imagery and metadata at varying levels of processing. There are currently
eight OrbView-3 products, from BASIC imagery to Orthorectified products (ORTHO). This paper focuses on
OrbView-3 BASIC Enhanced Panchromatic Imagery, which includes enhanced telemetry metadata.
Cooperation between the data provider and the photogrammetry software supplier is essential to understanding
and implementing the rigorous sensor model for a specific sensor. In the case of OrbView-3 imagery, agreements
were signed between BAE Systems and ORBIMAGE (now GeoEye), then technical material and sample imagery
were provided by ORBIMAGE. OrbView-3 BASIC Enhanced Imagery including enhanced telemetry data was sent
to BAE Systems along with ground control and documentation describing the imagery and metadata. Clearly,
cooperation in both business and technical aspects is mutually beneficial for the data provider (GeoEye) and data
user (BAE Systems) – GeoEye wishes to sell imagery products and BAE Systems wishes to sell image exploitation
software to mapping and intelligence agencies and commercial companies.
Support data includes information such as sensor location, velocity, orientation angles, focal length, time of
acquisition, and camera calibration data. Support data are often supplied as auxiliary files to the raw imagery. In the
past imagery and metadata were typically reformatted in the photogrammetry workstation into a common format for
convenient use by the image processing software solutions such as SOCET SET, but today the image providers are
typically delivering their imagery in common formats such as NITF and TIFF. Therefore, the imagery is often used
directly for photogrammetric processing in the software. The associated image metadata is normally stored in a
binary or ASCII format. Documentation provided by ORBIMAGE details the metadata definition and format. The
SOCET SET and SOCET GXP software products use the OrbView-3 imagery and metadata directly without the
need to reformat either input data source. Since triangulation is typically required by end-users of the
photogrammetry software, a solution providing updated sensor parameters must be provided. In the case of SOCET
SET and SOCET GXP, the updated (triangulated) sensor parameters are stored in a separate ASCII support file.
The sensor model converts ground space locations to image space locations, or vice versa. The ground space
coordinates are in the form of Earth-Centered-Earth-Fixed (ECEF) coordinates in meters. The image coordinates are
sub-pixel line and sample coordinates. The transformations are used in nearly every photogrammetric process, from
simply moving the cursor on an image to creating products such as orthophotos. The OrbView-3 sensor model
documentation provided by ORBIMAGE outlines the image-to-ground transformation. Therefore, the direct imageto-ground
transformation was implemented in SOCET SET and SOCET GXP with the ground-to-image
implemented as an iterative computation of the image-to-ground. The OrbView-3 documentation outlines the
projective image-to-ground transformation as
where
G 1 A k
O
= A + M a
(1)
G
A is the unknown object space coordinate triplet in the ECEF WGS84 coordinate system in meters
O
A is the instantaneous sensor position triplet in the ECEF WGS84 coordinate system in meters
interpolated from the ephemeris metadata
k is a scalar
M is the 3x3 image-to-ground rotation matrix based on instantaneous attitude information in the form of
quaternion values from the attitude metadata
ASPRS 2006 Annual Conference
Reno, Nevada ♦ May 1-5, 2006