three-line
CCD
camera in China supported by the National
High-Resolution Earth Observation System program. The key
components of the
GFXJ
camera include the optical imag-
ing system, large-capacity memory unit, operation display
system, pilot and navigation system, and gyro stabilization
platform Leica
PAV80
. The optical imaging system integrates
the lens system and the Leica
POS/AV
610 system together. The
Leica
POS/AV
610 system includes the inertial measurement
unit (
IMU
) unit, the Global Navigation Satellite System (
GNSS
)/
IMU
recoding and processing unit and the
GNSS
antenna,
which records the camera shooting position and attitude
parameter synchronously during flight. The
IMU
unit and the
GNSS
receiver and processing unit are rigidly connected to the
camera lens system. The positioning accuracy of Applanix
POS/AV
610 system is 0.05–0.30 m. The angular accuracy is
0.0025 degrees for pitch and roll angles, and 0.0050 degrees
for kappa angles. The large-capacity memory is mainly used
to store the acquired image and
GNSS
/
IMU
data, the operation
display system is mainly used to provide an interface for
human-computer interaction, and the navigation display sys-
tem is used to plan the navigation route. The Leica
PAV80
plat-
form provides angle compensation in three axes during flight.
The lens system, large-capacity memory unit, and operation
display system are self-developed in China. The
GFXJ
camera
uses the three-line array push-broom imaging mode. The same
ground target is imaged three times in forward, nadir and
backward views during aerial photography. The camera pro-
vides three panchromatic images with different view angles
and four-band multispectral (red, green, blue, near-infrared)
images. The nominal viewing angles of the forward, nadir and
backward
CCD
lines are 21, 0, and 27 degrees, respectively. For
panchromatic images, each imaging
CCD
line array consists
of 32 756 detectors, and the entire
CCD
array is an integrated
body instead of multichip splicing. This array is currently
the largest single
CCD
line array with the most detectors. The
pixel size is 5 µm, and the camera focal length is 130 mm.
The nominal viewing angle of the multispectral
CCD
line
is 13.5 degrees. The radiometric resolution is 16-bit. The
image line sampling rate is from 540 to 1080
Hz
. The imaging
spectrum for a panchromatic image is 465–680 nm, and those
of the multispectral bands are 608–662 nm (red), 533–587 nm
(green), 428–492 nm (blue), and 703–757 nm (near infrared).
Figure 1 shows the
GFXJ
camera and the gyro stabilization
platform Leica
PAV80
. An imaging principle sketch is outlined
in Figure 2. The global positioning system (
GPS
) sampling fre-
quency is 200
Hz
, and the
IMU
sampling frequency is 1000
Hz
.
Compared with the
POS/AV
510, Applanix uses a
GNSS
signal
receiver instead of a
GPS
receiver to simultaneously receive
and process satellite signals such as
GPS
, GLObal NAvigation
Satellite System (
GLONASS
), and Beidou (
BSD
). The
POS/AV
610
system mainly comprises an
IMU
,
GNSS
receiver, computer
system, and data postprocessing software
POSPac
. The
POSPac
postprocessing software provides optimal navigation data by
filtering the original
GNSS
and
IMU
data by combining the base
station receiver data. The
POSPac
postprocessing software can
only provide
GNSS
/
IMU
observations, and the
GFXJ
calibration
and triangulation work is realized by self-developed software.
Compared with the
ADS40
camera, the
GFXJ
has a longer
focal length, a larger
CCD
line size and a wider ground cover-
age range. To improve the positioning accuracy and meet
the requirements of 1:1000 scale topographic mapping, it is
necessary to carry out the aerial triangulation (
AT
) process on
the image. To further enhance the direct geopositioning (
DG
)
accuracy of the
GFXJ
camera and alleviate its dependence on
ground control points during triangulation, the inherent sys-
tematic error sources of the camera need to be calibrated. The
systematic error sources that lower the stereoscopic mapping
accuracy of airborne three-line
CCD
cameras and cause geomet-
ric deformation can be categorized into two parts: one is the
GNSS
antenna center offset (
GNSS
lever arms) and
IMU
boresight
misalignment, and the other is the distortion errors of the cam-
era lens and
CCDs
(Tempelmann 2000; Hinsken 2002; Kocaman
2006, 2008; Casella 2008a, 2008b; Fuchs 2010).
In this paper, a deep study is carried out on
GNSS
lever
arms and
IMU
boresight misalignment calibration and the
camera lens and
CCD
line distortion calibration of the
GFXJ
.
First, geometric models for
GNSS
lever arms and
IMU
boresight
misalignment are established, and corresponding calibration
models are put forward. Then, a piecewise self-calibration
model based on the
CCD
viewing angle is designed for the
GFXJ
lens and
CCD
line distortion. Subsequently, an iterative
two-step calibration scheme is designed for actual calibration.
Figure 1.
GFXJ
camera and the gyro stabilization platform.
Figure 2. Imaging principle sketch of the
GFXJ
.
644
September 2019
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
