airborne scenario, since careful mission planning and 
avoiding steep banking will usually assure a continuos 
lock to several satellites. The radio interference, 
however, is an important issue that can cause significant 
and extended losses of GPS lock, especially during the 
missions over heavily industrial areas, where signals 
from different active radio-sources is a primary reason 
for a very low signal-to-noise ratio, leading to virtually 
complete loss of GPS lock. The primary sources of GPS 
signal interference are amateur radio, mobile satellite 
communications, unlicensed transmitters, consumer 
transmitters, broadcast radio, and television harmonics 
(Krafcik and Guevara, 1999; Divis, 1999). 
The experiences gained in the presence of strong radio 
interference during the airborne mapping project of The 
Massachusetts Institute of Technology (MIT), Boston, 
campus area are presented in the sequel. In addition, 
some practical considerations related to the 
GPS/INS/CCD system calibration and some land-based 
mapping applications of the system are presented. 
2. DIRECT GEOREFERENCING: PRACTICAL 
EXPERIENCES 
2.1. System Calibration 
The Center for Mapping has conducted over 20 test 
flights using the integrated GPS/INS system for direct 
orientation of high-resolution digital frame imagery 
during the past two years. The system used during these 
test flights was the Airborne Integrated Mapping 
System (AIMS™), developed at the Center, which 
currently comprises two dual-frequency Trimble 
4000SSI GPS receivers, and a medium-accuracy and 
high-reliability strapdown Litton LN-100 inertial 
navigation system based on Zero-lock™ Laser Gyro 
(ZLG™) and A-4 accelerometer triad (0.8 nmi/h CEP, 
gyro bias - 0.003°/h, accelerometer bias - 25pg). The 
LN100 firmware, modified for the AIMS™ project, 
allows for access to the raw IMU data, updated at 
256 Hz. Estimation of errors in position, velocity, and 
attitude, as well as errors in inertial and GPS 
measurements, is accomplished by a centralized Kalman 
filter that processes GPS L1/L2 phase observables in 
double-differenced mode together with the INS 
strapdown navigation solution. The imaging component 
in the current configuration consists of a digital camera 
based on a 4,096 by 4,096 CCD with 60 by 60 mm 
imaging area (15-micron pixel size), manufactured by 
Lockheed Martin Fairchild Semiconductors. The 
imaging sensor is integrated into a camera-back 
(BigShot™) of a regular Hasselblad 553 ELX camera 
body. The system is described in more detail by (Da, 
1997; Grejner-Brzezinska, 1997 and 1998; Toth, 1997; 
Toth and Grejner-Brzezinska, 1998; Grejner- 
Brzezinska et al, 1998). 
The direct platform orientation, although very attractive 
from the theoretical standpoint, has several important 
practical implications, especially for the high-accuracy 
mission objectives. Our experiences indicate that one of 
the most important aspects is the proper and reliable 
system calibration. The direct platform orientation 
rotational components are naturally related to the INS 
body frame and must be transformed to the imaging 
sensor frame. The angular and linear misalignments 
between the INS body frame and the imaging sensor 
frame are known as boresight components. The 
boresight transformation is resolved by comparison of 
the GPS/INS positioning/orientation results with 
independent aerotriangulation solution, and must be 
determined with sufficiently high accuracy. Naturally, 
the boresight parameters should stay constant for the 
entire mission duration, thus no flex or rotation of the 
common mount of the imaging and the georeferencing 
sensors can occur during the airborne mission. 
Apart from the mechanical aspects of boresight 
calibration, another important component is the 
availability of high quality test range with very well 
signalized points that should be used for the calibration 
process. Our practical experiences indicate that even if 
the control points are surveyed at cm-level accuracy on 
the ground, their poor signalization may propagate to 
the projection centers’ positioning quality (in 
aerotriangulation procedure), immediately 
compromising the boresight performance (Grejner- 
Brzezinska, 1998). Another important aspect that 
determines the quality and reliability of the direct 
platform orientation methods is a reliable ambiguity 
resolution/cycle slip fixing procedure. Inability to 
restore the ambiguities after the loss of GPS lock, 
especially over the long baseline, limits the systems’ 
operability. This is especially important for the real 
time applications where the positioning results cannot 
be rectified by the benefits of post-processing. 
An additional, substantially important aspect of the 
system calibration is the proper estimation of the lever 
arm components. The offsets between the GPS antenna 
phase center and the center of the INS body frame are 
usually surveyed with high accuracy after the system is 
mounted on the aircraft or land-based vehicle, and 
considered constant until the system is dismounted. The 
knowledge of the lever arm offsets is very important for 
the high-accuracy applications, and crucial for the 
embedded systems, where the INS directly aids the 
carrier-phase-tracking loop. Our tightly coupled Kalman 
filter is able to estimate the corrections to the pre 
surveyed lever arm offsets as part of the state vector. 
Some tests performed on the speed of the lever arm 
recovery by the filter indicate that it is highly dependent 
on the dynamics of the trajectory. In other words, it 
takes much longer to estimate the lever arm offsets if 
the system is moving along a straight line, as compared 
to the curvilinear motion. The significant epoch-to- 
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