INS 
Attitude 
  
  
* Data Synchronization 
* Data Processing and Merging 
* Image Georefrencing 
* 
> 
> 
++ 
- 
a 
* 
> 
+» 
> 
- CCD cameras 
  
      
   
GPS 
Position 
  
  
   
  
Ee) 
  
  
  
  
Figure 1: The VISAT system 
GPS is the accuracy degradation due to poor satellite 
geometry, cycle slips, satellite outages, and dynamic lag 
during maneuvers. 
The INS measures linear acceleration and angular rates very 
accurately and with minimum time delay. For short time 
intervals, the integration of acceleration and angular rate 
results in extremely accurate velocity, position, and attitude 
with almost no noise or time lags. However, because the INS 
outputs are obtained by integration, they drift at low 
frequencies. To obtain very accurate outputs at all 
frequencies, the INS should be updated periodically using 
external measurements. One of the typical external 
measurements is the Zero-Velocity-Update (ZUPT) which is 
simply obtained by stopping the vehicle. Disadvantages of 
using ZUPTs are that: 
* the system will be limited to semi-kinematic 
applications; 
* on highways and high traffic roads, it is not possible 
to stop the vehicle without interrupting the traffic flow; 
* the production rate, which could be critical in many 
projects, will be reduced. 
The integration of GPS and INS, therefore, provides a 
navigation system that has a superior performance in 
comparison with a stand-alone system. For instance, GPS 
derived positions have approximately white noise 
characteristics over the whole frequency range. The GPS 
derived positions and velocities are therefore excellent 
external measurements for updating the INS, thus improving 
its long term accuracy. Similarly, the INS can provide 
precise position and velocity data for GPS signal acquisition 
and reacquisition after outages. This reduces the time and the 
search domain required for detecting and correcting cycle 
slips. To optimally combine the GPS and the INS data, a 
Kalman filtering scheme is used (Schwarz et. al., 1990). The 
University of Calgary has developed a decentralized Kalman 
filter softvare KINGSPAD (KINematic Geodetic System for 
Positions and Attitude Determination) for processing 
INS/GPS data. The GPS data are Kalman filtered to obtain 
estimates of position and velocity which are then used as 
quasi-observations for the INS Kalman filter. At the same 
time, the GPS data are continuously checked for cycle slips. 
For more details on the mathematical formulation and 
Kalman filtering alternatives, see Wei and Schwarz (1990a 
and 1990b). KINGSPAD can perform the following 
functions: 
* processing the data in three different modes, that is, 
pure GPS, pure INS, and hybrid INS/GPS. 
* defining which GPS data will be used to update the INS, 
namely, position, velocity, and position/velocity, 
* viewing individual space vehicle (SV) data, thus 
allowing the rejection of specific SV in the GPS processor, 
* selecting the GPS update rate according to a specific 
application (airborne, land application), 
* computing the updated INS position, velocity, and 
attitude at 1-64 Hz to suit different applications 
* applying rapid static integer ambiguity resolution 
242 
  
techniques 
s SPI 
kinematic | 
The outpu 
georeferer 
described | 
CCD came 
CCIR). Th 
provide th 
VISAT re 
center of c 
are chara 
radiometr: 
resolution 
to 580 line 
Determinit 
is in princ 
equivalent 
body mot 
rotations. 
velocities 
(velocities 
determinir 
siX parame 
In the vel 
PC-AT, 
programm 
and asser 
Solution I