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The University of Calgary has been involved with 
the development of GPS and GPS/INS systems for 
precise aircraft positioning and attitude 
determination aimed at the minimization or 
elimination of ground control in mapping 
applications. The accuracy requirements for such a 
system are generally dependent on the system in 
use as well as on the flying altitude. In general, 
the requirements are at the level of 10 cm or better 
for large-scale photogrammetry while they are 
relaxed to about 0.5 m for many remote sensing 
applications. GPS attitude is in general not 
sufficiently accurate for mapping applications, 
where the requirement may be in the order to 10-20 
arcseconds. A description of the approach to 
GPS/INS integration for remote sensing can be 
found in Schwarz et al. (1993) and Sun et al. (1994). 
Several flight tests have been carried out to 
confirm the feasibility of using GPS or GPS/INS 
for precise aircraft positioning. One of the 
difficulties is confirming the achievable accuracy 
during such tests since an independent system is 
required. One alternative is the use of inverse 
photogrammetry, where ground control is used to 
determine the coordinates of the GPS antenna 
which then can be compared to the estimated GPS 
coordinates, for example. Another concept is to 
build redundancy in the number of airborne and 
ground GPS receivers so independent aircraft 
trajectories can be compared. This concept is 
described in Shi and Cannon (1994) for the case 
when four receivers are used, two in the aircraft 
and two on the ground. In this test, baselines of 
over 100 km generated positioning accuracies at the 
10-20 cm level when dual frequency carrier phase 
measurements are used with precise ephemerides. 
[4 —20 m—»| 
  
Fig.8: Aircraft-to-Aircraft Positioning 
Concept with Four Receivers 
Many other applications in the airborne 
environment exist, with one being the precise 
positioning of one aircraft with respect to another. 
This work is being conducted between The 
University of Calgary and the U.S. for application 
in their magnetics program (Lachapelle et al., 
1994). The concept is shown in Figure 8. Several 
tests have been conducted with the system and 
results show that cm-level positioning can be 
achieved between the two aircraft if an on-the-fly 
algorithm is used in the data reduction. 
7. CONCLUSIONS 
This paper summarized the current status of GPS 
and outlined the various classes of receivers that 
are currently available. With the arrival of the 
operational phase of GPS, and the reduction of 
GPS receiver costs, the number and breadth of GPS 
applications is growing at a fast rate. Various 
modes of acquiring data with GPS were reviewed. 
Several applications using GPS were presented 
with the main focus being on kinematic 
applications for land, marine and airborne 
environments. Overall, the pace of GPS activities 
is accelerating and it is expected that it will play 
an important continued role in the development of 
GIS. 
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