ters 
  
  
problem 
on of a 
seperate 
le Sensor 
t for the 
center 
ude part 
(E 
a known 
)N OF 
NATES 
rojection 
has been 
recent 
R/Global 
;PS real- 
become 
irborne 
ications, 
for large 
scale the 
mapping 
table 1). 
  
  
  
  
  
  
  
  
  
  
map scale image scale required Ogps X, Ÿ required Ogps Z 
[m] [m] 
1:100000 1:100000 27 6,2 
1:50000 1:65000 11 2,4 
1:25000 1:40000 6,8 0,9 
1:10000 1:25000 1,7 0,5 
1:5000 1:12000 0,9 0,05 
  
  
  
Table 1 Required Accuracies for different Map Scales 
From table 1 it can be seen that the 
specifications for large scale mapping get more 
demanding as the map scale increases. The table 
has been mainly compiled for digital or 
analogue aerial cameras and it does not include 
the accuracy requirements for other airborne 
remote sensing sensors (e.g. CASI) or non- 
imaging sensors (eg. LASER). Nevertheless, the 
needed positioning accuracy will rarely be 
higher than the one for large scale (1:5000) 
mapping. 
As the real-time determination of the projection 
center coordinates with GPS can be affected by 
large systematic errors (e.g. orbit, system time, 
atmospheric effects, multipath) the handling of 
the systematic error effects in the ranging model 
has to be of special concern. The most effective 
Way of compensating these systematic errors is 
by using differential GPS (DGPS). For a 
detailed description of differential GPS 
applications please refer to BLACKWELL [1986]. 
The principle of DGPS is to observe the 
Systematic error effects on a stationary receiver 
Which is located over a known point and 
transmitting correction values to the moving 
receiver. For real-time applications the 
transmission of the correction data has to be 
done with telemetry links. Conventionally, these 
telemetry links are in the HF-UHF range due to 
legal constraints, transmission speed and line of 
sight problems. The information which is 
transmitted via the telemetry links can vary 
Significantly, and it can range from simple 
Coordinate corrections, over range corrections up 
to the full observation set on the reference 
station. An effort to standardize the transmitted 
values has been done by KALAFUS ET AL. [1986]. 
Using this technique most error sources related 
to the satellite and the signal transmission can be 
reduced. The size of the reduction is mainly 
dependent on the reference station receiver - 
moving receiver seperation. Receiver dependent 
error sources may be cancelled by differencing 
observations between satellites. The differencing 
principle can be used on all GPS observation 
types. In the remainder of this paper we refer to 
pseudorange and carrier phase observations in 
the sense of double differenced (between- 
station, between-satellite) observations. 
The GPS observation types which are available 
and suitable for real-time positioning purposes 
are pseudoranges on the C/A-Code frequency as 
well as carrier phase observations on the L1- and 
L2-frequencies. The P-Code pseudoranges can 
only be used for military users and should not be 
considered available for most of conventional 
mapping applications. Further a combination of 
the two signal types, pseudorange and carrier 
phase, can also be used for positioning. 
However, the suitability of the different signal 
types under real-time, airborne kinematic 
conditions has to be carefully analyzed, as the 
accuracy and the processing requirements for the 
observation types vary significantly. 
The usage of pseudorange data for real-time 
positioning is straight forward, as the 
pseudorange signal has been directly designed 
for this purpose. Using the observations from 
four satellites and the mathematical relation 
from equation 1 the position of a moving vehicle 
185