2. GPS OBSERVABLE USED IN PRECISE 
PHOTOGRAMMETRIC APPLICATIONS 
There are three types of positioning information which can be 
extracted from GPS satellite signals; pseudorange (code), 
carrier phase, and phase rate (Doppler Frequency). Due to the 
high accuracy required for aerotriangulation, GPS phase 
measurements are needed to meet the accuracy requirement. In 
order to eliminate the effects of systematic errors introduced by 
GPS, double difference GPS phase measurements are used. The 
reason is that most GPS errors affecting GPS accuracy are 
highly correlated over a certain area and can be eliminated or 
reduced. The observation equation for DGPS phase 
measurement is given as (Lachapelle et al., 1992): 
VA® = VAp + VA dy + AVAN — VA dion + VA duo + EVA® (1) 
where 
VA  isthe double difference notation, 
® is the carrier beat phase measurement in cycles, 
p is the distance from satellite to the receiver, 
dp is the orbital error, 
X is the carrier wave length, 
N is the integer carrier beat phase ambiguity, 
dion is the ionospheric error 
dwop 18 the tropospheric error 
€ is the receiver noise and multipath. 
The terms VA dy, VAdi,,, and VA dy, are generally small or 
negligible for short monitor-remote distances (e.g. «10-20 km). 
However, the term VAd, has become more significant due to 
Selective Availability (SA) and may have some negative effects 
on integer carrier ambiguity recovery. The satellite and receiver 
clock errors are eliminated using DGPS method but the receiver 
noise is amplified by a factor of 2. The phase observable is used 
extensively in kinematic mode where the initial ambiguity 
resolution can be achieved using static initialization or “ On 
The Fly “ methods. Accuracy at the centimeter level can be 
obtained if cycle slips can be detected and recovered (Cannon, 
1990). The accuracy of kinematic DGPS is a function of the 
following factors (Lachapelle, 1992): 
- Separation between the monitor and the remote station 
- The effect of Selective Availability 
- The receiver characteristics and ionospheric conditions 
3. GPS SUPPORTED AEROTRIANGULATION 
The main purpose of aerial triangulation (AT) is the 
determination of ground coordinates for a large number of 
terrain points and the exterior orientation parameters of aerial 
photographs using as few control points as possible. The best 
scenario in mapping projects is to have the exterior orientation 
parameters accurate enough so that the AT can be neglected. 
The GPS accuracy for attitude parameters is about 15 arc 
minutes and still far from what could be obtained from a 
conventional block adjustment. Therefore, aerial triangulation is 
still one of the important steps in mapping and can not be 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
avoided. 
The integration of GPS measurements with photogrammetric 
blocks allows for the accurate determination of the coordinates 
of the exposure stations resulting in a reduction of the number 
of ground control points to a minimum. The combined 
adjustment of photogrammetric data and GPS observations can 
be carried out by introducing GPS observation equations to the 
conventional block adjustment (Ebadi and Chapman, 1995). 
Empirical investigations (FrieB , 1991) showed that, in addition 
to the high internal accuracy of GPS aircraft positions, (0 =2 
cm) drift errors may occur due to the ionospheric and 
tropospheric errors, satellite orbital errors, and uncertainty of 
the initial ambiguity. Out of the mentioned remaining errors, 
incorrect carrier phase ambiguities contribute the majority of 
the drift errors to the exposure station position. 
4. GPS CONTROLLED STRIP TRIANGULATION WITH 
GEOMETRIC CONSTRAINTS OF MAN-MADE 
STRUCTURES 
The inherent geometry of a block and the common tie points in 
consecutive strips make it possible to recover all three rotation 
angles in the combined block adjustment. Unfortunately, this 
method can not be used for a single strip, since the GPS 
coordinates of the exposure stations do not recover the roll 
angle of the aircraft. As a consequence, control introduced by 
airborne-GPS leaves an ill-conditioned, if not singular system 
of normal equations. Ground control points can be used along 
the flight line to overcome this problem. 
A new technique for GPS controlled strip triangulation was 
developed based on geometric constraints of man-made 
structures (e.g. high voltage towers, high-rise buildings) located 
along the flight line. The observation equations for these 
constraints with proper weight are introduced to the combined 
strip adjustment. The constraint observation equation for a high 
voltage tower is written as: 
rx | [xi] [00] 
lv. Lv. foo] (2) 
where 
(X;,Y;) are the horizontal coordinates of the top of the 
structure, 
(X;,Y;) are the horizontal coordinates of the bottom of the 
structure. 
The weights for these equations should be appropriately chosen 
because the top and the bottom of the structure must have the 
same horizontal coordinates. However, the absolute ground 
coordinates of the structure are not required since the top and 
the bottom of the structure are similar to horizontal pass points. 
The main idea is to use a number of towers along the flight line 
in order to recover the roll angle of the aircraft and use these 
constraints to minimize the number of ground control points. 
154 
   
  
  
   
  
  
  
  
  
  
    
   
   
  
  
  
  
  
    
    
   
   
    
   
   
    
  
  
   
    
      
   
   
   
  
  
   
  
  
  
   
  
   
  
  
   
   
  
  
  
   
  
    
  
   
   
    
    
4. 1. Precisi 
The covari 
generally ta 
average thec 
The average 
check point 
where AX, 
and known 
m;, and U 
precision o 
respectively 
Reliability 
The ability 
referred by 
undetected 
is measurec 
the local ı 
determined, 
is the wei 
generally a: 
The intern: 
(Deren and 
where 
OL, is 
50 is 
9o; 18 
VoL, is 
Opi is 
with the sis 
B = 93%, t 
4. 2. Resul 
Many expe 
of the ne 
photograph 
were prede 
(pass point