y receiver 
nary air- 
lings had 
order to 
ently fast 
duce the 
he initial 
ed on the 
interrup- 
cause sig- 
vn as Cy- 
f a turn- 
Nation of 
y details 
ffice here 
the flight 
d. 
1ity solu- 
reference 
pted. Re- 
ceeded to 
and filter 
affect all 
| the help 
Ware pro- 
ing signal 
hase am- 
ns which 
5 possible 
' approx- 
mbiguity 
rer, linear 
ral prob- 
] tests on 
atic GPS 
der of 10 
amongst 
ift errors 
pted as a 
; may oc- 
turns. It 
GPS drift 
iguity so- 
her hand 
corrected 
it can be 
e to avoid 
with dur- 
onsidera- 
igulation. 
ces. If we 
t with the 
here is no 
s by sta- 
amended, 
ts, not to 
attempt any stationary base-line determination before take- 
off, i.e. to start flying and carry out the photo flight mission 
in the usual way, without particular care about GPS conti- 
nuity. The GPS recordings (at both receivers) need only be 
switched on a few minutes before the mission area is reached. 
In case of signal interruptions during flight turns (within a 
strip no serious interruptions are expected) the phase am- 
biguity solutions are redetermined in the post-processing by 
using the C/A-code or P-code pseudo-range positioning, and 
by considering a dynamic modelling of the aircraft move- 
ment. The solution may leave some systematic errors for 
the following stretch of the GPS trajectory. Such drift er- 
rors are practically linear, according to available experience. 
They may change after each major signal interruption. In 
the extreme case each strip may have its own linear drift 
error. It still needs to be investigated how large distances 
(several hundred km) between stationary receiver and mis- 
sion area and long flight missions (several hours) will affect 
the GPS drift behaviour. Linear drift errors can be assessed 
and compensated in combination with aerial triangulation, 
by including additional parameters into the combined block 
adjustment, as will be discussed in chapter 3. 
2.3 The datum problem, ground control 
GPS positioning refers generally to the earth-centered rect- 
angular GPS coordinate system WGS 84. The same is true, 
in principle, for relative positioning, although a transformed 
coordinate system may be locally tied to the reference point 
at the stationary receiver. It is to be stated clearly that aerial 
triangulation with combined blockadjustment can be carried 
out in that case, without any ground control point, provided 
the GPS trajectory is not interrupted over the complete flight 
mission. The result would refer to the original or a local 
transformation of the GPS reference system WGS 84. 
Normally, however, the results of aerial triangulation (and 
of mapping) are wanted in a national horizontal and verti- 
cal reference system. There are no absolute geodetic trans- 
formation formulae available, at present, which would pre- 
cisely enough link the WGS 84 to a national reference sys- 
tem. Hence, the datum transformation must be provided in 
the traditional photogrammetric way, i.e. by some ground 
control points which would preferably be given in both coor- 
dinate systems. The standard recommendation is, for aerial 
triangulation, to use 4 XYZ-ground control points, located 
at about the corners of a photo-block. They are sufficient 
to provide the datum transformation (in GPS blocks control 
points have no accuracy function any more), provided the 
GPS trajectory is continuous. They are even capable of cor- 
recting for overall drift errors. It is only the geoid reference 
which is not completely solved by 4 ground control points 
alone. If the geoid is known its undulations can be superim- 
posed in addition, or additional vertical control points could 
introduce the geoid indirectly. There have been discussions 
about using only one control point. It allows the determi- 
nation of the shift parameters of a datum transformation. 
All other parameters must be derived from the known geo- 
graphical position. The result can only be an approximate 
solution which may, however, be sufficient for low accuracy 
requirements. 
693 
Ground control points can be used in two ways in GPS sup- 
ported aerial triangulation. If the GPS trajectory is continu- 
ous the GPS-block may be adjusted without control or by us- 
ing them as GPS control only. The subsequent datum trans- 
formation (if necessary in 2 steps, or non linear because of 
map projection and spherical vertical reference) would then 
use the control point coordinates in the national system. It is 
preferable, however, and in case of different drift errors nec- 
essary, to include the control points directly in the combined 
block-adjustment. 
In the approach just described the linear datum transforma- 
tion parameters can be treated as additional unknowns and 
solved for in the combined blockadjustment. That solution 
would include the correction for overall linear drift errors. In 
fact, both effects cannot be separated completely from each 
other. Drift correction is identical with a datum correction, 
although it may include different parameters. 
In section 2.2 it was discussed that there may arise indepen- 
dent drift errors, even per strip. It means that there can be 
a datum problem for sections of the GPS trajectory, even 
per strip. To solve in that case for all unknown drift param- 
eters in combined blockadjustment additional information is 
required, in order to prevent singularities. The additional in- 
  
Fig.1 Control scenarios for GPS blocks 
/\ horizontal 
e vertical 
  
  
  
  
  
  
  
  
  
  
case c 
LS AM 
case a 
LS A 
case b