) is sufficient 
rticularly in 
. suitable for 
rowave rang- 
| accuracy of 
veen the con- 
n wave from 
nts, has now 
called Aero- 
tioned at the 
ison between 
(meter, phase 
sured). These 
t the Master 
nission power 
i. The ground 
nna beam. A 
-plus-Remote. 
d stations R, 
WN 
ust be known. 
od to the full 
SURVEY NAVIGATION, CORTEN 75 
Ground-to-air distances of about 100 miles can be measured. This can be done: 
1. by means of line crossing recording the sums of distances to two ground stations and 
establishing the minimum sum for reasons of trilateration; or 
2. by means of aircraft position determination determining the nadir position of each 
photo exposure station; or 
3. by means of determination of a third point’s position if two ground stations are 
known, figure 19. It may be done when the unknown point is separated from the 
known points by distances up to 150 miles; this method is called: position fixing by 
continuous trilateration. 
As continuously changing distances are measured in the moving airplane, all meas- 
urements must be recorded. It is necessary to have good radio line-of-sight conditions 
between aircraft and ground stations. 
The necessity of knowing the Aerodist ground stations' and aircraft stations' heights 
suggests the use of barometric height determinations. However, the application in com- 
bination with radar altimeter and with statoscope will probably allow for the most econ- 
omical operation. 
Performance. 
The over-all accuracy is stated to be = = 1 meter standard instrument error + 
standard error of 1 part in 100.000 of the measured distance. The limitation of accuracy 
is set by the knowledge of the variations of refractive index of the atmosphere. If the 
present tests would fulfill this expectation in actual survey operation, this method would 
become an important means of airborne trilateration and of determining the camera’s 
planimetric position at each exposure station. 
3. Navigation and orientation methods determining flying height, relative flying heights 
and terrain elevations, 
3.1. Barometric altimetry. 
There does not exist a physical phenomenon that can be used for correctly measuring 
the absolute altitude of the camera station over datum. One of the substitutes is the 
relation between static air pressure and elevation. The air pressure is not only dependent 
on the airplane’s height but also on a large number of other variables; consequently, 
there variables should be known and introduced into the measurement as corrections in 
order that static air pressure correctly expresses the flying height over datum. In prac- 
tice, many of these correcting data are known only partially, thus subjecting the altitude 
(scale) determination to large errors. 
It is reported that the U.S.A.F. Central air data computer system, handling all 
important data in flight, applies as many corrections as possible to the altimeters. One 
of these is the turbulence around the pitot tube or pressure probe; the problem has been 
solved to a great extent by using an angle of attack sensor and a compensation for static 
pressure and angle of attack. The altimeter indication is, under normal survey condi- 
tions, corrected to a value with only 1.2% H deviation from true altitude. 
DE 
3.2. Statoscope altimetry. 
Principle. 
Small differences in flying height can be measured barometrically by means of a 
microbarometer having an extremely high sensitivity and, consequently, a very limited 
range. These measurements refer to an isobaric surface which, in general, will not be 
horizontal nor flat. 
Archives 4