OBSERVATIONS ON POLARIS
OR OTHER STARS
For most land surveying, the determination of
astronomic azimuth by observing the sun is sufficient;
however, in some cases, the required degree of accuracy
may be such that observation of Polaris or another star
may be required. Several observation methods and
calculation procedures can be applied to determine
azimuth from Polaris; however, we will not discuss
them here. Instead you should refer to commercial
publications, such as Surveying Theory and Practice,
by Davis, Foote, Anderson, and Mikhail, or Elementary
Surveying, by Wolf and Brinker. You should also refer
to these or other similar publications for a more
thorough discussion of field astronomy in general.
This ends our discussion of field astronomy. Now
let’s take a brief look at a new development in surveying
that is related to field astronomy and to triangulation
which is the final topic in this chapter.
SATELLITE SURVEYING SYSTEMS
In the preceding discussion, you learned how the
location of a point on the earth can be determined from
observations taken on the sun or stars. A far more recent
development uses satellites.
Satellite surveying systems are an offshoot of the
space program and the U.S. Navy’s activities related to
navigation. Since their development, satellite surveying
systems have been successfully used in nearly all areas
of surveying and are capable of producing extremely
accurate results.
The first generation of satellite surveying systems
was the Doppler positioning systems. The success of
the Doppler systems led to the U.S. Department of
Defense development of a new navigation and
positioning system using NAVSTAR (Navigation
Satellite Timing And Ranging) satellites. This
development ushered in the second generation of
satellite surveying systems known as the Global
Positioning System (GPS).
In the Doppler system, a precisely controlled radio
frequency is continuously transmitted from a satellite as
it orbits past an observer’s station. As the satellite draws
nearer the receiver, the received frequency increases.
Then as the satellite passes the receiver, the frequency
decreases below the transmitted level. With the
transmitting frequency, satellite orbit, and precise
timing of observations known, you can then compute
the position of the receiving station.
The observer uses a specially designed receiver
system that is manufactured by one of several
commercial firms. Typically, the system is composed of
an antenna to receive the transmitted frequency; a
receiver to detect, amplify and decode the transmitted
signal; a recording medium, such as a paper or magnetic
tape; and a rugged carrying case.
To determine point locations using the Doppler
system, you can use three basic methods. They are
the point-positioning, translocation, and short-arc
methods.
In the point-positioning method a receiver located
at a single location of unknown position collects data
from multiple satellite passes. From the measured data,
the location of the receiver is determined using a
coordinate system that is relative to the position of the
satellite. Then the location is converted to a
conventional coordinate system used by surveyors.
In translocation, receivers located at two or more
stations track a satellite. The location of one of the
stations—the control station—must be known. The
control station, although its position is known, is first
treated as an unknown and its coordinates are deter-
mined using the point-positioning method described
above. The determined coordinates are then compared
to the known coordinates and differences indicate errors
in the system. Based on the errors, corrections are
determined and applied to the positions of the unknown
stations whose locations have also been determined
using the point-positioning method.
The short-arc method is the same as the trans-
location method, except that corrections are also made
for the orbital parameters of the satellite.
DOPPLER POSITIONING SYSTEMS
GLOBAL POSITIONING SYSTEMS
Imagine, if you will, the continuously changing
pitch of a train whistle as it approaches and passes you.
This is a classic example of the Doppler phenomenon
in which the change in frequency is a function of range
or distance. This phenomenon is the underlying
principle of the Doppler positioning systems.
Because of its superiority, the global positioning
system is phasing out the use of the Doppler positioning
system; however, like the Doppler system, the global
positioning system is based on observations of satellites.
GPS satellites are in near-circular orbits around the
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