PART A - GLOBAL POSITIONING SYSTEM OVERVIEW
5-1. Operating and Tracking Modes. There are two general operating modes from which GPS-
derived positions can be obtained--absolute and relative or differential positioning. Within each of these
two modes, range measurements to the satellites can be performed by tracking either the phase of the
satellite's carrier signal or PRN codes that are modulated on the carrier signal. In addition, GPS
positioning can be performed with the receiver operating in a static or dynamic (kinematic)
environment. This variety of operational options results in a wide range of accuracy levels, which may
be obtained from the NAVSTAR GPS. Accuracies can range up to 100 meters and down to less than a
centimeter. Increasing the accuracy to less than a centimeter requires additional observation time and
can be achieved in real time. Selection of a particular GPS operating and tracking mode (such as
absolute, differential, code, carrier, static, kinematic, or combinations thereof) depends on the user
application. Topographic survey applications typically require differential positioning using carrier
phase tracking. Absolute modes are rarely used for geodetic surveying applications, except when
worldwide reference control is established.
5-2. Absolute Positioning. Absolute positioning is the most commonly used military and civil
application of NAVSTAR GPS for real-time navigation. When operating in this passive, real-time
navigation mode, a single receiver, placed at a point where a position is desired, can observe ranges to
NAVSTAR GPS satellites. This receiver may be positioned to be stationary (static) over a point or in
motion (kinematic), such as on a vehicle, aircraft, missile, or backpack. Two levels of absolute-
positioning accuracy may be obtained from the NAVSTAR GPS--SPS and PPS. With certain
specialized GPS receiving equipment, data-processing refinements, long-term static observations, and
absolute positional coordinates can be determined to accuracy levels of less than one meter. These
applications are usually limited to worldwide geodetic reference surveys.
5-3. Differential Positioning. Differential positioning is a process of measuring the differences in
coordinates between two receiver points, each of which is simultaneously observing or measuring
satellite code ranges and carrier phases from the NAVSTAR GPS constellation. The process measures
the difference in ranges between the satellites and two or more ground observing points. The range
measurement is performed by a phase-difference comparison, using the carrier phase or the code phase.
The basic principle is that the absolute-positioning errors at the two receiver points will be about the
same for a given instant. The resultant accuracy of these coordinate differences is at the meter level for
code phase observations and at the centimeter level for carrier phase tracking. These coordinate
differences are usually expressed as 3D baseline vectors, which are comparable to conventional survey
azimuth and distance measurements. DGPS positioning can be performed in either a static mode or a
kinematic mode.
5-4. System Configuration. The NAVSTAR GPS consists of three distinct segments--the space
segment (satellites), the control segment (ground tracking and monitoring stations), and the user
segment (air-, land-, and sea-based receivers).
EN0593
5-2
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