surveying iii (topographic and geodetic surveys) - Modern Prepper

PART A - GLOBAL POSITIONING SYSTEM OVERVIEW5-1. Operating and Tracking Modes. There are two general operating modes from which GPSderivedpositions can be obtained--absolute and relative or differential positioning. Within each of thesetwo modes, range measurements to the satellites can be performed by tracking either the phase of thesatellite's carrier signal or PRN codes that are modulated on the carrier signal. In addition, GPSpositioning 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 obtainedfrom 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 beachieved 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 referencecontrol is established.5-2. Absolute Positioning. Absolute positioning is the most commonly used military and civilapplication of NAVSTAR GPS for real-time navigation. When operating in this passive, real-timenavigation mode, a single receiver, placed at a point where a position is desired, can observe ranges toNAVSTAR GPS satellites. This receiver may be positioned to be stationary (static) over a point or inmotion (kinematic), such as on a vehicle, aircraft, missile, or backpack. Two levels of absolutepositioningaccuracy may be obtained from the NAVSTAR GPS--SPS and PPS. With certainspecialized GPS receiving equipment, data-processing refinements, long-term static observations, andabsolute positional coordinates can be determined to accuracy levels of less than one meter. Theseapplications are usually limited to worldwide geodetic reference surveys.5-3. Differential Positioning. Differential positioning is a process of measuring the differences incoordinates between two receiver points, each of which is simultaneously observing or measuringsatellite code ranges and carrier phases from the NAVSTAR GPS constellation. The process measuresthe difference in ranges between the satellites and two or more ground observing points. The rangemeasurement 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 thesame for a given instant. The resultant accuracy of these coordinate differences is at the meter level forcode phase observations and at the centimeter level for carrier phase tracking. These coordinatedifferences are usually expressed as 3D baseline vectors, which are comparable to conventional surveyazimuth and distance measurements. DGPS positioning can be performed in either a static mode or akinematic mode.5-4. System Configuration. The NAVSTAR GPS consists of three distinct segments--the spacesegment (satellites), the control segment (ground tracking and monitoring stations), and the user segment(air-, land-, and sea-based receivers).EN0593 5-2