Leica GPS Basics- User Manual

5 GPS Basics -1.0.0en System Overview 4 5 1. What is GPS and what does it do ? GPS is the shortened form of NAVSTAR GPS. This is an acronym for NAV igation S ystem with T ime A nd R anging G lobal P ositioning S ystem. GPS is a solution for one of man’s longest and most troublesome prob- lems. It pr...

6 GPS Basics -1.0.0en System Overview 4 2. System Overview The total GPS configuration is com- prised of three distinct segments: • The Space Segment – Satellites orbiting the earth.• The Control Segment – Stations positioned on the earth’s equator to control the satellites• The User Segment – Anybo...

7 GPS Basics -1.0.0en System Overview 4 5 The satellites broadcast two carrier waves constantly. These carrier waves are in the L-Band (used for radio), and travel to earth at the speed of light. These carrier waves are derived from the fundamental frequency, generated by a very precise atomic clock...

8 GPS Basics -1.0.0en System Overview 4 2.2 The Control Segment The Control Segment consists of one master control station, 5 monitor sta- tions and 4 ground antennas distributed amongst 5 locations roughly on the earth’s equator.The Control Segment tracks the GPS satellites, updates their orbiting ...

9 GPS Basics -1.0.0en System Overview 4 5 2.3 The User Segment The User Segment comprises of anyone using a GPS receiver to receive the GPS signal and determine their position and/ or time. Typical applications within the user segment are land navigation for hikers, vehicle location, surveying, mari...

10 GPS Basics -1.0.0en How GPS works 4 3. How GPS works There are several different methods for obtaining a position using GPS. The method used depends on the accuracy required by the user and the type of GPS receiver available. Broadly speaking, the techniques can be broken down into three basic cl...

11 GPS Basics -1.0.0en How GPS works 4 5 6 3.1 Simple Navigation This is the most simple technique employed by GPS receivers to instanta- neously give a position and height and/ or accurate time to a user. The accuracy obtained is better than 100m (usually around the 30-50m mark) for civilian users ...

13 GPS Basics -1.0.0en How GPS works 4 5 6 3.1.2 Calculating the distance to the satellite Calculating the Time The satellite signal has two codes modulated upon it, the C/A code and the P-code (see section 2.1). The C/A code is based upon the time given by a very accurate atomic clock. The receiver...

14 GPS Basics -1.0.0en How GPS works 4 3.1.3 Error Sources Up until this point, it has been assumed that the position derived from GPS is very accurate and free of error, but there are several sources of error that degrade the GPS position from a theoretical few metres to tens of metres. These error...

15 GPS Basics -1.0.0en How GPS works 4 5 6 a. Satellite elevation. Signals from low elevation satellites will be affected more than signals from higher elevation satellites. This is due to the increased distance that the signal passes through the atmosphere. b. The density of the ionosphere is affec...

16 GPS Basics -1.0.0en How GPS works 4 2. Satellite and Receiver clock errors Even though the clocks in the satellite are very accurate (to about 3 nanosec- onds), they do sometimes drift slightly and cause small errors, affecting the accuracy of the position. The US Depart- ment of Defense monitors...

17 GPS Basics -1.0.0en How GPS works 4 5 6 4. Dilution of Precision The Dilution of Precision (DOP) is a measure of the strength of satellite geometry and is related to the spacing and position of the satellites in the sky. The DOP can magnify the effect of satellite ranging errors.The principle can...

18 GPS Basics -1.0.0en How GPS works 4 5. Selective Availability (S/A) Selective Availability is a process applied by the U.S. Department of Defense to the GPS signal. This is intended to deny civilian and hostile foreign powers the full accuracy of GPS by subjecting the satellite clocks to a proces...

22 GPS Basics -1.0.0en How GPS works 4 3.3 Differential Phase GPS and Ambiguity Resolution Differential Phase GPS is used mainly in surveying and related industries to achieve relative positioning accuracies of typically 5-50mm (0.25-2.5 in). The technique used differs from previously described tech...

24 GPS Basics -1.0.0en How GPS works 4 3.3.4 Ambiguity and Ambiguity Resolution 1. 2. an approximate position. The precise answer must lie somewhere within this circle.The wavefronts from a single satellite strike both within and outside of the circle. The precise point must lie somewhere on one of ...

26 GPS Basics -1.0.0en Geodetic Aspects 4. Geodetic Aspects Since GPS has become increasingly popular as a Surveying and Navigation instrument, surveyors and navigators require a basic understanding of how GPS positions relate to standard mapping systems.A common cause of errors in GPS surveys is th...

29 GPS Basics -1.0.0en Geodetic Aspects 4.3 Local coordinate systems Just as with GPS coordinates, local coordinates or coordinates used in a particular country’s maps are based on a local ellipsoid, designed to match the geoid (see section 4.4) in the area. Usually, these coordinates will have been...

30 GPS Basics -1.0.0en Geodetic Aspects P H N h 4.4 Problems with height The nature of GPS also affects the measurement of height.All heights measured with GPS are given in relation to the surface of the WGS84 ellipsoid. These are known as Ellipsoidal Heights.Existing heights are usually orthometric...

32 GPS Basics -1.0.0en Geodetic Aspects Helmert Transformations The Helmert 7 parameter transformation offers a mathematically correct transfor- mation. This maintains the accuracy of the GPS measurements and local coordinates.Experience has shown that it is common for GPS surveys to be measured to ...

33 GPS Basics -1.0.0en Geodetic Aspects Other transformation approaches Whilst the Helmert transformation approach is mathematically correct, it cannot account for irregularities in the local coordinate system and for accurate heighting, the geoid separation must be known.Therefore, Leica also makes...

34 GPS Basics -1.0.0en Geodetic Aspects 4.6 Map Projections and Plane Coordinates Most Surveyors measure and record coordinates in an orthogonal grid system. This means that points are defined by Northings, Eastings and orthometric height (height above sea level). Map Projections allow surveyors to ...

36 GPS Basics -1.0.0en Geodetic Aspects 0 N E The False Easting and False Northing are defined in order that the origin of the grid projection can be in the lower left hand corner as convention dictates. This does away with the need for negative coordinates.The Latitude of Origin defines the Latitud...

38 GPS Basics -1.0.0en Surveying with GPS 5. Surveying with GPS Probably even more important to the surveyor or engineer than the theory behind GPS, are the practicalities of the effective use of GPS.Like any tool, GPS is only as good as it’s operator. Proper planning and prepara- tion are essential...

39 GPS Basics -1.0.0en Surveying with GPS 5.1 GPS Measuring Techniques There are several measuring techniques that can be used by most GPS Survey Receivers. The surveyor should choose the appropriate technique for the appli- cation. Static - Used for long lines, geodetic networks, tectonic plate stu...

44 GPS Basics -1.0.0en Surveying with GPS 5.1.3 Kinematic Surveys The Kinematic technique is typically used for detail surveying, recording trajectories etc., although with the advent of RTK it’s popularity is diminishing.The technique involves a moving Rover whose position can be calculated relativ...

45 GPS Basics -1.0.0en Surveying with GPS 5.1.4 RTK Surveys RTK stands for Real Time Kinematic. It is a Kinematic on the Fly survey carried out in real time.The Reference Station has a radio link attached and rebroadcasts the data it receives from the satellites.The Rover also has a radio link and r...

46 GPS Basics -1.0.0en Surveying with GPS 5.2 Pre-survey preparation Before heading out into the field, the surveyor needs to prepare for the survey. Items that must be considered are:1. Radio Licences2. Power - charged batteries3. Spare cables4. Communication between survey parties 5. Coordinates o...

47 GPS Basics -1.0.0en Surveying with GPS Field Sheet Point Id Sensor Serial No Operation Type Antenna Type Height Reading Start Time Stop Time No. of Epochs No. of Satellites GDOP Date Operator Notes

48 GPS Basics -1.0.0en Glossary Glossary Almanac Library of coarse satellite orbital data used to calculate satellite position, rise time, elevation, and azimuth. Ambiguity The unknown integer number of cycles of the reconstructed carrier phase contained in an unbroken set of mea- surements from a s...

52 GPS Basics -1.0.0en Glossary Flattening Relating to Ellipsoids. f = (a-b)/a = 1-(1-e 2 ) 1/2 where a ... semimajor axis b ... semiminor axise ... eccentricity Fundamental frequency The fundamental frequency used in GPS is 10.23 MHz. The carrier frequencies L1 and L2 are integer multiples of the f...

59 GPS Basics -1.0.0en Glossary Further Reading GPS Theory and Practice - B. Hofmann-Wellenhof, H. Lichtenegger and J. Collins. ISBN 3-211-82839-7 Springer Verlag. GPS Satellite Surveying - Alfred Leick. ISBN 0471306266 John Wiley and Sons. Satellite Geodesy: Foundations, Methods and Applications - ...

60 GPS Basics -1.0.0en Glossary A Almanac 48 Ambiguity 24, 45, 48 Ambiguity Resolution 22, 24 Anti-Spoofing (A-S). See Error Sources Atmospheric propagation delay 48 Azimuth 48 B Bandwidth 48 Baseline 48 Bearing 48 Beat frequency 48 Binary biphase modulation 48 C C/A code 7, 13, 18, 22, 49 Carrier 4...

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