Surveying with the Global Positioning System Code Pseudo-Ranges

Outline of Session 1 GPS Fundamentals Types of GPS Positioning Levels of Accuracy Pseudo-Ranges DOPs GLONASS

Before GPS There Was... TRANSIT (Doppler Shift) - 16 or less fixes a day - Sub-meter accuracy in about 3 days - Worldwide coverage - Lat/Long/Height LORAN (Triangulation) - Continuous position fixes - Accurate to 300 meters - Limited coverage - Lat/Long

...Now GPS - Continuous position fixes - Worldwide coverage - Lat/Long/Height - Centimeter accuracy in seconds TRANSIT (Doppler Shift) - 16 or less fixes a day - Sub-meter accuracy in about 3 days LORAN (Triangulation) - Accurate to 300 meters - Limited coverage - Lat/Long

What is GPS? A super accurate system Developed and maintained by US Department of Defense Satellite-based Sold US Congress on the idea that other applications would follow Signals are free Unlimited users

and one Master Control Station 3 Segments of GPS Space Segment 4 Monitor Stations Control Segment and one Master Control Station User Segment

GPS Satellite Constellation 24 Satellites 6 orbital planes 20,000 km high 12 hour orbits At least 4 Satellites in view 24 hours per day Any weather

Launched on Delta Rockets

The GPS Satellite 3 atomic clocks L Band Radio Signals - 19 -24 cm Wavelength Codes on the signal Distance to SV by time signal takes to reach the receiver or other method Satellite Position Also give health of system

The GPS Satellite

3 Types of GPS Positioning Point Positioning Differential GPS GPS Surveying

3 Levels of GPS Accuracy Point Positioning - 100m 95% of the Time for Civilian Users - With SA About 10-20m without SA Differential GPS - 0.5m to 10m GPS Surveying - 5mm per km typical and centimetres in thousands of km possible Surveyors can and should be involved in all 3 levels

1 Receiver - “Point Positioning” Satellites are saying My Time is … My Position is ... Basic technique for which GPS was designed Basic Civilian Receivers < $ 500

3 4 2 5 1 Code Range GPS in 5 Steps 4 SVs to solve Based on Trilateration 1 Use message from satellite for its location Correct for Troposphere & Ionosphere 5 Distance from satellites (SV) using speed of light 2 3 4 SVs to solve for X,Y,Z,t

Trilateration From Satellites 1 Trilateration From Satellites By measuring distance from several satellites you can calculate your position

Trilateration Assume for now we can measure a distance to a SV One measurement narrows down our position to the surface of a sphere 20,000 Km We're somewhere on the surface of this sphere.

Trilateration Second measurement narrows it down to intersection of two spheres Intersection of two Spheres is a circle

Trilateration Third measurement narrows to just two points

Trilateration In theory 3 measurements are enough because We can discard one point because it will be a ridiculous answer Out in space Or moving at high speed But we do need the 4th measurement to cancel out clock errors

Trilateration 4 Ranges to resolve for Latitude, Longitude, Height & Time It is similar in principle to a resection problem 16

2 Satellite Ranging Measuring the distance from a satellite Done by measuring travel time of radio signals

Speed-of-Light Measurement Measure how long it takes the GPS signal to get to us Multiply that time by 300,000 km/sec Time (sec) x 300,000 = km If you've got a good clock in the receiver, all you need to know is exactly when signal left satellite

Outline Principle : Range Xll Vl Xl lll l ll lV V Vll Vlll X lX 8

Outline Principle : Range Xll Vl Xl lll l ll lV V Vll Vlll X lX 9

Outline Principle : Range Xll Vl Xl lll l ll lV V Vll Vlll X lX 10

Outline Principle : Range Xll Vl Xl lll l ll lV V Vll Vlll X lX Range = Time Taken x Speed of Light R = Dt x c 11

How Do We Know When the Signal Left the Satellite? One of the Clever Ideas of GPS: Use same code at receiver and satellite Synchronize satellites and receivers so they're generating same code at same time Then we look at the incoming code from the satellite and see how long ago our receiver generated the same code from satellite from ground receiver measure time difference between same part of code

3 Accurate Clocks Whole system depends on very accurate clocks Necessary to measure travel time Ensures receiver and satellite are synchronized Satellites have atomic clocks Accurate but expensive Ground receivers need consistent clocks Secret is in extra satellite measurement that adjusts receiver clock

Knowing Where the Satellites Are 4 Knowing Where the Satellites Are 20,000 km up - high orbit Very stable orbits No atmospheric drag Survivability Earth coverage Monitored by US Defense Department DOD transmits corrections back to satellite Corrections transmitted from satellites to us Status message

Knowing Where the Satellites Are - Ephemeris Monitor stations Diego Garcia Ascension Island Kwajalein Hawaii Current ephemeris is transmitted to users Space Segment GPS Control Colorado Springs

Atmospheric Corrections 5 Atmospheric Corrections Apply estimated corrections The signals are delayed by the ionosphere and troposphere Receiver makes estimated corrections for these delays Ionosphere Troposphere

Satellite Geometry

Dilution of Precision (DOP) A measure of Satellite geometry Indicates the quality of position fix Can be expressed in different dimensions for example: PDOP, HDOP, VDOP, TDOP PDOP less than 6 is best

Dilution of Precision (DOP) Relative position of satellites can affect error 4 sec 6 sec idealized situation

Dilution of Precision (DOP) Real situation - fuzzy circles 4 ‘ish sec 6 ‘ish sec uncertainty Point representing position is really a box

Dilution of Precision (DOP) Even worse at some angles Area of uncertainty becomes larger as satellites get closer together

Selective Availability “SA” Military Users - Precise Positioning Service (PPS) +/- 15m for Military Users Uses P Code which has high resolution Good accuracy Satellite Positions (Ephemeris) Civilian Users - Standard Positioning Service (SPS) +/- 100m 95% of the time (+/- 150m vertical) SA on +/- 10-20m 95% of the time SA off C/A code 10 times less resolution than P code Ephemeris accuracy deliberately downgraded Future of SA? Now turned off - Presidential Directive For how long?

GLONASS Russian Federation’s GLObal NAvigation Satellite System First launch Oct 1982 Uses 3 orbital planes rather than 6 with GPS GPS - same frequency but different codes GLONASS - same code, different frequency Channel of Standard Accuracy (CSA) 60m horizontal, 75m vertical (99.7% confidence) Restricted access to Channel of High Accuracy (CHA) Gallelio emerging

GLONASS Receivers with both GPS and GLONASS More satellites available = more robust solution Can work despite significant satellite obstruction Advantages for surveying; especially for techniques which use minimal observation time 2 significant system level issues: different timing systems used different underlying geodetic reference systems, WGS84 for GPS vs PZ90 for GLONASS. IGEX98 - observe both systems around the world using geodetic quality equipment Station at DNR’s Landcentre in Brisbane

Conclusion to Session 1 GPS Fundamentals Types of GPS Positioning Levels of Accuracy Pseudo-Ranges DOPs GLONASS