1. Surveying with the Global Positioning System Code Pseudo-Ranges

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

3. 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

4. ...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

5. 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

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

7. 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

8. Launched on Delta Rockets

9. The GPS Satellite 3 atomic clocks
L Band Radio Signals 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

10. The GPS Satellite

11. 3 Types of GPS Positioning
Point Positioning
Differential GPS
GPS Surveying

12. 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

13. 1 Receiver - “Point Positioning”
Satellites are saying
My Time is …
My Position is ...
Basic technique for which GPS was designed
Basic Civilian Receivers < $ 500

14. 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

15. Trilateration From Satellites1
Trilateration From Satellites
By measuring distance from several satellites you can calculate your position

16. 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.

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

18. Trilateration
Third measurement narrows to just two points

19. 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

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

21. 2 Satellite Ranging Measuring the distance from a satellite
Done by measuring travel time of radio signals

22. 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

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

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

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

26. 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

27. 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

28. 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

29. Knowing Where the Satellites Are4
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

30. 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

31. Atmospheric Corrections5
Atmospheric Corrections
Apply estimated corrections
The signals are delayed by the ionosphere and troposphere
Receiver makes estimated corrections for these delays
Ionosphere
Troposphere

32. Satellite Geometry

33. 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

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

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

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

37. 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
+/ m 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?

38. 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

39. 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

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