GPS Accuracy Comparison. Tom Biernacki Florida Department of Environmental Protection

Transcription

1 GPS Accuracy Comparison Tom Biernacki Florida Department of Environmental Protection

2 What Effects the accuracy of GPS? Atmospheric delay Multipath bounce Weak receiver algorithms

3 Atmospheric Delay Signal is bent

4 How does one correct for atmospheric delay? Atmospheric delay is caused by signals being delayed in getting to the receiver. Differential Corrections will correct for most of this. Base station must be within 300 miles.

5 Atmospheric Delay Differential Corrections can be either real time or post processed. Post processing often gives better corrections since averaging times may be much shorter.

6 Elevation Mask Elevation mask specifies what region of the sky to look for SVs 15 degrees Base Station: 10 Rover: miles or less

7 How does one correct for Multipath errors? Multipath errors are caused by bouncing signals Measuring the signals strength can help prevent suffering from this phenomenon. High quality antennas are the best way to prevent multipath errors.

8 Antenna Types Bigger is better and it cost more! Small, inexpensive, and not very accurate Big, expensive, and very accurate

9 Multipath Errors GPS Receiver Building or most any object

10 Expensive Antenna have SNR Mask Filters SNR Mask is the Signal-to-Noise Ratio Mask A ratio of the signal to noise is a measure of how strong a signal is.» The higher the ratio number, the stronger the signal.» Normal setting is 6, but higher is better.

11 SNR Mask Signals Noise

12 Receiver Algorithms What does the receiver do with the data? Accept everything or filter out the bad?

13 GPS Receiver Settings Include: Elevation Mask SNR Mask PDOP Minimum Positions

14 PDOP PDOP is calculated using RMS (Root Mean Squares) RMS: An expression of the accuracy of a point measurement. It is the radius of the error circle, within which approximately 70% of the position fixes are to be found.

15 PDOP PDOP: Point Dilution Of Precision A measure of the strength of the satellites geometry. Better accuracy is achieved when SVs are a greater distance apart.

16 WHY?

17 Which Well? 5 meters... Well A Well B Well C

18 Which Stream is it? Stream A Stream B X 10 meter or 33 foot radius Stream C

19 Accuracy Standards GPS Standard Operating Procedures

20 Accuracy Standards Accuracy standards ensure: Spatial data meets a specified accuracy level. Protocols are followed for repeatability. High quality databases.

21 Accuracy Standards Accuracy standards specify: How to collect data. Internal settings. How to process and store data.

22 What are DEP s Accuracy Standards Division of Water Resource = 12.2 meters Program = 5 meters

23 Accuracy Settings Internal Settings: PDOP = 6 or less SNR = 6 or more Elevation Mask = 15 o Minimum number of positions = 30 Minimum number of satellites = 4

24 GPS Accuracy Comparison 1. Hammerhead 3 with Garmin AGPS With filter settings 2. Trimble Pro XR DGPS with filter settings

25 Ginger - Second order Benchmark

26 5 meter circles

27 Hammerhead AGPS

28 Hammerhead AGPS

29 Trimble in AGPS mode

30 Trimble in DGPS mode

31 Hammerhead using Trimble in DGPS mode

32 Post-processed Trimble in AGPS mode using Tally CBS

33 Base Station Data during same time

34 Questions?