1. SVY207: Lecture 17 Third party products and GPS processing procedures
Motivation
Ordnance Survey has established a National Network of over 30 Continuously Operating GPS Reference Stations (CORS)
Data from these stations is served, free of charge, across the internet and can be used by anyone
GPS is now a standard technique in topographic and engineering surveying
Aim
To understand what GPS products are available, their format, and how/when to use them

2. Available GPS Products
What kind of products are available?
Data from CORS GPS Stations
Static
Real Time
Data types
RINEX
sp3
GPS satellite orbit products
Final
Rapid
Predicted/Real Time

3. What is a CORS? The principle components of a CORS
Geodetic dual frequency receiver
Choke ring antenna (Optional ray-dome)
Communications to central data collection unit

4. Typical CORS site monuments
Wall Mount
Drilled Gournd Marker

5. Who uses CORS and for what?

6. What are the advantages of CORS?
Provide 3-dimensional data 24hrs/365 days per year
The are permanent
Users do not need to reconnaissance control points
Users do not need to set up instruments at control points
Saves time and money!
Data is often available for FREE
Saves money!
CORS positional coordinates are more precise than those of other control points
Positions and velocities are available together with metadata (more later)
Positions are continuously monitored and will be updated if station moves
CORS provide a direct tie to National Reference System

7. What are the disadvantages of CORS?
Distances to sites is currently excessive
UK Ordnance Survey active network of CORS provides one station per 100km on average
Receiver hardware at CORS site will may differ from users hardware
Antenna phase centre offsets
Is data recorded in the same way?
Fritz Brunner (2001), found that mixing of the Trimble 4000 SST with the Trimble 4000 SSE and Trimble 4000 SSI geodetic receivers over short observation times (less than a few hours) can cause vertical height errors of up to five centimeters.
These errors appear to be caused by differences in how each receiver internally processes the incoming carrier phase signals.

8. CORS sites in the UK

9. CORS sites in Europe

10. CORS sites USA

11. Metadata for CORS sites
Metadata should include
Coordinates (positions and velocities)
Data availability profiles
Charts showing time for which data has been collected
Data sheets
Information about receiver type, antenna type, station construction, raydome, log files
Site photos
Time series of positional coordinates

12. A typical Time Series for a CORS

13. Surveying Applications using CORS
Precise Positioning
Full processing of code and phase
Double differencing ++
Tie in surveys to Ordnance Survey Reference System
Establish GPS base station for local survey
Rapid Static
Static
RTK
Blunder detection - resolving conflicting results
Base station for monitoring
NEW data provision for one man RTK
Requires high CORS data rate + communication of data to users in real-time

14. Surveying Applications using CORS
Sub-meter Applications
Process code only
GIS asset surveys
Environmental inventories
Tree locations
Species locations
Precision Agriculture (after the event)
NEW - Real time via communications link to user

15. Who provides CORS data? http://igscb.jpl.nasa.gov/

16. Summary of CORS
To position points at the cm level over 20km use precise orbit products (see later) and phase data
Ensure that you record >4hrs data
Use ion-free solutions for long baselines
Ensure that either estimate or model Troposphere
Remember final orbit products have a significant time delay (see later)
Ensure that all antenna information is correct - otherwise large height errors will result
Sub-meter positioning is possible with shorter data spans

17. Summary of CORS

18. RINEX data format Receiver INdependent EXchange Format
Motivation for development
All manufacturers have developed their own ‘BINARY’ file formats for data storage specific to their processing software
Problems occur when processing data from another manufactures receiver
TWO potential solutions
Front-end software to interpret and convert the raw data for each receiver type
Unique data exchange format + software to convert the raw data into this exchange format
RINEX developed by the Astronomical Institute of the University of Berne to allow easy exchange of the GPS data
Principle driver was the large European GPS campaign EUREF 89 - involved more than 60 GPS receivers of 4 different manufacturers.

19. RINEX data format cont’d
Receiver INdependent EXchange Format
Includes formats for observation, navigation and meteorological data
Observation and Navigation are the most important
Each file type consists of a header section and a data section
Header section contains global information for the entire file and is placed at the beginning of the file.
Contains header labels in columns for each line contained in the header section
These labels are mandatory and must appear exactly as per format description

20. RINEX data format cont’d
RINEX VERSION / TYPE
PGM / RUN BY / DATE
COMMENT (Opt)
MARKER NAME
MARKER NUMBER (Opt)
OBSERVER / AGENCY
REC # / TYPE / VERS
ANT # / TYPE
APPROX POSITION XYZ
ANTENNA: DELTA H/E/N
WAVELENGTH FACT L1/2
# TYPES OF OBSERV
INTERVAL (Opt)
TIME OF FIRST OBS
TIME OF LAST OBS (Opt)
LEAP SECONDS (Opt)
# OF SATELLITES (Opt)
PRN / # OF OBS (Opt)
END OF HEADER
RINEX Observation file header labels
e.g. NEWC067a.02o

21. RINEX data format cont’d
An example of RINEX Observation data

22. RINEX data format cont’d
RINEX filename convention:
SSSSDDDT.YYo
Four site SSSS
On day-of-year DDD
Observations session T
Year YY
o = Observation
Other options - n = Navigation, g = GLONAS, m = Meteorological data
E.g., NEWC067a.02o is observation data for CORS site Newcastle, day 067, session a, year 2002.

23. RINEX data format cont’d
RINEX Navigation data
RINEX VERSION / TYPE
PGM / RUN BY / DATE
COMMENT (opt)
ION ALPHA (opt)
ION BETA (opt)
DELTA-UTC: A0,A1,T,W (opt)
LEAP SECONDS (opt)
END OF HEADER
RINEX Navigation file header labels
e.g. NEWC067a.02n

24. RINEX data format cont’d
An example of RINEX Navigation data

25. Orbit Products Types of obit product
Variety of orbit products are available from a variety of providers
Precise orbit (Final)
Rapid orbit
Ultra Rapid orbit
These orbits tend to be more accurate
Typically not available for real-time applications
BUT don’t forget about Broadcast orbit
Braodcast orbit - transmitted from GPS satellite in real-time

26. Orbit Products cont’d Orbit Product Providers
NOTE: Predicted, Rapid and Final Orbit Products often come in .sp3 format

27. Orbit Products cont’d Which orbit type should I use? Rule of Thumb:
Where d is baseline length in km and the errors are in metres.
Errors at different baselines lengths:
<<1mm
Precise
<1mm
Rapid
2.5mm
Ultra-rapid
20-100mm
5-25mm
1-5mm
Navigation
200km
50km
10km