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

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

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

Typical CORS site monuments Wall Mount Drilled Gournd Marker

Who uses CORS and for what?

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

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.

CORS sites in the UK

CORS sites in Europe

CORS sites USA

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

A typical Time Series for a CORS

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

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

Who provides CORS data? http://igscb.jpl.nasa.gov/ http://www.epncb.oma.be/ http://www.ngs.noaa.gov/CORS/ http://swepos.lmv.lm.se/english/index.htm http://www.ordsvy.gov.uk/

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

Summary of CORS

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.

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 61-80 for each line contained in the header section These labels are mandatory and must appear exactly as per format description

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

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

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.

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

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

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

Orbit Products cont’d Orbit Product Providers http://igscb.jpl.nasa.gov/components/compindex.html http://www.ngs.noaa.gov/ http://sopac.ucsd.edu NOTE: Predicted, Rapid and Final Orbit Products often come in .sp3 format

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