1 The U.S. Continuously Operating Reference Station (CORS) System Richard Snay NOAA’s National Geodetic Survey Corbin, Virginia June 2008.

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The U.S. Continuously Operating

Reference Station (CORS) System

Richard SnayNOAA’s National Geodetic Survey

Corbin, VirginiaJune 2008

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Everyone is able to know where they are and

where other things are anytime, anyplace!

NOAA’s National Geodetic Survey

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The Global Positioning System (GPS)

Unaugmented GPS enables positioning with accuracies ranging from 1 to 10 meters.

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The CORS network enables differential GPS positioning with accuracies from 1 to 10

centimeters, or better.

Continuously Operating Reference Stations (CORS)

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Sample CORS Sites

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CORS Information

• CORS network contains over 1,300 stations as of June 2008.

• Growing at rate of about 200 stations per year.

• Each station collects GPS signals, and NOAA makes these data freely available to the public via the Internet for post-processing applications.

• More than 200 organizations participate in the CORS program by sponsoring and operating one or more stations.

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CORS Partners

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Access to CORS Data

In Silver Spring, Maryland (CORS-East)• Anonymous File Transfer Protocol (FTP)

ftp://cors.ngs.noaa.gov• UFCORS - User Friendly CORS

http://www.ngs.noaa.gov/UFCORS

In Boulder Colorado (CORS-West)• Parallel and independent data collection and on-line

storage at NOAA’s National Geophysical Data Center Anonymous FTP ftp://wwwest.ngs.noaa.gov

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CORS Supports Precise Positioning

After CORS: Accurate differential GPS positioning with one-person field crew.

Before CORS: Accurate differential GPS positioningwith multi-person field crew.

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Online Positioning UserService (OPUS)

• Submit data to www.ngs.noaa.gov/OPUS/

• Data are processed automatically using NOAA computers & software

• Corresponding positional coordinates computed with respect to 3 suitable CORS or IGS sites

• Computed coordinates returned via email (usually in minutes)

● Collect at least 15 minutes of dual-frequency GPS data

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Positioning Error vs. Duration of the Observing Session

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CORS enables sub-meter differential positioning with less than a minute of GPS data

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CORS Enables Users to Determine the Travel Path of a Moving

Platform

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Aircraft Positioning with CORS

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0 100 200 300 400 500

Distance from CORS to Aircraft, km

3D R

MS

Err

or, c

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3D RMS error = 15 cm + 0.6 ppm

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CORS for MonitoringHorizontal Crustal Motion

Horizontal velocities in the western U.S.relative to the North American Datum of 1983as derived from geodetic observations.

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CORS for Monitoring Vertical Crustal Motion

Vertical velocities associated with Glacial Isostatic Adjustment

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Tide Gauges Located Near a CORS Whose Vertical Crustal Velocity Has Been Accurately Determined

CORS-calibrated tide gauge dataindicate that the rate of sea level rise

around North America was 1.8 +/- 0.2 mm/yr

during the 1900-1999 time interval

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CORS for MonitoringPrecipitable Water Vapor

NOAA’s Earth Systems Research Laboratory uses CORS data to produce nowcasts of precipitable

water vapor, once every 30 minutes.

Transitioning to operational status starting in 2010.

NOAA will seek to install “robust” CORS on Caribbean islands and on oil platforms in the Gulf of Mexico to better anticipate the effect of an active hurricane or tropical storm.

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CORS for Monitoring Space Weather

NOAA’s Space Weather Prediction Center uses CORS data to map the distribution of free electrons in

the ionosphere, once every 15 minutes.

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Just Around the Corner

Starting later in 2008, the CORS system will:

• Provide GPS L2C data

• Provide GLONASS data

• Broadcast GNSS data via the Internet in real-time (on an experimental basis). (For selected sites only.)

Red dots identify locations of CORS sites that collect both GPS and GLONASS data.

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CORS ADVANTAGES OVERPASSIVE REFERENCE STATIONS

• 3-dimensional (Lat., Long., & Ellipsoid Ht. or X, Y, & Z)

• Eliminates control points reconnaissance (time and money).

• Eliminates needing people and equipment at a control points.

• Direct tie to National Spatial Reference System (NSRS).

• CORS positions are of the highest accuracy.

• CORS positions and velocities are available in both NAD 83 and ITRF coordinate systems.

• CORS positions are continuously monitored and will be updated if the site moves.

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CORS DATA QUALITY

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Common Question

• CORS GPS hardware differs from our GPS hardware. Do we have to use only CORS with the same hardware?

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GPS Data: The Receiver INdependent

EXchange (RINEX) Format • Data file spans

• hourly, daily, customized (UFCORS)

• Data collection rates

• 1sec, 5sec, 10sec, 15sec, and 30sec

• Data file life-time

• hourly: 2 days + today

• daily: permanently

• Data are decimated to a 30-second rate after 30 days

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Common Question

• The closest CORS to our project area is 50 miles away. How can we use CORS at this distance?

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TWO TESTS FOR POSITIONING PRECISION

• Dual Frequency Carrier Phase Data

• Single Frequency Code Data

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TEST DESIGN:DUAL FREQUENCY CARRIER

PHASE DATA

• Dual Frequency Geodetic Receivers

• Post-Processed with a Precise Orbits

• Pairs of CORS sites forming 11 Baselines

• Baseline lengths ranging from 26 to 300 km

• Various Observation Session Duration

(1, 2, 4, 6, 8, 12, and 24 hours)

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Positioning error versus distance

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Positioning Error vs. Duration of the Observing Session

Dual-frequency GPS carrier-phase observations

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East Offset from 24 hr. Mean (m)

4 Hr/160 K

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East Offset from 24 hr. Mean (m)

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East Offset from 24 hr. Mean (m)

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East Offset from 24 hr. Mean (m)

24 Hrs/160 K

Time Scatter Plots (Horizontal)

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Vertical Precision Using Dual-FrequencyGPS Carrier Phase Observations

95% Confidence Level

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TEST DESIGN:SINGLE-FREQUENCY CODE DATA

• Positioned 12 points relative to each of seven CORS sites• Baseline lengths of 18, 23, 132, 165, 170, 253, and 292 kilometers

• Observed 1-minute sessions at a 5-second record rate (interpolated CORS data from 30 to 5 seconds)• Repeated experiment 4 times over a 2-day period

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00.10.20.30.40.50.60.70.80.9

0 100 200 300 400

Baseline Length, km

RM

SE, m

eter

sPositioning Accuracy with Code Data

red = east-westblue = north-south

Observation Time = 1 minute

RMS error = 25 cm + 2 ppm

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COMMENTS

•Sub-Meter precision is possible with baseline lengths up to 300 kilometers

•This precision is possible using interpolated CORS data

•Most CORS data is available within 1-hour of the survey

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SURVEYING METHODS

Static Positioning (code)

Static Positioning (carrier phase)

Kinematic GPS

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ACCESSING CORS DATAACCESSING CORS DATA

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ACCESSING CORS DATA & METADATA

• Web address = http://www.ngs.noaa.gov/CORS/

• Metadata = data about data

• CORSAGE = CORS Amiable Geographic Environment

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CORSAGECORS Web Page

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CORSAGENetwork Map

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CORSAGE Regional Map

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METADATA FOR A CORS SITE

• Coordinates (positions & velocities) • Data availability profiles (charts showing

times for which data has been collected)• Data sheets (descriptive information)• Log files (descriptive information)• Site photos• Time series of positional coordinates• Google Maps

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BILLINGS 1 (BIL1), MONTANA

Retrieved from NGS DataBase on 01/10/01 at 09:45:16.

____________________________________________________________________________ | || NAD_83 POSITION (EPOCH 1997.0) || Transformed from ITRF97 (epoch 1997.0) position in Sep. 2000. || X = -1372156.022 m latitude = 45 58 16.23742 N || Y = -4223946.947 m longitude = 107 59 47.29949 W || Z = 4563650.156 m ellipsoid height = 874.381 m || || NAD_83 VELOCITY || Transformed from ITRF97 velocity in Sep. 2000. || VX = 0.0000 m/yr northward = 0.0000 m/yr || VY = 0.0000 m/yr eastward = 0.0000 m/yr || VZ = 0.0000 m/yr upward = 0.0000 m/yr ||____________________________________________________________________________|

CORS POSITION & VELOCITY (NAD 83)

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BILLINGS 1 (BIL1), MONTANA

Retrieved from NGS DataBase on 09/25/00 at 12:27:27. ____________________________________________________________________________ | || Antenna Reference Point(ARP): BILLINGS 1 CORS ARP || ------------------------------------------------- || PID = AI7658 || || || ITRF97 POSITION (EPOCH 1997.0) || Computed in Sept., 2000 using 12 days of data. || X = -1372156.567 m latitude = 45 58 16.26213 N || Y = -4223945.695 m longitude = 107 59 47.34150 W || Z = 4563650.195 m ellipsoid height = 873.698 m || || ITRF97 VELOCITY || Predicted with HTDP_2.4 in Aug., 2000. || VX = -0.0184 m/yr northward = -0.0106 m/yr || VY = -0.0020 m/yr eastward = -0.0169 m/yr || VZ = -0.0074 m/yr upward = 0.0000 m/yr || |

CORS POSITION & VELOCITY (ITRF)

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BIL1 STATION LOG FILE

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BIL1 STATION LOG FILE

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CORS SITE PHOTO

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Position Time Series (long-term)

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POSITION TIME SERIES (last 60 days)

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PRIMARY DATA FILES

• GPS observations at a CORS site

• Satellite orbits (ephemerides)

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GPS Data – RINEX Format

• Data file spans

• hourly, daily, customized (UFCORS)

• Data collection rates

• 1sec, 5sec, 10sec, 15sec, and 30sec

• Data file life-time

• hourly: 2 days + today

• daily: permanently

• Data are decimated to a 30-second rate after 30 days

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The RINEX file naming convention is as follows:{SSSS}{DDD}{H}.{YY}{T} where SSSS is the four character site identifier, DDD is the day of year,

H is a letter which corresponds to an hour long UTC time block,

YY is the year,T is the file type.

For daily files, the format would be {SSSS}{DDD}0.{YY}{T}.Hour long UTC time block identifier (H):

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 a b c d e f g h i j k l m n o p q r s t u v w x

File type Ending (T)Meteorological mObservation oNavigation nSummary s

RINEX FILE NAMING CONVENTION

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NGS is one of the eight International GNSS Service

(IGS) Analysis Centers (AC) participating in the production of accurate GPS orbits:

Final Precise (~ 13 days latency)[accuracy < 4 cm] Rapid (17 hours latency) [accuracy < 5 cm]

Ultra-Rapid (real-time) [accuracy < 10 cm] Satellite positions in SP3 format are given (once

every 15 minutes) in current ITRFxx frame

IGS Satellite Ephemerideshttp://www.ngs.noaa.gov/orbits/

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Three ways to download CORS Information

• Web-based User-Friendly CORS (UFCORS)

• Web-based “Standard” download

• Anonymous FTP (File Transfer Protocol)

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UFCORS/STANDARD

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UFCORS: a Web utility enabling users to

• Obtain CORS data for an exact time interval• Choose a sampling rate for the requested

data• Specify how the data files should be

compressed• Receive all associated data & metadata

(coordinates, descriptive information, orbits)• Receive information as soon as it is posted

(GPS data are usually posted within an hour of the time these data are received by NOAA)

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UFCORS - page 1

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UFCORS - page 2

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Standard Download

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ACCESS TO CORS ARCHIVEVIA FTP

To access the CORS public directories, follow the steps below.

Type the “ftp” command followed by the Internet address as follows

ftp cors.ngs.noaa.gov

Respond to the following:

Name(cors.ngs.noaa.gov): anonymous

Password: [email protected]

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FTP is a user interface to the File Transfer Protocol. FTP copies filesover a network connection between the local ``client'' (user) computerand a remote ``server'' computer. FTP runs on the client computer.

The user's system must have access to the INTERNET and support the File Transfer Protocol (FTP). Some useful ftp commands are given below.

ascii set ascii transfer type binary set binary transfer type bye terminate ftp session and exit cd change remote working directory dir list contents of remote directory get retrieve one file help print local help information mget retrieve multiple files mput send multiple files prompt force interactive prompting on multiple commands put send one file quit terminate ftp session and exit show display the contents of an ASCII file

* Actual commands may vary among operating systems.

FILE TRANSFER PROTOCOL (FTP)

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You will arrive at the ftp command level indicated by the prompt “ftp>”. If you have trouble, type “help” to print local help information or review the section FILE TRANSFER PROTOCOL for help with additional commands.

The following sub-directories contain additional files and information

- coord NAD83 and ITRF positional information.

- graphics CORS network maps.

- itrf Files on the IERS Terrestrial Reference Frame.

- rinex Rinex data files.

- station_log Station information, antenna specifications, and site contacts.

- utilities Programs for manipulating the RINEX files.

FTP DIRECTORIES

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File Transfer Protocol (FTP)

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File Transfer Protocol (FTP)

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RINEX files on the CORS file server are stored in a gzip compressed mode.These compressed files will have the extension .gz . An example is given below.

ais12330.98o.gz

All compressed files and executables should be transferred in binary mode.Text files should be transferred in ascii mode.

Before downloading files using the FTP protocol, set the transfer mode bytyping “binary” or “ascii” at the ftp prompt. Then use “get” or “mget” toretrieve the files.

FILE COMPRESSION FORMAT

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Several DOS based utility programs are available to manipulate the RINEXdata files. Versions also exist for other platforms such as SiliconGraphics (sgi), Sun Microsystems (sun), and Hewlett Packard (hp).

decimate.exe Utility program to decimate 5 second data to a user specified rate.

gzip386.exe Executable file which contains the utility "gzip.exe".

inflate.exe Self-extracting utility program to uncompress files with the ".Z" extension.

interpo.exe Utility program to interpolate between data epochs. Please read the documentation for this utility for more details.

join24pc.exe Utility program to join two or more hourly RINEX observation or navigation files.

SOFTWARE / RINEX UTILITIES

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INTERPO Interpolate RINEX observational data at faster ratesusing Neville’s algorithm for polynomial interpolation.

interpo -i <input file> -o <output file> [ -s <start time>-e < end time>] -n <interpolation interval>

* Fields between [ ] are optional.

interpo -i ais1030a.96o -o ais1030a.out -n 5

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GLOBAL NAVIGATION SATELLITE SYSTEMS (GNSS)

POTENTIAL DEVELOPMENTS

(2008 - 2020)

US GPS – Second (L2C) and third civil codes US GPS MODERNIZATION - BLOCK III RUSSIAN GLONASS ENHANCEMENTS

EUROPEAN UNION - GALILEO CHINESE – COMPASS (BEIDOU)

90+ Satellites

More Robust Signal Transmissions

Real-Time Unaugmented Accuracy = 1 Meter (or better!)