ftp://ftp.ngs.noaa.gov/dist/whenning/GLRHMC/

REAL-TIME ACTIVITIES AT THE NGS

I. OPERATE AN NTRIP CASTER. (Fed. Owned/operated – currently 8. RTCM 2.3 & 3.0, From Foundation CORS. NO CORRECTORS)

II. DEVELOP AND PUBLISH GUIDELINES DESCRIBING BEST PRACTICES IN RTK & RTN .(RTK Users draft, RTN Operators draft, etc.)

III. PARTICIPATE IN MEETINGS, FORUMS, WORKSHOPS, ETC., CONCERNING REAL-TIME NETWORKS. SEEK LEADERSHIP ROLES.(FIG, FGCS, ESRI, ACSM, RTCM, etc.)

IV.RESEARCH PHENOMENA AFFECTING ACCURATE REAL-TIME POSITIONING. (Orbits, refraction, multipath, antenna calibration, geoid separations, gravity, crustal motion, etc.)

SINGLE-BASE USERS GUIDELINES

WHY/HOW?

• Legacy equipment

•Closest base networks

•Areas with no cell coverage

•Empirical methods

•Source for background info.

•Dynamic technology

SINGLE BASE GUIDELINES –MEANT AS A BEST METHODS AND ALSO AS A

BACKGROUND REFERENCE FOR RT:• EQUIPMENT• GNSS SIGNAL BASICS• PRINCIPLES OF RT POSITIONING• ATMOSPHERIC ISSUES• PLANNING• BEST METHODS• ACCURACY/PRECISION• ATMOSPHERIC ISSUES• MULTIPATH• METADATA• COMMUNICATION• OFFICE CHECKS• RT GLOSSARY

EVERY EPOCH OF OBSERVATION

MUST COMPUTE CHANGE IN

POSITION DUE TO EARTH ROTATION, SATELLITE ORBIT

CHANGE AND RELATIVITY!

AT “C”, 1 NANOSECOND = 30 CM!

SUNSPOT CYCLE• Sunspots follow a regular 11

year cycle• We are just past the low

point of the current cycle• Sunspots increase the

radiation hitting the earth's upper atmosphere and produce an active and unstable ionosphere

http://www.swpc.noaa.gov/

2013

WWW.SWPC.NOAA.GOV

IONOSPHERIC EFFECTS ON POSITIONING

HIGH IONO- NO NETWORK

AVERAGE IONO- NO NETWORK

WITH NETWORK

(SOURCE-BKG- GERMANY)

DISTANCE TO REFERENCE STATION (KM)

2D

PR

EC

ISIO

N/A

CC

UR

AC

Y (

CM

)

SINGLE BASE

RTK @ 10 KM

NETWORK SOLUTION

@ 30 KM

(1994-1995)(2000-2002)

IONO, TROPO, ORBIT CONTRIBUTE TO PPM ERROR

REMEMBER GNSS EQUIPMENT MANUFACTURERS’ SPECS!

Ionospheretroposphere

TROPOSPHERE DELAY

The more air molecules, the slower the signal (dry delay)High pressure, Low temperature 90% of total delayrelatively constant and EASY TO CORRECT FOR

The more water vapor in the atmosphere the slower the signal (wet delay)High humidity10% of total delayHighly variable and HARD TO CORRECT FOR

THE NGS AND REAL-TIME NETWORKS

REAL TIME NETWORKS (RTN)- A NEW INFRASTRUCTURE!

• PERHAPS OVER 80 RTN EXIST IN THE USA WITH MANY IN THE PLANNING STAGES

• HOW ARE THEY ESTABLISHED? • HOW ARE THEIR COORDINATES

COMPUTED? ARE THEY CONSISTENT? • HOW IS THE NETWORK ADJUSTED? • **HOW DOES THE RTN ALIGN TO THE

NSRS? • CAN USERS USE ANY MANUFACTURERS’

EQUIPMENT IN THE RTN?• DO OVERLAPPING NETWORKS GIVE THE

SAME COORDINATES?• WHAT ARE THE FIELD ACCURACIES?

?

NGS RT WEB PAGES – OPERATIONAL PROTOTYPE

CURRENT POLICY: NGS WILL NOT RESTREAM RT DATA FROM NON-FEDERAL SITES

RADIO TECHNICAL COMMISSION FOR MARITIME SERVICES

DIFFERENTIAL GNSS POSITIONING

= RTCM SC-104• INTERNATIONAL STANDARDS BY ORGANIZATIONS- non-profit scientific, professional and educational, governmental and non-governmental

•RTCM format is OPEN SOURCE, GENERIC

•RECOMMENDED STANDARDS FOR DIFFERENTIAL GNSS POSITIONING •ALL MAJOR RT GNSS GEAR CAN USE THIS FORMAT

•IN USA, MEMBERS INCLUDE: FCC, USCG, NGS, MAJOR GNSS MANUFACTURERS

•NTRIP STANDARDS (An application-level protocol that supports streaming (GNSS) data over the Internet)

NGS RTCM 3.X STREAMS

“FOUNDATION” CORS CONCEPT (SHOWN AT NOMINAL 500 KM SPACING)

USCG86 TOTAL -38 INLAND (DOT)

DEVELOPING COOPERATIVE PARTNERS

USCG DGPS TEST SITES

DRIVER, VAUPPER KEWANEE, MINEW BERN, NCMORICHES, NYANNAPOLIS, MDENGLISH TURN, LACARD SOUND, FLTAMPA, FLKODIAK, AKPENOBSCOT, ME

PBO STATIONS

BINEX, RTCM 2.4, RTCM 3.0

0.6 to 2.0 seconds latency

SERVER IN BOULDER, CO

97 RT STATIONS +/-

http://pboweb.unavco.org/?pageid=107

PBO RT STREAMS

http://www.ntrip.org/155 STREAMS TO

http://www.igs-ip.net/home

2002 2003 2004 2005 2006 2007 2008 2002 2003 2004 2005 2006 2007 2008

2002 2003 2004 2005 2006 2007 2008 2002 2003 2004 2005 2006 2007 2008

Position Time Series (long-term)

2002 2003 2004 2005 2006 2007 2008

2002 2003 2004 2005 2006 2007 2008

earthquake

seasonal variation

EXAMPLE OF WHY RTN REFERENCE STATIONS SHOULD BE MONITORED

≈ 6 MM / YEARENGLISH TURN CORS

SUBSIDENCE

POSSIBLE REASONS FOR CYCLICAL MOVEMENT

FLUID WITHDRAWAL/INFUSION

OCEAN LOADING

ATMOSPHERIC LOADING

RECEIVERS

PROCESSING

IONO MODELING

VOLCANIC “BREATHING”

INTERMITTENT ELECTRICAL INTERFERENCE

SNOW

27

PURPOSE OF RTN GUIDELINES

Promote consistency of RTN-generated coordinates with current realizations of both the North American Datum of 1983 (NAD 83) and the International Terrestrial Reference System (ITRS).

Note that NAD 83 is the official spatial reference system for geometric positioning in the United States.

OPERATING A RTN- CHAPTER ONE DRAFT

#1 Include a subnetwork of the RTN into the National CORS network. This would be three stations If RTN has less than 30 stations, 10% of RTN with greater than 30 stations.

#2 Align all RTN reference stations coordinates to the CORS network at 2-cm horizontal and 4-cm vertical

#3 For each reference station in the RTN, use the Online Positioning User Service (OPUS) at http://www.ngs.noaa.gov/OPUS/ to test for the continued consistency of its adopted positional coordinatesand velocity on a daily basis, and revise the station’s adopted coordinates and/or velocity if the tests reveal a need to do so.

REFERENCE STATION COORDINATE DERIVATION:

ALL CORS FIXED

ALL CORS WEIGHTED

OPUS

OPUS + HARN

BEST FIT TO ONE MASTER STATION

30

Suggestions for Determining RTN Station Coordinates

Option 1: Submit 24 hours of GPS data from each station to OPUS for each of at least 10 days and compute the arithmetic mean of the daily OPUS-generated coordinates.

Option 2: Process at least 10 days of GPS data from all RTN stations using a simultaneous network adjustment while “constraining” several CORS coordinates with weights of 1 cm in each horizontal dimension and 2 cm in the vertical dimension.

NGS recommends Option 2.

31

Suggestions for Determining Velocities for RTN Stations

• Use the HTDP (Horizontal Time-Dependent Positioning) software to predict velocities for new RTN stations. (The predicted vertical velocity will be zero.)

• After 3 years, use GPS data from the RTN station to produce a time series of the station’s coordinates, then use this time series to estimate a velocity for the RTN station.

“OPUS-LIKE” GENERATED GRAPHIC OF RTN STATIONS- SIMILAR TO CORS 60-DAY PLOT

BEST METHODS FORPOSITIONING WITH RT

BEST METHODS FROM THE GUIDELINES:THE 7 “C’s”

• CHECK EQUIPMENT• COMMUNICATION• CONDITIONS• CONSTRAINTS(OR NOT)• COORDINATES• COLLECTION• CONFIDENCE

THE CONTROL IS AT THE POLE

http://www.ngs.noaa.gov/SEARCH: CLASSICAL REAL TIME

FROM NGS SINGLE BASE DRAFT GUIDELINES CHAPTER 5 - FIELD

PROCEDURES, AND USERS CHAPTER OF RTN GUIDELINES:

RT = single base, either active or passive

B = Both Single base and RTN

ACHIEVING ACCURATE, RELIABLE POSITIONS USING GNSS REAL TIME

TECHNIQUES

• B BUBBLE- ADJUSTED?• RT BATTERY- BASE FULLY CHARGED 12V?• B BATTERY – ROVER SPARES?• RT USE PROPER RADIO CABLE (REDUCE SIGNAL

LOSS)• RT RADIO MAST HIGH AS POSSIBLE? (5’ = 5 MILES,

20’ = 11 MILES, DOUBLE HEIGHT=40% RANGE INCREASE). LOW LOSS CABLE FOR >25’.

• RT DIPOLE (DIRECTIONAL) ANTENNA NEEDED?• RT REPEATER?• RT CABLE CONNECTIONS SEATED AND TIGHT?• B“FIXED HEIGHT” CHECKED?• RT BASE SECURE?

• RT UHF FREQUECY CLEAR?• B CDMA/CELL - STATIC IP FOR COMMS?• B CONSTANT COMMS WHILE LOCATING• RT BATTERY STRENGTH OK?• B CELL COVERAGE?

• RT WEATHER CONSISTENT?• B CHECK SPACE WEATHER?• B CHECK PDOP/SATS FOR THE DAY?• RT OPEN SKY AT BASE?• RT MULTIPATH AT BASE?• B MULTIPATH AT ROVER?• B USE BIPOD?

• B ≥ 4 H & V, KNOWN & TRUSTED POINTS?• B CALIBRATION RESIDUALS-OUTLIERS?• B DO ANY PASSIVE MARKS NEED TO BE

HELD?• RT BASE WITHIN CALIBRATION?• B SAME OFFICE & FIELD CALIBRATION

USED?

CALIBRATIONS/VERTICAL LOCALIZATIONS

• B TRUSTED SOURCE?• B WHAT DATUM/EPOCH ARE NEEDED?• RT GIGO• B ALWAYS CHECK KNOWN POINTS.• B PRECISION VS. ACCURACY• B GROUND/PROJECT VS. GRID/GEODETIC• B GEOID MODEL QUALITY• B LOG METADATA

• B CHECK ON KNOWN POINTS!• B SET ELEVATION MASK• B ANTENNA TYPES ENTERED OK?• B SET COVARIANCE MATRICES ON (IF NECESSARY).• B RMS SHOWN IS TYPICALLY 68% CONFIDENCE (BRAND

DEPENDENT)• B H & V PRECISION SHOWN IS TYPICALLY 68% CONFIDENCE• B TIME ON POINT? QA/QC OF INTEGER FIX• B MULTIPATH? DISCRETE/DIFFUSE• B BUBBLE LEVELED?• B PDOP?• B FIXED SOLUTION?• B USE BIPOD?• B COMMS CONTINUOUS DURING LOCATION?• B BLUNDER CHECK LOCATION ON IMPORTANT POINTS.

MULTIPATHMULTIPATH

θ

θ

= EXTRA DISTANCE

MULTIPATH = NOISESPECULAR(DISCRETE) & DIFFUSE

INSIDE GNSS

NOVEMBER-DECEMBER 2008

“MULTIPATH-MITIGATION

TECHNIQUES USING MAXIMUM-LIKELIHOOD

PRINCIPLE”

MOHAMED SAHMOUDI AND

RENE JR. LANDRY

WWW.INSIDEGNSS.COM

Ncc relmultiiontroprs

• B CHECK KNOWN BEFORE, DURING, AFTER SESSION.

• B NECESSARY REDUNDANCY?• B WHAT ACCURACY IS NEEDED?• RT REMEMBER PPM• RT BASE PRECISION TO NEAREST

CALIBRATION POINT• B AVERAGE REDUNDANT SHOTS –

PRECISION DIFFERENCE WITHIN NEEDS OF SURVEY

• B BE AWARE OF POTENTIAL INTERFERENCE (E.G., HIGH TENSION TOWER LINES)

Comparison of 30 Minute Solutions - Precise Orbit; Hopfield (0); IONOFREE(30 Minute solutions computed on the hour and the half hour)

MOLA to RV22 10.8 Km

Day 264dh (m)

Hours Diff.

Day 265dh (m)

Day 264 minus

Day 265 (cm)

* diff >2 cm

Mean dh (m)

Mean dh minus "Truth" (cm)

* diff >2 cm

14:00-14:30 -10.281 27hrs 17:00-17:30 -10.279 -0.2 -10.280 -0.514:30-15:00 -10.278 27hrs 17:30-18:00 -10.270 -0.8 -10.274 0.215:00-15:30 -10.281 27hrs 18:00-18:30 -10.278 -0.3 -10.280 -0.415:30-16:00 -10.291 27hrs 18:30-19:00 -10.274 -1.7 -10.283 -0.716:00-16:30 -10.274 27hrs 19:00-19:30 -10.274 0.0 -10.274 0.216:30-17:00 -10.287 27hrs 19:30-20:00 -10.276 -1.1 -10.282 -0.617:00-17:30 -10.279 27hrs 20:00-20:30 -10.261 -1.8 -10.270 0.617:30-18:00 -10.270 27hrs 20:30-21:00 -10.251 -1.9 -10.261 1.518:00-18:30 -10.277 21hrs 15:00-15:30 -10.270 -0.7 -10.274 0.218:30-19:00 -10.271 21hrs 15:30-16:00 -10.276 0.5 -10.274 0.219:00-19:30 -10.277 21hrs 16:00-16:30 -10.278 0.1 -10.278 -0.219:30-20:00 -10.271 21hrs 16:30-17:00 -10.286 1.5 -10.279 -0.320:00-20:30 -10.259 18hrs 14:00-14:30 -10.278 1.9 -10.269 0.720:30-21:00 -10.254 18hrs 14:30-15:00 -10.295 4.1 * -10.275 0.1

"Truth"14:00-21:00 -10.275 14:00-21:00 -10.276 0.1 -10.276

Two Days/Same Time -10.254 -10.251 > -10.253

Difference = 0.3 cm

“Truth” = -10.276Difference = 2.3 cm

Two Days/Different Times

-10.254-10.295 > -10.275

Difference = 4.1 cm

“Truth” = -10.276

Difference = 0.1 cm

THE IMPORTANCE OF REDUNDANCY

BESIDES ATTRIBUTE FIELDS, THE RT PRACTICIONER MUST KEEP RECORDS OF ITEMS

NOT RECORDED IN THE FIELD,FOR INSTANCE:

WHAT IS THE SOURCE OF THE DATA?WHAT IS THE DATUM/ADJUSTMENT/EPOCH?

WHAT ARE THE FIELD CONDITIONS?WHAT EQUIPMENT WAS USED, ESPECIALLY-

WHAT ANTENNA?WHAT FIRMWARE WAS IN THE RECEIVER &

COLLECTOR?WHAT REDUNDANCY, IF ANY, WAS USED?

QUICK FIELD SUMMARY:

•Set the base at a wide open site•Set rover elevation mask between 12° & 15°

•The more satellites the better•The lower the PDOP the better

•The more redundancy the better•Beware multipath

•Beware long initialization times•Beware antenna height blunders

•Survey with “fixed” solutions only•Always check known points before, during and after new

location sessions•Keep equipment adjusted for highest accuracy

•Communication should be continuous while locating a point•Precision displayed in the data collector can be at the 68 percent level (or 1σ), which is only about half the error spread to get 95

percent confidence•Have back up batteries & cables

•RT doesn’t like tree canopy or tall buildings

•Set the base at a wide open site•Set rover elevation mask between 12° & 15°

•The more satellites the better•The lower the PDOP the better

•The more redundancy the better•Beware multipath

•Beware long initialization times•Beware antenna height blunders

•Survey with “fixed” solutions only•Always check known points before, during and after new

location sessions•Keep equipment adjusted for highest accuracy

•Communication should be continuous while locating a point•Precision displayed in the data collector can be at the 68 percent level (or 1σ), which is only about half the error spread to get 95

percent confidence•Have back up batteries & cables

•RT doesn’t like tree canopy or tall buildings

THE QUICK SUMMARY BOILED DOWN:

FURTHER WORK IN THE OFFICE

•Antenna heights (height blunders are unacceptable and can even produce horizontal error - Meyer, et.al, 2005).•Antenna types •RMS values•Redundant observations•Horizontal & vertical precision•PDOP•Base station coordinates•Number of satellites•Calibration (if any) residuals

USING OPUS-S OR OPUS –RS WITH REAL TIME POSITIONING

FOR SMALL PROJECTS

ESTIMATED VERTICAL STANDARD ERRORS – A CHECK ON RT ORTHOMETRIC HEIGHTS

Estimated Horizontal Standard Errors – divide by 3.6

http://igs.bkg.bund.de/index_ntrip.htm

http://pboweb.unavco.org/?pageid=107

http://www.ntrip.org/

http://sopac.ucsd.edu/cgi-bin/dbShowArraySitesMap.cgi

http://sopac.ucsd.edu/cgi-bin/somi4i

http://www.rtigs.net/

http://csrc.ucsd.edu/maps/mapBrowser.html

LINKS FOR BROWSING:

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

WWW.SWPC.NOAA.GOV

ftp://ftp.ngs.noaa.gov/dist/whenning/GLRHMC/