Post on 27-Dec-2015
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/