1. NGS AND REAL TIME POSITIONING
GREAT LAKES REGION
HEIGHT MODERNIZATION CONSORTIUM
NGS AND REAL TIME POSITIONING
Bill Henning
Senior Geodesist, PLS.
x 111,
ftp://ftp.ngs.noaa.gov/dist/whenning/GLRHMC/

2. REAL-TIME ACTIVITIES AT THE NGS
OPERATE AN NTRIP CASTER. (Fed. Owned/operated – currently 8. RTCM 2.3 & 3.0, From Foundation CORS. NO CORRECTORS)
DEVELOP AND PUBLISH GUIDELINES DESCRIBING BEST PRACTICES IN RTK & RTN . (RTK Users draft, RTN Operators draft, etc.)
PARTICIPATE IN MEETINGS, FORUMS, WORKSHOPS, ETC., CONCERNING REAL-TIME NETWORKS. SEEK LEADERSHIP ROLES. (FIG, FGCS, ESRI, ACSM, RTCM, etc.)
RESEARCH PHENOMENA AFFECTING ACCURATE REAL-TIME POSITIONING. (Orbits, refraction, multipath, antenna calibration, geoid separations, gravity, crustal motion, etc.)

3. SINGLE-BASE USERS GUIDELINES
WHY/HOW?
Legacy equipment
Closest base networks
Areas with no cell coverage
Empirical methods
Source for background info.
Dynamic technology

4. 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
MULTIPATH
METADATA
COMMUNICATION
OFFICE CHECKS
RT GLOSSARY

5. 8400 MPH 8400 MPH 8400 MPH 8400 MPH 800 MPH +/- SVN 31 SVN 8 SVN 14
EVERY EPOCH OF OBSERVATION MUST COMPUTE CHANGE IN POSITION DUE TO EARTH ROTATION, SATELLITE ORBIT CHANGE AND RELATIVITY!
8400 MPH
800 MPH +/-
AT “C”, 1 NANOSECOND = 30 CM!

6. SUNSPOT CYCLE http://www.swpc.noaa.gov/
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
2013

8. IONOSPHERIC EFFECTS ON POSITIONING
AVERAGE IONO- NO NETWORK
HIGH IONO- NO NETWORK
2D PRECISION/ACCURACY (CM)
SINGLE BASE
10 KM
( )
( )
NETWORK SOLUTION
@ 30 KM
WITH NETWORK
DISTANCE TO REFERENCE STATION (KM)
(SOURCE-BKG- GERMANY)

9. IONO, TROPO, ORBIT CONTRIBUTE TO PPM ERROR
REMEMBER GNSS EQUIPMENT MANUFACTURERS’ SPECS!

10. TROPOSPHERE DELAY Ionosphere troposphere 10 KM GREATER THAN 10 KM
The more air molecules, the slower the signal (dry delay)
High pressure, Low temperature
90% of total delay
relatively constant and EASY TO CORRECT FOR
The more water vapor in the atmosphere the slower the signal (wet delay)
High humidity
10% of total delay
Highly variable and HARD TO CORRECT FOR
Ionosphere
troposphere
10 KM
GREATER THAN 10 KM

11. DRAFT GUIDELINES- 95% CONFIDENCE

12. THE NGS AND REAL-TIME NETWORKS

13. 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? ?

14. NGS RT WEB PAGES – OPERATIONAL PROTOTYPE
CURRENT POLICY: NGS WILL NOT RESTREAM RT DATA FROM NON-FEDERAL SITES

15. 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)

16. NGS RTCM 3.X STREAMS

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

18. DEVELOPING COOPERATIVE PARTNERS
USCG
86 TOTAL -38 INLAND (DOT)

19. USCG DGPS TEST SITES DRIVER, VA UPPER KEWANEE, MI NEW BERN, NC
MORICHES, NY
ANNAPOLIS, MD
ENGLISH TURN, LA
CARD SOUND, FL
TAMPA, FL
KODIAK, AK
PENOBSCOT, ME

20. PBO STATIONS BINEX, RTCM 2.4, RTCM 3.0 0.6 to 2.0 seconds latency
SERVER IN BOULDER, CO
97 RT STATIONS +/-

21. PBO RT STREAMS

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

23. Position Time Series (long-term)
earthquake
Here the vertical coordinate clearly shows an annual variation.
seasonal variation

24. EXAMPLE OF WHY RTN REFERENCE STATIONS SHOULD BE MONITORED
SUBSIDENCE
≈ 6 MM / YEAR
ENGLISH TURN CORS

25. POSSIBLE REASONS FOR CYCLICAL MOVEMENT
FLUID WITHDRAWAL/INFUSION
OCEAN LOADING
ATMOSPHERIC LOADING
RECEIVERS
PROCESSING
IONO MODELING
VOLCANIC “BREATHING”
INTERMITTENT ELECTRICAL INTERFERENCE
SNOW

26. DRAFT

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.

28. 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 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.

29. 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.
IDOP – Interpolative Dilution of Precision is a measure of the suitability of the geometry between the network stations and the rover (or the VRS). It tells us how well we can interpolate quantities such as tropospheric refraction from know values at the network stations to the rover (or the VRS). The best situation occurs when the network stations are evenly spaced and the rover falls exactly in the middle. The value of IDOP grows progressively larger as the rover moves away from the centroid of the network stations, becoming quite large when the rover is outside the polygon enclosing the reference stations. Unfortunately, this can happen when the surveyor is working close to the coast and all the reference stations are inland.

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.

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

33. BEST METHODS FOR POSITIONING WITH RT

34. BEST METHODS FROM THE GUIDELINES: THE 7 “C’s”
SEARCH: CLASSICAL REAL TIME
CHECK EQUIPMENT
COMMUNICATION
CONDITIONS
CONSTRAINTS(OR NOT)
COORDINATES
COLLECTION
CONFIDENCE
THE CONTROL IS AT THE POLE

35. ACHIEVING ACCURATE, RELIABLE POSITIONS USING GNSS REAL TIME TECHNIQUES
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

36. CHECK EQUIPMENT 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?

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

38. CONDITIONS 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?

39. CONSTRAINTS (OR NOT) 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?

40. CALIBRATIONS/VERTICAL LOCALIZATIONS

41. COORDINATES 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

42. COLLECTION 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.

43. MULTIPATH θ θ
= EXTRA DISTANCE

44. MULTIPATH = NOISE SPECULAR(DISCRETE) & DIFFUSE
INSIDE GNSS
NOVEMBER-DECEMBER 2008
“MULTIPATH-MITIGATION TECHNIQUES USING MAXIMUM-LIKELIHOOD PRINCIPLE”
MOHAMED SAHMOUDI AND
RENE JR. LANDRY

45. CONFIDENCE 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)

46. THE IMPORTANCE OF REDUNDANCY
Two Days/Same Time
THE IMPORTANCE OF REDUNDANCY
>
Difference = 0.3 cm
“Truth” =
Difference = 2.3 cm
Two Days/
Different Times
Dh is computed for a series of 30 minute sessions over 2 days and the average dh is found
Taking the observation from the same time each day (when the satellite geometry is similar), the difference in the dh is only .3 cm, but the difference from the average is 2.3 cm.
Now if you take the dh from different times on the 2 days (different geometry), the difference between the 2 vectors is 4.1 cm, but the difference from the average is only .1 cm
>
Difference = 4.1 cm
“Truth” =
Difference = 0.1 cm

47. DRAFT GUIDELINES- 95% CONFIDENCE

48. METADATA !
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?

49. 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

50. THE QUICK SUMMARY BOILED DOWN:
COMMUNICATIONS: THE KEY TO SUCCESS
CHECK SHOT: FIRST BEFORE NEW WORK
REDUNDANCY: FOR CONFIDENCE
≥200 RTN WORLDWIDE
≥80 RTN IN THE USA
≥35 DOT WITH STATEWIDE NETWORKS PLANNED ?

51. 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

52. USING OPUS-S OR OPUS –RS WITH REAL TIME POSITIONING FOR SMALL PROJECTS

55. Estimated Vertical Standard Errors – A CHECK ON RT ORTHOMETRIC HEIGHTS
Estimated Horizontal Standard Errors –
divide by 3.6

56. ftp://ftp.ngs.noaa.gov/dist/whenning/GLRHMC/
LINKS FOR BROWSING:
ftp://ftp.ngs.noaa.gov/dist/whenning/GLRHMC/