FROM: Charles W. Challstrom
Acting Director, National Geodetic Survey

SUBJECT: INSTRUCTIONS: Charleston Bridge Project
Task Numbers: 8K6D5000 (HEIGHT MODERNIZATION)
 

GENERAL:

The National Geodetic Survey (NGS), Oceanographic Products and Services Division (OPSD) and the Coast Survey (CS), in accordance with NOAA's goal to Promote Safe Navigation and the Physical Oceanographic Real-Time System (PORTS) are working toward implementation of real-time, using GPS, sensors that will determine the maximum height of a vessel at the lowest point of a bridge. This project plan is for measuring the vertical clearance for the Cooper River Bridges, known as the Grace Memorial Bridge and Silias Pearman Bridge. The vertical clearance is critical for ensuring safe navigation for the Port of Charleston.

The project consists of 4 different parts as follows:

1. Determine the ellipsoid height of the two bridges using three different GPS receivers on the two bridges, one receiver at the pilot house, and a CORS site.
2. Determine the difference between Geoid96 with NAVD 88 by observing a minimum of four NAVD 88 bench marks including one primary bench mark at the tidal station.
3. Determine the height from the bottom of the bridges to each of the antenna mounts on the bridges using classical and trig leveling.
4. Perform a pseudo-kinematic survey of at least three NAVD 88 bench marks that includes the primary tidal bench mark station to demonstrate the capabilities of GPS to determine relative heights at tidal stations. (Note this will be done only if time permits.)

SPECIFICATIONS:

Project requirements for the GPS bridge observations are to ensure 2-centimeter local accuracy in the horizontal component, as well as 2-centimeter local accuracy for the ellipsoid heights.

Data from the Charleston (char1) CORS in the region are to be used in the processing. Position and data are available from the NGS web site.

General specifications for the project are as follows.

1. The observations on the two bridges consist of seven to thirty days of continuous operation.
2. Observing a minimum of three bench marks and one bench mark at the tidal station for at least two one-hour sessions. The observing scheme shall be arranged so that the start time of one observing session shall be at least 4 hours different than the other one. This ensures different atmospheric conditions (different days) and significantly different satellite geometry (different times) for the two base line measurements. Fixed-height tripods are required for all the receivers.
3. Special trig leveling procedures will be used to determine the height from the antenna mounts to the bottom of the two bridges.

In general, station occupation and observing procedures must be carried out according to appropriate sections of the "NGS Operations Handbook" and the current applicable receiver field manuals. Data formats and digital file definitions are given in "Input Formats and Specifications of the National Geodetic Survey Data Base," Volume I. Horizontal Control Data, Federal Geodetic Control Subcommittee, September 1994. Success in meeting the accuracy standards will be based on repeatability of measurements, analysis of loop misclosures, and adjustment residuals.

Data Acquisition - Data collection must be accomplished as specified in the appropriate dual-frequency receiver field manuals in the compressed mode at a 15-second epoch collection interval. The GPS receivers must be dual-frequency and full-wavelength. Track satellites down to a 10-degree elevation angle.

Record weather data if possible just before, immediately after, and at the mid-point of each session. Meteorological data shall also be collected immediately after an obvious weather front passes during a session and immediately before it passes, if possible. Pressure and relative humidity measurements must be made near and at about the height of the GPS antenna phase center. Indicate in the log the location of the barometer and psychrometer.

Survey operations shall be conducted with due regard to the safety of personnel and equipment.

Vector Computations - Data management, quality review of collected data, and final vector processing for the bridge survey will be accomplished by the NGS using PAGE4 and OPSD using Ashtech program "Ashtech Office Suite for Survey".

NGS will process the vectors observations on the two bridges consisting of seven to thirty days. The vectors shall be computed in the International Terrestrial Reference Frame (ITRF) system, using the most current epoch and precise IGS ephemerides. Use 30-second epoch intervals for data processing. Antenna reference point (ARP) positions will be used for the CORS. For stations where weather data are not available, or are suspect, predicted values computed based on the station's latitude, height above mean sea level, and time and day of year will be used. Use 15 degrees as the cutoff elevation angle in data processing. A cutoff angle of 10 degrees may be used when necessary to improve results.

In general, vectors greater than 5 km in length are to be computed in an ion-free fixed, or partially-fixed, solution. In all cases, integer ambiguities will be fixed for each vector whenever possible.

For each set of CORS reference marks, the CORS antenna at the site will be used as the reference station in the data processing.

The quality of collected data shall be determined from the plots generated from PAGE4, by analysis of repeated vectors and/or comparison of station positions, and free adjustment residuals and/or loop misclosures.

OPSD using "Ashtech Office Suite for Survey" with the guidance of NGS will perform the vector processing of all the GPS bench mark ties and all quality checks for conformance with NGS format standards such as executing software programs COMPGB, OBSCHK, and OBSDES. The final ITRF vectors will be assessed and transformed to the NAD 83 coordinate system using program ADJUST.

The type of final solution, L1 versus ion-free, will depend on the length of the vectors. For vectors which are less than 5 km in length, the final reduction will consist of a L1 fixed solution.

The all the data and results from NGS and OPSD will be submitted to the Observation and Analysis Division. All B-files and G-files must be complete, including *25* and *27* records.

Station Descriptions - Station recovery notes must be submitted in computer-readable form using DDPROC software. Include the name, address, and telephone number of a property owner or other contact.

Special Requirements - Antenna set-up is critical to the success of this project. Fixed-height tripods will be used for all receivers. The plumbing bubbles on the antenna pole of the fixed-height tripod must be shaded when plumbing is performed. They must be shaded for 3 minutes before checking and/or re-plumbing. Also, the perpendicularity of the poles must be checked at the beginning of the project and any other time there is suspicion of a problem.

A rubbing of the stamping of the mark must be made at each visit to a station. If it is impossible to make a rubbing of the mark, a plan sketch of the mark must be substituted, accurately recording all markings.

For each station visited, a visibility obstruction diagram must be prepared and the TO-REACH description carefully checked for errors or omissions.

The following must be recorded at each occupation of a station:

(1) receiver manufacturer,
(2) antenna manufacturer,
(3) receiver model number (part number),
(4) antenna model number (part number),
(5) the complete serial number of the receiver, and
(6) the complete serial number of the antenna.

GPS DATA:

Visibility tables and plots of the present satellite constellation for February 2, 1999 through February 7, 1999 have been reviewed and the observations can be performed at any time of the day. For operational use, current data must be generated with Trimble, Ashtech mission planning software or from program SATMAP.

Project report and data listed in Annex L of "Input Formats and Specifications of the NGS Data Base" and in the attached addendum for the adjustment portion must be transmitted. Any data considered suspect as to quality in achieving accuracy standards should be sent via FedEx immediately for office review. Backup of transmitted data must be held until notified by Edward Carlson at the Project Development Branch, N/NGS21.

The data set collected during the project shall be named "scbr029a.086". All records in connection with this project shall be titled "CHARLESTON BRIDGE PROJECT, 1999". The project number (accession number) is GPS-1368.
 

LEVELING DATA:

Leveling data will be submitted in "Blue Book" format (see "Input Formats and Specifications of the National Geodetic Survey Data Base," Volume I. Horizontal Control Data and Volume II. Vertical Control Data, Federal Geodetic Control Subcommittee, September 1994) to Project Development Branch of NGS on floppy disks. Floppy disks shall contain the final version of the following files: HGF, HGZ, and HA in direct access format and RPT and ABS in sequential format. Data shall be submitted within 30 days of completion of the project.

All bench marks established and/or recovery bench marks descriptions shall be written in accordance with the "Blue Book" (unified descriptions), except do not submit incomplete descriptions (i.e. descriptions with Recovery Type Codes of "RM"), unless the bench mark is destroyed or not recovered.
 

LIAISON:

Liaison must be maintained with designated offices at the National Geodetic Survey headquarters located at:
 

1315 East-West Highway
Silver Spring, Maryland 20910-3282

Questions and problems concerning survey field operations, adjustment processing, and vector processing should be directed to:

Edward E. Carlson
Project Development Branch
N/NGS21
Telephone: 301-713-3196, ext. 108
Fax: 301-713-4316 or 301-713-4324
e-Mail: edc@ngs.noaa.gov

Questions and problems concerning using CORS data in processing should be directed to:

Neil Weston
Geosciences Research Division
N/NGS6
Telephone: 301-713-2847, ext. 202
Fax: 301-713-4475
e-Mail: nweston@ngs.noaa.gov

Questions and problems which could affect the technical adequacy of the project should be directed to:

Edward E. Carlson
Project Development Branch
N/NGS21
Telephone: 301-713-3196, ext. 108
Fax: 301-713-4316 or 301-713-4324
e-Mail: edc@ngs.noaa.gov

For tide site information, contact:

Brooks Widder
Oceanographic Products and Services Division
Requirements and Engineering Branch, N/CS41
Telephone: 301-713-2902, ext. 184

The contact for the project is:

Commander John D. Wilder, NOAA
Navigation Advisor for SE United States
Telehone: 843-740-1236
e-Mail: John.Wilder@noaa.gov

Names and telephone numbers of local contacts are given in the station description material.
 

PUBLICITY:

See "NGS Operations Handbook," Section 1.4.1.
 

EXPENSES:

Expenses for this project will be charged to task number 8K6D5000.
 

TRAVEL:

Travel and per diem are authorized in accordance with Federal Travel Regulations, Part 301-7, Per Diem Allowances. Current per diem rates were effective January 1, 1999.
 

ACKNOWLEDGMENT:

Please acknowledge receipt of these instructions in your Monthly Report.

Attachment

cc:
N/CS - N. Prahl*
N/CS - R. Barazotto*
N/CS3 - J. Wilder
N/CS4 - J. Dixon*
N/CS41 - B. Widder
N/CS42 - J. Oyler
N/NGS - S. Misenheimer*
N/NGS - D. Zilkoski*
N/NGS1 - G. Mitchell*
N/NGS11 - S. Cofer*
N/NGS11 - A. Murray
N/NGS2 - J. Till*
N/NGS21 - S. Frakes*
N/NGS21 - E. Carlson
N/NGS22 - T. Soler*
N/NGS3 - E. Allen*
N/NGS4 - E. Wade*
N/NGS5 - R. Snay*
N/NGS6 - N. Weston*
FGCS Members*

* first page only

DATA TO BE SENT TO HEADQUARTERS RELATING TO THE ADJUSTMENT PORTION OF THE PROJECT

Note that all except specified files or listings can be paper printouts or ASCII files on disk; be sure to keep backups until notified that the data base is loaded.
Free adjustment.
[For A- and B-order projects, the data are not scaled. First- order projects would also have a free adjustment using the modified G-file.]
Constrained horizontal adjustment holding previously published A- and B-order stations and ellipsoid heights (optional).
Constrained vertical adjustment holding NAVD 88 heights.
Final combined Blue Book file (ASCII required).
Free adjustment with accuracies.
Accuracy listing (output of BBACCUR). (Paper required).
ELLACC output.
OBSCHK output*.
CHKDESC output*.
OBSDES output*.

*any errors or warning messages must be explained.

Copies of data used to determine fixed control, both horizontal and vertical.
Final description file (ASCII required).
Final G file (ASCII required).