Marti Ikehra
California State Advisor
Hector Mine Earthquake Survey Project Coordinator

Charles W.Challstrom
Director, National Geodetic Survey

INSTRUCTIONS: California FBN/CBN Re-observations As a Result of the Hector Mine Earthquake of 1999 (GPS-1460)
Task Number: 8K6D2000

On October 16, 1999, a 7.1 magnitude earthquake, designated the Hector Mine Earthquake, occurred in the Mojave Desert. The National Geodetic Survey (NGS), in cooperation with the California Department of Transportation (CALTRANS) Districts 8 and 11, Riverside County, San Bernadino County, US Geological Survey, Las Vegas Valley Water District, and Mohave County, AZ, will re-observe the portion of the California Federal Base Network (FBN) and Cooperative Base Network (CBN) that was affected by this earthquake: 12 FBN and 18 CBN stations in California and one CBN station in Nevada that were affected will be re-observed. One FBN station in Arizona and one FBN and one CBN station in Nevada will also be occupied as ties.

In order to meet America's accelerating positioning and navigation needs, the existing coordinate reference system must be continually enhanced to provide the accessibility and high accuracy required for use with GPS. The digital revolution in mapping, charting, and surveying requires a National Spatial Reference System (NSRS) consisting of, among other components, a network of monumented points having four-dimensional positions. NGS is tasked with the responsibility of maintaining the FBN component of the NSRS, as the user community involved is tasked with maintaining the CBN.

This GPS project will provide new coordinates for the affected FBN/CBN relative to the portion of FBN/CBN that was unaffected by the earthquake.

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

General specifications for the project are as follows. At each site, three sessions of 5½ hours duration shall be observed. The observing scheme shall be arranged so that for each station, the start time of one of the observing sessions shall be at least 4 hours different than the other two. The observing scheme shall be arranged to ensure that adjacent stations are directly connected in at least one observing session, and at least half of all base lines are repeated.

In general, station occupation and observing procedures must be carried out according to appropriate sections of the "NGS Operations Handbook" and the current applicable GPS 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, revised and reprinted November 1998. Success in meeting the accuracy standards will be based on repeatability of measurements, analysis of loop misclosures, and adjustment residuals.

General specifications for the project are given in "Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques," Version 5.0: dated May 11, 1988, reprinted with corrections August 1, 1989. Specific project criteria are given in the following sections.

Data from all CORS within the project area are to be used in processing. The CORS information is included in the attached Table 2. Further, the CORS of the Southern California Integrated GPS Network (SCIGN) are also to be used in processing. The SCIGN information will be found in Table 3.

A list of stations have been provided in the attached Table 1. A sketch of stations has also been provided.

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. Vectors between the project stations shall be measured by single sessions consisting of continuously and simultaneously tracking for 5½ hours.

Each station must be occupied at least three times - twice at one observing window and once at the other. Adjacent stations must be directly connected in at least one observing session, and at least one-half of all base lines must be repeated. All available CORS data will be used throughout the project.

Record weather data 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.

Data management, quality review of collected data, and final vector processing for the survey will be accomplished by the Spatial Reference System Division (SRSD) using PAGES. 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. 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.

The data will be processed in 24-hour sessions (or slightly longer if the observation session crosses 0000 UTC) in order to utilize the 24-hour data sets collected at the CORS. The "fixed baseline" option in PAGES will be used to compute the baseline between the CORS. 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 less than 10 km in length, the final reduction will consist of a L1 fixed solution. Compute vectors greater than 10 km in length in a separate processing session from the short (less than 10 km) vectors. In all cases, integer ambiguities will be fixed for each vector whenever possible. The quality of collected data shall be determined from the plots generated from PAGES and by analysis of repeated vectors and free adjustment residuals and/or loop misclosures.

The data and results will be submitted to the Global Positioning System Branch of SRSD. All B-files and G-files must be complete, including *25* and *27* records.

Station recovery notes must be submitted in computer-readable form using DDPROC software. Include the name, address, and, if public ownership, telephone number of the responsible party. Do not include the telephone numbers of private property owners. The California Geodetic Advisor will be responsible for all quality checks for conformance with NGS format standards as pertains to descriptions and recoveries.

Antenna set-up is critical to the success of this project. Fixed-height tripods are preferred 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 a suspicion of a problem.

When a fixed-height tripod is not used, the height of the antenna must be carefully measured to prevent station set-up blunders from occurring. Tribrachs used for these set-ups must be checked and adjusted when necessary. Totally independent measurements of the antenna height above the mark in both metric units and English units must be made before and after each session. Someone other than the observer must check the measurement computations by carefully comparing measurements and then entering his/her initials on the log.

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.

Success of this project requires that the highest quality GPS data be collected. Therefore, during each station occupation, the operators shall carefully monitor the operation of the receivers. Any irregularities in the data due to equipment malfunction, DOD adjustment of the satellite orbit, obstructions, etc., must be reported to the Project Development Branch, N/NGS21, as soon as possible and noted on the observing log. If the quality of observations for an observing session is questionable, notify the Project Development Branch immediately.

The survey team shall not depart the project area until they have quality reviewed all data and advised N/NGS21.

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 the Project Development Branch, N/NGS21.

The data set collected during the project shall be named "caro040d.746". All records in connection with this project shall be titled "CALIFORNIA FBN/CBN RE-OBSERVATIONS, 2000". The project number (accession number) is GPS-1460.

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

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

The Arizona Geodetic Advisor is:

CDR Dave Minkel, NOAA
Office of the State Cartographer
1616 W. Adams St.
Phoenix, AZ 85007
Telephone: 602-542-1569
Cellular: 602-549-4744
FAX: 602-542-2600
e-Mail: Dave.Minkel@noaa.gov

The Nevada contact is:

Timothy J. Wolf
District Surveyor
1001 South Valley View Boulevard
Las Vegas, NV 89153
Telephone: 702-258-7163
Mobile: 702-375-9430
FAX: 702-258-3811
e-Mail: timothy.wolf@lvvwd.com

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

Steven J. Frakes
Project Development Branch
Spatial Reference System Division
Telephone: 301-713-3194, ext. 111
FAX: 301-713-4316
E-mail

Questions and problems concerning adjustment processing should be directed to:

Maralyn L. Vorhauer
Observation and Analysis Division
Telephone: 301-713-3176, ext. 104
FAX: 301-713-4327
E-mail

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

Neil Weston
Geosciences Research Division
Telephone: 301-713-2847, ext. 202
FAX: 301-713-4475
E-mail

See "NGS Operations Handbook," Section 1.4.1.

Expenses for this project will be charged to Task Number 8K6D2000.

Travel and per diem are authorized in accordance with Federal Travel Regulations, Part 301-11, Per Diem allowances. Current per diem rates were effective January 1, 2000.

Attachments
 

cc: N/NGS - D.Zilkoski
N/NGS - S. Misenheimer*
N/NGS1 - G. Mitchell
N/NGS1x1 - Dave Minkel
N/NGS21 - S. Frakes
N/NGS22 - D. Pursell
N/NGS4 - E. Wade
N/NGS4 - M. Vorhauer
N/NGS4 - D. Hoar
N/NGS41 - W. McLemore
N/NGS41 - J. Blackwell
N/NGS413 - G. Sowell
N/NGS5 - R. Snay
N/NGS6 - N. Weston
FGCS Members*
T. Wolf, District Surveyor, Las Vegas, Nevada
W. Young, CSRC
G. Helmer, CSRC
M. Duffy, CSRC
D. D'Onofrio, CSRC

*First page only.