I. GPS SURVEY EQUIPMENT

 -Page 1 of 3-
[HOME]
[2.OBSERVATIONS]
[3.DATA]
DRAFT - DRAFT - DRAFT.  Comments? e-mail Joe Evjen

TABLE OF CONTENTS:

A. Equipment Specifications, Calibration, and Care

B. Troubleshooting Guide

C. Equipment Checklist
 

EQUIPMENT LINKS:
  • UNAVCO GPS Equipment Support website
  • SECO Technical Information

    •  A. Equipment Specifications, Calibration, and Care:
    .    		 [top] [home]
    Basic instrumentation for a GPS network survey includes multiple sets of receivers, antennas, fixed-height tripods, and meteorological instruments.  Identical equipment should be used whenever possible to minimize the effect of equipment biases. The compatibility of mixing different instrument models or brands should be demonstrated by performing a validation survey.

    Survey equipment, like all scientific instrumentation, should be handled with care, maintained according to manufacturer specifications, and calibrated on a regular basis.  Equipment calibrations should be performed at the start and end of a project, before and after any maintenance, and at sufficient intervals to maintain data integrity.  Any data not bracketed by successful calibrations are suspect.  To prevent the invalidation of good data, frequent calibrations are recommended. The entire system of GPS equipment, personnel, and processing procedures should be proven with a validation survey as a final check to ensure all components interact properly.

    The following equipment is described below:

    1. Receiver Specifications
    2. Antenna Specifications
    3. Tripod Specifications
    4. Tribrach Specifications
    5. Meteorological Equipment Specifications
    6. Personnel Specifications
     A-1. Receiver Specifications: [receiver operation] [top] [home]
  • GPS receiverThe receivers used for network surveys should record the full wavelength carrier phase and signal strength of both the L1 and L2 frequencies, and track at least eight satellites simultaneously on parallel channels.  Dual frequency instruments are required for all baselines longer than 10 km.
  • Ensure that the receivers should have sufficient memory and battery power for the entire field campaign.
  • Receiver test reports are available from the Federal Geodetic Control Subcommittee (FGCS).

    • Receiver Calibration and Care:

    Ensure that your receiver contains the latest manufacturer's firmware upgrades.  A zero-baseline test can measure receiver internal noise if the performance is suspect.  Consult your user's manual for additional specifications.

     A-2. Antenna Specifications: [antenna height measurement] [top] [home]
    GPS antennaThe antennas should have stable phase centers and choke rings or large (> 16 cm) ground planes to minimize multipath interference, and a common orientation indicator (e.g., an arrow) to point north during observations.

    Antenna Calibration and Care:

    All antenna models used shall have undergone Antenna Calibration by the National Geodetic Survey (NGS).  Consult your user's manual for other specifications.


     A-3. Tripod Specifications: [tripod setup] [top] [home]
    Fixed-height TripodThe tripods used must facilitate precise offset measurements between the mark datum point and the Antenna Reference Point (ARP).  Fixed height tripods are preferable, due to the decreased potential for antenna centering and height measurement errors.

    Tripod Calibration and Care:

  • All tripods shall be examined for stability with each use.  Ensure that hinges, clamps, and feet are secure and in good repair.
  • Fixed-height tripods shall be tested for stability, plumb alignment, and height verification at the start and end of each project.


    • Link - SECO GPS Tripod Length Test

     A-4. Tribrach Specifications: [tribrach operation] [top] [home]
    TribrachTribrachs used shall be of suitable quality and condition for high-accuracy surveys.  Consult with your Project Coordinator for details.

    Tribrach Calibration and Care:

    The optical plummet alignment shall be tested at the start and end of each project.

    Link - Using SECO Tribrach Adjusters

     A-5. Meteorological Equipment Specifications: [weather observations] [top] [home]
    ThermometerMeteorological equipment includes a psychrometer (wet-bulb and dry-bulb thermometers) to measure temperatures and a barometer or altimeter to measure atmospheric pressure.  Relative humidity can either be measured with a hygrometer or computed (the preferred method) from psychrometer and barometer readings.

    Meteorological Equipment Calibration and Care:

    Meteorological equipment should be calibrated at the beginning and end of a project.  Use the Meteorological Equipment Comparison Form to compare your instruments with standard instruments, available at National Weather Service offices.

     A-6. Personnel Specifications: [personnel operation] [top] [home]
    All field personnel should be trained in the avoidance of systematic errors and blunders during field operations.  Field personnel often work alone and must be prepared to make wise, on-the-spot decisions regarding mark identification and stability, equipment use and troubleshooting, and antenna setup.  Office personnel should be familiar with geodetic concepts and least-squares adjustments.

    Personnel Calibration and Care:

    Personnel should participate in any available certification and training activities.


     B. Equipment Troubleshooting Guide:
    .    		 [top] [home]

    Battery Failure:

    If you encounter a battery failure during your session, attempt to restore power as soon as possible.  Note power outage time and duration in the "Remarks" section of the observation log, and continue to record data through the end of the session.  Notify your Project Coordinator if the outage lasted more than 5 minutes.

    Antenna Out of Plumb:

    If at any time during the observation session you notice that the antenna is out of plumb (not centered on station mark), record the current time and the last time you know it was plumb in the "Remarks" section of the observation log.  Describe the magnitude of the eccentricity by including a sketch and measurements, noting the alignment from true north. Re-plumb without removing the antenna, being careful not to disturb the setup, and finish the session. Notify the Project Coordinator.

    Run Out of Memory:

    Terminate the session and download the data into your computer.  Delete saved data (older sessions first) from the receiver and restart. Notify the Project Coordinator.

    If All Else Fails:

    The safest way to solve unexpected problems is to reobserve an additional session when other observers are adjacent to your location. Contact the Project Coordinator for rescheduling information.


     C. Equipment Checklist:
    .    		 [observer checklist] [top] [home]
    ____ GPS antenna
    ____ GPS receiver, with operating manual, data collector, power supply, and data cables
    ____ Fixed-height tripod, or slip-leg tripod and tribrach, with a shade for the leveling bubbles
    ____ Sandbags or chain or spikes for setting up the tripod
    ____ HI measuring rods, tapes, or other devices in both feet and meters for measuring antenna height
    ____ Calculator with squaring and square-root functions for computing antenna height and converting measurements
    ____ Survey tape and digging tools for recovering the monument and checking descriptions
    ____ Magnetic compass and inclinometer for orienting the antenna and checking diagrams and descriptions
    ____ Station datasheet with "to-reach" description
    ____ Forms: Observation Log, Pencil Rubbing, Station Description, and Visibility Obstruction Diagram
    ____ Indelible black (fine ball point) pen for forms, and a carpenter's pencil for station rubbings
    ____ Psychrometer (wet-bulb and dry-bulb thermometers) or Hygrometer
    ____ Barometer or altimeter
    ____ Cell phone or radio for communication
    ____ Digital camera for documenting observation conditions
    ____ Keys for access to station site
    ____ Food, water, shelter, sunscreen, bug spray, reading material, etc.
    ____ Data downloading equipment (computer, software, cables, diskettes) and shipping materials (labels, envelopes, stamps, etc.)
    ____ Battery recharging equipment
    ____ Other:

     Return to the National Geodetic Survey [top] [observations] [data] [home]