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Frequently Asked Questions about:

  1. What is GPS?
  2. What is a base station?
  3. Why is a GPS base station needed?
  4. How can I retrieve GPS base station data from NJDEP's base station?
  5. Will NJDEP's base data work with my GPS receiver?
  6. Where can I go if NJDEP's base data is not compatible with my system?
  7. What does each base file contain?
  8. How do I identify the correct name(s) of the base files I need?
  9. What are the regular hours of data logging for NJDEP's GPS base station?
  10. Are there other base stations that collect similar data as NJDEP's?
  11. How was the station's reference position (antenna location) determined?
  12. How is the NJDEP base station currently configured?


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  1. What is GPS?
  2. GPS stands for Global Positioning System, and is a satellite based navigation system. The system consists of satellites, operated by the US Department of Defense, which orbit the Earth and transmit coded signals with information on satellite positions and time. Earth-based GPS receivers obtaining signals from no fewer than four satellites can calculate accurate horizontal and vertical position. Four GPS satellite signals are needed to compute positions in three dimensions and correct the time offset in the receiver clock. Additional information about GPS can be obtained through our list of GPS links.

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  3. What is a base station?
  4. A base station is a GPS receiver that collects GPS measurements at a known location. Its main components are an antenna, a GPS receiver, and a device to which the GPS data is logged - most often a personal computer. The antenna's location is determined very accurately (to within a few centimeters or less) by surveying methods. A base station provides reference data that can be used to increase the accuracy of GPS data collected in the field.

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  5. Why is a GPS base station needed?
  6. Under normal circumstances a GPS receiver should be able to determine a fix for a user's location to within 10 meters. Very often, the results are in the 5 to 7 meter range. To consistently achieve better than 5 meter accuracy, GPS measurements from a base station are needed to differentially correct GPS measurements determined in the field.

    Several sources of error related to timing, satellite orbits, and the atmosphere affect GPS measurements. The GPS measurements collected by a base station are used to model these error sources at any given time. Since the measurements at the base station are collected at a known location, these sources of error can be determined and used for improving or correcting field data that was/is collected at the same time.

    The differential correction can be performed on field data after the data has been downloaded to a PC running GPS data processing software (post-process correction), or performed in real time while the field data is being collected. Generally the post-process correction solution is more accurate, and is favored over the real time solution when requirements dictate achieving higher accuracy. Real time correction still typically provides better than 5 meter accuracy, but users might not always be able to receive the correction signals in all environments. Naturally, the real time solution has advantages when using GPS for navigation.

    Base stations are ground based, and collect and store GPS measurements for later retrieval by users. Some base stations also transmit correction data that are used for real time differential GPS. Many newer GPS receivers have WAAS (Wide Area Augmentation System) correction capabilities. WAAS is a satellite based real time correction solution that was initially designed to aid aircraft navigation.

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  7. How can I retrieve GPS base station data from NJDEP's base station?
  8. The Trenton GPS base station files may be downloaded from our GPS page.

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  9. Will NJDEP's base data work with my GPS receiver?
  10. The base station is a Trimble Navigation Community Base Station. It generates base files in Trimble's .ssf format. It produces files compatible with Trimble's GIS/mapping line of receivers including the Pro XR, Pro XRS, Pro XL, GeoExplorer, GeoExplorer II, GeoExplorer 3, and GeoExplorer CE. Most other manufacturer's receivers and processing software work with a GPS file format called RINEX. The GPS processing software produced by other manufacturers often does not support the use of .ssf base files. If your receiver's processing software allows you to import a .ssf file or select a .ssf file as an acceptable base file format, you should be able to use the base station files.

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  11. Where can I go if NJDEP's base data is not compatible with my system?
  12. If your system requires RINEX format, your best option is to acquire base data from one of the National Geodetic Survey (NGS) Continuously Operating Reference Stations (CORS). The closest stations are located at Sandy Hook, NJ; Newark, NJ; Warminster, PA; West Chester, PA; Dover, DE; Reedy Point, DE; Palisades, NY; and East Moriches, NY. Base data from these sites can be downloaded over the Web by accessing the NGS CORS site.

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  13. What does each base file contain?
  14. Among other things, each file consists of GPS positions, range measurements (pseudoranges to all satellites in view), and carrier phase data collected over the course of one hour. The files are compressed to an executable file. Depending on your GPS processing software, you may have to uncompress the files. To do this, double click on the file's icon. This should create a decompressed version of the file. The decompressed file will have the same filename as the compressed file, but with an .ssf extension.

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  15. How do I identify the correct name(s) of the base files I need?
  16. The naming convention is tYMMDDHH.exe

    Where:
    t=Trenton
    Y=last digit of the year in which the file was logged
    MM=month in which the file was logged (01 = January, etc.)
    DD=day in which the file was logged
    HH=hour in which the file was logged
    .exe=filename extension (file is compressed)

    The value used for the hour often causes some confusion because the value is not consistent with our Eastern time zone. GPS uses Coordinated Universal Time (UTC) which is equivalent to Greenwich Mean Time (GMT) to reference the hours. While on Eastern Standard Time (last Sunday in October through the last Sunday in March) the GPS hour should be 5 hours ahead (9am EST = 14:00 UTC). During Eastern Daylight Time the difference is 4 hours (9am EDT = 13:00 UTC). The 7 - 8 pm file during Standard time is actually 00:00 hours (midnight), the following day.

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  17. What are the regular hours of data logging for NJDEP's GPS base station?
  18. The station logs base data every day, from 6 AM to 8 PM local time. New files should become available for downloading fifteen minutes after the close of the hour.

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  19. Are there other base stations that collect similar data as NJDEP's?
  20. File can be accessed via the internet at a base station maintained by the US EPA in Edison, NJ.

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  21. How was the station's reference position (antenna location) determined?
  22. The reference position was determined by the NGS NJ State Advisor working with NJDOT's Geodetic Control Section in 1993, using single frequency carrier phase receivers. In addition to the receiver that was set up at NJDEP, four other receivers collected data at first order control points (4 with first order horizontal control and 3 with first order vertical control) within 3.5 km of the base station antenna. Data was collected in two (morning and afternoon) two-hour logging sessions. The location is in the NGS National Spatial Reference System (NSRS) database, and is considered to be accurate to within a few centimeters.

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  23. How is the NJDEP base station currently configured?
  24. Reference position (NAD83 - 1992)

    Latitude:

    40 13' 14.0134" North

    Longitude:

    74 45' 24.6418" West

    Elevation:

    18.443 meters Height Above Ellipsoid (HAE)

     

     

    Base Station Receiver:

    Trimble Navigation Pathfinder Pro XR (12 Channel)

     

     

    Firmware

     

    Navigation Proc:

    1.11

    Signal Proc:

    249.10

     

     

    Antenna Type:

    Compact L1 with ground-plane

     

     

    Base Station Software:

    Trimble Navigation PFCBS software version 2.68

     

     

    Operational Settings:

     

     

     

    Dynamic Code:

    Static

    Position fix mode:

    Manual 3D (Over-determined)

    Elevation Mask:

    5

    PDOP Mask:

    6

    SNR Mask:

    6

    Logging rates-

     

    Position:

    5 seconds

    Synchronized meas.:

    5 seconds

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