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- What is GPS?
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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- What is a base station?
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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- Why is a GPS base station needed?
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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- How can I retrieve GPS base station data from NJDEP's base station?
The Trenton GPS base station files may be downloaded from our
GPS page.
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- Will NJDEP's base data work with my GPS receiver?
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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- Where can I go if NJDEP's base data is not compatible with my system?
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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- What does each base file contain?
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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- How do I identify the correct name(s) of the base files I need?
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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- What are the regular hours of data logging for NJDEP's GPS base station?
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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- Are there other base stations that collect similar data as NJDEP's?
File can be accessed via the internet at a
base station maintained by the US EPA in Edison, NJ.
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- How was the station's reference position (antenna location) determined?
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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- How is the NJDEP base station currently configured?
Reference position (NAD83 - 1992)
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Latitude:
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40° 13' 14.0134" North
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Longitude:
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74° 45' 24.6418" West
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Elevation:
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18.443 meters Height Above Ellipsoid (HAE)
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Base Station Receiver:
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Trimble Navigation Pathfinder Pro XR (12 Channel)
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Firmware
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Navigation Proc:
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1.11
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Signal Proc:
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249.10
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Antenna Type:
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Compact L1 with ground-plane
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Base Station Software:
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Trimble Navigation PFCBS software version 2.68
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Operational Settings:
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Dynamic Code:
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Static
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Position fix mode:
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Manual 3D (Over-determined)
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Elevation Mask:
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5
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PDOP Mask:
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6
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SNR Mask:
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6
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Logging rates-
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Position:
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5 seconds
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Synchronized meas.:
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5 seconds
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