EndRun Tempus LX GPS, User Manual

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T e m p u s L X G P S U s e r M a n u a l 73
"Smarter Timing Solutions"
Appendix
D
GPS Reference Position
Your Tempus LX is capable of operation from either an automatically determined GPS reference
position or a manually entered GPS reference position. If your Tempus LX is unable to automati-
cally determine this information itself, this appendix describes the needed background information
and procedures for determining an acceptably accurate GPS reference position in the proper World
Geodetic Survey of 1984 (WGS-84) geodetic datum . Refer to the Geodesy and WGS-84 Positions
sections of this appendix for details on some of the jargon contained herein.
Obtaining Reference
Positions
If you need to provide an accurate (< 100 meter error) reference position to your Tempus LX because
you are using a window-mounted antenna with inadequate satellite visibility, there are two good ways
to do it: 1) use a handheld GPS receiver to obtain a position near the location of your Tempus LX
antenna or 2) reference a geodetic database to obtain a position for your street address. The first way
is the easiest and probably the best:
Using a Handheld GPS Receiver
Obtain an inexpensive, handheld GPS receiver. Use it outside of the building to determine a position
that is within 100 meters of the installed Tempus LX antenna. Make sure that the handheld GPS re-
ceiver is configured to report its positions in the WGS-84 datum. Record the position and then make
any adjustments to the height that might be necessary if the antenna is installed in a high-rise build-
ing. Input it to the Tempus LX via the
setgpsrefpos
command.
Using Geodetic Databases
Many users will not feel confident in determining their own reference position via this technique. For
those users, EndRun Technologies technical support will be happy to assist you. We are familiar with
the procedure and can convert your street address and zipcode information to the proper WGS-84 co-
ordinates for you. The following provides the necessary background information needed to interpret
the geodetic database and then describes the procedure:
Geodesy: Geodesy is the science of mathematically describing the earth’s surface. To do this, a
model or
geodetic datum
is used to fit the shape of the earth. These models are flattened spheres
called ellipsoids . The earth’s shape is accurately modeled using such an ellipsoid, with the equator
being a circle around the fattest part and with the north and south poles corresponding to the com-
pressed top and bottom of the ellipsoid. Some of these models are intended only for localized regions
of the earth’s surface. The GPS uses a model that is called the WGS-84 ellipsoid. It is intended to
model the entire earth, and is currently the best global model available.
What these ellipsoids are actually attempting to approximate is the
geoid
. The geoid is a gravitation-
ally equipotential surface surrounding the earth that is everywhere perpendicular to the gravitational
field and approximates the surface of the oceans. The height of the surface of the geoid relative to the