GPS Unit Accuracy by Time of Day

1
GPS Unit Accuracy by Time of Day
Thomas M. Smith, Virginia Tech
Introduction
Conclusion
Global positioning systems, also known as GPS,
are tools to quickly and precisely determine the
position of an object anywhere on earth and was
initially developed and maintained by the United
States Department of Defense. The system relies
on 24 orbiting satellites, three of which must be
in range by a GPS receiver at the site you wish
to measure. Of course, the more satellites the
receiver can communicate with, the better the
reading. The receiver communicates with the
satellites and forms a triangulation using the
concepts of the speed of light and the atomic
clock to determine distance. One of the problems
associated with the GPS receivers is that most
only use quartz clocks, which are less reliable,
but this is compensated for by use of
self-corrected algorithms. Other issues
associated with varied readings on a single
location could be attributed to the lay of the
land (hills vs flat), buildings (obstruct or
reflect signal), weather conditions possible
(clouds, fog, storms). Thus, GPS receivers hold
a potential for variation in the accuracy and
precision of readings. Therefore, the objective
of this study is to determine whether the time of
day (AM vs PM) affects the accuracy (in meters
Universe Transverse Mercator (UTM) mode) of GPS
receiver readings of the geographic coordinates
of a stationary object.
This study shows that there is some variation
between readings of a GPS receiver when the same
object is measured consecutively. While it is
not significant, the average AM readings for the
ten day period varied 2.67 meters more than the
PM readings compared to the actual location of
the concrete bench (AM ten day average variation
6.97 PM ten day average variation 4.30).
Results
The standard deviation from the objects true
position was found, assuming that the true
position of the object was the average of the
five AM readings on the first day. The standard
deviation was found using the Pythagoreans
theorem (a-f)2 (b-g)2 c2 with a easting, f
true easting, b northing, g true northing,
and c variation. When variation is plotted by
measure number the AM readings (blue) showed two
readings of substantial variation on day 5 and
day 10, while the PM readings (pink) showed no
substantial variation from the rest of the
readings (see graph entitled GPS Reading
Variation). This large variation in the AM
reading on day 5 could be attributed to a
slightly foggy/overcast condition that existed
that morning. There was no different
weather/environment condition on day 10, though.
References
Material and methods
The Garmin GPS III plus (SN92118406) was used to
record the geographic coordinates of a concrete
bench on the Ag Quad of Virginia Tech Campus.
Readings were taken in UTM (Universe Transverse
Mercator) mode twice daily, approximately between
900-1030AM and 300-430 PM for ten days
(non-consecutive). Five readings were taken each
in AM and PM, with two minutes time between
readings. Each reading gave an Easting and a
Northing measure. Results were recorded, noting
any major differences in weather or environment,
if they existed.
Aknowledgement

• 1. Virginia Polytechnic Institute and State
  University
• 2. Carlyle C. Brewster, Ph.D., Assistant
  Professor, Department of Entomology
• 3. Donald E. Mullins, Ph.D., Professor,
  Department of Entomology