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					                                                Does Time of Day Affect GPS Readings?
 Summary                                                                                    Justin Tanner, Virginia Tech
 Global Positioning System (GPS) is a way                          Introduction
                                                                     Global Position System (GPS) is a satellite-based radio navigation                                                                                                                                            Discussion
 that a precise position can be obtained for                                                                                                                                     Results                                                                                             Based on the results of this experiment it can be said that
                                                                   system, initially developed in the early 1960s and operated by the
 any location with an open view of the sky.                        U.S. Department of Defense (DOD) since then. GPS consists of                                                                                                                                                    the morning and evening readings have an equal amount of
                                                                                                                                                                                 Charts 1 shows the Standard Deviation                                                             variability from the true reading. The deviation graph (Chart
 This study is attempting to determine if                          three segments - the satellite constellation, ground control network,                                         of the morning and evening readings
                                                                   and user equipment (FAA, 2005). Each one of these segments is                                                                                                                                                   3) shows that morning and evening readings have roughly
 readings taken in the morning have a                                                                                                                                            from the true reading.                                                                            the same variability from each other. There may be slightly
                                                                   important to the accuracy of GPS.
 different amount of variability from the true                                                                                                                                                                                                                                     more variability in the morning but not a significant amount.
                                                                     The satellite constellation consists of 24 satellites positioned in six
 reading than readings taken in the evening.                       earth-centered orbital planes with four operation satellites and a                                                    Devi ati on of GP S Read in gs Fro m GP S T rue Read i ng                                 Charts 1 and 2 show that both morning and evening readings
 The data collected in this study suggest that                     spare satellite slot in each orbital plane. With the baseline satellite                                                                                                                                         exhibit variability from the true reading but also show that
                                                                   constellation, users with a clear view of the sky have a minimum of
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                                                                                                                                                                                                                                                                                   neither is more variable than the other. This data leads to the
 morning and evening readings have an                                                                                                                                                                                                                                              rejection of the original hypothesis that morning and evening
                                                                   four satellites in view. It’s more likely that a user would see six to                                  5 .00 0 0

 equal amount of variability thereby                                                                                                                                                                                                                                               readings would have a different amount of variability.

                                                                                                                                               Sta n da r d De via t i n
                                                                   eight satellites (FAA, 2005).

                                                                                                                                                                           4 .00 0 0
 rejecting the hypothesis that they had a                            The ground control network consists of a series of monitoring                                                                                                                                                   This experiment was most likely affected by weather
 different amount of variability. Weather                          stations around the globe that track satellites and update their exact
                                                                                                                                                                           3 .00 0 0
                                                                                                                                                                                                                                                                                   conditions which can change over the course of the day
                                                                   position in the sky. The user equipment varies greatly but generally                                    2 .00 0 0                                                                                               leading to false readings at one time or the other. This can
 conditions may have played a factor in this                                                                                                                                                                                                                                       only be overcome by a larger data set which would require
                                                                   consists of a handheld tracking unit that receives signals from the                                     1 .00 0 0
 experiment so future studies would contain                                                                                                                                                                                                                                        more readings over a longer time period.
                                                                   satellites in view overhead. The satellites send a time-encoded
 more readings taken over a longer time                            signal that the receiver then uses to calculate distance based on its
                                                                                                                                                                           0 .00 0 0

                                                                                                                                                                                                                     n             n
                                                                                                                                                                                                              R ea d i g (5 R ea d i g s = 1 D a y)
 frame.                                                            own internal clock.
                                                                     The internal clock of the GPS unit is generally a quartz crystal                                                                    M or n ing De v iatio n            Eve n ing De v iatio n

                                                                   clock which causes some degree of error in the measurement
                                                                   because the satellite clock is generally a super accurate atomic                                                                                                         M orning and Ev e ning De v iation
                                                                   clock. Timing errors exist nominally due to the nature of most
                                                                   (imprecise) receiver clocks, which must be calibrated using GPS. In                                                                                                    6.0000

                                                                                                                                                                                                                       Evening Standard
                                                                   addition to this, however, residual errors in the satellite clock may                                                                                                  5.0000

                                                                   exist. Also, the satellite clock may fail and cause a particular range
 Materials & Methods                                               solution to drift off until the problem is corrected by the Ground                                                                                                     2.0000
   A GPS unit was obtained from Dr. Mullins for use in this        Control Segment (Phelts, 2004). Also weather conditions can affect                                                                                                     1.0000
 experiment. The GPS used in this study was a Garmin               the speed at which the signal travels which will affect the                                                                                                            0.0000
 Unit, Garmin GPSIII 1997. The unit was set to the UTM             measurement the unit produces (Brewster, 2005).                                                                                                                             0.0000                2.0000      4.0000         6.0000
 zone 17S. All measurements were taken using this UTM              A major source of error arises from the fact that radio signal speed is                                                                                                                       Mor ning Standar d Deviation
 zone.                                                             constant only in a vacuum. Water vapor and other particles in the
   Readings were taken two times a day each weekday for            atmosphere can slow signals down, resulting in propagation delay.                                                                                 Chart 2 shows the morning and evening deviations when plotted against
 10 days. The morning reading was taken around 11:00am             Errors due to multipath fading, which occurs when a signal bounces                                                                                one another. The line represents a perfect correlation. The data points are
 each morning and the evening reading was taken around             off a building or terrain before reaching the receiver’s antenna, also                                                                            almost evenly distributed across the line.
 6:00pm each evening. Each time the unit was turned on             can reduce accuracy. In addition, distance measurements are less
 and allowed to warm up for 5 minutes prior to taking the          reliable when the satellites a receiver locks on to are closely oriented
 first reading. A set of 5 readings was then taken over the        with respect to each other. Atomic clock discrepancies, receiver noise,
 next 10 minutes in both the morning and the evening.              and interruptions to ephemeris monitoring can result in minor errors                                                                                                                                  References
   The average of the first day of readings, both morning          (Bajaj, 2002).
 and evening, was taken to be the true GPS coordinates for           Selective Availability (SA) was a technique implemented by the                                                                                                                                      - Bajaj, R. Ranaweera, S. Agrawal, D. GPS: Location Tracking
 the location chosen. The location chosen was the Prices           Department of Defense (DOD) to intentionally degrade a user’s                                                                                                                                         Technology. Computing in Science and Engineering. April
 Fork Road Bike Trail sign on the edge of campus near the          navigation solution. The net result of SA was about a five-fold                                                                                                                                       2002.
 corner of West Campus Drive and Prices Fork Road.                 increase in positioning error. DOD achieved signal degradation by                                                                                                                                     - Brewster, C. Geospatial Technology. Presented in ALS5204
                                                                   altering (also known as dithering) the satellite clock (FAA, 2005). SA                                                                                                                                during the Fall Semester 2005.
                                                                   was discontinued in May, 2000 by President Bill Clinton.
                                                                     This study was done in order to determine the accuracy of a                                                                                                                                         - Federal Aviation Administration: Satellite Navigation. FAQ:
                                                                                                                                                                                       This is the location that all the                                                 Global Positioning System (GPS).
                                                                   handheld GPS unit during two different times of the day. The                                                        readings where taken at. All
Acknowledgements                                                                                                                                                                                                                                                         gps.htm Visited October 26, 2005.
                                                                   working hypothesis for this study is that the accuracy of GPS readings                                              readings where taken with the unit
  I would like to thank Dr. Mullins and Dr. Brewster for the       taken on a stationary object is affected by the time of day that the                                                placed on top of this sign in the
instruction and support during this study and for the use of the   readings are taken. The readings from the morning and evening will                                                                                                                                    - Phelts, R. Akos, D. Nominal Signal Deformations: Limits on
                                                                                                                                                                                       middle. The unit was left in place                                                GPS Range Accuracy. Presented The 2004 International
GPS equipment.                                                     have a different amount of variability.                                                                             during the entire 10 minute reading                                               Symposium on GNSS/GPS. Sydney, Australia. 6-8 December
                                                                                                                                                                                       period each time.                                                                 2004.