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 6 .00 0 0 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). o 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 Deviation 4.0000 exist. Also, the satellite clock may fail and cause a particular range 3.0000 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). http://gps.faa.gov/FAQ/faq- 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.