A GPS receiver on earth that detects the signals can calculate its distance from each satellite based on the travel times of pulse sequences encoded in the radio signal. The position of the receiver is then estimated by triangulation U. EPA, Originally, the U. The effect of selective availability was to superimpose a time-dependent dithering pattern with a radius of about m on the apparent location of the GPS receiver.
To calculate the location of a mobile GPS receiver more precisely, albeit with some delay, the user could mathematically remove this dithering by applying differential corrections that were determined from the measured dithering in the location of a GPS base station of known coordinates. Selective availability was turned off by presidential order on May 1, , after the U. The end of selective availability resulted in an estimated fold improvement in the real-time precision of GPS for civilian users.
Although GPS has been widely used in applications such as land surveying, mapping, and navigation, it has limitations that potentially reduce its usefulness in some spheres of human activity. To calculate coordinates i. The radio waves transmitted by the satellites can pass through clouds and vegetation without significant attenuation, but they are blocked by conductive materials such as earth and metal.
Due to the Faraday cage effect, the signals typically cannot penetrate through steel-reinforced structures. Partial blocking of the sky by embankments or tall buildings may result in reduced precision of location measurements. Precision of GPS location estimates is highest when signals are received from satellites that are widely separated in azimuth and elevation.
The reduction in precision due to poor relative geometry of satellites is termed positional dilution of precision PDOP U. GPS technology has been used for wildlife tracking and for tracking of fleet vehicles, such as taxicabs and emergency vehicles, but use of GPS for tracking individual humans during their daily activities appears to be quite limited to date.
The U. Forest Service has reported on the research use of backpack-mounted GPS recorders to track elk hunters over 6-h periods in Montana Lyon and Burcham, A lightweight GPS receiver was also used to measure the velocity of a human volunteer engaged in short-term outdoor athletic activity Schutz and Chambaz, The primary objective of OUATS is to develop generalized models for estimating personal exposure in broad populations, based on an investigation of how temporal, personal, and urban factors influence the relationship between personal and area exposure measurements.
Exposure is estimated from two sequential h nighttime and daytime integrated air samples for volatile organic compounds collected simultaneously in the personal breathing zone of the research subject, in the main living area of the subject's home, and in the subject's backyard.
Activity patterns of the research subject will serve in the data analysis both as potential determinants of exposure and as potential outcome variables determined by independent factors such as weather conditions, size of urban area, occupation of the research subject, and the presence of young children in the household. The specific purpose of the GPS study was to test the application of currently available GPS data recorders as a means of validating time—location data recorded in participants' time—activities diaries in community-based exposure assessment studies. The primary means of recruitment were flyers posted in public areas, news media reports, and personal contacts in workplaces and public gathering places.
Participation in the study was limited to adults between 21 and 55 years of age who lived in nonsmoking households in detached single-family dwellings. The GPS data recorders used in this study had to be capable of performing their data-logging function with a minimum of inconvenience and effort on the part of the research subject.
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In brief, the device had to be capable of logging positions automatically every 5 to 15 min or more frequently, if possible and storing 16 to 24 h of data. The battery life had to be sufficient to allow continuous logging with no more than one or two battery changes in a h period. The unit had to be small enough to be worn comfortably around the waist, without impeding normal activities.
Triangulation | National Geographic Society
The antenna had to be configured so that it would not be damaged during use and its appearance would not be unacceptable to the research subject. Finally, because selective availability was in effect at the beginning of this project, the GPS unit had to be capable of storing data in a form that could be postprocessed with differential corrections. Product information was solicited from a number of GPS manufacturers. In accordance with institutional requirements, the purchase of GPS data recorders followed a competitive bidding process, subject to the performance specifications described above and the constraints of the project budget.
The unit was powered by an internal rechargeable lithium metal hydride battery, which was designed to provide 4 to 5 h of charge. Operating time was extended by the use of an external rechargeable lead—acid battery, designed to provide an additional 12 h of charge. The unit included a built-in antenna, but for improved reception, an external patch antenna was used. The external antenna was a flat oblong disk measuring 8 cm by 6. The total weight of the unit with battery was about 2 kg.
The data storage capacity was 2 MB, which was claimed to be sufficient to hold 14, data points. The h monitoring period generally began and ended in the early evening. The GPS data recorder was set to log every 4 to 8 s. After it was verified that the unit was receiving satellites and starting to log data, the unit was secured inside a nylon hip pack, the external battery was plugged into a nine-pin com port on the unit, and the hip pack was fastened around the subject's waist.
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The external antenna was magnetically mounted on a large steel button that was pinned to the shoulder of the subject's clothing. The subject was instructed to wear the GPS unit at all times except when bathing or sleeping and to replace the external battery in the morning, after 12 h.
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A photograph of the monitoring equipment as worn by the subject is presented in Figure 1. Personal monitoring equipment worn in the study. The external battery is contained in a separate pouch worn on the belt of the fanny pack. The external patch antenna, attached to the data recorder by a thin cable, is positioned at the subject's shoulder. The air-sampling pump is also typically worn at the waist, attached by flexible tubing to a sorbent tube in the subject's breathing zone, as shown. The GPS data points could be imported into MapInfo and superimposed on a street map, but generally the locations of the logged points were determined by visually comparing the plotted points to a separate printed street map.
It should be noted that location data had to be secured as confidential information because it could be used to identify the residence of the study participant.
Navstar: GPS Satellite Network
The time—activity diary used in this study was based on the Multinational Time Budget Research Project method Harvey et al. The standard diary form employed in time budget research was modified to elicit information on exposure to environmental tobacco smoke and use of consumer products, chemicals, equipment, etc. A sample page of the diary is provided in Figure 2. Subjects were given oral and written instruction on how to complete the diaries.
The subjects were instructed to:. After the diary and the GPS data were reviewed by an investigator, the subject was called by telephone to clarify diary entries, fill information gaps, and resolve any apparent discrepancies between the diary and the GPS tracking data. No study participants objected to wearing the GPS monitors. Several participants of wide girth found it more comfortable to wear their GPS unit in a minibackpack than in a hip pack.
In preliminary trials, it was found that some individuals had poor reception of satellites when the built-in antenna was used with the GPS unit worn at the waist.
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Use of the shoulder-mounted external antenna dramatically improved reception for these individuals and was therefore made part of the standard procedure. In the best case, satellite reception was sufficient to allow the GPS unit to log position data nearly continuously every 4 to 8 s throughout all outdoor activities. In addition, reception was typically good enough to allow logging inside dwellings.
As expected, however, partial blocking of the sky by the metal body panels of some vehicles and by urban canyons often reduced the fraction of outdoor time for which position data was successfully logged. Logging usually stopped inside commercial buildings due to the shielding effect of metal-reinforced construction. After exiting a shielded location, the unit often took 5 to 10 min to start logging again.
A fourth signal is used to synchronize the low accuracy clock in the GPS receiver with the high accuracy atomic clocks in the satellites. To get the best accuracy, surveyors use two GPS receivers to measure between two points.
One of the GPS receivers is set up on a point with a known position. This allows the surveyor to calculate the position of the second point. Ready for a quiz? This is an artist's impression of a GPS satellite, used to transmit location and time signals used to calculate a position. Image: NASA. GPS devices are useful for activities such as tramping and climbing where it is important to know where you are. Image: LINZ.
GPS: The Global Positioning System
Many drivers now rely on car navigation systems which use GPS to give directions. Skip to main content.
Listen to this page. Who has heard of GPS? A third person tells you that you are 3km from Redwood.