文章資訊


原始檔案

Journal/Conference: Transactions in GIS

Year:2009

Authors:

Units:
  1. Department of Geography University of New Mexico

摘要

支援3G的iPhone手機,是第一隻能夠支援無縫式整合3種定位技術(AGPSWiFiCellular Network )的手機。 本研究主要便是在實驗iPhone 3G上這三種定位技術的精準度。AGPS定位技術是透過參考點及低成本的GPS接收器來進行定位。而使用WiFi以及Cellular來進行室內定位的方式則是使用高分辨率的正射影像技術。

實驗的結果顯示,AGPS定位的情快較原本的GPS差(平均誤差值達8m),但仍足夠位置為基礎的服務(Location Based Service, LBS)所使用,而WiFi定位技術的誤差更大(平均誤差值可達74m)且不符合已發佈的精確度標準。Cellular定位技術是誤差值最大的定位方式(平均誤差值達600m),本研究主要針對在iPhone上這三種定位技術的涵蓋範圍、精準度、可信賴程度進行深入探討。


看完心得


認同之處


懷疑與問題


改進建議




介紹


願景與概念


欲解決的問題


研究的內容


解決方案


研究方法


研究過程


Data and Methods

  • A 3G iPhone was used to collect locations using three different modes: Assisted GPS, WiFi and cellular
positioning. The location service provides coordinates in the format of latitude/longitude and also an altitude when the A-GPS mode is employed.

  • A-GPS locations were collected at outdoor sites under ideal conditions, i.e. excellent satellite visibility. WiFi and cellular positions were collected at indoor sites where A-GPS position fixes were not available. Switching between these two positioning modes was accomplished by turning the iPhone’s WiFi receiver on and off – when no A-GPS or WiFi is available, the iPhone’s location service defaults to cellular positioning.

A-GPS Position

  • Locations in A-GPS mode were collected outdoors at 10 different surveyed benchmarks within the Albuquerque, NM metropolitan area. All benchmarks (n = 853) were plotted and overlaid on 6-inch color orthophotos from 2006.

  • At each of the 10 benchmarks, position fixes were recorded using both the 3G iPhone in A-GPS mode and a Garmin GPSMAP 60Cx unit (with a SiRF III chipset) in autonomous mode. Both units were placed vertically in a mount attached to the top of a survey tripod. The units were placed at the same height, at opposite sides of the survey pole, approximately 5 inches apart horizontally. The horizontal displacement of the antennae relative to the center of the survey pole was not considered in the analysis since it contributed very little to the overall positional error, expected to be in the order of several meters.

WiFi and Cellular Positions

  • WiFi and cellular positions were collected at indoor sites where no A-GPS position fix could be adopted. To determine suitable locations for indoor sites, the following sampling strategy was obtained.
    • First, a data file of address points was obtained from the City of Albuquerque – this includes the location of every occupied building within the city limits. A random sample of 65 properties was obtained, with the additional conditions that no two locations could be closer together than 300 m. These 65 locations were visited in the field. If access to the particular building was restricted or impractical, a new random location was selected. If an A-GPS position fix could be obtained inside the building, preventing the use of WiFi and cellular positioning on the iPhone, a new random location was chosen. In total 87 buildings were visited with 22 resulting in an A-GPS position fix. Only the results for WiFi and cellular positions at the remaining 65 locations were used in the analysis.
    • Within each building a location was selected that could easily be recognized on the 6-inch color orthophotos from 2006, for example, near an entrance, window, or corner. A laptop preloaded with the orthophotos was used in the field to digitize the estimated location of the indoor sites. At each location a single WiFi and cellular position fix was recorded. In the initial testing phase multiple position fixes were recorded at 5 second intervals, but these turned out to be identical (to 6-decimals in lat/long format) for each positioning mode. As a result, only a single position fix was recorded in the final field data collection effort. The field laptop was also used to confirm the availability of WiFi positioning at each indoor site. To access the WiFi positioning system without having to rely on connections to potentially weak or encrypted WiFi networks, the laptop was equipped with a cellular broadband Internet connection. The WiFi positions obtained using the laptop were not used in the analysis, but only served to confirm the availability of the WiFi positioning system. When A-GPS and WiFi positioning are not available, the iPhone defaults to cellular positioning and the logged positions of WiFi and cellular positioning on the iPhone are indistinguishable (in contrast to A-GPS positions).
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研究成果


總結

  • The performance of A-GPS on the iPhone at outdoor locations was substantially less than that achieved using dedicated consumer-grade GPS receivers. This can likely be attributed to the concessions that are made in the design of the A-GPS hardware on the iPhone, including antenna, power and other considerations.
  • The performance of WiFi positioning on the iPhone at indoor locations was substantially less than the performance of A-GPS outdoors and in fact was far below expectations based on published performance measures by Skyhook Wireless (2008).
    • Claims that WiFi positioning is able to acquire a location nearly 100% of the time in urban settings could not be confirmed and only 87.7% availability was achieved.
    • Claims that WiFi positioning is able to achieve a median horizontal accuracy of 20 to 30 m could not be confirmed and a much larger median error of 74 m was found based on 57 observations.
  • Availability of cellular positioning on the iPhone at indoor locations was 98.5%, substantially higher compared to WiFi positioning at the same locations. Positional accuracy was much lower than for WiFi positioning, but similar to the results published by previous studies on cellular positioning (e.g. Mohr et al. 2008).

後續


後續的工作


後續的挑戰




相關資料


相關專案


相關研究


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