手機定位技術 (Mobile Location)

2001年筆者在美國Spirent出差的時候, 正好是Sprint-Nextel委託Spirent針對E911手機定位準確度的測試設備的開發最後階段，當時Spirent成功提供了全球第一套符合IS-916的測試設備.

以下分享我對手機定位技術的粗淺認識:
1. Mobile Location技術可分為三種:
- Mobile based, 如GPS
- Network based, 如cell ID, AOA, TOT
- Mobile and Network co-work: 如A-GPS, AFLT, E-OTD

2. GPS技術:
- 由手機尋找GPS衛星 --> it would take a long time for first time power on in a new area
- 由手機量測與GPS衛星的距離
- 由手機計算自己的X, Y, Z

3. A-GPS技術:
- 由網路告知手機該尋找哪些GPS衛星 --> it would take only few seconds
- 由手機量測與GPS衛星的距離
- 由網路計算手機的X, Y, Z

4. 無論GPS or A-GPS, 都需要至少4顆GPS衛星:
X, Y, Z, delta T; X= 經度, Y= 緯度, Z= 高度, delta T = time difference
--> 以上四個未知數, 需要四個方程式來解.
--> 因此需要四顆GPS衛星.

5. IS-801-1 signal基本上走control plan for E911, 但無法傳送太多服務訊息(如地圖資訊), 因此建議走user plan技術上是正確的.

6. E911是美國FCC的要求, 台灣沒有還無此規定.

7. FCC關於E911的要求:
E911 Issues (E911 stands for electronic 911 and refers to the requirement by cellular network carriers to provide location information for emergency calls from cellular telephones)

E911 is primarily an FCC (federal communications regulator in USA) initiative. Success of E911 has faced several hurdles. Hatfield report in USA has highlighted several concerns and issues. We are highlighting some of these issues for professionals to deal with:

- Accuracy of various location identification technologies - GPS, E-OTD, etc - Go to the relevant location-based page on our site to understand this. FCC has set compliance expectations - phase one (1998) and phase 2 (October 2001) - 100 metres for 67% of calls, and 300 metres for 95 % of calls

- Lukewarm support of wireless carriers to meet the compliance requirements

- Availability of interoperable standardized handsets - A number of proprietary handsets are available now

- Retrofitting of PSAPs (Public Safety Answering Points) - various agencies that receive these calls - mostly emergency dispatch centers.

- State of Existing PSAP infrastructure - Somewhat antiquated in many places

- Inadequacy of funding at local level to retrofit hardware and software - it is not a trivial effort- billions of dollars are required. Somebody must foot the bill - federal government, homeland security agency, state agencies or local jurisdictions.

參考文獻:
[1] MOBILEINFO.com: Vertical Applications- E911 - An Important Public Safety Requirement-
: http://www.mobileinfo.com/applications_vertical/public_safety/e911.htm

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