wireless, cellular, geolocation harry lewis april 14, 20111harvard bits
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Harvard Bits 1
Wireless, Cellular, Geolocation
Harry Lewis
April 14, 2011
Harvard Bits 2
Inverse Square Law• Signal strength at distance D is proportional
to 1/D2
April 14, 2011
So a radio signal is only 1/10,000th as strong at distance 100 miles as it is at distance 1 mile
Harvard Bits 3
Cell Phone Technology• Cells are about a mile across• Tower in each cell• Phones can have low power because distances are small• More frequency bands can be allocated to simultaneous calls
because the “guard bands” can be narrower• Frequencies can be reused in different cells because with fading,
calls on the same frequency will not interfere• As phone moves from cell to cell frequency may have to be
switched• Phone that is turned on constantly reports its location to nearby
towers so incoming calls can be directed to it
April 14, 2011
Harvard Bits 4
Cell Phone Technology• Cells are about a mile across• Tower in each cell• Phones can have low power because distances are small• More frequency bands can be allocated to simultaneous calls
because the “guard bands” can be narrower• Frequencies can be reused in different cells because with fading,
calls on the same frequency will not interfere• As phone moves from cell to cell frequency may have to be
switched• Phone that is turned on constantly reports its location to nearby
towers so incoming calls can be directed to it
April 14, 2011
Harvard Bits 5
Global Positioning System
• 21-29 satellites• 12,000 mi (~2 orbits/day)• Orbit @ 55o to equator• From anywhere on earth,
≥ 4 at least 15o above horizon
• Each transmits– Its own ID– Clock time– Its own location
April 14, 2011
Harvard Bits 6
Global Positioning System
• GPS receiver calculates its distance to 3 or 4 satellites
• (Tgps-Tsat)*c• Infers its own location
(trigonometry)• Passive device!• Civilian use
April 14, 2011
Harvard Bits 7April 14, 2011
Harvard Bits 8April 14, 2011
Harvard Bits 9April 14, 2011
Harvard Bits 10April 14, 2011
Harvard Bits 11April 14, 2011
Harvard Bits 12April 14, 2011