1. Wireless, Cellular, Geolocation Harry Lewis April 14, 20111Harvard Bits

2. Inverse Square Law Signal strength at distance D is proportional to 1/D 2 April 14, 2011Harvard Bits2 So a radio signal is only 1/10,000th as strong at distance 100 miles as it is at distance 1 mile

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, 2011Harvard Bits3

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, 2011Harvard Bits4

5. Global Positioning System 21-29 satellites 12,000 mi (~2 orbits/day) Orbit @ 55 o to equator From anywhere on earth, ≥ 4 at least 15 o above horizon Each transmits – Its own ID – Clock time – Its own location April 14, 2011Harvard Bits5

6. Global Positioning System GPS receiver calculates its distance to 3 or 4 satellites (T gps -T sat )*c Infers its own location (trigonometry) Passive device! Civilian use April 14, 2011Harvard Bits6

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