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==Where y'at? An introduction to Global Positioning Systems (GPS)==

# Where y'at?
## New Orleans response
# Who knows their address?
# Who knows how to get home?
# Who knows the latitude and longitude of their home to 3 decimal places of precision?

==Introduction to measurement of location==
# Question has plagued humanity for ages
## Location of home
## Location of resources
### Water
### Food
### Dangers
# Knowing location of one place implies knowing distances '''between''' places
## How are distances measured?
# Ancient wonders were measured simply: distance and direction (measuring tapes, protractors, compasses)
## Great Wall
## Pyramids
## Topographic survey of India, US, etc.
#: Measurement methods did not change until the electronic age

=="Modern" location measurements==
# WW2: Radio at the forefront of electronic technology
## RADAR for detecting objects (friendly and enemy vehicles)
## Development in to ground-based LORAN system
### More about LORAN technology later: basics are the same as GPS
# Cold war
## Missile launching, silo-to-silo
#: knowledge of ground resources possible (high-altitude spy photo, satellite images)
## Most efficient killing machine: nuclear submarine
#: Problem: how to surface quickly, get location, fire missile, and dive
#: Solution: GPS, an extension of previously developed technologies

==How does it work?==
# d = r * t
## speed of light =~ 300,000,000 m/s
## satellite signals include satellite ID number and time of origin of signal
# demonstration
#: one transit time = 1 distance = surface of a sphere, etc.
#: simultaneous solution of 4 xyz equations
#: demonstration (string)
# Precision timing: 1/1000 s error => 300,000 m error
#: Receivers can keep good time over short intervals
#: Receivers add or subtract very small amounts of time until a very precise estimate of location is made, time is fixed.

==Parts of the system==
# Space segment
#: 32 satellites in 6 orbital planes
# Control segment: US DOD at AF bases
# User segment: GPS units (base stations, cars, cell phones)

==Sources of error==
# low quality receivers
# insensitive electronics, errors in processing software & hardware
# cheap clocks
# multipathing (signals bouncing off buildings, etc.)
# atmospheric effects
# blunders (user error)
# selective availability (removed May 1, 2000)
#: Added up to 100 m horizontal error by introducing clock errors
# Correcting errors
## more sensitive equipment
## more advanced on-board software (filtering multipath errors, e.g.)
## differential correction
### real-time differential correction
# Accuracy of typical handheld systems: 15 m at the worst, 1-5 m at best, varying under different conditions

==Uses==
# Military applications
## smart bombs
## personnel, facilities, and equipment management
# Civilian applications
## Transportation (location and timing)
## Safety (personal, e-911, etc.)
## Find your friends
# Science
## Mapping
## precision timing
## Continental drift, mountain building

==Future of GPS==
# Already in many new cars
# Telephones
# wrist watch size
# more advanced satellites, more advanced receivers
# additional locational technologies to augment GPS (WiFi, phone cells)
# Privacy issues

==Where y'at?==
* Third Place Books
*: 122°18'22.785"W  
*: 47°40'33.193"N