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

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

Introduction to measurement of location

1. Question has plagued humanity for ages
   1. Location of home
   2. Location of resources
      1. Water
      2. Food
      3. Dangers
2. Knowing location of one place implies knowing distances between places
   1. How are distances measured?
3. Ancient wonders were measured simply: distance and direction (measuring tapes, protractors, compasses)
   1. Great Wall
   2. Pyramids
   3. Topographic survey of India, US, etc.

   Measurement methods did not change until the electronic age

"Modern" location measurements

1. WW2: Radio at the forefront of electronic technology
   1. RADAR for detecting objects (friendly and enemy vehicles)
   2. Development in to ground-based LORAN system
      1. More about LORAN technology later: basics are the same as GPS
2. Cold war
   1. Missile launching, silo-to-silo

   knowledge of ground resources possible (high-altitude spy photo, satellite images)

   1. 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?

1. d = r * t
   1. speed of light =~ 300,000,000 m/s
   2. satellite signals include satellite ID number and time of origin of signal
2. demonstration

   one transit time = 1 distance = surface of a sphere, etc.

   simultaneous solution of 4 xyz equations

   demonstration (string)

3. 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

1. Space segment

   32 satellites in 6 orbital planes

2. Control segment: US DOD at AF bases
3. User segment: GPS units (base stations, cars, cell phones)

Sources of error

1. low quality receivers
2. insensitive electronics, errors in processing software & hardware
3. cheap clocks
4. multipathing (signals bouncing off buildings, etc.)
5. atmospheric effects
6. blunders (user error)
7. selective availability (removed May 1, 2000)

   Added up to 100 m horizontal error by introducing clock errors

8. Correcting errors
   1. more sensitive equipment
   2. more advanced on-board software (filtering multipath errors, e.g.)
   3. differential correction
      1. real-time differential correction
9. Accuracy of typical handheld systems: 15 m at the worst, 1-5 m at best, varying under different conditions

Uses

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

Future of GPS

1. Already in many new cars
2. Telephones
3. wrist watch size
4. more advanced satellites, more advanced receivers
5. additional locational technologies to augment GPS (WiFi, phone cells)
6. Privacy issues

Where y'at?

• Third Place Books

  122°18'22.785"W

  47°40'33.193"N