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GPS 101October 2, 2015

Victor Quinn, VP of Engineering

GPS 101

• Time to first fix

• Assisted GPS

• Estimated Accuracy

• Location Point

• Environmental Factors

• Raw GPS data is alarmingly poor

• LocationKit

Time to first fix

• In the GPS world, this is the time to first figure out where the device is in the world

• With old school/traditional GPS devices, this often takes 30s or more

• Surely we can do better…

Assisted GPS

• Standard for Smartphones

• Uses local intelligence (cell towers, wifi, bluetooth, etc.) to help quickly get device location, shortening time to first fix

• While in parallel spinning up the GPS chip to get a solid lock

Assisted GPS• In the background, Apple

uses cell tower triangulation, knowledge of wifi networks, etc. to “assist” and get your location quickly

• But that first location before a GPS lock is not terribly accurate

Estimated Accuracy• Since the accuracy varies widely, Apple and

Google both provide a measure of the accuracy of the points they provide

• It is a quantity, in meters, of the margin of error of the location points

• For Google it is the radius of the confidence of the point being within one standard deviation. For Apple it’s likely the same (though they’re less forthcoming in their docs)

Location Point• Latitude/Longitude

• Accuracy

• Horizontal

• Vertical

• Altitude

• Bearing or Course

• Speed

• Time

• Reference

• Google Location

• Apple CLLocation

Environmental Factors• Buildings — GPS signal indoors is

very poor, buildings reflect GPS signal

• Weather — On cloudy, rainy days (like today here in DC) the GPS signal is noticeably worse

• Time of Day — Orbit of one satellite takes 12 hours

• Accuracy of clock — GPS requires sub-millisecond clock accuracy

• Device — We notice differences in accuracy from one device to another. Could be chip manufacturing inconsistencies or antenna, not entirely sure

• Motion — Whether device is still or moving

• Atmosphere — Ionospheric distortion

Raw GPS is alarmingly poor

• Video

LocationKit

• We built LocationKit and baked in all our expertise on the subject so you don’t have to think about it

• We’re able to take that crappy raw GPS and make it useful to developers

GPS 101

• Time to first fix

• Assisted GPS

• Estimated Accuracy

• Location Point

• Environmental Factors

• Raw GPS data is alarmingly poor

• LocationKit

Questions?