This document discusses the use of GPS technology in vehicle accident reconstruction. It begins by explaining how GPS works through satellites and receivers to determine location and velocity via trilateration. The document then discusses how GPS is used for navigation, emergency services, and location-based applications. It provides an example of how GPS data from a bicycle involved in a collision was used to determine the cyclist's speed. The document concludes by discussing privacy issues regarding location data and the evidentiary value of GPS devices in reconstructing accidents.
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The Global Positioning System
What is GPS?
Global Positioning System: a navigational
system involving satellites and computers that
can determine the latitude and longitude of a
receiver on Earth by computing the time
difference for signals from different satellites
to reach the receiver.
~ Princeton WordNet 3.0
Layman’s Terms: GPS knows where you are
(Lat, Long, Elev.), and how fast you’re going
(Instant Speed).
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How GPS Works
Satellites (Maintained by U.S. Government):
Orbiting constellation (31 in 2008), altitude of
12,550 miles
Satellite emit identical signals
Receivers (both Military and Civilian):
Observes satellite signals, analyzing them
Time Difference – Position
Doppler Effect – Velocity
Trilateration ~= Triangulation
Minimum of 3 Signals necessary
More “visible” signals = more accuracy
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How GPS Works
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Psuedo-Random Code (PRC)
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Trilateration
Similar to Triangulation,
using distances in lieu
of angles
Minimum Satellites = 3
Assumed to be on
surface of the Earth
More Satellites result in
better accuracy
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GPS Use
Military/Civilian:
Navigation
Missile, aircraft, land vehicle
Vehicle-based, handheld units, cell phones
Emergency
FCC (2002) mandated in order for emergency
services to locate 911 callers (tower-based)
Location
Geofencing: Kids, pets, phones, etc
Geotagging: Photo location embedded
Surveying (Waymarking)
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GPS In The News
http://www.abc15.com/dpp/entertainment/celebrity/Man-arrested-after-Lil-Wayne-video-shoot-theft_04082332
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GPS In The News
http://www.huffingtonpost.com/2011/02/21/anaheim-gps-truant-students_n_826100.html
GPS Receivers
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GPS Accuracy
Accuracy varies HIGHLY by GPS brand
Proprietary calculations
Typical Published values (GPS):
Position: +/- 150 meters (492 feet)
Velocity: +/- 2.5 meters/second (5.5 MPH)
Typical Published values (DGPS):
Differential GPS (assisted by ground-based
reference stations)
Position: +/- 3 meters (10 feet)
Velocity: +/- 0.2 meters/second (0.5 MPH)
Velocity data tends to be underestimated
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Broadening the Horizons
In the typical vehicle collision investigation,
there are lots of unknowns:
Which lane were they in?
Where were they going?
How fast were they driving?
Who’s the “at-fault” driver?
Etc, etc
GPS data may be able to answer some of
these questions
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Broadening the Horizons
GPS data may shed light on:
Which lane were they in?
Where were they going? Directions?
How fast were they driving?
Who’s the “at-fault” driver?
GPS data can provide:
Position (Lat, Long, Elev)
Speed (mph, orientation)
…at discrete times
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Sample GPS Data Set
NOT Collision-Related… Thankfully
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Viewing GPS Data Sets
Viewable in software applications
Numerous Web-based viewers
Google Maps/Earth
Sample GPS Data Set: Use in AR
Vehicle/Bicycle Collision
Front of vehicle collides with rear of
bicycle
Liability disputed
Many issues, one of which was the pre-
collision speed of the two vehicles
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Scene
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Bicycle GPS Data
Cyclist wearing a Garmin device for
logging/training purposes
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Bicycle GPS Data
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Bicycle GPS Data
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Bicycle GPS Data
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Bicycle GPS Data
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Sample GPS Data Set: Use in AR
Summary:
GPS data depicts the motion history of
the cyclist
The GPS Data
CANNOT place the cyclist in a specific lane
of travel (or a portion of the lane)
CAN depict the instantaneous speed of the
Cyclist
Critical piece of evidence in this case
Side notes: “Extra” data also captured…
Post-collision movement
Cycling data XX-days after collision
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Elephant in the Room:
Current Topics
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Location, location, location…
Recall that your electronic device has the
capability of identifying your location, with
or without your knowledge.
Photo-taking devices CAN embed the
location of the photograph into the image
data (Cameras, Cell Phones, etc)
Is there a privacy issue?
Most consumers do not know about
“Geotagging”
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Location, location, location…
Recall that your electronic device has the
capability of identifying your location, with
or without your knowledge.
Photo-taking devices CAN embed the
location of the photograph into the image
data (Cameras, Cell Phones, etc)
Is there a privacy issue?
Most consumers do not know about
“Geotagging”
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Location, location, location…
http://www.nytimes.com/2010/08/12/technology/personaltech/12basics.html
More Unknowns…
Your (vehicle, phone, handheld device)
may contain evidence related to an incident
Difficult/Impossible to obtain if the user is
unaware
Example: Cyclist Collision
Spoliation issues?
While most consumers probably recognize the
ability of their GPS device to navigate them
through uncharted highways and byways, few
realize that their device may play a serious
role in the event of a crash
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Solutions…
Solution: User Beware
Understand what the devices are doing
Is it recording a path?
How is the path downloaded?
Most do not record paths under the factory
settings
Case Application:
What data “the other side” might have
Discovery language Re: EDR
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Analogies
The GPS is a silent witness, capable of
providing an un-biased perspective on
the events surrounding a collision.
Analogous to an event data recorder
(“black box”); Technology that has been
widely utilized in collision reconstruction
CRS 12-6-401 and 12-6-402
“ ’Event data’ means… vehicle speed,
vehicle direction, vehicle location…”
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