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GPS
GPS BASICS
Satellite Constellation
Global Positioning System Network of 24 satellites (with spares) Developed by Department of Defense Operational 24 hours/day Available worldwide Land, sea and air Works in all weather conditions It is FREE!!
Navigation Tools of the Past Sextant - make precise observation of the sun, stars, and planets to chart their course Chronometer allowed navigators to know what time it was in two places at once, which helped to determine longitude.
Beginnings
TRANSIT developed by Navy in 1960s – 3 satellites used to track nuclear submarines
Low accuracy – not available 24/7
GPS System – 3 segments
Space Segment – Satellite Constellation
Control Segment– Monitoring Stations and Ground Antennas
Air Force created own system Used to locate vehicles on land or in air
master control station located at Schriever Air Force Base in Colorado
User Segment – Receivers
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How Does GPS Work?
How it works
Trilateration
Satellite circles the Earth and transmits a radio signal called a psuedo random code.
Signal contains time it was sent and its location
All satellites send their signal at the same time
Difference in time to reach receivers is used to determine location
Trilateration
Process of measuring the distance from at least three satellites
Three satellites calculate 2D position (Latitude and Longitude)
Four or more satellites calculate 3D position (Latitude, Longitude, and Altitude)
Trilateration
You don’t know where you are, you ask three people and get the following responses:
Where are you?
125 miles Wichita, KS
250 miles Springfield, MO
Stillwater, OK
65 miles Oklahoma City, OK
One satellite can locate a receiver’s position somewhere on a sphere
Two satellites can locate a receiver’s position to a circle representing the intersection of two spheres
Three satellites can locate a receiver’s position to one of two points represented by the intersection of three spheres.
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Latitude
Latitude lines run horizontal Equator is 0° North and South Poles are 90°
Longitude
Also known as meridians, run vertical The Prime Meridian in Greenwich, England is 0° Lines range from 0° to 180° International Date Line is 180°
Accuracy Problems
Limitations and Accuracy with GPS
Accuracy Problems
Number of Satellites Visible - The more satellites available or visible, the accuracy increases. At all times there should be at least 3 satellites visible.
Satellite Geometry and shading – Another variable is whether the satellites are bunched up or spread out. This is called satellite geometry and shading. The ideal positioning is when the satellites are spread out.
Selective Availability – adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.
Atmosphere Delay - As the signal passes through the ionosphere and troposphere, the water vapor and particles can slow a signal down
Signal Multipath - caused by the satellite signal reflecting off of buildings, rocks, water, trees, etc. Accuracy tends to be better in open areas where the likelihood of reflection is decreased.
Receiver Clock Errors - clocks within a receiver are not as accurate as an atomic clock. Satellites however are equipped with an atomic clock.
Differential Correction
Developed by the Coast Guard
Also known as DGPS
Ten meter or less accuracy
Full Operational Capacity March 15, 1999
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Uses of GPS
Differential GPS
Locating Tracking ItThe hasstationary no idea which satellites the roving receiver then works receiver is using to compute its location. backwards to reference compute the errors. It the The stationary receiver gets Therefore the stationary receiver computes how long it should take to signals from the satellites as does computes the error corrections for all visible satellites. It receive a signal versus what it actually mobile receiver (boat). then this correction to took.sends The difference is theinformation error correction. the satellites and mobile receivers. Correction Signal
Navigating Mapping Timing Stationary Receiver
you
Using GPS
Geocaching
What are uses you can think of? Boating
Fishing
Hunting
Camping
Hiking
Biking Rafting
Compass Format of Waypoint Name of Waypoint GOTO
BRG = Bearing DST = Distance in miles TRK = Track taken ENTER
SPD = Speed in miles/hour ETE = Estimated Time Enroute
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Point 1
Point 1 m i DST
212º
.21
BRG
+ S
W
+
+
E
N
+ TRK
SPD
210º
2.0
m h
ETE 06:17
Point 1 .21 i
BRG
DST
+
S
DST
+
S
If arrow is not pointing straight ahead, veer in the direction that the arrow is pointing.
+ W
E +
NN
+
TRK
SPD
183º
2.0
m h
In this example, veer right or toward the southwest.
ETE 06:17
Realistically it may get only to .01 miles, maybe even .02 miles.
W NN
.21 i
BRG
+
E +
m
212º
Goal is to work the distance to the Waypoint down to .00 miles.
m
212º
In order to get bearing, distance, etc…, you need to start walking in any direction. After about 2030 feet, the unit will triangulate your direction of travel with your starting point.
+
TRK
SPD
183º
2.0
m h
ETE 06:17
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