Lecture 4: Navigation Systems Flashcards
Name the 4 Global Navigation Satellite Systems(GNSS)
- United States’ Global Positioning System (GPS)
- Russia’s GLONASS
- European Union’s Galileo
- China’s BeiDou
What is GPS?
A system that uses satellites to provide geographic positioning
How does GPS get a location?
small electronic receivers determine their longitude, latitude, and elevation using radio signals transmitted from satellites
What is selective ability?
adds intentional, time varying errors of up to 100 meters to the publicly available navigation signals(to deny enemy the use of civilian GPS receivers for precision weapon guidance)
What is Trilateration?
method of determining relative positions using the geometry of triangles
What is a fix?
An absolute position of a receiver
What does Trilateration need for GPS?
- known locations of two or more satellites
- measured distance between the GPS receiver and each satellite
What does accurate distance calculation need for Trilateration?
both the GPS receiver & satellite clocks must be precisely and continually synchronized
Why is accurate distance calculation for Trilateration impossible?
The clocks in GPS receivers are not as accurate as the atomic clocks in the satellites
How do you eliminate the clock errors in accurate distance calculation for trilateration
A fourth satellite is required
What does each GPS satellite constantly send to the Earth
Coded radio signals (pseudorandom code)
What do GPS satellite signals contain?
The satellite that is sending the info
Where the satellite is at any given time
Date and time it sent the signal
What does a GPS receiver have?
- antenna tuned to the radio frequencies transmitted by the satellites
- very accurate clock
- a processor to decode the satellite’s signal
Name some GPS applications
Navigation Autonomous Vehicles Tracking Recreation (fun) Mapping Location based social media Mapping Accurate time
Satellite signals cannot pass through…
Objects that contain a lot of metal or water
Satellite signals can pass through…
clouds, glass, and plastic.
What decreases the power of satellite signals?
solid objects such as buildings
What is the minimum number of satellites for a full constellation?
24 operational 95% of the time
Why 24 satellites for a full constellation?
ensures there are at least four satellites in view from virtually any point on the planet
What do monitoring stations do?
track the location and health of each satellite (e.g., clock error, malfunctions, correct orbit)
Synchronize the atomic clocks onboard the satellites to within a few nanoseconds of each other
Name some accuracy problems for a GPS
Atmosphere delay Signal multipath error Receiver clock errors Number of satellites visible Satellite geometry Orbital Errors
What is a Multipath error?
GPS signal bounces off of objects (building, rocks) before it reaches GPS receiver.
(increases the travel time of signal)
What is Satellite Geometry?
the relative position of the satellites at any given time
When does the ideal satellite Geometry exist?
- satellites are located at wide angles relative to each other
- when overhead the earth
