Lecture 10 - GPS and Inertial Sensors Flashcards
Define a GPS
A system which enables us to know where we are, what time it is and where we are going (PSDT; Position, Speed, Direction, Time information)
Define the Doppler Effect
The apparent change in frequency that occurs when a source and a target are in relative motion
What can the doppler effect be used to determine?
It can be used to determine velocity (compression of the frequency)
What is the general expression for velocity from doppler?
v = c (1 - f(t) / f(r))
What is the meaning of the doppler general expression variables?
1) f(t) = The transmitted frequency
2) f(r) = Received frequency
3) c = Speed of light
4) v = Relative velocity
What to the doppler effect velocity signs represent?
+ sign means that the transmitter is approaching the observer while - means they are going away
What happened at the launch of SPUTNIK-1 ?
On October 4 1957, doppler was employed to determine the entire satellite orbit
What is the first basic concept of a GPS system?
GPS satellited broadcast radio signals provide the precise transmission time (t1) from on-board atomic clock
What is the second basic concept of a GPS system?
GPS devices receive the radio signals and record their exact time of arrival (t2)
What formula would be used to calculate the distance using signal transmissions?
Distance = c x (t2 - t1); where c is the light
What are the three GPS segments which make a GPS system?
1) Space segment (satellites)
2) Control segment (antennae, control station, etc)
3) User segment (transportation vehicles)
What are five details that can be said about the space segment of a GPS system?
1) Consists of at least 24+ GPS space vehicles (SVs)
2) Orbit at 20,350 km above the surface of the earth
3) 6 orbital planes are inclined at 55 degrees with the equator
4) Every orbit contains 4 unevenly distributed SVs
5) SVs orbit the earth in 12-hrs, twice daily
Why are signals for the north and south pole not as accurate as the rest of the world?
Due to the inclination of the orbitals who were inclined in favor of whatever isn’t either in the north or south pole (inhabitated areas)
What do SVs broadcast radio signals contain?
They contain their location, status, clock corrections and precise transmission time
What are the nine main GPS error sources?
1) Sattelite clock error
2) Receiver clock error
3) Inospheric delay (atm delay; biggest)
4) Tropospheric delay (atm delay)
5) Multipath error
6) Satellite orbital (ephemeris) error
7) Receiving noice
8) Satellite geometry
9) Jamming
What are the two types of differential GPS (DGPS) which are used to enhance a GPSs accuracy?
1) Local Area Differential GPS (LADGPS)
2) Wide Area Differential GPS (WADGPS)
What happens in LADGPS?
There is a reference station with a known distance which is compared by the GPS distance for the error which is then removed from the user’s interface; works best when the distance between the reference and the user is relatively small
What happens in WADGPS?
Same concept as LADGPS except that there are multiple references which allows multiple errors which will be transmitted to the user’s satellite; works in large distance situations
What does INS stand for?
Inertial Naviguation System
What does GPS stand for?
Global Positioning System
What is INS?
Also known as the dead reckoning system, INS is a system where, if you know your location, speed and travel direction, then based on the lapsed time, you can know where you are going
What are three components which INS gives?
1) Position
2) Velocity
3) Attitude
What is attitude?
Pitch, roll and yaw moment
What are the two instruments which make a INS?
1) Accelerometer
2) Gyroscope
What is a gyroscope?
A gyroscope stays in the same direction even if you move the thing it’s in, thanks to the way it spins; gives the roll, pitch and yaw
What happens when one takes the integration (antiderivative) of a gyroscope’s rotation rate?
Attitude = Pitch, roll and yaw
What does an IMU consist of (Inertial Measurement Unit)?
1) 3x Accelerometers: For acceleration, hence speed and position
2) 3x Gyroscopes: For rotational angles, hence, direction
What process converts IMU data into a graph?
Mechanization
Pros/Cons of GPS
Pro: Good long term accuracy
Con: Subject to outages and jamming
Pros/Cons of INS
Pro: Self contained and good short term accuracy
Con: Slow but increasing error if left unaided
What happens when GPS and INS are combined (GPS & INS Integration)?
1) Improves INS long term accuracy
2) Fills in for GPS during outages (GPS ‘calibrates’ the INS)
