Lecture 10 - GPS and Inertial Sensors

Question 1

Define a GPS

Answer 1

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)

Question 2

Define the Doppler Effect

Answer 2

The apparent change in frequency that occurs when a source and a target are in relative motion

Question 3

What can the doppler effect be used to determine?

Answer 3

It can be used to determine velocity (compression of the frequency)

Question 4

What is the general expression for velocity from doppler?

Answer 4

v = c (1 - f(t) / f(r))

Question 5

What is the meaning of the doppler general expression variables?

Answer 5

1) f(t) = The transmitted frequency
2) f(r) = Received frequency
3) c = Speed of light
4) v = Relative velocity

Question 6

What to the doppler effect velocity signs represent?

Answer 6

+ sign means that the transmitter is approaching the observer while - means they are going away

Question 7

What happened at the launch of SPUTNIK-1 ?

Answer 7

On October 4 1957, doppler was employed to determine the entire satellite orbit

Question 8

What is the first basic concept of a GPS system?

Answer 8

GPS satellited broadcast radio signals provide the precise transmission time (t1) from on-board atomic clock

Question 9

What is the second basic concept of a GPS system?

Answer 9

GPS devices receive the radio signals and record their exact time of arrival (t2)

Question 10

What formula would be used to calculate the distance using signal transmissions?

Answer 10

Distance = c x (t2 - t1); where c is the light

Question 11

What are the three GPS segments which make a GPS system?

Answer 11

1) Space segment (satellites)
2) Control segment (antennae, control station, etc)
3) User segment (transportation vehicles)

Question 12

What are five details that can be said about the space segment of a GPS system?

Answer 12

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

Question 13

Why are signals for the north and south pole not as accurate as the rest of the world?

Answer 13

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)

Question 14

What do SVs broadcast radio signals contain?

Answer 14

They contain their location, status, clock corrections and precise transmission time

Question 15

What are the nine main GPS error sources?

Answer 15

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

Question 16

What are the two types of differential GPS (DGPS) which are used to enhance a GPSs accuracy?

Answer 16

1) Local Area Differential GPS (LADGPS)
2) Wide Area Differential GPS (WADGPS)

Question 17

What happens in LADGPS?

Answer 17

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

Question 18

What happens in WADGPS?

Answer 18

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

Question 19

What does INS stand for?

Answer 19

Inertial Naviguation System

Question 20

What does GPS stand for?

Answer 20

Global Positioning System

Question 21

What is INS?

Answer 21

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

Question 22

What are three components which INS gives?

Answer 22

1) Position
2) Velocity
3) Attitude

Question 23

What is attitude?

Answer 23

Pitch, roll and yaw moment

Question 24

What are the two instruments which make a INS?

Answer 24

1) Accelerometer
2) Gyroscope

Question 25

What is a gyroscope?

Answer 25

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

Question 26

What happens when one takes the integration (antiderivative) of a gyroscope’s rotation rate?

Answer 26

Attitude = Pitch, roll and yaw

Question 27

What does an IMU consist of (Inertial Measurement Unit)?

Answer 27

1) 3x Accelerometers: For acceleration, hence speed and position
2) 3x Gyroscopes: For rotational angles, hence, direction

Question 28

What process converts IMU data into a graph?

Answer 28

Mechanization

Question 29

Pros/Cons of GPS

Answer 29

Pro: Good long term accuracy
Con: Subject to outages and jamming

Question 30

Pros/Cons of INS

Answer 30

Pro: Self contained and good short term accuracy
Con: Slow but increasing error if left unaided

Question 31

What happens when GPS and INS are combined (GPS & INS Integration)?

Answer 31

1) Improves INS long term accuracy
2) Fills in for GPS during outages (GPS ‘calibrates’ the INS)