GNSS

Question 1

What is GNSS?

Answer 1

Global navigation satellite system

Satellites orbit the Earth
Distance to satellites allow us to pinpoint our position on earth

Question 2

What are the three segments of GNSS?

Answer 2

Space segment
Control segment
User segment

Question 3

What is the space segment of GNSS?

Answer 3

Orbiting gps satellites

Emit signals controlled by highly accurate atomic clocks

Question 4

What is the control segment of GNSS?

Answer 4

Monitors and controls the orbit of the satellites

Question 5

What is the user segment of GNSS?

Answer 5

Antenna tuned to the frequency transmitted by the satellites

Number of channels to monitor satellites 10-20

Question 6

What are the types of GNSS systems?

Answer 6

GPS
GLONAS
BeiDou
Galileo
IRNSS

Question 7

GPS?

Answer 7

NAVSTAR - USA
31 satellites
24 required
20,180 km

Question 8

GLONAS

Answer 8

Russia
28 satellites
24 required
19,130 km

Question 9

BeiDou

Answer 9

China
5 geostationary satellites
30 medium earth orbit
21,150 km

Question 10

Galileo

Answer 10

European Union
8 test bed satellites
22 operational satellites budgeted
23,222 km

Question 11

IRNSS

Answer 11

India
3 geostationary satellites
4 geosynchronous satellites
36,000 km

Question 12

How do satellites work?

Answer 12

They send out signals at different frequencies

Question 13

How do you measure position from a satellite?

Answer 13

Trilateration
Distance from a point to 3 different satellites is recorded
Results in 2 possible locations one of which can usually be rejected

Question 14

How do you measure distance to a satellite?

Answer 14

Distance = velocity x time
3.0x10^8
Timing is difficult
Travel time = aprox 6s

Question 15

What is travel time?

Answer 15

Difference in sync of the satellite time - receiver time

To overcome this both satellite and receiver generate a pseudo random code - comparing lag enables travel time to be determined precisely

Question 16

What is a pseudo random code?

Answer 16

A digital code consisting of a sequence of on and off pulses

Question 17

Code phase vs carrier phase

Answer 17

Random code - 3-6m of potential error

Carrier phase - random code is piggy backed on a higher frequency signal
3-4 mm of potential error (theoretically)

Question 18

What are some GNSS error sources?

Answer 18

Timing issues 
Ephemeris errors - orbitial position 
Atmosphere - ionosphere / troposphere (weather) 
Multipath
Intentional errors
System noise in receivers

Question 19

Atmospheric errors

Answer 19

Ionosphere - ionised particles / 50-500 km / most significant error source - most can be removed through mathematic modelling

Troposphere - weather layer - water vapour - varies in temperature and pressure - relatively low error

Question 20

Multi path error

Answer 20

Reflection of the GPS signal from the satellite to the receiver off objects
Signals end up arriving at different times
GPS is more reliable in open areas - the more satellites the better

Question 21

GNSS modernisation

Answer 21

Improvements in atomic clocks, satellite signals and strength and reliability
Improved atmospheric modelling, in orbit accuracy and additional monitoring stations

Question 22

Differential GPS

Answer 22

Use of 2 receivers - 1 fixed and 1 that revolves
Fairly close - few hundred km
Able to eliminate errors common to both receivers

Question 23

Dilution of precision

Answer 23

Specify the multiplicative effect
SD position = DOP * SD inputs

Standard deviation of the positional error is proportional to the standard deviation of the errors associated with the satellites multiplied by a factor

Small values preferred