Gps

Question 1

Gps errors

Answer 1

Ian Eats Sweets Makes Teeth Rot + selective availability and jamming

Ionospheric error: can cause up to 5m inaccuracy

Ephemeris error: satellite position not accurate

Satellite clock bias error: caused by atomic clocks being very slightly out

Multipathing: signal reflected between surfaces

Tropospheric error: can cause up to 1m inaccuracy

Horizontal dilution of precision: pseudo ranges have very shallow angle of cut and therefore not very accurate.

Horizontal datum: charts may not be wgs84

Gps Jamming

Selective availability: US government ability to decrease accuracy of system

Question 2

What are the HDOP numbers

Answer 2

HDOP 1 - 6

1 very accurate
6 not very accurate but still usable

Anything more than 6 is unacceptable

Question 3

Accuracy of GPS

Answer 3

With all the errors taken into account the accepted accuracy is 33m 95% therefore 95% of all readings will be with in 33m accuracy. This is HDOP 1

HDOP 5 would be 33 x 5 = 165 m 95%

Question 4

How does gps work

Answer 4

32 satellites, 24 in operation, 8 standby
6 orbits. 55* to horizon
Space segment, control segment, user segment
Satellites have atomic clocks and send signal with position info and time of signal transmission
GPS receivers have own clocks but not as accurate
Receiver works out distance from satellite, not accurate so known as PSEUDO RANGE
Receiver calculates pseudo ranges form at least 3 satellites
Pseudo ranges create cocked hat
Time error is the same between each satellite therefore time error can be worked out and the error removed and accurate fix created.

Control station will correct time error and position error when satellite pass over

Question 5

What is SBAS

Answer 5

Satellite based augmentation system.
Improves accuracy of position via other geostationary satellites and ground station.
Corrections sent to GPS satellites