GPS

Question 1

who created gps and why

Answer 1

US army to aid with navigation

Question 2

how many satellites orbit

Answer 2

24

Question 3

how many satellite orbits are there

Answer 3

6

Question 4

what intervals do the orbits occur

Answer 4

60degrees

Question 5

what angle does the orbital plane make

Answer 5

55 degrees to the equatorial plane

Question 6

what are the period of the orbits

Answer 6

12 sidereal hours

Question 7

what is the altitude of the orbits

Answer 7

20200km

Question 8

how many earth control stations are there

Answer 8

4, 2 in the pacific one in the indian and one in the atlanitic ocean

Question 9

where is the master control station and what does it do

Answer 9

colorado springs, uses information transmitted there to predict future orbits

Question 10

what does a gps receiver do

Answer 10

it records the time it receives the digital signal and is able to calculate the distance/psuedorange from the satellite

Question 11

what does increasing the number of satellites measured do

Answer 11

one satellite means the position lies on the surface of a sphere, two means the position lies in a circular region, three will show two alternative positions

Question 12

what are the main sources of error

Answer 12

clock error in the receiver and satellite, error in the position, effects of the atmosphere on the speed of light

Question 13

how can accuracy be improved

Answer 13

by taking values for a hour and averaging the values accuracy can be improved to less than a meter.

Question 14

is horizontal or vertical accuracy better

Answer 14

Horizontal as the planes of the observed satellites are always above the plane of the horizon

Question 15

what is GDOP

Answer 15

geometric dilution of precision factor is determined from the relative position of the satellites to the receiver

Question 16

what are the two frequencies and what are their values

Answer 16

L1 = 1575.42MHz, L2 = 1227.60 MHZ

Question 17

why are two frequencies used

Answer 17

to calculate the erros caused by the ionosphere

Question 18

what is the CA code

Answer 18

coarse acquisition, or standard S code which has the length of 1023 bits released at a rate of 1.023 Mb/s, repeating every 1ms. Each bit corresponds to approximately 300m

Question 19

what is the p code

Answer 19

Precision code released at a rate pf 10.23Mb/s but only repeats every 38 weeks. Each satellite has a separate week, effectively giving each satellite their own p code

Question 20

what is a y code

Answer 20

an encrypted us military p code

Question 21

how is the distance between the satellite and the receiver calculated

Answer 21

incoming signals cause the receiver to generate CA code identical to the satellite code. the two codes are compared and shifted until in phase with each other. the amount shifted is equal to the travel time. multiply by the speed of light for the distance

Question 22

what is the navigation message

Answer 22

a transmitted digital signal with codes every 30 seconds. the message contains information on the ephemeris and the approx positions of the satellites

Question 23

what is carried by the L1 and L2 frequencies

Answer 23

navigation message and P/Y codes are carried on both while CA codes are only carried on L1

Question 24

how can you minimize error from the position of the satellites

Answer 24

if two receivers are within 10km, the error should be the same for both receivers and the difference will be correct to within 0.1m

Question 25

what is DGPS

Answer 25

differential GPS, technique of using two satellites and standard procedure for satellite surveying