GPS Flashcards
(25 cards)
who created gps and why
US army to aid with navigation
how many satellites orbit
24
how many satellite orbits are there
6
what intervals do the orbits occur
60degrees
what angle does the orbital plane make
55 degrees to the equatorial plane
what are the period of the orbits
12 sidereal hours
what is the altitude of the orbits
20200km
how many earth control stations are there
4, 2 in the pacific one in the indian and one in the atlanitic ocean
where is the master control station and what does it do
colorado springs, uses information transmitted there to predict future orbits
what does a gps receiver do
it records the time it receives the digital signal and is able to calculate the distance/psuedorange from the satellite
what does increasing the number of satellites measured do
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
what are the main sources of error
clock error in the receiver and satellite, error in the position, effects of the atmosphere on the speed of light
how can accuracy be improved
by taking values for a hour and averaging the values accuracy can be improved to less than a meter.
is horizontal or vertical accuracy better
Horizontal as the planes of the observed satellites are always above the plane of the horizon
what is GDOP
geometric dilution of precision factor is determined from the relative position of the satellites to the receiver
what are the two frequencies and what are their values
L1 = 1575.42MHz, L2 = 1227.60 MHZ
why are two frequencies used
to calculate the erros caused by the ionosphere
what is the CA code
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
what is the p code
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
what is a y code
an encrypted us military p code
how is the distance between the satellite and the receiver calculated
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
what is the navigation message
a transmitted digital signal with codes every 30 seconds. the message contains information on the ephemeris and the approx positions of the satellites
what is carried by the L1 and L2 frequencies
navigation message and P/Y codes are carried on both while CA codes are only carried on L1
how can you minimize error from the position of the satellites
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
