64 GPS 2

Question 1

GPS clock

Answer 1

MCS controls GPS time using Caesium clocks
Constantly updates the satellites clock to the correct time
Clock drift in between updates is predicted and passed to users to allow more accurate positioning
Each satellite has four atomic clocks (2 caesium and 2 rubidium)
One nanosecond clock error = one foot position error

Question 2

Receiver lock

Answer 2

Satellite atomic clocks are very expensive
The receiver clock uses an inexpensive crystal oscillator (quartz)
Pseudo-Ranging calculates difference in timing accuracy between satellite and receiver

Question 3

C/A code

Answer 3

C/A code or coarse acquisition code is available to all users of GPS. Also known as clear access code or Gold Code
Total of 37 codes assigned to GPS satellites
Each satellite has its own unique C/A code to allow for satellite identification by receivers
C/A code is recycled as satellites are replaced
C/A code is 1.023bits long and is transmitted at the rate of 1.023 Mega bits/sec
Repeated every millisecond
Provides position accuracy of 7.8 meters 95% of the time
Transmitted on the L1 carrier frequency only

Question 4

P code (military)

Answer 4

Precision or P code
267 day long code sequence
Each satellite has a unique 7 day portion of the code
10.23 mega bit/sec transmission rate
Broadcast on both L1 and L2 frequencies
Accuracy is less than or equal to 5.9 meters 95% of the time

Question 5

Unique features of P code

Answer 5

US military can encrypt the P code which is known as Y code (selective availability capability)
P code requires a much more sophisticated receiver than that required for C/A code
Y code requires crypto capability
Encryption for the P code is available to US military and limited allies only

Question 6

GPS operating frequencies

Answer 6

the satellite clock frequency operates at 10.23MHz and all other frequencies are tied to this clock rate
The satellite transmit on two frequencies
L1 is 154 times 10.23MHz = 1575.42MHz
L2 is 120 times 10.23MHz = 1227.6MHZ

Question 7

Future GPS capabilities

Answer 7

Civilian L2 (L2C)
Military (M-Code)
Safety of Life (L5)
New civilian L1 (L1X)

Question 8

Pseudo-Random codes

Answer 8

Satellites and receivers are synchronized
generating the same coded signal at the same time
receiver then compares the received signal to its self-generated signal to come up with a time difference and thus a range
Peudo-random sequences are repeated every millisecond
referred to as pseudo-random code (or noise)

Question 9

Navigation Message

Answer 9

Contains the info required by the receiver to perform the operations and computations required to navigate using GPS
The message is superimposed over the C/A and P codes
It is divided into 5 sub-frames and takes 30 seconds to receive

Question 10

Ephemeris

Answer 10

the predictions/updates of current position of an individual satellites orbital position, that is transmitted to the user in the Nav message. Usually updated hourly by the MCS, transmitted to that particular satellite, wh

Question 11

Almanac

Answer 11

a catalogue of ephemeris data, for all the satellites, continually updated in the user receiver, as new individual satellite ephemeris data is received. The almanac tells the user where in the sky, the approximate position of each satellite is.

Question 12

Navigation message length

Answer 12

Takes 12.5 minutes to receive entire navigation message.