L2 Carrier
This is a carrier frequency
The L1 carrier is modulated by both the C/A and P codes
L1 carrier cycles have a wavelength of 19 cm.
Transmit timing information by the GPS satellites
L2 Carrier
This is a carrier frequency
L2 carrier is only modulated by the P code
L1 carrier cycles have a wavelength of 24.4 cm
Transmit timing information by the GPS satellites
GPS Satellite signal to Positioning
When a GPS
Satellite is rotating around the earth it will transmit continuously satellite
signal to the earth.
Measure
arrival time of GPS signals from several satellites simultaneously
To determine a position on earth, the receiver has to calculate its
range (distance) to several satellites.
Decode the GPS signal and figure out the signal propagation
Decode the navigation message and convert it into satellite positions.
Use at least 4 pseudoranges acquired at the same time from 4 different
satellites to compute a position
GPS Satellite distance or range
There are two types to calculate distance or range to satellites:
Code Ranging by Code Matching
Carrier Phase Ranging by Shift measurements
Code Ranging by Code Matching
Code rang or pseudo range is the distance between satellite and the
receiver.
Code range calculated by (Di)
= CL X ∆t (CL=Speed
of the light, ∆t=Signal
travel time)
Synchronized the satellite and receiver to generate same frequency
(Code) same time.
P code is more accurate than the C/A code
Carrier Phase Ranging by Shift
measurements
Measure the wavelengths from satellite
to receiver to calculate range or distance.
The wavelengths of the carrier waves are
very short, L1 - 19cm and L2 -24cm.
More accurate than P code distance calculation.
Carrier phase can be measured to
millimeters.
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L1 and L2
Atomic clock
The atomic clocks are synchronized to
Coordinated Universal Time (UTC), the atomic clocks on the satellites are set
to GPS time
Atomic standards clocks are using in
monitoring stations.
Stability of at least 10-13 sec
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