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GPS Frequencies and signals

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					GPS Frequencies and signals
1      Frequencies

Each GPS satellite transmits unique ranging code signals on two frequencies: 1575.42
MHz (L1) and 1227.60 MHz (L2). The Fundamental oscillator frequency of GPS system, Fo
is 10,23 MHz. The carrier L1 = 154∙Fo. There are three main digital signals or codes
modulated into carrier frequencies.

Near time enhancements include new signals implementations L1C, L2C (gives second
carrier frequency for civil users), L5 (“Safety Of Life” signal, mainly for civil aviation!

The modulation in use for digital signal transmission from space is phase modulation-PM,
also known as phase-shift keying (PSK). At each phase shift, the bit is flipped from 0 to 1
or vice versa. This is the method used in GPS.

The Coarse Acquisition (C/A) code is transmitted on L1 and can be received by any type
of GPS receiver. The C/A code consists of 1023 bits and is repeated every millisecond.
The Precision (P-code) code is transmitted on L1 and L2. P-code is encrypted and
available only to users with appropriate decryption equipment provided by the USA
Department of Defense. The P-code is transmitted at 10.23 MHz and repeats every 267
days. Both codes are synchronized to the satellite’s atomic clocks. C/A and P codes are
specific for every single GPS satellite i.e. satellites are distinguished via personal code
called pseudo random noise code (PRN code). In view satellites PRN codes or numbers
appear on the GPS receiver screen after turning on the receiver. Navigation message
(includes Almanac data) with relatively low bit rate - 50 bps. Refer to Figure 2-1 for GPS
signals.

A GPS Navigation Data Message is combined with each ranging code and transmitted on
both L1 and L2 frequencies.
2.1    GPS Navigation Data Message

The GPS Navigation Data message consists of 25 frames, each 1500 bits long,
transmitted at as a streem of digital data with 50 Hz (50 bps) rate. The complete
message requires 12.5 minutes for transmission and contains the transmitting (in view)
satellite’s clock correction data and satellite’s predicted path (Ephemeris). Remaining part
of Navigation data Message contains information about all satellites in the constellation
(Almanac).

2.2    Ephemeris and clock data

Clock data contains particular satellite’s on board atomic clock drift information from GPS
time. GPS time is kept on Colorado at MCS.

Ephemeris data contains precise orbital parameters of SV, taking into account SV drift on
theoretical orbit. This information comes from MCS and it permits the receiver to
estimate the exact position of the satellite at any time. This is critical for computing the
receiver’s position.

2.3    Almanac Data

The satellites also transmit almanac data, which contains an indicator of the health of all
the satellites and coarse orbital data, atmospheric delay parameters, ionosphere model
data and the current GPS time and offset from UTC time.

Each satellite transmits almanac data for the entire constellation. The entire almanac is
broadcast over a period of 12.5 minutes. When a receiver is new, or has not been
operated for a long time, it has to acquire a new almanac before it can begin to compute
a position.

2.4    Range Determination

The PRN code transmitted by each satellite is also generated in the receiver. The receiver
uses code matching techniques to determine the time it took the signal to travel from the
satellite to the receiver. Refer to Figure 2-4.




Figure 2-4:   PRN Code Comparison
The speed of the signal is closely approximated by the speed of light, with variations
resulting from ionospheric and atmospheric effects modeled from parameters contained
in the almanac.

The distance from the receiver to the satellite, referred to as a pseudo range, is
computed by multiplying the signal travel time and the average speed of the signal.

When computing position, the receiver also requires the position of the tracked satellite
which is provided by the Navigation message (ephemeris data).



3   GPS signal strength, frequency domain and filtering.

The strength of the transmitted GPS signals is very low and cannot be easily viewed on a
spectrum analyzer. For this reason, it is susceptible to both intentional and unintentional
interference. The minimum received power levels at the surface of the earth are as
follows:

L1 C/A code           -160 dBW       or -130 dBmW

L1 P code             -163 dBW       or -133 dBmW

L2 P code             -166 dBW       or -136 dBmW



The received signals are at least 16 dB below the noise level of the receiver and require
code     matching       (correlation)
technique to recover the PRN code.
Spread spectrum techniques are
used by satellite transmitters to
reduce the effects of noise and
improve signal to noise ratio
without increasing the transmitter
power.

C/A code is below noise
level. Shown on the drawing on the
right side. Signal is multiplied in the
receiver by the internally calculated
code to allow tracking.
C/A-code chip is 1.023 Mhz
P-code chip is 10.23 Mhz
The calculated power spectrum
derives from the Fourier
transform of a square wave
of width 2π and unit amplitude. Common function in DSP called the “sinc” function.

Filtering allows to remove some portion of the frequency spectrum that contains
unwanted signal:

       Low Pass Filter: lets all frequencies below a cutoff frequency through.
      High Pass Filter: lets all frequencies above a cutoff frequency through.
      Band Pass Filter: lets all frequencies within a specified frequency window pass
       through. The window is called the pass band



C/A code acquisition is shown on the
right side. C/A-code is 1023 chips
long and repeats every 1/1000 s,
therefore it is inherently ambiguous
by 1 msec or ~300 km. Modulo-2
must add the transmitted and
received codes after correlation to
increase SNR and narrow bandwidth.

				
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