GPS signals

Hello everyone. I have question related to gps signals. At antenna signal from different satellites will be mixed signal or not? If yes how satellite despreads and identify individual signal from that mixed signal?

 

GPS signals are on a common frequency. The signals are transmitted using a code division multiple access or CDMA method. They DO NOT use a time division multiple access or TDMA method. For more details about CDMA see

https://en.wikipedia.org/wiki/Code-division_multiple_access

and

https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum#Uses

and

https://en.wikipedia.org/wiki/Global_Positioning_System#Demodulation_and_decoding

A further consideration is that at a particular receiver location, the frequency of the signals from GPS satellites will be altered by their apparent motion to the receiver. This is known as Doppler Shift. Thus the actual frequency received for each satellite will be slightly different due to the effect of Doppler Shift.

 

The GPS satellites all transmit their L1 signal on the same frequency, 1575.42-MHz. At any particular receiver location, due to the apparent motion of the satellite relative to a stationary receiver, the signals arrive at slightly different frequencies due to Doppler Shift. A GPS receiver must be able to be slightly tunable so it can adjust to the changed frequency of the arriving signal.

A GPS receiver must be capable of simultaneous reception of at least three or four signals. Modern receivers can receive dozens of signals simultaneously.

The CDMA method is a form of a general method called SPREAD SPECTRUM. The signal is not on just one frequency, but it is spread across a wide band of frequencies. By careful selection of the CDMA modulating digital codes, all the GPS signals can be sent on one frequency and not interfere with each other.

 

There is no contradiction in what I said. You should undertake a study of radio signals. In brief, a radio signal consists of a carrier wave and modulation applied to the carrier wave. Any radio signal is not a phenomenon of a single frequency. Even the on-off keying of an unmodulated carrier causes spreading of the frequency. When a carrier wave is modulated by a wideband modulation signal, the resulting signal is spread over a bandwidth that is proportional to the modulating signal's bandwidth. The GPS L1 signal is modulated by a wideband signal, so the resulting signal is wideband.

If you study radio signals, modulation, and CDMA methods, you will be able to understand this better. A good textbook on radio and various forms of modulation will give you a background that will allow you to understand how my statements that you have taken out of context are not contradictions.

The GPS receiver adjusts its center frequency to match the frequency of the signal that is arriving. You would have to do the same thing in your car radio if your car were travelling toward a broadcast station at a speed of 14,000-kilometers-per-hour. To receive a station that was sending on 100.000-MHz, you would have to tune slightly higher in frequency to receive it due to Doppler shift. The signal would appear to you to be arriving at 100.131-MHz.

You can calculate Doppler Shift with this handy calculator:
http://www.calctool.org/CALC/phys/default/doppler

 

Also regarding GPS signals:

The signal level of the L1 signal from a GPS satellite as received on Earth is extraordinarily low, and would generally be impossible to receive and demodulate if it were not for the property of CDMA demodulation known as CORRELATOR GAIN. The detection of GPS signals in the receiver with a correlator provides a substantial improvement in signal-to-noise ratio, an improvement of more than one-hundred times, and this correlator gain is crucial in the detection of the the signal and demodulation of it.

The more one studies the technology, science, and mathematics involved in the design of the GPS system, the more beautiful it becomes.

 

Also, it should be noted that the Russian GNSS system called GLONASS works differently. In GLONASS, all satellites send the SAME code for their CDMA modulation, but each satellite operates on a different carrier frequency. The identity of the satellite is established by the carrier frequency, not by the CDMA code.