C/A (coarse/acquisition)

C/A (Coarse/Acquisition) is a code division multiple access (CDMA) signal that is transmitted by the Global Positioning System (GPS) satellites. The C/A code is a binary sequence that is modulated onto the carrier wave of the GPS signal to provide timing and location information to GPS receivers. This code is used by the receiver to acquire the GPS signal and to determine the time and position of the receiver.

The C/A code is a pseudo-random binary sequence that is generated by a shift register that is clocked at a high rate. The C/A code is a binary sequence of 1023 bits that repeats every millisecond. The C/A code is modulated onto the carrier wave of the GPS signal by a process called spreading. Spreading is the process of multiplying the carrier wave by the C/A code.

The C/A code is called coarse because it has a relatively low rate of repetition compared to the P code (precision code) which is another code used by the GPS system. The P code has a repetition rate of 267 days, while the C/A code has a repetition rate of 1 millisecond.

The C/A code is also called acquisition because it is used by the receiver to acquire the GPS signal. The receiver uses the C/A code to synchronize its clock with the GPS clock and to determine the time of arrival of the GPS signal. The receiver also uses the C/A code to determine the distance between the receiver and the GPS satellite.

The C/A code is transmitted by the GPS satellite on the L1 frequency which has a carrier frequency of 1575.42 MHz. The C/A code is transmitted at a power level of approximately 27 dBW which makes it easy for the receiver to acquire the signal.

The C/A code is designed to be resistant to interference and jamming. The C/A code is a spread spectrum signal which means that it is spread over a wide frequency range. This makes it difficult for an adversary to jam the GPS signal by transmitting a signal on the same frequency.

The C/A code is also designed to be resistant to multipath interference. Multipath interference is a phenomenon where the GPS signal is reflected off of objects such as buildings, trees, and other obstacles. The reflected signal can cause the receiver to receive multiple copies of the signal which can result in errors in the position calculation. The C/A code is designed to have a long code length and a high rate of repetition which makes it less susceptible to multipath interference.

To acquire the GPS signal using the C/A code, the receiver must first synchronize its clock with the GPS clock. The receiver does this by searching for the C/A code sequence. The receiver searches for the C/A code sequence by correlating the received signal with a locally generated replica of the C/A code sequence.

The correlation process involves multiplying the received signal with the locally generated C/A code sequence and then integrating the result over a period of time. If the received signal is the same as the locally generated C/A code sequence, then the integration will produce a large positive value. If the received signal is different from the locally generated C/A code sequence, then the integration will produce a small or negative value.

The receiver searches for the C/A code sequence by varying the delay of the locally generated C/A code sequence. The delay of the locally generated C/A code sequence is varied by shifting the code sequence in time. The receiver performs the correlation process for each possible delay of the locally generated C/A code sequence until it finds the correct delay. The correct delay is the delay that produces the largest positive correlation value.

Once the receiver has found the correct delay of the C/A code sequence, it can determine the time of arrival of the GPS signal. The time of arrival of the GPS signal is determined by comparing the time of the locally generated C/A code sequence with the time of the received C/A code sequence. The time difference between the two sequences represents the time it took for the GPS signal to travel from the satellite to the receiver.

The receiver can also use the C/A code to determine the distance between the receiver and the GPS satellite. The distance between the receiver and the GPS satellite is determined by measuring the time it took for the GPS signal to travel from the satellite to the receiver and multiplying that time by the speed of light.

The C/A code is also used in conjunction with other information from the GPS system to determine the position of the receiver. The position of the receiver is determined by measuring the distance between the receiver and multiple GPS satellites and using that information to calculate the position using trilateration.

In summary, the C/A code is a critical component of the GPS system that provides timing and location information to GPS receivers. The C/A code is designed to be resistant to interference and jamming and is used by the receiver to acquire the GPS signal, synchronize its clock with the GPS clock, and determine the time of arrival and distance to the GPS satellite. The C/A code is a fundamental technology that enables many applications such as navigation, surveying, and geolocation.