GNSS Receiver: Decoding the Pseudo-Random Code and Calculating Time Delay

Global Navigation Satellite Systems (GNSS) form the backbone of our modern GPS and other positioning technologies, allowing us to determine precise locations and times. At the heart of these systems lies the Pseudo-Random Code (PRN), a unique sequence of bits transmitted by each satellite. This article explores how a GNSS receiver identifies the PRN code, calculates the time delay, and ultimately determines the location and time.

Pseudo-Random Code Generation

Each GNSS satellite transmits a unique Pseudo-Random Code (PRN) sequence that appears random but is actually deterministic. This code is pre-programmed into the receiver, ensuring that both the satellite and the receiver have a reference for the same PRN sequence.

Signal Reception

The GNSS receiver receives signals from multiple satellites, picking up both the PRN codes and the navigation data embedded within the signals. This step is crucial, as it allows the receiver to cross-correlate the received PRN codes with its own locally generated codes.

Cross-Correlation

The receiver generates a local copy of the PRN code for each satellite it is tracking. The process of cross-correlation involves shifting this locally generated code in time and comparing it to the incoming signal. This comparison is done at different time shifts to find the best match.

Time Delay Calculation

The position where the cross-correlation function reaches its maximum indicates the time delay between when the signal was transmitted by the satellite and when it was received by the receiver. This time delay is crucial because the speed of light is well-known, allowing the receiver to calculate the precise distance to the satellite.

Multiple Satellites

By repeating this process for multiple satellites, the receiver can triangulate its position in a three-dimensional space, calculating latitude, longitude, and altitude. Additionally, the receiver derives the precise time by considering the information from the satellites and the receiver's internal clock. The precise timing is adjusted based on the calculated time delay from each satellite, ensuring accurate results.

Integration and Fractional Parts

The parameter ‘epoch’ is vital in understanding the timing of the received signal. The 'epoch' of the received waveform and that of the internally generated sequence help in determining both the integral and fractional parts of the time delay. The integral part provides the whole number of units, while the fractional part is calculated by performing the cross-correlation to find the precise time shift.

In summary, the GNSS receiver uses pre-programmed PRN codes to identify the transmitted signal, calculates the time delay, and thus determines its position and the precise time. This intricate process ensures the accuracy and reliability of GNSS-based location and timing services.