GNSS Signal Processing & Receiver Design

Receiver Architecture & Signal Flow

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Figure 1: A generic GNSS receiver architecture. The antenna feeds an RF front-end that down-converts, amplifies, and digitizes the satellite signals (producing in-phase I and quadrature Q samples). Dedicated digital signal processing channels then acquire and track each satellite’s signal in parallel, and a navigation processor computes the Position/Velocity/Time (PVT) solution (Meas. Data = measurements output).

A typical GNSS receiver consists of several building blocks:

• Antenna: Usually a Right-Hand Circularly Polarized (RHCP) antenna tuned to the GNSS L-band frequencies (~1–2 GHz). It captures the direct satellite signals (Signals In Space, SIS) while minimizing multipath and interference. Many GNSS antennas include a low-noise preamplifier and filtering stage to boost the very weak signals (~–130 dBm) and reject out-of-band noise.
• RF Front-End: This analog front-end down-converts and digitizes the incoming RF signals. This is a critical first step, as it translates the high-frequency radio waves into a digital stream suitable for processing. The output of the front-end is a digitized baseband stream containing the spread-spectrum signals of all visible satellites.
• Baseband Signal Processing (Channels): The digital baseband section contains multiple parallel channels, each devoted to acquiring and tracking one GNSS satellite signal. Each channel generates a replica of a specific satellite’s code and carrier, then mixes it with the incoming I/Q samples to acquire the signal and maintain lock with it. The outputs of these channels are raw measurements—such as pseudoranges, carrier phases, and Doppler frequencies—which are then passed to the navigation processor.
• Navigation/Applications Processor: The receiver’s navigation processor uses the measured pseudoranges, carrier phases, Doppler frequencies, and decoded satellite messages to compute the user’s position, velocity, and time solution (the PVT). This is the “brain” of the receiver, where complex algorithms like the Kalman filter are employed for fusing measurements and providing a smooth and accurate solution.

Signal Acquisition and Tracking (DLLs, PLLs)

All GNSS receivers perform two fundamental processing steps: acquisition and tracking.

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• Signal Acquisition: This is the initial process of detecting a satellite’s signal and coarsely aligning its code and frequency. It involves a two-dimensional search: correlating a local code replica with the incoming signal across all possible code phase shifts and Doppler frequency offsets. Once a strong correlation peak is found, the signal is considered “acquired,” and the receiver has a rough estimate of the satellite’s pseudorange and Doppler shift.
• Signal Tracking: Once acquired, the receiver continuously fine-tunes the alignment of its local signal replica to remain locked onto the satellite’s signal. This is achieved using feedback loops such as the Delay-Locked Loop (DLL) for pseudorange measurement and the Phase-Lock Loop (PLL) for carrier frequency/phase measurements. These loops work together to continuously track the incoming signal, even as the receiver moves and the signal strength changes.

These loops continuously produce two types of measurements: pseudorange measurements and carrier phase measurements.

• Pseudorange: The pseudorange is the apparent distance from the satellite to the receiver, calculated by multiplying the time it takes for the signal to travel by the speed of light.
• Carrier Phase: The carrier phase is a measure of the phase of the carrier wave. Carrier phase tracking offers a significantly higher level of precision compared to pseudorange measurements, often by two to three orders of magnitude.

Correlation Techniques & Code Tracking Strategies

Correlation is the process of comparing the incoming satellite signal with a locally generated replica to determine the time alignment. The correlation process is fundamental to both signal acquisition and tracking. The output of a correlator is a measure of the similarity between the two signals as a function of their relative delay, producing a characteristic "correlation peak."

• Early-Late Correlators: This is the most common tracking strategy. The receiver uses three correlators: a Prompt correlator locked onto the signal, an Early correlator slightly ahead, and a Late correlator slightly behind. By comparing the power of the Early and Late correlators, the receiver can determine if it is tracking the signal correctly.
• Multicorrelators: These use a bank of correlators with fine spacing to achieve more robust and accurate code tracking, especially in environments with high multipath.

Carrier Phase Tracking & Ambiguity Resolution

While pseudorange measurements offer meter-level accuracy, carrier phase measurements provide significantly higher precision, often at the millimeter level. Carrier phase measurements are based on the number of carrier cycles between the satellite and the receiver. The primary challenge in using carrier phase is **ambiguity resolution**, which is the process of determining the exact integer number of full carrier cycles between the satellite and the receiver at the start of the measurement.

Anti-Multipath and Interference Mitigation

Multipath is a major source of error in GNSS, where signals reflect off surfaces before reaching the receiver, creating a delayed, corrupted signal. Interference, both intentional (jamming) and unintentional, can also degrade or completely deny GNSS signals. Modern receivers use several techniques to combat these issues.

• Multipath Mitigation: Techniques include using more advanced correlation functions (e.g., Narrow Correlator Spacing, Strobe Correlators), antenna design to reject reflected signals, and signal quality monitoring to exclude corrupted signals.
• Jamming and Spoofing Mitigation: This can be achieved through digital signal processing techniques, such as adaptive filtering, which can detect and suppress interfering signals.

Software-Defined GNSS Receivers (SDR)

A Software-Defined Radio (SDR) replaces most of the traditional hardware-based signal processing with software algorithms running on a general-purpose processor (e.g., a powerful computer or a Field Programmable Gate Array, FPGA). This offers significant advantages:

• Flexibility: The receiver's functionality can be easily modified or updated with a simple software change.
• Scalability: A single hardware platform can be configured to track multiple GNSS constellations and signals simultaneously.
• Research and Development: SDRs are an invaluable tool for researchers to test new algorithms for signal processing, such as advanced multipath or anti-jamming techniques, without having to develop new hardware.