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Last updated on 2024年8月1日

What are the best methods to estimate the carrier phase and code delay of GNSS signals?

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GNSS signals are the basis of modern navigation and positioning systems, such as GPS, GLONASS, Galileo, and BeiDou. They consist of radio waves that carry information about the location and time of the satellites that transmit them. To use these signals for accurate and reliable navigation, you need to estimate two key parameters: the carrier phase and the code delay. These parameters measure the difference between the received and the expected signal, and they depend on various factors, such as the satellite orbit, the receiver clock, the atmospheric conditions, and the multipath effects. In this article, you will learn about the best methods to estimate the carrier phase and code delay of GNSS signals, and how they can improve your navigation performance.

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Wahyudin P. Syam

Technical project manager in global navigation satellite system

1 Carrier phase estimation

The carrier phase is the fraction of a complete cycle of the signal waveform that the receiver observes at a given time. It is a very precise parameter, as it can be measured with sub-millimeter accuracy. However, it is also ambiguous, as it can have any integer value added to it without changing the observed phase. To estimate the carrier phase, you need to use a phase-locked loop (PLL), a feedback system that tracks the phase of the incoming signal and adjusts the receiver's local oscillator accordingly. The PLL can also estimate the Doppler frequency shift of the signal, which is caused by the relative motion of the satellite and the receiver. The PLL needs to be robust and adaptive to cope with the dynamics and noise of the signal.

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2 Code delay estimation

The code delay is the time difference between the arrival of the signal at the receiver and the expected arrival based on the satellite position and clock. It is a less precise parameter than the carrier phase, as it can be measured with meter-level accuracy. However, it is also unambiguous, as it has a unique value for each satellite-receiver pair. To estimate the code delay, you need to use a delay-locked loop (DLL), a feedback system that aligns the receiver's local replica of the signal code with the incoming signal. The DLL can also estimate the code phase, which is the position of the code bit within a code period. The DLL needs to be fast and sensitive to acquire and track the signal.

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Wahyudin P. Syam

Technical project manager in global navigation satellite system
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One of the most powerful code delay estimation is GNSS signal tracking is by using normalised “early-minus-late power”. There are some advantages of using this discriminator: 1. Since it is normalised, it can be used for various signals with different signal strength and C/N0. 2. Since it uses both I and Q arm, the code delay tracking will be less dependent to PLL tracking. 3. It is robust for code delay tracking in noisy signal.

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3 Combined estimation

The carrier phase and code delay estimation are not independent, as they both depend on the same signal parameters. Therefore, it is beneficial to combine them in a unified estimation scheme, such as a vector tracking loop (VTL). A VTL uses a common filter that processes the measurements from all the channels and estimates the receiver state vector, which includes the position, velocity, clock bias, and clock drift. A VTL can improve the accuracy, robustness, and continuity of the navigation solution, especially in challenging environments, such as urban canyons, forests, or tunnels.

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4 Error sources and mitigation

The estimation of the carrier phase and code delay is not perfect, as there are various error sources that affect the signal quality and integrity. For example, the signal can travel slower in the ionosphere or troposphere, which can be mitigated by using dual-frequency receivers or applying ionospheric or tropospheric models or corrections. The signal can also reflect or diffract from objects near the receiver, creating multiple copies of the signal that interfere with each other and cause errors in the measurements. This can be mitigated by using antenna designs or signal processing techniques that suppress or reject the multipath signals. Additionally, thermal noise or quantization noise in the receiver’s components can reduce the signal-to-noise ratio and affect the estimation accuracy and precision. This can be mitigated by using high-quality hardware or applying noise filtering or smoothing techniques.

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Wahyudin P. Syam

Technical project manager in global navigation satellite system
举报内容
There are two main sources in GNSS positioning: 1. Error related to pseudorange estimations. These includes satellite orbit error, clock bias, multipath, ionospheric and tropospheric delays, receiver noise affecting signal processing. Possible mitigation: accurate satellite orbit and clock-bias estimations, advanced signal processing algorithms, using low-noise receiver, etc. 2. Error related to in-view satellite configurations or formations (also called dilution of precision). Possible mitigation: processing PNT from as many satellite as possible (constellation combinations), etc.

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5 Best practices and recommendations

To get the best results when estimating the carrier phase and code delay of GNSS signals, it is important to follow some best practices and recommendations. These include using a high-performance receiver that supports multiple constellations, frequencies, and signals, as well as a suitable estimation method that matches your application requirements. Additionally, you should use a high-gain antenna with a wide beamwidth and low multipath response, as well as appropriate models or corrections for the error sources. Quality control or integrity monitoring techniques should also be employed to detect and exclude faulty or unreliable measurements.

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6 Further learning resources

If you're interested in learning more about the methods to estimate the carrier phase and code delay of GNSS signals, you can explore a variety of further learning resources. For example, Shuanggen Jin's textbook, "Global Navigation Satellite Systems: Signal, Theory and Applications", covers the fundamentals and applications of GNSS signal processing and analysis. ETH Zurich offers an edX course called "GNSS Signal Processing" to teach the principles and techniques of GNSS signal processing and analysis. Additionally, "GNSS-SDR" is an open-source software-defined radio project that provides a software tool to simulate and analyze GNSS signals and measurements.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the best methods to estimate the carrier phase and code delay of GNSS signals?

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1 Carrier phase estimation

2 Code delay estimation

3 Combined estimation

4 Error sources and mitigation

5 Best practices and recommendations

6 Further learning resources

7 Here’s what else to consider

Global Navigation Satellite System (GNSS)

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