GNSS Signal, Correction, and Constellations

Posted by Luke Wilson on Sep 23, 2020 5:43:55 PM

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Purpose: Global Navigation Satellite Systems (GNSS) such as GPS or BeiDou can sometimes have confusing terminology. This knowledge base document will explain some of the most commonly used terms regrading GNSS

Last Updated: September 2020

Constellation: A constellation is the collection of satellites that make up a particular system. Some examples of constellations are: GPS, BeiDou, Galileo, and GLONASS.

Signal: A satellite signal is comprised of a carrier frequency, ranging code, and navigation data. Individual satellites will usually transmit multiple signals. For instance, a GPS satellite transmits 8 different signals: L1C/A, L1C, L2C, L5, L1P(Y), L1M, L2P(Y), and L2M.

Carrier Frequency: A carrier frequency is the transmission of an altered or modulated fixed frequency. The frequency is measured in Hertz. When discussing satellite signals specifically, we are referring to the precise carrier frequency of a GNSS signal. For example, in GPS L1C/A signal, the carrier frequency is 1575.42 MHz.

Frequency Band: GNSS frequencies are typically referred to as bands. All GNSS frequencies fit into either the L1, L2, L5, or L6 bands. Through a lot of work and international cooperation, these signals are packed close enough together so that one receiver can receive all of the signals, but far enough apart so that the signals do not interfere with each other. For example, GPS, GLONASS, Galileo, and BeiDou all have signals that operate within the the L1 band even though they have different carrier frequencies.

PRN Code: Short for Pseudo-Random Noise Code, PRN is also known as a ranging code or spreading code, and is the sequence of phase shifts that it takes to generate the spread spectrum of the GNSS signals. The PRN Code is used by the receiver to measure the distance to the satellite and to recover the original signal. With several types of codes, PRN codes are used across different constellation and signals.

Modulation Types: Phase modulation is used in GNSS to carry both the PRN code and navigation message data on the carrier frequency to the receiver. Binary phase shift keying (BPSK) is the most commonly used modulation technique for GNSS signals. However, newer signals have begun to use a binary offset carrier (BOC) modulation scheme.

Services: Different signals or a combination of different signals from a constellation may be used for different services. GPS uses two different services, SPS for public use, and PPS for military applications. Galileo has four different services, Open Service, High Accuracy Service, Public Regulated Service, and Search and Rescue. This use of multiple services is consistent amongst other constellations such as BeiDou and GLONASS.

Topics: Constellation, GPS, GNSS, BeiDou, GLONASS, Galileo, Signals, Frequencies, Correction

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