GNSS Terminology Explained
When it comes to Global Navigation Satellite Systems (Global navigation satellite system (GNSS): A general term describing any satellite constellation that provides positioning, navigation, and timing (PNT) services on a global or regional basis. See also) such as Global Positioning System is a navigation satellite system. See also or Galileo, terminology can be confusing. There are several GNSS and regional systems, with more on the horizon. This blog post will explain and clarify some of the most commonly used terms.
The GNSS Spectrum Poster, available from Orolia, is a printable poster that shows the different GNSS signals, their constellations, carrier frequencies, modulation schemes, and how all these signals relate to each other within the same L-Band frequency bands. It is a necessary tool for any GNSS professional, including engineers who design and develop GNSS systems, and engineers who test GNSS systems.
Now for the definitions:
- Constellation – The set of satellites that make up a particular system. GPS, GLONASS, Galileo, BeiDou, NAViC, and QZSS is a regional satellite system in the Asia/Pacific Region. Developed and maintained by the Japanese government, this satellite constellation provides similar services as GPS: location, navigation and timing. are all considered independent constellations.
- Signal – A signal is comprised of a carrier frequency, a ranging code and navigation data. A single satellite typically transmits several signals. A single GPS III satellite, for example, transmits eight different GPS signals (L1C/A, L1C, L2C, L5, L1P(Y), L1M, L2P(Y), L2M).
- Carrier Frequency – The transmission of a fixed frequency that has been altered, or modulated, to “carry” data. The frequency is measured in Hertz (cycles per second). For our usage, we are referring to the precise carrier frequency of a GNSS signal. For GPS L1C/A signal, the carrier frequency is 1575.42MHz.
- Frequency Band – GNSS frequencies are commonly referred to as bands. All GNSS frequencies fit into either L1, L2, L5, or L6 bands, as defined by the International Telecommunications Union (ITU). A lot of work and international cooperation went into packing all these signals close enough together so that one single receiver can receive all these signals, but not so close together that they interfere with each other. GPS L1C/A signals are in the L1 band, as are GLONASS G1, Galileo E1, and BeiDou B1, even though they may have different carrier frequencies.
- PRN Code –A Pseudo-Random Noise Code, also called a ranging code or a spreading code, is the sequence of phase shifts used to generate the spread-spectrum of the GNSS signals. The PRN Code is used by the receiver to recover the original signal and measure the distance to the satellite. There are several types of codes used across different constellations and signals.
- Modulation Types – Phase modulation is used in GNSS to carry the PRN code and navigation message data on the carrier frequency to the receiver. The most commonly-used modulation technique for GNSS signals has been binary phase shift keying (BPSK), although newer signals are using a binary offset carrier (BOC) modulation scheme.
- Services – Different signals or combination of signals from a constellation may be used for different services. For GPS, there is the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). SPS is available for all users, and PPS is reserved for military applications. Galileo has four services, Open Service (OS), High Accuracy Service (HAS), Public Regulated Service (PRS), and Search and Rescue (Search and Rescue). Other constellations offer different services as well.
Lisa Perdue is a world-leading expert in testing critical GPS and GNSS systems. She has trained hundreds of engineers and technicians who are responsible for high-reliability positioning, navigation and timing (PNT) applications. She took a lead role in the development of the first GNSS Vulnerability Test System and speaks widely on the topic at many industry conferences. Lisa Perdue is currently the Simulation Product Line Director at Orolia, directing the organization's GNSS simulation activities and contributing to its entire portfolio of resilient PNT solutions. She has more than 15 years of navigation and RF systems experience, which includes 10 years of service with the U.S. Navy, where she was a certified master training specialist.