What is the L6 GNSS Frequency Band?

What is the GNSS L6 Band? Who uses it and where is it used?

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- everything RF

Jul 2, 2024

The L6 GNSS Frequency band refers to the GNSS frequency corresponding to a frequency of 1278.75 MHz that is used to carry correction information from GNSS constellations like Galileo high-accuracy service (HAS) and QZSS centimeter-level augmentation service (CLAS). A GNSS antenna configured for the frequency of this L6 signal (1278.75 MHz) is required to receive and decode these correction messages. 

Achieving centimeter-level positioning with GNSS requires correction for various errors in GNSS observations, such as ionospheric and tropospheric delays, satellite orbit errors, and satellite clock errors. This correction information is obtained from geostationary satellite systems deployed specifically for this purpose by using L-band frequencies like L6 to achieve high-precision positioning.

Though there are services that are fee-based and require separate agreements with private companies, there also exist free-of-charge GNSS correction information distribution services like MADOCA-PPP5 (Multi-GNSS Advanced Orbit and Clock Augmentation – Precise Point Positioning) which provides correction information via QZSS, and CLAS which provides correction information for PPP-RTK by the Japanese QZSS. The MADOCA-PPP service is an L6 augmentation service independent from CLAS. The European Galileo satellite system’s HAS also provides this service free of charge. The CLAS service provides high-precision positioning compatible with the higher-precision E6 service of Galileo.  

All these services provide PPP or PPP-RTK correction information from a geostationary satellite using the L6 band (1278.75 MHz). The QZSS MADOCA, CLAS, and Galileo HAS messages are transmitted over signals called L6E, L6D, and E6 respectively. 

The Quasi-Zenith Satellite System (QZSS) operated by Japan transmits L6 signals (also called LEX) intended for high-precision navigation and timing applications and is more resistant to interference and multipath. As QZSS is designed to work as a regional system, it complements GPS and other global navigation satellite systems, by providing more precise positioning and timing services in the Asia-Oceania region. This way even the most popular GNSS systems like GPS can utilize correction information from L6 signals from this GNSS constellation. 

A GNSS antenna corresponding to the L6 band frequency and a GNSS receiver capable of tracking the L6D/E and E6 signals are required to use this transmitted correction information. As the L6 signal is close in frequency to the conventionally used GNSS L2 signal, the L6 signal can often be received at a high signal strength (at high elevation angles) on an antenna capable of receiving L2. However, some antennas cannot receive L6 signals at low elevation angles even though they are designed to support the L6 band. PPP-RTK with QZSS CLAS can provide centimeter-level positioning, even when using small, lightweight L6-compatible antennas. This is useful for applications with space and weight limitations, such as drones and robots.

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