3.4.4. Signals of GNSS

The GNSS system sends its information through various signals. Broadly, it works with three types of signals: L1, L2, and L5.

The GNSS system broadcasts its signals for communication with receivers using three main frequencies: L1, L2, and L5. Each signal has specific applications and purposes, enabling the system to cater to different user needs, including civilian and military.

Think of the three signals as different radio stations, each offering a unique type of music or talk show. L1 is like a pop station that's widely available, L2 is a more specialized talk radio for specific topics, and L5 is like a high-fidelity station meant for critical listening, such as safety messages for aircraft.

The L1 signals operate at 1575.42 MHz, and carry both the status message and a pseudo random code (PRC) for timing.

The L1 signal operates at a frequency of 1575.42 MHz. It transmits important information like status messages, which inform the receiver about the signal's quality and health, and it carries a pseudo-random code that helps in precise timing for location calculations.

Imagine the L1 signal as a class teacher who sends messages to students about their homework (status messages) while also using a unique code system to ensure that everyone understands exactly when the homework should be submitted (pseudo-random code).

The L2 signals operate at 1227.60 MHz, and are used for more precise military work.

Operating at a frequency of 1227.60 MHz, the L2 signal is primarily utilized for military applications requiring higher accuracy. This signal provides additional data to improve positioning accuracy, which is critical for tasks such as targeting.

You can think of L2 as a highly specialized toolkit that a mechanic uses for precision repairs, ensuring every task is done with the utmost accuracy, unlike a general multi-tool that can do many things but might not be as precise.

The L5 signal operates at 1176.45 MHz which was turned on April 2009. The L5 frequency is used to improve accuracy for civilian use, such as aircraft precision approach guidance.

Launched in April 2009, the L5 signal operates at 1176.45 MHz and is designed to enhance accuracy for civilian applications. This signal is particularly vital for aircraft during precision approaches, ensuring safety and reliability.

Just like having a high-definition camera versus a basic one can make a huge difference in capturing important moments clearly, the L5 signal’s advanced technology significantly improves the precision in positioning aircraft during landing maneuvers.

In good satellite visibility, the GNSS device with L5 will give a 6 ft accuracy, and 10 ft with GPS only.

When there is good visibility of satellites, receivers using the L5 signal can achieve remarkable accuracy of about 6 feet. In contrast, traditional GPS signals can only achieve accuracy within 10 feet, showcasing the advancements in newer GNSS technology.

It’s like using a laser measuring tool in a construction project, which provides measurements within a fraction of an inch, versus a standard yardstick that could be off by a foot, emphasizing the importance of technology in achieving precision.

The SPS accuracy is intentionally degraded by the DoD by the use of Selective Availability (SA). The SPS provides 100 m horizontal, 156 m vertical, and 340 nanoseconds time accuracy.

The Standard Positioning Service (SPS) accuracy is intentionally limited by the U.S. Department of Defense through Selective Availability (SA). This means civilian users receive less precise data, with horizontal accuracy at 100 meters and vertical at 156 meters.

This can be compared to restricting access to an exclusive swimming pool where only a few members can enjoy the best facilities, while the general public gets access to a smaller, less maintained pool.

The signals traveling from satellite to receivers will pass through clouds, dust, gas, particles etc., but will not travel through solid objects, such as buildings and mountains.

GNSS signals can penetrate through atmospheric phenomena such as clouds and dust, allowing them to reach the receiver as long as there are no solid obstructions. This characteristic is important for ensuring consistent signal availability for users in various weather conditions.

Think of GNSS signals like sunshine; on a cloudy day, the light still manages to come through but is blocked completely by a tall building, demonstrating how obstructions affect signal reception.

The pseudoranges, which are derived from signal travel time to the receiver, use two pseudorandom noise (PRN) codes. These codes are modulated onto the carrier frequencies.

To calculate distances known as pseudoranges, the GNSS receivers use two pseudorandom noise (PRN) codes. These PRN codes are modulated onto carrier frequencies, ensuring unique identification of signals from different satellites.

Imagine listening to several different musical tracks on your playlist; each track represents a satellite, while the unique codes are like the distinct melodies that help you identify them, allowing you to enjoy each song without confusion.

Dual frequency receivers get signals in L1 and L2 carrier wave at 24.45 cm wavelength. But today many of the multi-band receivers operate in L1, L2 and L5.

Dual frequency receivers can capture signals from both L1 and L2, effectively reducing systematic range errors caused by ionospheric effects. Advanced receivers today work across multiple bands (L1, L2, and L5) for enhanced accuracy.

Think of dual frequency receivers as a telephone that allows you to talk on two different lines simultaneously, enhancing clarity in communication that would otherwise be muddled if only one line was open.

The structure of the future operational GPS L5 signal will offer a two carrier components signal.

The future GPS L5 signal is designed to provide two carrier components, which will help in achieving greater signal reliability and accuracy. Both components will be synchronized, allowing for advanced positioning capabilities.

This can be compared to having two smartphones that can work together to ensure you never lose connection, thus providing a stronger and more reliable service than a single device alone.

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