How to Navigate the L1, L2, L5, E5a, E5b, and G2 Alphabet Soup of GNSS Constellations and Signals

The GNSS ecosystem has become more complex in recent years. More bands and more constellations are enabling use cases previously out of reach of civilians. But in the process, all of these additional options are making it even more complicated when developing a positioning application. Let’s break this down.

New Constellations

In the beginning, there was one system: the U.S. Global Positioning System (GPS). Not long after, the Russian Federal Space Agency developed and deployed GLONASS, a fully independent system with a focus on providing positioning in high latitudes. Both systems have been operational since the 1990s.

Four additional global systems are now deployed:

• China’s BeiDou (BDS or Compass), which started as a regional system and but now provides full global coverage with 35 satellites.
• The European Space Agency’s GALILEO, which became fully deployed globally in 2018 with 24 satellites launched, plus six spares. GALILEO is the only fully civilian global advanced positioning system.
• Japan’s Quasi-Zenith Satellite System (QZSS), a regional system serving eastern Asia-Oceania. It became fully operational in 2018 with four satellites.
• India’s IRNSS, or NavIC, a regional system focused on India and surrounding areas. It became fully operational in 2018 with seven satellites.

Every constellation has multiple signals, each operating at a different frequency. The main reason for and advantage of providing multiple signals is to provide enhanced accuracy (really, reduced error). Receivers can use two frequencies to minimize the error from the ionosphere (one of the layers of the atmosphere). Second frequencies can also provide enhanced availability because the signals are more likely to be there when you need them. Other frequencies (second or third in some systems) can be used to provide other types of correction data, further enhancing accuracy. Here’s an overview of the key signals:

GPS L1

Nearly every civilian receiver supports GPS’s L1 signal at 1575.74 MHz. Technically there are two signals: Coarse/Acquisition (C/A), which is commonly used but limited in precision (as the name implies) and the Precision (P(Y)) code, which is encrypted and thus fully available only to authorized users.

The U.S. is in the process of modernizing the L1 signal by adding L1C, an improved signal for civilian users (intended to provide better availability) and L1M, an improved signal for military users. These new signals are expected to be available by the end of this decade.

GPS L2

GPS’s L2P(Y) signal, at 1227.6 MHz, has long been used for precision applications, especially by authorized users. Civilian users have been able to use L2P(Y) in a “codeless” fashion, where the receiver finds the L1 signal and then uses some of the L2 information to improve accuracy.

The GPS modernization program is adding a new signal, L2C. It’s not a high-precision signal like the legacy L2 signal or the P(Y) or M signals, but rather a stronger, slower signal for more challenging environments. A receiver can receive L2C without first receiving L1.

The L2M signal is also new and available only to authorized users.

GPS L5

GPS’s L5 signal, at 1176.45 MHz, has been developed and deployed with aviation safety in mind. It’s the most advanced civilian signal available from GPS, due to being faster like the Precision codes at L1 and L2, its higher power, and its lower frequency. L5 is largely available (from 12 satellites) and is expected to be fully available (from 24 satellites) in 2024.

GLONASS L1

GLONASS has long had its primary signal near L1, at 1602 MHz. Also known as G1, it and its L2 cousin are unique among all the modern positioning systems in how it is provided: FDMA instead of CDMA, which detracts from very-high-accuracy applications. Even so, it has been successfully used for civilian applications for decades.

GLONASS L2

Also known as G2, GLONASS L2 is located at 1246 MHz and is FDMA like its L1 cousin.

GLONASS L3

The Russian Federal Space Agency announced a plan for future satellites to transmit at a new frequency, termed L3 and located at 1201 MHz (right next to GALILEO E5b). This is not yet deployed.

BeiDou B1

BeiDou B1 is near L1, centered at 1561.098 MHz. A second signal is planned — but not yet implemented — directly above L1, at 1589.742 MHz. The latest BeiDou satellites also include a signal at 1575.42 MHz that is practically identical to GPS’s L1C.

BeiDou B2

BeiDou’s lower frequency for its dual-frequency operation is at 1207.14 MHz. Much like the modernized GPS L2 signal, a narrower signal is openly available, and a wider, higher-precision signal is available only to authorized users.

BeiDou B3

Similar to GALILEO, BeiDou has a third signal located above B2, at 1268.52 MHz. An open signal and authorized-user-only signal are available.

GALILEO E1

GALILEO’s satellites transmit the E1 signal on the same frequency as GPS’s L1 (1575.42 MHz). This signal has been designed to coexist with this and other nearby signals. This signal is very similar to GPS’s L1C.

GALILEO PRS

Although GALILEO is purely a civilian system, it also has a set of signals only available to authorized users called Public Regulated Service (PRS). There are two PRS signals: one centered at E1 and the other centered at E6. These signals are wider in bandwidth than the Open Service signals typically used.

GALILEO E5

GALILEO’s E5 signal is actually two signals: E5a is centered at 1176 MHz, which is co-located with GPS’s L5, while E5b is centered at 1207 MHz. Both are 20.46 MHz wide and can be used independently or together. E5 is intended to provide a similar function as GPS’s L5: higher precision with higher availability.

GALILEO E6

GALILEO’s E6 signal is centered at 1278.75 MHz. It’s co-located with and similar in use to QZSS’s L6 signal, transmitting correction data for High Accuracy Services, typically to provide Precise Point Positioning (PPP). E6’s data signal provides a higher data rate than most signals and thus is well-positioned to provide such information for global applications. Applications that require global, high-accuracy positioning could make use of this signal.

IRNSS/NavIC

IRNSS has two signals: one co-located with GPS L5 at 1176.75 MHz and the other at 2492.028 MHz. The latter signal (in the S-band) is currently unique among positioning systems. Receivers can use the L5 signal along with GPS, GALILEO, BDS, or GLONASS signals in the L1 band to provide the benefits of the dual-frequency operation. Both signals can be used independently to provide a single-frequency position.

The NavIC system also plans to transmit ionospheric correction data for the coverage region, providing improved accuracy.

QZSS

QZSS has four signals that are practically identical to GPS at L1, L2, and L5, plus a new signal at L6 (co-located with E6) at 1278.75 MHz. Like GALILEO’s E6, the L6 (also known as LEX) signal provides data at a faster rate, enabling distribution of new types of data. This is now being used to provide free, open correction data, allowing for free PPP in the region — something previously only available from subscription L-band services.

Typical Combinations

For GNSS receivers, there are some common combinations that are appearing:

Single-Band

In the past, a dual-band receiver might simply be one that could receive both GPS and GLONASS L1. Nowadays, this level of functionality is expected, so a modern single-band receiver typically supports GPS L1, GLONASS L1, and BeiDou B1 (which is really three different frequencies). Many or most modern receivers will also support GALILEO E1.

Multi-Band

With the drive for improved accuracy and more robust locating performance, true multi-band receivers are becoming more commonplace. These integrate at least one other “significantly different” frequency from the L1/B1/E1 band set. Again, in the past this often meant strictly supporting GPS L1, GPS L2P(Y) (codeless or not), and maybe GLONASS L1. Modern multi-band receivers have higher expectations placed on them and will typically fall into a few categories:

Commercial/Industrial:

• GPS L1C/A, L1C, L2C
• GLONASS L1, L2OF
• BDS B1 (possibly also B2)
• GALILEO E1, E5b

Consumer/Commercial:

• GPS L1C/A, L1C, L5
• GLONASS L1
• BDS B1
• GALILEO E5a

Consumer/Commercial – India Subcontinent Only:

• GPS L1C/A, L1C
• GLONASS L1
• BDS B1
• GALILEO E5a
• NAVIC/IRNSS L1C, L5

High Precision/Reference:

• GPS L1C/A, L1C, L2P(Y) (codeless), L2C
• GLONASS L1, L2
• BDS B1, B2, B3
• GALILEO E1, E5, E6
• QZSS L1C, L6
• NAVIC/IRNSS L1C, L5

That’s a lot to consider. Taoglas’ Engineering experts are available to help navigate all of those options and choose the ideal constellation(s) and signals to meet your application’s unique coverage and performance requirements. We’ve developed a helpful workflow to explore our extensive range of antenna design and testing services, accessible via the button below.