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Location adds a lot of value - and not just for real estate. Whether it’s asset tracking, turn-by-turn navigation, fraud prevention or fitness, the ability to pinpoint a person, place or thing is now a key part of most consumer and business applications.
Satellites are a common way to enable location-based services (LBS), which is why the global GNSS chip market is on track to grow from $2.7 billion in 2019 to nearly $3.8 billion by 2026, according to the research firm QYR. When developing a GNSS-based solution, device OEMs, IoT service providers and systems designers need to carefully consider how their spectrum band and constellation choices directly affect accuracy, consistency and more.
The constellation selection has increased significantly over just the past decade, with China’s BeiDou and the European Space Agency’s GALILEO joining the US GPS and Russian GLONASS global systems. Two regional systems also became fully operational in 2018: Japan’s Quasi-Zenith Satellite System (QZSS) and India’s IRNSS/NavIC.
Every constellation has multiple signals, each operating at its own frequency. This design maximizes accuracy because receivers can use two frequencies to minimize errors created by the ionosphere. Two frequencies also increase the likelihood that signals will be available when the receiver needs them. Some systems use the second or a third frequency to provide correction data to further enhance accuracy. For higher accuracy the receiver needs to receive signals from as many satellites as possible.
GNSS Constellation Types and Spectrum Bands
GPS
Nearly every civilian (consumer and enterprise) receiver supports GPS’s L1 signal at 1575.74 MHz, which includes the Coarse/Acquisition (C/A) code, as well the encrypted Precision (P(Y)) code, which is only authorized users can access. In the future, the L1 signal will be augmented with L1C to increase availability for civilian users and L1M for military users.
GPS’s L2P(Y) signal at 1227.6 MHz has long been used for precision military applications. Civilian users also can use it in a “codeless” fashion, where the receiver finds the L1 signal first and then uses some of the L2 signal’s information to improve accuracy. As with L1, the GPS modernization program is adding two L2 signals: L2C, which is not high precision but rather a stronger and slower signal designed to be available in more challenging environments. A receiver can access L2C without first receiving L1. The other new signal, L2M, is available only to authorized users.
GPS’s L5 signal at 1176.45 MHz was developed for aviation safety. It’s the most advanced civilian signal available from GPS because it’s faster like the precision codes at L1 and L2, and for its higher power and lower frequency. L5 currently is widely available (from 12 satellites) and is expected to be fully available (24 satellites) in 2024.
GLONASS
GLONASS’s primary signal, sometimes called G1, is near L1, at 1602 MHz. It’s unique among all the modern positioning systems because it uses FDMA instead of CDMA, thus undermining its accuracy. Even so, civilian applications have successfully used it for decades.
GLONASS L2 (G2) is located at 1246 MHz and also uses FDMA. Plans call for future satellites to transmit at a new frequency, called L3, at 1201 MHz, right next to GALILEO E5b).
BeiDou
BeiDou B1 is near L1, centered at 1561.098 MHz. A second signal is planned for 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’s lower frequency, B2, for its dual-frequency operation is at 1207.14 MHz. Much like the modernized GPS L2 signal, a narrower signal is openly available, while a wider, higher-precision signal is available only to authorized users.
Similar to GALILEO, BeiDou has a third signal, B3, located above B2, at 1268.52 MHz. It’s available in open and authorized-user-only versions.
GALILEO
GALILEO satellites transmit the E1 signal on the same 1575.42 MHz frequency as GPS’s L1. E1 is designed to coexist with this and other nearby signals. It’s also very similar to GPS’s L1C.
Although GALILEO is purely a civilian system, it also has a set of signals, called Public Regulated Service (PRS), exclusively for authorized users. One is centered at E1 and the other at E6. These signals are wider in bandwidth than Open Service signals.
GALILEO’s E5 signal is split into E5a and E5b, each 20.46 MHz wide. E5a is centered at 1176 MHz, which is co-located with GPS’s L5, while E5b is centered at 1207 MHz. They can be used independently or together. Like GPS’s L5, E5 is designed to provide higher precision and higher availability.
GALILEO’s E6 signal is centered at 1278.75 MHz. Co-located with and similar in use to QZSS’s L6 signal, E6 transmits correction data for high-accuracy services, typically to provide precise point positioning (PPP). E6 also provides a higher data rate, making it ideal for applications that require global, high-accuracy positioning.
IRNSS/NavIC
IRNSS has two signals: one co-located with GPS L5 at 1176.75 MHz and the other at 2492.028 MHz. This latter signal (in 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. Three are nearly identical to GPS at L1, L2 and L5, while the fourth is 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 that previously was available only from subscription L-band services.
Typical Receiver Combinations
In the past, a dual-band receiver might simply be one that could receive both GPS and GLONASS L1. Today, 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 modern receivers also support GALILEO E1.
In response to demand for greater accuracy and more robust locating performance, true multi-band receivers are becoming more common. These integrate at least one other “significantly different” frequency from the L1/B1/E1 band set. 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 typically fall into a handful of groups:
All of this is a lot to consider. That’s why device OEMs, IoT service providers and systems designers frequently need another type of navigation: an experienced partner to help them navigate all of their GNSS options.
One example is an antenna vendor with decades of experience overcoming a wide variety of challenging use cases. This kind of partner can help identify how spectrum band and constellation choices directly affect a solution’s accuracy, consistency and more. Taoglas is one such company. Click here to learn about Taoglas.
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