Location-based positioning capabilities bring a lot of value - and not just to indoor location applications. 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 critical 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 expected to grow from $2.7 billion in 2019 to nearly $3.8 billion by 2026. When developing GNSS-based solutions, device OEMs, IoT service providers, and system designers need to carefully consider how their choice of frequency bands and constellations directly impacts accuracy, consistency, and more.
Constellation options have increased significantly in the last decade alone, with China's Beidou and the European Space Agency's Galileo joining the U.S. GPS and Russian GLONASS global systems. Two regional satellite positioning systems also became fully operational in 2018: Japan's Quasi-Zenith Satellite System (QZSS) and India's IRNSS/NavIC.
Each constellation has multiple signals, each operating on its own frequency. This design maximizes positional accuracy because the receiver can use two frequencies to minimize errors from the ionosphere. Two frequencies also increase the likelihood that a signal will be available when the receiver needs it. Some systems use a second or third frequency to provide correction data to further improve accuracy. For greater accuracy, the receiver needs to receive signals from as many satellites as possible.
Global Positioning System (GPS)
Almost all residential (consumer and business) receivers support GPS's L1 signal at 1575.74 MHz, which includes coarse/acquisition (C/A) code, as well as encrypted precision (P(Y)) code that can only be accessed by authorized users. In the future, the L1 signal will be augmented by 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 can also use it in a "code-free" manner, where the receiver first finds the L1 signal and then uses some information from the L2 signal to improve accuracy. As with L1, the GPS modernization program is adding two L2 signals: L2C, which is less accurate, but a stronger, slower signal designed for more challenging environments. Receivers can access L2C without first receiving L1. The other new signal, L2M, is available only to authorized users.
The GPS L5 signal at 1176.45 MHz was developed for aviation safety. It is the most advanced civil signal offered by GPS because it is faster than the L1 and L2 precision codes, and has higher power and lower frequency. L5 is currently widely available (from 12 satellites) and is expected to be fully available (24 satellites) by 2024.
GLONASS.
The primary signal of GLONASS, sometimes referred to as G1, is located near L1 at 1602 MHz. it is unique among all modern positioning systems in that it uses FDMA rather than CDMA, which diminishes its accuracy. even though civilian applications have been successfully using it for decades, GLONASS L2 (G2) is located at 1246 MHz and also uses FDMA. plans call for future satellites to operate on GLONASS. Even so, it has been used successfully for decades for civilian applications. glonass L2 (G2), located at 1246 MHz, also uses FDMA. plans call for future satellites to transmit at a new frequency of 1201 MHz (called L3) next to the GALILEO E5b. the GLONASS L2 (G2) is located at 1246 MHz and also uses FDMA.
BeiDou
Beidou B1 is close to L1, centered at 1561.098 MHz. The second signal is planned to be directly above L1 at 1589.742 MHz. the latest BeiDou satellites also include a signal at 1575.42 MHz, which is almost the same as L1C for GPS. BeiDou's dual-frequency operation operates at the lower B2 frequency of 1207.14 MHz. Much like the modernized GPS L2 signal, the narrower signal is publicly available, while the 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, which is available in both open and authorized user-only versions.
GALILEO (Galileo)
The GALILEO satellite transmits the E1 signal at the same frequency of 1575.42 MHz as GPS's L1. E1 is designed to coexist with this and other nearby signals. It is also very similar to GPS's L1C. Although GALILEO is a purely civilian system, it also has a set of signals, called the Public Regulatory Service (PRS), which are available exclusively to authorized users. One is centered on E1 and the other on E6. These signals have a wider bandwidth than the Open Service signals.
GALILEO's E5 signal is divided into E5a and E5b, each 20.46 MHz wide. E5a is centered at 1176 MHz, the same location as the GPS's L5, while E5b is centered at 1207 MHz. They can be used individually or together. Like GPS's L5, E5 is designed to provide higher accuracy and higher availability.GALILEO's E6 signal is centered at 1278.75 MHz. Co-located and similar in use to QZSS's L6 signal, E6 transmits calibration data for high-precision services and is typically used to provide precise single-point positioning (PPP). E6 also offers higher data rates, making it ideal for applications requiring global, high-precision positioning.
IRNSS/NavIC
IRNSS has two signals: one 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. The receiver can use the L5 signal - together with GPS, GALILEO, BDS or GLONASS signals in the L1 band - to provide the benefits of dual-band operation. The two signals can be used independently to provide a single-frequency position, and the NavIC system is also planned to transmit ionospheric correction data for the coverage area to improve accuracy.
QZSS
QZSS has four signals. Three are virtually identical to the GPS of L1, L2 and L5, while the fourth is a new signal for L6 (in the same location as E6) at 1278.75 MHz. like GALILEO's E6, the L6 (also known as LEX) signal delivers data at a much faster rate, which enables distribution of new types of data. This is now being used to provide free, open calibration data, allowing for free PPP in the region, which was previously only available through a subscription to the L-band service.
Typical Receiver Combinations
In the past, a dual-frequency receiver might have been simply a receiver that could receive both GPS and GLONASS L1. Today, this level of functionality is expected, so modern "single-frequency" receivers typically support GPS L1, GLONASS L1, and Beidou B1, which are actually three different frequencies. Many modern receivers also support GALILEO E1.
To meet the demand for higher accuracy and more robust positioning performance, true multi-band receivers are becoming more common. They integrate at least one frequency that is "significantly different" from the L1/B1/E1 band set. In the past, this usually meant strict support for GPS L1, GPS L2P(Y) (codeless or not), and perhaps GLONASS L1. Modern multi-band receivers have higher expectations and are typically categorized as follows:
Commercial/Industrial:
GPS L1C/A, L1C, L2C.
GLONASS L1, L2OF
BDS B1 (and possibly B2)
Galileo E1, E5b
Consumer/Commercial:
GPS L1C/A, L1C, L5
GLONASS L1
Beidou B1
Galileo E5a
Consumer/Commercial (Indian subcontinent only):
GPS L1C/A, L1C
GLONASS L1
Beidou B1
Galileo E5a
NAVIC/IRNSS L1C, L5
High Accuracy/Reference:
GPS L1C/A, L1C, L2P(Y) (no code), L2C
GLONASS L1, L2
Beidou B1, B2, B3
Galileo E1, E5, E6
QZSS L1C, L6
NAVIC/IRNSS L1C, L5
All of these require a lot of consideration. That's why device OEMs, IoT service providers, and system designers often need another way to navigate: an experienced partner to help them navigate all the GNSS options. One example is an antenna provider with decades of experience overcoming a variety of challenging use cases. Such a partner can help determine how band and constellation choices directly impact the accuracy, consistency, etc. of the solution. JunoTech is one such company.
