Introduction
The four major GNSS constellations - GPS, Galileo, GLONASS, and BeiDou - are all undergoing significant modernisation programmes. New civilian signals are being added, signal power is increasing, and new modulation schemes are being adopted. These changes are not incremental improvements; they represent a fundamental transformation of the signal landscape that will substantially improve accuracy, availability, and resilience for all users over the coming decade.
GPS Modernisation
The United States GPS system has been transmitting civilian signals on L1 (1575.42 MHz) since its operational introduction, and L2 P(Y) has been available to authorised users. The modernisation programme adds new civilian signals designed for improved accuracy and interoperability:
L2C (L2 Civil)
Transmitted by Block IIR-M and later satellites (from 2005 onward), L2C provides the first civilian GPS signal on the L2 frequency (1227.60 MHz). It uses a more advanced modulation scheme (CM+CL code structure) that makes it easier to track and more resistant to interference than the original L1 C/A code. Critically, L2C enables direct dual-frequency ionosphere-free combinations with L1, allowing civilian users to measure and remove the first-order ionospheric delay - worth 1–5 metres of range error in typical conditions.
L5 (Safety-of-Life)
The L5 signal at 1176.45 MHz was designed from the outset for safety-of-life applications including civil aviation. Transmitted by Block IIF and later satellites (from 2010), L5 has 10 times the chipping rate of L1 C/A, a pilot channel for longer coherent integration, and operates in a protected aeronautical radio navigation services (ARNS) frequency band - providing stronger interference rejection. L5 is now fully operational and is the recommended primary ranging signal for high-accuracy applications.
L1C (Interoperability Signal)
L1C is a next-generation civilian signal at 1575.42 MHz designed with international interoperability in mind - it shares frequency and modulation characteristics with Galileo E1 OS, BeiDou B1C, and QZSS L1C. Transmitted by GPS Block III satellites (from 2018), L1C includes a pilot channel for improved weak-signal tracking and a new MBOC(6,1,1/11) modulation for better code tracking precision. As Block III gradually replaces older satellites, L1C will become increasingly available.
Galileo Full Operational Capability and E6
Europe''s Galileo system achieved Initial Operational Capability in 2016 and Full Operational Capability with 24+ operational satellites broadcasting Open Service signals on E1, E5a, E5b, and E5 (AltBOC). Galileo''s signals are transmitted at higher power than GPS (minimum received power -152 dBW vs -158.5 dBW for GPS L1) and use modern modulation schemes designed from lessons learned from GPS.
The E6 signal at 1278.75 MHz is Galileo''s high-accuracy service channel. It carries the Galileo High Accuracy Service (HAS) - a free PPP correction service broadcast directly from Galileo satellites, enabling sub-decimetre accuracy globally without a NTRIP connection. The E6-HAS service entered initial service in January 2023 and is progressively improving. E6 also forms part of the Commercial Authentication Service (CAS) and Public Regulated Service (PRS) for higher-security applications.
BeiDou-3 Global Signals
China''s BeiDou Navigation Satellite System completed its global constellation (BeiDou-3) in 2020 with 30 operational satellites. BeiDou-3 introduces entirely new civilian signals alongside the legacy BeiDou-2 signals:
| Signal | Frequency (MHz) | Interoperability | Purpose |
|---|---|---|---|
| B1C | 1575.42 | GPS L1C, Galileo E1 | Open service, interoperability |
| B2a | 1176.45 | GPS L5, Galileo E5a | Open service, high accuracy |
| B2b | 1207.14 | Galileo E5b | PPP-B2b corrections (Asia-Pacific) |
| B3A | 1268.52 | Galileo E6 | Authorised service |
BeiDou-3''s B2a signal is particularly notable for its very wide bandwidth and clean AltBOC modulation. BeiDou also provides the PPP-B2b service - a free PPP correction signal broadcast on the B2b frequency for the Asia-Pacific region, enabling sub-decimetre positioning without a ground correction network.
GLONASS CDMA Modernisation
Russia''s GLONASS is unique among major GNSS constellations in using Frequency Division Multiple Access (FDMA) for its legacy signals - each satellite transmits on a slightly different frequency, complicating receiver design and limiting multi-GNSS interoperability. The GLONASS modernisation programme is introducing new Code Division Multiple Access (CDMA) signals that operate on standard GNSS frequencies:
- L1OC: Open CDMA signal at 1575.42 MHz - same frequency as GPS L1, Galileo E1, and BeiDou B1C
- L2OC: Open CDMA signal at 1242.0 MHz
- L3OC: Open CDMA signal at 1202.025 MHz
GLONASS CDMA signals will enable standard multi-frequency receiver designs to work with GLONASS alongside GPS, Galileo, and BeiDou without the frequency-specific hardware previously required. Legacy FDMA signals will continue operating for the foreseeable future to maintain backward compatibility.
What New Signals Mean for Accuracy
The primary accuracy benefit of multi-frequency signals is the ability to form ionosphere-free (IF) combinations. The ionosphere delays GNSS signals in proportion to their frequency squared (dispersive medium). By combining measurements on two frequencies, the first-order ionospheric delay - responsible for 1–5 metres of range error at the zenith - is eliminated without modelling. Multi-frequency receivers that were once restricted to geodetic survey equipment are now becoming standard in agricultural, construction, and industrial GNSS receivers, democratising centimetre-level positioning.