# Detecting and Mitigating Attacks on GPS Devices
## 3 GNSS Overview
### Other GNSS Systems  
While GPS is the oldest and most mature GNSS system, there are other GNSS systems that have emerged since the GPS system was initially fielded. The first is the Russian Federation's GLObalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS), with the first satellites launched in 1982 and reaching full operational capability in 1995. GLONASS consists of 24 satellites in medium-earth orbit (MEO). The constellation operates in three orbital planes, with eight evenly spaced satellites on each plane. For a GLONASS receiver to calculate an accurate position estimate, the receiver must be in range of at least four satellites. providing the system originally. GLONASS satellites broadcast two types of signals: (1) an open standard-precision signal and (2) an obfuscated high-precision signal available only to authorized users. The L1 and L2 carriers utilize Frequency Division Multiple Access (FDMA), where individual satellites transmit at slightly different frequencies. The most common GLONASS signals, L1 and L2, are centered at 1602 MHz and 1246 MHz, respectively, with both standard-precision and obfuscated high-precision signals provided at both L1 and L2. Starting with newer GLONASS-K1 satellites, a new L3 channel centered at 1207 MHz was introduced, which is also available on modern GLONASS-M+ satellites launched in the past decade. Similar to GPS, GLONAS uses Code Division Multiple Access (CDMA) and BPSK modulation. New CDMA-based signals have also been defined for L1 and L2 channels that are available in the newest GLONASS-K2 satellites. Galileo, developed and maintained by the European Space Agency (ESA), is the European Union's (EU's) GNSS that consists of 28 satellites, all but two of which are positioned in three MEO orbital planes. The remaining two satellites were placed in incorrect orbits and are currently used for search and rescue purposes only but are not considered an operational part of the constellation. Galileo began providing an initial operational capability in late 2016. It broadcasts signals in three primary bands: E1, E6, and E5 ab. The E1 carrier offers open service (OS) using a composite binary offset carrier (CBOC) modulation designed for reduced ranging noise and enhanced multipath performance. This E1 OS signal was also designed to provide improved interoperability with other GNSSs, sharing a common spectrum with the GPS L1C and BeiDou-3 B1C carriers. Dual-frequency receivers can also utilize the E5a and E5b signals, which share the same spectrum as the GPS L5 and BeiDou B2a carriers. The E5a and E5b carriers utilize an Alternative Binary Offset Carrier (AltBOC) modulation and multiplexing scheme. The E6 carrier includes a fully encrypted signal component for authorized users. The Chinese BeiDou-3 system provides global GNSS service through its 24-satellite constellation in MEO across three orbital planes. Precedesor BeiDou systems (e.g., BeiDou-2) were regional systems and did not provide global service. BeiDou-3 began offering initial services in late 2018, with a full operational capability announced in 2020. This BeiDou-3 system offers four primary signals for navigation. The B1I and B3I carriers provide open services and were retained from BeiDou-2 for backwards compatibility. It also introduced the B1C and B2A carriers. B1C operates in the same spectrum as the GPS L1 and Galileo E1 carriers (1575.42 MHz), and the B2A carrier operates in the same spectrum as the GPS L5 and Galileo E5a carriers (1176.45 MHz). These spectrum choices were made to facilitate multi-constellation receivers. The BeiDou-3 system uses a range of modulation and multiple access approaches that are quite different than GPS, Galileo, or GLONASS systems. BeiDou-3 uses Constant Envelope Modulation via Intermodulation Construction (CEMIC) in the B1 band to generate the legacy B1I carrier. The B2A carrier is generated through Asymmetric Constant Envelope Binary Offset Carrier (ACE-BOC) modulation, while the BIC carrier uses Quadrature Multiplexed BOC (QMBOC).