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Principles of Compatibility among GNSS
|Title||Principles of Compatibility among GNSS|
|Year of Publication||2011|
Within the last decade, several new (and modernized) global and regional navigation satellite systems have been announced. One of the main technical reasons behind this phenomenon is that a single GNSS system is often not enough to guarantee the target user performances, especially in challenging conditions such as urban environments. Therefore the emergence (and modernization) of GNSS systems entails discussions on compatibility and interoperability among the different service providers.
The ICG (International Committee on Global Navigation Satellite Systems) Forum defines compatibility as: “Compatibility refers to the ability of global and regional navigation satellite systems and augmentations to be used separately or together without causing unacceptable interference and/or other harm to an individual system and/or service.”
Two aspects are often considered:
- Radiofrequency compatibility (RFC), including factors of cross-correlation properties and affordable receiver noise floor
- Spectral separation between authorized signals and other signals, and if overlapping is unavoidable, then a close discussion among providers should be undertaken to guarantee the required service
Radio Frequency Compatibility
Radio Frequency Compatibility (RFC) involves consideration of technical factors such as the protection of user equipment against RF interference from other systems, effects on receiver noise floor and cross-correlation amongst signals. GNSS signals are transmitted at a very low power and they arrive at a terrestrial receiver antenna below the noise floor (e.g. around -160 dBW for GPS L1). However, when increasing the number of satellites, these signals are added together - especially in the most populated frequency bands, such as L1. Simulations show that three constellations (i.e. more than around 70 MEO satellites) could already contribute to increase the receiver noise floor, hence affecting the signal to noise ratio at GNSS receivers.
Regarding user equipment, it should be noted that there exist standards only for maritime and aviation users. In fact, there are no standards available for land vehicles and rail in what concerns RFC. Moreover, these requirements (e.g. on minimum signal to noise ratio, masking angles and affordable external interference) are highly dependent on the application at hand. The civil transportation community – maybe the largest GNSS market segment – recommends that the service providers establish compatible technical system characteristics so as to allow the definition of user equipment standards.
Spectral separation between authorized and non-authorized signals is the most desirable situation for national security purposes. Nevertheless, spectral separation is not always possible (and even sometimes desirable for interoperability purposes). GNSS providers have therefore conducted a series of bilateral and multilateral initiatives in order to guarantee compatibility (and interoperability).
Some examples of these coordination initiatives are reported in the ICG GNSS Report:
- 1998: US and Japan commit to make QZSS/ MSAS and GPS interoperable and compatible
- 2004: US-Russia agreement was reached to provide foundation for cooperation between GPS and Glonass
- 2004: EU-US agreement was reached to provide foundation for cooperation between GPS and Galileo
- 2007: India and US issue a joint statement on GNSS cooperation
- EU-Russia Radio Frequency coordination on GLONASS-Galileo compatibility and interoperability (since Galileo Definition Phase 1999-2001)
- Coordination between Galileo/ EGNOS and GPS/ WAAS as well as the first QZSS satellite has been completed.
- ^ “QUO VADIS? Where are We Going in Satellite Navigation?”, G.Hein, PNT Symposium, Stanford, CA, USA, 9 Nov 2010
- ^ “GNSS Compatibility and Interoperability – Civil Transportation Perspectives”, Christopher J. Hegarty, 2008
- ^ “Current and Planned Global and Regional Navigation Satellite Systems and Satellite-based Augmentation Systems”, ICG Forum, United Nations office for Outer Space Affairs, 2010, http://www.oosa.unvienna.org/pdf/publications/icg_ebook.pdf