Comparison of existing and emerging GNSSs

3.7 Comparison of existing and emerging GNSSs

A GNSS is massive and complex architecture containing typically three seg-ments, i.e. space, ground, and user segseg-ments, each of which consists of a large number of systems, subsystems and interfaces. A GNSS is characterized by many functional and technical features as well as implements multiple modes of operation, in a highly dynamic environment. Consequently, the complexity associated with the comparison of such systems is so high that typically, scien-tists, researchers, or experts focus only on one or few aspects. As a detailed comparison of the functionalities of current and emerging GNSS would be out-side the scope of this thesis, in this section, we present a high level comparison of some of the key features present in the GNSSs described in the previous sub-sections (i.e. Galileo, GPS, GLONASS, and Compass) with emphasis on the provided services.

In terms of system architecture, the biggest difference among the four GNSSs is in their space segment. Specifically, each GNSS has not only a different number of operational satellites (in-use and spares) but also the configuration of the constellation differs greatly in terms of number of orbits, orbital inclination, dis-tribution of satellites among the orbits, and altitude. For instance, the satellites of Galileo and Compass systems are located at a higher altitude than the GPS and GLONASS satellites. Moreover, because the inclination of the satellite or-bits in GLONASS are higher than in other systems, it is more suitable for posi-tioning in higher latitudes. Also, the technical characteristics (e.g. modulation scheme, frequency, data bit rate, etc.) of the transmitted navigation signals dif-fer greatly among the four systems. In terms of services, all global systems are offering or plan to offer at least two types of services; a free-of-charge open ac-cess service for civilians with basic performance characteristics and a restricted service for military or governmental authorities with enhanced performance.

Among the four GNSSs described earlier, future Galileo and modernized GPS systems promise additional civilian services. Specifically, Galileo will offer in total five distinctive services (including the search and rescue service under COSPAS – SARSAT program), two of which can be openly accessed, while in the modernized GPS three additional civil signals aimed at improving the per-formance for civilian users. Also, the GLONASS signals will be modernized but there are currently no plans on providing additional services. In terms of the systems’ governance, GPS and GLONASS are operated by the military while Galileo and Compass are civil operated. Table 1 summarizes some of the key features of current and emerging GNSSs.

By year 2020 there will be in total four GNSSs, several regional satellite naviga-tions systems, and more than 100 navigation satellites in the space. From the user point of view, the availability of more satellites can to lead to enhanced performance in terms of flexibility (i.e. more visible satellites at a given location), reliability (i.e., the more diverse the maintenance of the components of GNSS the less chance of overall system failure), faster positioning (i.e. more

measure-ments in shorter time can lead to shorter observation periods without degrad-ing accuracy), faster initialization (i.e. the time to first fix can be reduced), and higher positing accuracy (e.g. better correction of ionospheric errors with in-creasing number of signal frequencies) (PSU, 2013).

However, with an increasing number of GNSSs, the concepts of compati-bility and interoperacompati-bility become increasingly relevant. In particular, the Inter-national Committee on Global Navigation Satellite Systems (ICG) forum has defined compatibility as the ability of global and regional navigation satellite systems and augmentations and the services they provide to be used separately or together without causing unacceptable interference and/or other harm to an individual system and/or service. In turn, interoperability refers to the ability of these systems to be used together to provide better capabilities at the user level than would be achieved by relying solely on the open signals of one sys-tem (OOSA, 2007). All in all, it is important to seek common understanding on appropriate methods to determine compatibility among all GNSS and it is de-sirable open signals and services to be interoperable to the maximum extent possible, in order to maximize benefit to all GNSS users (OOSA, 2008; OOSA, 2011).

Table 1 Comparison of existing and emerging GNSSs

Features GNSS

Type of operating entity

Coverage Constellation

size Services Key characteristics Access

Galileo Civil Global 27 MEO & 3

spares Open Service Standard performance comparable to GPS SPS Open & Free Safety-of-Life Integrity, guaranteed enhanced performance Open & Free Commercial Service Higher positioning accuracy than OS, authentication,

service guarantee Restricted &

Fee-based Public Regulated

Service Encrypted, resistant to jamming, high continuity Restricted GPS Military Global 24 MEO & 4-6

spares Standard Positioning

Service (L1 C/A) Standard PVT service Open & Free

Precise Positioning

Service Encrypted, better performance than SPS Restricted

L2C (& L1 C/A) Faster signal acquisition, enhanced reliability, greater

operating range Open & Free

L5 Improved accuracy and robustness (reserved

exclu-sively for aviation safety services) Restricted L1C Interoperability with other GNSSs, enhanced

perfor-mance in cities and other challenging environments Open & Free

M-code Improved anti-jamming performance Restricted

GLONASS Military Global 21 MEO & 3

spares Standard Precision Standard PVT service Open & Free

High Precision Enhanced characteristics, better performance than SP Restricted COMPASS Civil Global 5 GEO & 30

GSO Open service Standard PVT service Open & Free

Authorized service Encrypted, better performance than the open Restricted