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GNSS: Navigation for all

Dr. S. V. Kibe


Dr. S. V. Kibe
Brahmprakash Professor
Indian Space Research Organisation
[email protected]


With total annual sales of GPS chipsets in excess of USD 20 billion and new application areas being discovered, the future growth scenario for GNSS is very robust

GNSS stands for global navigation satellite system. Currently, the US GPS and the Russian GLONASS are the only operating systems in the radionavigation- satellite service (RNSS) bands (space-to-Earth, space-tospace) and 1 164 1 215 MHz, 1 215- 1 300 MHz and 1 559-1 610 MHz. The European Galileo and the Chinese Compass systems are under development.

GLOBAL SYSTEMS
Most of the navigation satellites mentioned earlier transmit L-band EIRPs of the order of 27 – 28 dBW from 19,000 to 24,000 km altitude in such a way that the received signal power at the input of the receiver is about 155 dBW. Brief technical characteristics of the four systems are given in Table 1.

Each constellation has at least 24 satellites in the medium earth orbit (MEO). Regional and augmentation systems have been planned by India, Japan and Nigeria. More than 100 navigation satellites are expected to be in orbit by 2014. These constellations of navigation satellites are loosely termed as GNSS. User receivers capable of operating with one or more systems are called GNSS receivers. It is literally a “more the merrier” case. A GNSS receiver capable of operating with more than one system should outperform any receiver capable of operating with only one satellite system. However, this is easier said than done.

GLONASS. The Russian GLONASS system had 22 satellites by 1996. Soon after, the number of satellites declined to 6 in 2000 and then 14 in 2004. The GLONASS constellation has now been built up and the number of satellites currently is 24. GLONASS uses frequency division multiple access (FDMA) for signal transmission from satellites and has 500 KHz and 5 MHz PN codes for ranging. GLONASS system uses 12 anti-codal frequencies for 24 satellites.

GPS. Navstar Global Positioning System (GPS) of the US is an operational, 31-satellite GNSS system with downlinks in the L1, L2 and L5 bands. Low cost GPS receivers capable of operating with the L1 Standard Positioning Service (SPS) provide position, navigation and timing services anywhere in the world. The typical position accuracy available to a user is better than 30 metres. Better accuracies are available through advanced receivers capable of working with augmentation satellites. The original system configuration for GPS had 24 satellites. This number has now been officially increased to 30. GPS has recently introduced the L5 downlink frequency.

Galileo. The Galileo system has been under planning by the European Commission since 2008. The GIOVE-A and GIOVE-B satellites were launched in 2005 and 2007 as experimental satellites. The initial phase of Galileo consists of four satellites followed by 26 operational satellites to be launched in the next 2-3 years. The constellation is expected to be completed by 2013-14. To be self reliant in the GNSS technology appears to be the main motive of the Europeans in implementing a EUR 4 billion Galileo system. The Galileo signals have been well coordinated with the GPS signals in the last 8 to 10 years. Once complete, Galileo would enhance the capabilities of a joint GPS-Galileo receiver.

Compass (Beidou). The Chinese Compass system consists of a constellation of 30 non-geostationary satellites and five geostationary satellites with positions at 58.75° E, 80° E, 110.5° E, 140° E and 160° E. Each satellite transmits the same four carrier frequencies for navigational signals. These navigational signals are modulated with a predetermined bit stream, containing coded ephemeris data and time and having a sufficient bandwidth to produce the necessary navigation precision without recourse to two-way transmission or Doppler integration. The system provides accurate position determination in three dimensions anywhere on or near the surface of the Earth.

The frequency requirements for the Compass system are based upon an assessment of user accuracy requirements, space-to-Earth propagation delay resolution, multipath suppression and equipment cost and configurations. Three initial channels used for the Compass operations are 1 575.42 MHz, 1 191.795 MHz, and 1 268.52 MHz. This frequency diversity and the wide bandwidth used by Compass will increase the range accuracy for space-to-Earth propagation delay resolution and will improve the multipath suppression to increase the total accuracy. Telemetry and maintenance signals are accommodated in an allocated telemetry band.

CIVIL AVIATION AND GNSS
The use of GPS and its augmentation systems is widespread in civil aviation. The International Civil Aviation Organization (ICAO) defined GNSS as “a worldwide position and time determination system that includes one or more satellite constellations, aircraft receivers and system integrity monitoring, augmented as necessary to support the required navigation performance for the intended operation,” and developed the International Standards and Recommended Practices (SARPs) for seamless worldwide air navigation service. The requirements of accuracy, integrity, availability, continuity during the approach and landing phases are very exacting and drive the system performance requirements for GNSS systems in the world.

GNSS navigation service will be provided using various combinations of the following GNSS elements installed on the ground, the space and/or the aircraft:

  • GPS
  • GLONASS
  • Aircraft-Based Augmentation System (ABAS)
  • Satellite-Based Augmentation System (SBAS)
  • Ground-Based Augmentation System (GBAS)
  • Aircraft GNSS receiver


REGIONAL SYSTEMS
Indian Regional Navigation Satellite System (IRNSS). IRNSS is a continuous space-based all-weather radio navigation system for positioning, navigation and timing service for any user equipped with a suitable receiver, anywhere in the service area. IRNSS is an approved project being implemented by ISRO/DOS. IRNSS constellation will consist of 7 or 11 geo synchronous satellites – 3 in geostationary orbit and the rest in an inclined geosynchronous orbit. All satellites will downlink two signals in L5 and S-band (2483.5 – 2500 MHz) – a BPSK signal and a BOC signal (binary offset carrier). The IRNSS coverage is mainly over India and adjacent regions. IRNSS ground segment will consist of a host of ranging stations, master control facility, a timing centre etc. The constellation is expected to be operational by 2013-14.


QZSS. The Quasi-Zenith Satellite System (QZSS) consists of three satellite positions with one satellite position in each of the three 45° inclined equally spaced orbital planes. Each satellite transmits the same four carrier frequencies for navigational signals. These navigational signals are modulated with a predetermined bit stream, containing coded ephemeris data and time and having a sufficient bandwidth to produce the necessary navigation precision without recourse to two-way transmission or Doppler integration. The frequency requirements for the QZSS system are based upon an assessment of user accuracy requirements, space-to-Earth propagation delay resolution, multipath suppression and equipment cost and configurations. The three initial channels used for QZSS operations are 1 575.42 MHz (L1), 1 227.6 MHz (L2) and 1 176.45 MHz (L5). An experimental signal (LEX) will be added, centred at 1 278.75 MHz (LEX).

GPS AUGMENTATIONS
The first GPS augmentation system was implemented by the US. It is called Wide Area Augmentation System (WAAS). It has been operational since 2008 with Approach Vertical (APV) level 2 service. It covers North America and Canada. This is the most advanced GPS augmentation system in today’s world.

Modified GPS receivers capable of working with GPS augmentation are required to benefit from higher accuracies achievable. WAAS is attempting to achieve Cat.I level service levels in the near future. Augmentation systems offer the uniform level of system performance over a wide area/airspace and guarantee availability, continuity and integrity as required by International Civil Aviation Organisation (ICAO).


The European Geostationary Overlay System (EGNOS) has been under development since 1998. EGNOS was declared operational with APV -1 level of service this year. Both WAAS and EGNOS provide seamless navigation to any aircraft fitted with a WAAS receiver. Both WAAS and EGNOS have three geostationary GPS L1 and L5 payloads as part of the system.

The Japanese MTSAT (multifunctional transport satellite) satellite- based augmentation system (MSAS) is an SBAS, defined as “a wide coverage augmentation system in which the user receives augmentation information from a satellitebased transmitter.”

The MSAS plays the role of the RNSS function in the MTSAT. MSAS utilises two MTSATs to enhance the system reliability and robustness. Each MTSAT transmits one carrier frequency for GPS augmentation signals (RNSS signals). These signals include the following information: ranging, GPS satellite status, basic differential correction (GPS satellite ephemeris and clock corrections) and precise differential correction (ionospheric corrections).

The Indian SBAS is called GAGAN. GAGAN is working towards achieving APV-2 level of position accuracy by 2012. Thereafter, the system would undergo a certification process. Once certified, GAGAN would be one of the most advanced air navigation systems over the Indian airspace. ISRO and AAI are implementing GAGAN which is in its last phase of implementation.

GNSS APPLICATIONS
The global annual sales of GPS receivers worldwide are in excess of USD 20 billion. This number is growing exponentially due to rapid integration of GPS receivers in mobile phones and other mass market vehicles.

GPS chipsets are being integrated into mobile phones as a matter of routine these days. Many of the highend mobile phones available in India have a GPS chipset and local maps of roads of most Indian cities. The integration of satellite communication and satellite navigation is evident. However, there are a few technical issues which need to be solved before the GNSS chipsets becomes an integral part of mobile phones. These issues are:

  • The ability of mobile phone with GNSS to operate inside buildings which is currently not possible,
  • The use of terrestrial navigation aids to assist the GNSS chipsets within a mobile which requires a large network of terrestrial differential GPS transmitters in a country which is currently not available in India and
  • Advances in technology with powerful forward error correction techniques to enable indoor positioning without the assistance of either ground transmitters or terrestrial transmitters such as FM transmitters.

It is difficult to predict whether the GNSS chipsets will be integrated as a standard feature into each and every mobile phone that is marketed in the coming years. However, there are other markets to be exploited for GNSS as indicated in Fig. 1.


FUTURE OF GNSS
With over 100 navigation satellites in the world, the total expenditure on space systems is in excess of around USD 10 billion.

With total annual sales of GPS chipsets in excess of USD 20 billion and new application areas being discovered, the future growth scenario for GNSS is very robust. From shipping ports, long bridges, intelligent highway systems, location-based services in mobile phones to mass market vehicles (rail and road), applications of GNSS extend to earth sciences, tectonic plate movements, radio occultation techniques for weather prediction and temperature profiles of Earth’s atmosphere have pervaded all engineering and science disciplines.

The reason for such high profile growth is that this service can be provided only through space systems which are known for their reliability, reach, low cost receivers and high standards of performance.