REVIEW 1

An output of the ff. topics * GNSS * GPS * GLONASS * QZSS * GALILEO * IRNSS

Simon Nichole B. Gabutan

GNSS or Global Navigation Satellite Systems

The primary purpose of GNSSs is to provide positions. Different methods of positioning with GNSS exist with the achieved positioning accuracy varying from 10 meter to the millimeter level. Generally we distinguish between the usage of GNSS code observations and GNSS code and phase observations.
GNSS positioning with code observations
The basic principle of obtaining positions using GNSS is based on the observation of distances to the satellites. Satellite positions are broadcasted in navigation messages coded into the satellite signals, the time of transmission of the signal is also coded into the satellite signal. The difference between time of transmission and time of reception of the coded GNSS satellite signal gives the traveling time. The distance to a satellite can be obtained by multiplying the traveling time of the GNSS signal from the satellite to the users GNSS receiver by the speed of the GNSS signal (approximately 300,000 km/s).
Examples of GNSS positioning with code observations are:
Single point positioning
Differential positioning with code observations

GPS or Global Positioning Systems
GPS Operating Principles
1. Position Fixing
The GPS receiver determines its position using three satellites to triangulate its 3D position. For this the receiver needs to know the:
− satellites position
− distance from each satellite.
Measuring the distance to three satellites allows triangulation to a 3D-position fix since the receiver must be at a position where three spheres intersect. The ambiguity is solved by the receiver since one of the two intersecting points leads to a ridiculous answer.

− Satellite position
The receiver obtains satellite almanac, ephemeris and clock data from the digital data words that comprise the navigation message broadcasted by all satellites. The almanac (or time table) consists of generic satellite constellation data from which the receiver determines which satellites are in view and could be used for navigation. Ephemeris data provides detailed data about the orbit of the selected satellite and together with the clock data the receiver computes the precise satellite X, Y, Z, position relative to the earth centre at the moment the navigation solution is computed.
− Satellite ranging
GPS works by measuring how long it takes a radio signal to travel from the satellite to the receiver and the distance is determined by the formula:
S = V.t (distance (S) equals velocity (V) times time (t)
In this formula the constant (V) represents the speed of light (and radio waves) in vacuum (~300.000 km/s). Thus, an accurate clock with nanosecond resolution is needed to measure the travel time of the radio wave. A measurement error of 1/1000 of a second would mean an error of 300 km.
Measuring the distance to three satellites only would lead to an error since the cheap quartz clock in the receiver is simply not as good as the atomic clocks in space. The receiver clock will have some, yet unknown, error and the ranges found initially are called pseudo ranges.
To refer to the exact moment the signal left the satellite, the receiver clock offset or bias must be determined. This now requires a fourth measurement. Thus four spheres should be intersecting in one single point when all the measurements were exact and all the clocks in sync. The computer inside the receiver starts adding or subtracting time, called ‘clock bias’, to the measurements until all four spheres intersect in one point. This is a simple task for the computer, so the clock bias is quickly calculated. Mathematically the receiver has to solve four equations with four unknowns for the 3 satellite ranges and the common receiver clock-bias determination:
For a 3-D position and time fix a minimum of 4 satellites must be received.

GLONASS or Global Navigation Satellite System
Global navigation system (GLObal NAvigation Satellite System), called GLONASS, which is a summary of the former Soviet Union in the first generation of satellite navigation systems on the basis of CICADA, absorbing part of the U.S. GPS system, the experience, since 12 October 1982 launched the beginning of the second generation navigation satellite systems. On January 18, 1996 to complete the design of satellite data (24), and start the whole operation. GLONASS's primary role is to achieve global, all-weather navigation and positioning, real-time, the other, but also for global time transfer. Currently, GLONASS by Russia responsible.
GLONASS system and the composition and working principle is very similar to GPS, but also into space satellites, ground control and user equipment 3:
1, the space satellite parts. Space satellites in part by the 24 GLONASS satellites, including the work of the satellite 21, three spare satellites in orbit, evenly distributed in three orbital planes. Three orbital planes cross into the 120 degree angle, uniformly distributed on each track 8 satellites, orbit altitude of about 19100km, orbital eccentricity was 0.01, orbital inclination of 64.8 degrees. This ensures that the distribution of any place on Earth at any one time can be received at least four satellite navigation information for the user's navigation and positioning to provide protection. GLONASS satellites are each equipped with satellite stability of cesium atomic clock, and receives the ground control station and control the navigation information and instructions, on-board computer on which the navigation information for processing to generate the navigation message broadcast to the user, control information is used to control the operation of the satellite in space.
2, the ground monitoring part. Ground monitoring part of the GLONASS satellites to achieve the overall maintenance and control. It includes the system control center (located in Moscow Golitsyn Novo) and scattered throughout the territory of Russia, the tracking controller network. Ground control equipment is responsible for collecting, processing GLONASS satellites in orbit and signal information to each satellite launch control commands and navigation information.
3, the user segment. GLONASS GLONASS users to receive satellite signal receiver, and measure the pseudorange or carrier phase, combined with the satellite ephemeris for the necessary processing, the user can get the 3-dimensional coordinates, velocity and time.
GLONASS positioning principle is Intersection. GLONASS satellite position at any one time can be calculated by the satellite ephemeris, in theory, as long as users know the distance to three satellites, can calculate the user's location, but it requires a satellite and the user and the precision of time synchronization between the satellite and high, is still not fully satisfied, but to introduce a time parameter. As much an unknown quantity, so the actual positioning receiver to at least four satellite signals. GLONASS satellites simultaneously launch coarse code (C / A code) and precision code (P code), C / A codes are used to provide to the civil standard positioning while the P code for the Russian military precision positioning or scientific research. GLONASS and GPS in addition to using different systems and coordinate system of time outside the biggest difference between the two is: all GPS satellite signal transmission frequency is the same, and different GPS satellite launch code pseudo-random noise (PRN) is different , the user in order to distinguish the satellite, known as Code Division Multiple Access (CDMA); and all GLONASS satellites pseudo-random noise code is the same, different satellites transmitting frequency is different, to distinguish the different satellite, called for the frequency division multiple access (FDMA). In addition, with the different GPS, GLONASS does not reduce the accuracy of any artificial measures.
QZSS or Quasi- Zenith Satellite System
The Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional time transfer system and Satellite Based Augmentation System for the Global Positioning System, that would be receivable within Japan. The first satellite 'Michibiki' was launched on 11 September 2010. Full operational status was expected by 2013. In March 2013, Japan's Cabinet Office announced the expansion of the Quasi-Zenith Satellite System from three satellites to four. The $526 million contract with Mitsubishi Electric for the construction of three satellites is slated for launch before the end of 2017.
Authorized by the Japanese government in 2002, work on a concept for a Quasi-Zenith Satellite System (QZSS), or Juntencho (準天頂?) in Japanese, began development by the Advanced Space Business Corporation (ASBC) team, including Mitsubishi Electric, Hitachi, and GNSS Technologies Inc. However, ASBC collapsed in 2007. The work was taken over by the Satellite Positioning Research and Application Center. SPAC is owned by four departments of the Japanese government: the Ministry of Education, Culture, Sports, Science and Technology, the Ministry of Internal Affairs and Communications, the Ministry of Economy, Trade and Industry, and theMinistry of Land, Infrastructure and Transport.
QZSS is targeted at mobile applications, to provide communications-based services (video, audio, and data) and positioning information. With regards to its positioning service, QZSS can only provide limited accuracy on its own and is not currently required in its specifications to work in a stand-alone mode. As such, it is viewed as a GNSS Augmentation service. Its positioning service could also collaborate with the geostationary satellites in Japan's Multi-Functional Transport Satellite (MTSAT), currently under development, which itself is a Satellite Based Augmentation System similar to the U.S. Federal Aviation Administration's Wide Area Augmentation System (WAAS).

GALILEO or Europes Global Satellite Navigation System

Satellite navigation is based on the principle of triangulation: If I know my distance from three different points, I can calculate my exact position. The Galileo Global Navigation Satellite System (GNSS) is built on the same basic principle. Four satellites in view are necessary to determine your exact position on or above the Earth – however the more satellites in view/used to calculate your position, the greater the accuracy will be.
Determining precise location depends on measuring accurately the distances between receiver and satellite, and that depends on very accurate measurement of signal travel time. As signals travel at the speed of light, travel times are tiny fractions of a second. Your receiver determines your distance from each of the satellites by measuring the time taken for the signal to travel from the satellite to your receiver antenna (the signal travels at the speed of light). For this you need extremely accurate timing, hence the reason for the extremely precise atomic clocks in the Galileo constellation. The receiver measures travel times by comparing ‘time marks’ imprinted on the satellite signals with the time recorded on the receiver’s clock. The time marks are controlled by a highly accurate atomic clock on board each satellite which provides the time marks for your receiver to compare and calculate.
Of course, it is only possible to determine a location on Earth if you know the location of the navigational satellites very precisely. This is achieved by placing the satellites in highly stable Medium Earth Orbits (MEOs) at an altitude of about 20 000 km. MEOs are the orbits of choice for a number of reasons: their stability enables exact orbit predictions; the satellites travel relatively slowly and so can be observed over several hours, like a fixed star; and, the satellites can be arranged in a constellation so that at least four are visible from any point on the Earth’s surface at any time.
What is Galileo?
The Galileo program is Europe's initiative for a state-of-the-art global satellite navigation system, providing a highly accurate, guaranteed global positioning service under civilian control. Discussions on a European system started in the late nineties and in 1999 the Council called on the Commission to develop a global system managed by public civil authorities.1 After the failure of negotiations on a public-private partnership, the Parliament and the Council in 2008 decided to complete the constellation using EU budget.2
While providing autonomous navigation and positioning services, the system established under the Galileo program will at the same time be interoperable with GPS and GLONASS, the two other global satellite navigation systems. The fully deployed system will consist of 30 satellites and the associated ground infrastructure.
Based on the award of the contracts for the first order of satellites, the launch services, the system support services and the operations, the European Commission announced that three initial services will be provided from 2014 onwards: an initial Open Service, an initial Public Regulated Service and an initial Search and Rescue Service.

IRNSS or Indian Regional Navigation Satellite System

The Indian Regional Navigation Satellite System envisages establishment of a constellation made up of a combination of geostationary Earth orbit (GEO) and geosynchronous orbit (GSO) spacecraft over the Indian region.
The IRNSS constellation will consist of seven satellites —three in GEO orbit (at 34º E, 83º E and 131.5º E) and four in GSO orbit inclined at 29 degrees to the equatorial plane with their longitude crossings at 55º E and 111.5º E (two in each plane) as shown in Figure 2. All the satellites will be continuously visible in the Indian region for 24 hours a day.
The IRNSS is expected to provide position accuracy (two sigma) of better than 20 meters over India and a region extending outside the landmass to about 1,500 kilometers. The system will provide two types of services, a Standard Positioning Service hereafter referred to as SPS, and a Restricted/Authorized Service or RS. Both of these services will be provided at two frequencies, one in the L5 band and the other in S-band.
The IRNSS signal structure has one group delay differential correction parameter (TGD). TGDis to correct for S- and L5- band RS signal group delays. To obtain better position accuracy, other single-frequency users require inter-signal group delay correction parameters (ISCL5-SPS, ISCS-SPS). For space navigation users with off-nadir angles greater than 8.4 degrees with respect to an IRNSS satellite, an SUD correction is required. The SUD bias will provide additional improvement on the order of three nanoseconds to space users. These will be transmitted in navigation data in the future.