06-03-2013, 10:59 AM
Global Navigation Satellite System (GNSS)
[attachment=52726]
INTRODUCTION
Navigation experts worldwide have been discussing for many years about the concept of one navigational system that is available everywhere on globe, at all the time with extreme accuracy, trusted and easy to use, which overcomes the limitations of existing conventional navigationalaids. The concept is Global Navigation SatelliteSystem (GNSS).such a system could be used as sole means of navigation system and could eventually replace most, if not all, of the costly ground based infrastructures. Satellite navigation and positioning system represent the most important technological breakthrough in civil aviation navigation,surveillanceand air traffic management since radar was introduced over half a century agoGPS ,developed by us is currently approved for use in all weather conditions during en-route,terminal air navigation.For civil aviation community whose requirements are stringent,GPS constellations alone fail to meet such requirements. Thus, the need for augmenting these constellations arises to meet the required navigation performance for aviation use as navigational system covering various phases of flight.
Global Navigation Satellite System (GNSS)
GNSS (Global Navigation Satellite System) is a satellitesystem that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). A third, Europe's Galileo, is slated to reach full operational capacity in 2008. Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a
Network of ground stations.
Satellite-based navigation systems use a version of triangulationto locate the user, through calculations involving information from a number of satellites. Each satellite transmits coded signals at precise intervals. The receiver converts signal information into position, velocity, and time estimates. Using this information, any receiver on or near the earth's surface can calculate the exact position of the transmitting satellite and the distance (from the transmission time delay) between it and the receiver. Coordinating current signal data from four or more satellites enables the receiver to determine its position.
Working Of Satellite Navigation
Navigation satellites can tell you where you are to the nearest few metres or better, whatever the weather. Enhanced instruments can even pinpoint the position of a stationary object to within a few centimetres by measuring the object’s position many thousands of times over several hours and then working out the average of the measurements.
All you need to take advantage of this sophisticated new technology is a receiver to pick up
signals transmitted by navigational satellites. The receiver technology is small enough to be
Incorporated into the electronics of a car or mobile phone, and all our lives are changing as a
Result.
To pinpoint your location accurately, your receiver needs to receive signals from at least four navigational satellites. The receiver determines your distance from each of the satellites by measuring the time taken by the signal to travel from the satellite to your receiver
antenna.
Distance measurements from two satellites tell you that you are situated somewhere on the circle where two spheres intersect. The spheres each have one of the two satellites at their centre and their radii are the satellite-receiver distances. Knowing your distance from a third satellite fixes your position at one of the two points where the circle intersects the third sphere. One of the intersection points can usually be discounted – for instance, it may be thousands of kilometres above the Earth’s surface.
In practice, a fourth satellite is needed to synchronise your receiver’s clock with a common time standard which is strictly adhered to by the clocks on board all the satellites. The use of a fourth satellite also resolves the position ambiguity that occurs with only three satellites. In general, the more satellites used, the greater the positioning accuracy. Many receivers have channels for receiving signals from up to 15 satellites.
GPS Signals
is modulated using the two carriers (L1 and L2) at a chipping rate of 50 The generated signals on board the satellites are based or derived from generation of afundamental frequency ƒo=10.23 MHZ . The signal iscontrolled by atomic clock and has stability in the range of 10−13 over one day. Two carriersignals in the L-band, denoted L1 and L2, are generated by integer multiplications of fo. Thecarriers L1 and L2 are biphase modulated by codes to provide satellite clock readings to thereceiver and transmit information such as the orbital parameters. The codes consist of a sequence with the states +1 or -1, corresponding to the binary values 0 or 1.
Modernized GPS
Due to the vast civil applications of GPS technology during the past decade or so and due to the new technologies used in the satellite and receivers, the U.S government has decided to extend the capabilities of GPS to give more benefits to the civil community. In addition to the existing GPS signals, new signals will be transmitted by GPS satellite; see Figure 5. Moreover, this will increase the robustness in the signals and improve the resistance to signal interference. This definitely will lead to a better quality of service (QoS). The new signals added to the GPS are: (i) a new L5 frequency in an aeronautical radio navigation service (ARNS) band with a signal structure designed to improve aviation applications, (ii) C/A code to L2C carrier (L2 civil signal ), and (iii) a new military (M) code on L1 and L2 frequency for the DoD has been added. It has the potential to track signal even in poor conditions where the C/A code tracking on L1 would not be possible. The new military code will be transmitted from the Block IIR-M and IIF satellites.
SATILLITE BASED AUGMENTATION SYSTEM (SBAS)
A satellite based augmentation system is a vital component in aviation systems across the globe. It provides reliability and accuracy in existing globalnavigation systems. This is an article that contains more information about the SBAS.
A satellite based augmentation system, or SBAS, is an augmentation system that uses additional messages from satellite broadcasts to supportregional and wide area augmentation. A satellite based augmentation system is usually made up of several ground stations that can be located atexact surveyed points. The ground stations measure satellite signals and other environmental factors that can impact the satellite signal that is beingreceived by users. These measurements make it possible for navigation data to be created and sent for broadcast via multiple satellites.
There are several satellite based augmentation system implementations and designs. SBAS implementations include the Wide Area Augmentation System from (WAAS)from US, the European Geostationary Navigation Overlay Service, or EGNOSfrom Europe, and the Multi-functional SatelliteAugmentation System, or MSAS from Japan. These systems and the other augmentation systems in the world follow a specific frequency andmessage format set by the International Civil Aviation Organization.
SBAS CONCEPT
A satellite-based augmentation system (SBAS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. Such systems are commonly composed of multiple ground stations, located at accurately-surveyed points. The ground stations take measurements of one or more of the GNSS satellites, the satellite signals, or other environmental factors which may impact the signal received by the users. Using these measurements, information messages are created and sent to one or more satellites for broadcast to the end users. It employs a ranging function to generate GPS-like signals.Differential correction function provides ranging error data to each user.
3 SBAS CONCEPT OF OPERATION
Operation of SBAS can be explained as follows
SBAS reference stations are deployed throughout the region of service at pre-surveyed locations to measure pseudo ranges and carrier phases onL1 and L2 frequencies from all visible satellites.
The reference stations send these measurements to SBAS Master Station, which calculate clock and ephemeris corrections for each GPS satellite monitored, ephemeris information for each GEO and ionospheric vertical delays on agrid.The grid consists of fixed ionospheric grid points(IGPs) at an altitude of 350 km above the earth’s surface.
In addition to the corrections,the master station calculates error bounds for ionospheric corrections called grid ionospheric vertical errors (GIVEs) at each IGP,and also combined error bounds for clock and ephemeris corrections for each visible satellite,called user differential range errors (UDREs).
The master station sends these corrections and error bounds to the user through GEO communication satellites with a data rate of 250 bps. User avionics apply these connections to their pseudo ranges obtained from GPSmeasurements,in order to improve the accuracy of their positionestimates. They also use UDREs and GIVEs and other information to calculate error bounds on position error called the vertical protection level (VPL) and horizontal protection level (HPL).for the integrity of the system, these protection levels must bound the position errors with probability greater or equal to 0.999 in one hour for en-route through NPA operations and for PA in 150s.
Ionospheric Correction Model
Ionospheric models are usually computed by determining the TEC in the direction of all GPS satellites in view from a ground GPS network. The line of sight TEC measurements are scaled to the vertical direction (TECV). These TECV are attributed to the intercept point of the slant path with an infinitesimally thin shell that is typically placed at a height of 350400 km. Next all the TECV values observed over some time are fit by a surface in a Sun-fixed reference frame. The Sun-fixed frame is chosen because the ionosphere rotates with the Sun. Modeling of the ionosphere in this way (called the standard model in this paper) does not capture small scale and high frequency ionospheric disturbances. We model these fluctuations as small correction planes to the standard ionospheric model as illustrated in Figure 2.
[attachment=52726]
INTRODUCTION
Navigation experts worldwide have been discussing for many years about the concept of one navigational system that is available everywhere on globe, at all the time with extreme accuracy, trusted and easy to use, which overcomes the limitations of existing conventional navigationalaids. The concept is Global Navigation SatelliteSystem (GNSS).such a system could be used as sole means of navigation system and could eventually replace most, if not all, of the costly ground based infrastructures. Satellite navigation and positioning system represent the most important technological breakthrough in civil aviation navigation,surveillanceand air traffic management since radar was introduced over half a century agoGPS ,developed by us is currently approved for use in all weather conditions during en-route,terminal air navigation.For civil aviation community whose requirements are stringent,GPS constellations alone fail to meet such requirements. Thus, the need for augmenting these constellations arises to meet the required navigation performance for aviation use as navigational system covering various phases of flight.
Global Navigation Satellite System (GNSS)
GNSS (Global Navigation Satellite System) is a satellitesystem that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). A third, Europe's Galileo, is slated to reach full operational capacity in 2008. Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a
Network of ground stations.
Satellite-based navigation systems use a version of triangulationto locate the user, through calculations involving information from a number of satellites. Each satellite transmits coded signals at precise intervals. The receiver converts signal information into position, velocity, and time estimates. Using this information, any receiver on or near the earth's surface can calculate the exact position of the transmitting satellite and the distance (from the transmission time delay) between it and the receiver. Coordinating current signal data from four or more satellites enables the receiver to determine its position.
Working Of Satellite Navigation
Navigation satellites can tell you where you are to the nearest few metres or better, whatever the weather. Enhanced instruments can even pinpoint the position of a stationary object to within a few centimetres by measuring the object’s position many thousands of times over several hours and then working out the average of the measurements.
All you need to take advantage of this sophisticated new technology is a receiver to pick up
signals transmitted by navigational satellites. The receiver technology is small enough to be
Incorporated into the electronics of a car or mobile phone, and all our lives are changing as a
Result.
To pinpoint your location accurately, your receiver needs to receive signals from at least four navigational satellites. The receiver determines your distance from each of the satellites by measuring the time taken by the signal to travel from the satellite to your receiver
antenna.
Distance measurements from two satellites tell you that you are situated somewhere on the circle where two spheres intersect. The spheres each have one of the two satellites at their centre and their radii are the satellite-receiver distances. Knowing your distance from a third satellite fixes your position at one of the two points where the circle intersects the third sphere. One of the intersection points can usually be discounted – for instance, it may be thousands of kilometres above the Earth’s surface.
In practice, a fourth satellite is needed to synchronise your receiver’s clock with a common time standard which is strictly adhered to by the clocks on board all the satellites. The use of a fourth satellite also resolves the position ambiguity that occurs with only three satellites. In general, the more satellites used, the greater the positioning accuracy. Many receivers have channels for receiving signals from up to 15 satellites.
GPS Signals
is modulated using the two carriers (L1 and L2) at a chipping rate of 50 The generated signals on board the satellites are based or derived from generation of afundamental frequency ƒo=10.23 MHZ . The signal iscontrolled by atomic clock and has stability in the range of 10−13 over one day. Two carriersignals in the L-band, denoted L1 and L2, are generated by integer multiplications of fo. Thecarriers L1 and L2 are biphase modulated by codes to provide satellite clock readings to thereceiver and transmit information such as the orbital parameters. The codes consist of a sequence with the states +1 or -1, corresponding to the binary values 0 or 1.
Modernized GPS
Due to the vast civil applications of GPS technology during the past decade or so and due to the new technologies used in the satellite and receivers, the U.S government has decided to extend the capabilities of GPS to give more benefits to the civil community. In addition to the existing GPS signals, new signals will be transmitted by GPS satellite; see Figure 5. Moreover, this will increase the robustness in the signals and improve the resistance to signal interference. This definitely will lead to a better quality of service (QoS). The new signals added to the GPS are: (i) a new L5 frequency in an aeronautical radio navigation service (ARNS) band with a signal structure designed to improve aviation applications, (ii) C/A code to L2C carrier (L2 civil signal ), and (iii) a new military (M) code on L1 and L2 frequency for the DoD has been added. It has the potential to track signal even in poor conditions where the C/A code tracking on L1 would not be possible. The new military code will be transmitted from the Block IIR-M and IIF satellites.
SATILLITE BASED AUGMENTATION SYSTEM (SBAS)
A satellite based augmentation system is a vital component in aviation systems across the globe. It provides reliability and accuracy in existing globalnavigation systems. This is an article that contains more information about the SBAS.
A satellite based augmentation system, or SBAS, is an augmentation system that uses additional messages from satellite broadcasts to supportregional and wide area augmentation. A satellite based augmentation system is usually made up of several ground stations that can be located atexact surveyed points. The ground stations measure satellite signals and other environmental factors that can impact the satellite signal that is beingreceived by users. These measurements make it possible for navigation data to be created and sent for broadcast via multiple satellites.
There are several satellite based augmentation system implementations and designs. SBAS implementations include the Wide Area Augmentation System from (WAAS)from US, the European Geostationary Navigation Overlay Service, or EGNOSfrom Europe, and the Multi-functional SatelliteAugmentation System, or MSAS from Japan. These systems and the other augmentation systems in the world follow a specific frequency andmessage format set by the International Civil Aviation Organization.
SBAS CONCEPT
A satellite-based augmentation system (SBAS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. Such systems are commonly composed of multiple ground stations, located at accurately-surveyed points. The ground stations take measurements of one or more of the GNSS satellites, the satellite signals, or other environmental factors which may impact the signal received by the users. Using these measurements, information messages are created and sent to one or more satellites for broadcast to the end users. It employs a ranging function to generate GPS-like signals.Differential correction function provides ranging error data to each user.
3 SBAS CONCEPT OF OPERATION
Operation of SBAS can be explained as follows
SBAS reference stations are deployed throughout the region of service at pre-surveyed locations to measure pseudo ranges and carrier phases onL1 and L2 frequencies from all visible satellites.
The reference stations send these measurements to SBAS Master Station, which calculate clock and ephemeris corrections for each GPS satellite monitored, ephemeris information for each GEO and ionospheric vertical delays on agrid.The grid consists of fixed ionospheric grid points(IGPs) at an altitude of 350 km above the earth’s surface.
In addition to the corrections,the master station calculates error bounds for ionospheric corrections called grid ionospheric vertical errors (GIVEs) at each IGP,and also combined error bounds for clock and ephemeris corrections for each visible satellite,called user differential range errors (UDREs).
The master station sends these corrections and error bounds to the user through GEO communication satellites with a data rate of 250 bps. User avionics apply these connections to their pseudo ranges obtained from GPSmeasurements,in order to improve the accuracy of their positionestimates. They also use UDREs and GIVEs and other information to calculate error bounds on position error called the vertical protection level (VPL) and horizontal protection level (HPL).for the integrity of the system, these protection levels must bound the position errors with probability greater or equal to 0.999 in one hour for en-route through NPA operations and for PA in 150s.
Ionospheric Correction Model
Ionospheric models are usually computed by determining the TEC in the direction of all GPS satellites in view from a ground GPS network. The line of sight TEC measurements are scaled to the vertical direction (TECV). These TECV are attributed to the intercept point of the slant path with an infinitesimally thin shell that is typically placed at a height of 350400 km. Next all the TECV values observed over some time are fit by a surface in a Sun-fixed reference frame. The Sun-fixed frame is chosen because the ionosphere rotates with the Sun. Modeling of the ionosphere in this way (called the standard model in this paper) does not capture small scale and high frequency ionospheric disturbances. We model these fluctuations as small correction planes to the standard ionospheric model as illustrated in Figure 2.