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Augmentation of a GNSS Global Navigation Satellite System is a method of improving the navigation system's attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process. There are many such systems in place and they are generally named or described based on how the GNSS sensor receives the external information.

Some systems transmit additional information about sources of error such as clock drift, ephemeris, or ionospheric delay , others provide direct measurements of how much the signal was off in the past, while a third group provides additional vehicle information to be integrated in the calculation process. The SBAS Satellite-Based Augmentation Systems is a civil aviation safety-critical system that supports wide-area or regional augmentation — even continental scale - through the use of geostationary GEO satellites which broadcast the augmentation information.

While the main goal of SBAS is to provide integrity assurance, it also increases the accuracy with position errors below 1 meter 1 sigma. One GBAS supports up to 48 approaches and covers many runway ends with more installation flexibility than an ILS with localizer and glideslope antennas at each end. A GBAS can provide multiple approaches to reduce wake turbulence and improve resilience, maintaining availability and operations continuity.

Some background on the navigation signal delays in Earth's atmosphere. Satellite navigation is based on measuring the signal delay between transmission at the satellite and reception by the user receiver.

Therefore, the modeling and correction of the additional delay due to the propagation through the atmosphere and mitigation of signal disturbances in the user environment play an important role for the accuracy of the derived position solution. The positioning with satellite navigation systems like GPS or Galileo is based on time of arrival measurements of the signals transmitted from the satellite at the receiver. The measured signal delays are converted to pseudo ranges by multiplication with the speed of light.

For synchronization of the receiver clock to the system time measurement data from at least four satellites must be available. Then the receiver clock bias can be solved for as a fourth unknown together with the three unknown co-ordinates of the position x, y, z by triangulation.

While system errors can normally not be corrected in real time, propagation errors can be reduced by models or advanced signal processing, where the remaining error depends on the type of measurements e. Satellite navigation signals are extremely weak when they arrive at the user antenna; the nominal GPS power received at ground is dBW, which is below the noise level. Therefore, the signals are sensitive to shadowing by buildings or vegetation.

Navigation signal are spread spectrum signals, which are recovered from noise by dispreading in the receiver, i. By this correlation process also the time of arrival, i. If the direct signal is superposed with reflected signals from the user environment due to multipath propagation, the peak of the correlation function becomes less unique and the accuracy of the delay measurement is reduced.

Note, that these are the residual errors after applying corrections. In the table the main error contributions besides orbit and satellite clock errors are by propagation effects in the atmosphere and multipath propagation in the user environment. For single frequency measurements in L-band the ionosphere error is even the dominant contributor in the whole error budget.

The UERE budget demonstrates the importance of propagation modeling and improvement of correction models for high accuracy navigation, in particular when orbit and clock errors will be reduced by the technological progress in future systems like Galileo and modernized GPS. Therefore, the objective of this paper is to provide an overview about the characteristics of the main propagation effects and common correction and mitigation methods.

Propagation path through the atmosphere: Figure 1 shows the propagation path of the signals through the different layers of the atmosphere. Within these layers the signal path and the propagation speed is influenced by the variation and gradient of the refractive index. Starting from the top, the signals first reach the ionosphere, where due the existence of free electrons the refractive index is smaller than one.

The effect is that the group velocity of the signals is lowered compared to the vacuum but the phase velocity is enhanced. The concentration of free electrons increases with lower altitudes, where the highest concentration is around km above the earth surface, and then decreases again.

The corresponding gradient of the refractive index causes a curvature of the propagation path as shown in the figure as dotted line. Between 50 and km above ground the free electrons disappear and the signals start to propagate through the neutral atmosphere which is composed of different atmospheric gases. With lower altitudes the air pressure and density of the atmospheric gases increase and the refractive index grows accordingly to values larger than 1. The gradient of the refractive index again causes a curvature of the propagation path, which however is in the opposite direction as in the ionosphere.

Here both the group velocity and the carrier velocity are slowed down compared to the vacuum speed of light. The lower part of the neutral atmosphere, the troposphere extends from 0 to about 8 km at the poles, from 0 to about 13 km in mid latitudes and from 0 up to 18 km at the equator. It has the main contribution to the signal delay because it is the densest layer and contains most of the air mass and practically the total amount of water vapor of the atmosphere.

Therefore, in the navigation community the delay in the neutral atmosphere is usually just called troposphere delay. However, due to the high temporal and local variability of the water vapor, it is the most crucial component for modeling and error correction.

The troposphere is also the region where the daily weather phenomena take place. In the lower troposphere additionally clouds, rain and fog exist. However, their influence on the amplitude and propagation velocity of the signals is negligible in L-band, where current and planned satellite navigation systems operate. Figure 1: Structure of the atmosphere and influence on propagation path image credit: DLR.

Therefore, the ionosphere state is coupled to the 11 year cycle of solar activity, to day and night time and to the season. The relevant layer which contains most of these particles extends between 80 and km above the earth surface. The key parameter for navigation is the TEC, which is the integral of the total electron content in a column of 1m 2 from the observation point to the satellite.

Table 2 shows typical values of the ionosphere excess path dependent on the carrier frequency for low and high TEC values. The vertical excess path lies normally between these values. Table 2: Ionosphere excess path. Multipath: Signals which are reflected by the ground, buildings or other obstacles and superpose with the direct signal at the user antenna disturb the correlation function and reduce the ranging accuracy.

The error depends on the amplitude, phase and the delay of the echo relative to the line-of-sight signal. These systems derive the ionospheric error in a grid from dual frequency measurements in a regional ground station network and transmit differential corrections and integrity data via geostationary satellites.

In satellite navigation the modeling and correction of the additional delay due to the propagation through the atmosphere and errors due to further propagation phenomena play an important role for the accuracy of the derived position solution. The largest additional signal delay occurs within the ionosphere. Because the ionosphere delay depends on frequency, it can be precisely determined and nearly completed eliminated by dual frequency measurements.

However, for single frequency receivers, like most commercially available mass market GPS-receivers, the ionosphere delay must be corrected by modeling, and a significant error can remain.

The troposphere delay can be separated in a wet component due to water vapor and a dry component due to other atmospheric gases. This may change for next generation systems in the future, if additionally higher frequency bands are utilized, e.

C-Band, which is already allocated for Galileo. However, also in L-band, occasionally, fast amplitude and phase scintillations can occur due to fast variations of the total electron content in the ionosphere or tropospheric turbulences.

Strong scintillations occur only rarely, but then they are critical and can even lead to complete loss of the navigation signals by the receiver. Due to the extreme low signal power of the satellite navigation signals when arriving at the Earth, the signals can be easily attenuated and shadowed by buildings or vegetation, e. In these environments, additionally, multipath propagation by reflections of the signals by the ground, buildings and other obstacles before they arrive at the user antenna can significantly degrade the ranging and positioning accuracy.

Multipath propagation is difficult to correct by models, because it depends strongly on the local user environment. Different techniques exist to mitigate the effect of multipath signals as far as possible either by advanced signal processing in receiver or by specially designed antennas, but multipath mitigation is still a hot topic of research. Status reports about the varies SBAS implementations.

Augmented satnav teams work together for safer flying. March All satellite navigation is not created equal. These signals are generated by regional systems based around the globe.

The teams responsible for them met recently at Delhi in India to coordinate their future development. For more than 20 years, the IWG has been coordinating the introduction of satellite-based augmentation systems for satnav — today there are 10 either in operation or in active development. For this meeting of the group, SBAS developers and operators were joined for the first time by users of the systems, with representatives of airlines, aircraft makers and avionics manufacturers.

They operate by using additional ground stations to enhance the accuracy and reliability of existing satnav services across given geographic regions. Signal error measurements and integrity warnings highlighting reliability problems are then relayed to geostationary satellites for rebroadcast to users — messages available to any suitably equipped receiver.

The first of these was the U. July WAAS is a satellite technology that has become essential in making air travel safer and more efficient for both private and commercial travelers.

WAAS represents an enormous leap forward in air navigation. That means that pilots can use WAAS alone to navigate and land, in most instances. It is important that pilots have as accurate information as possible when landing; WAAS collects, processes, and corrects the GPS information to ensure that the data the pilot receives can be trusted. Pilots fly into airports with the guidance of either ground-based navigational aids such as Instrument Landing Systems ILS or or satellite-based navigation, i.

For the past 60 years, the Category-1 Instrument Landing System ILS has been used at airports throughout the National Airspace System to guide aircraft to as low as feet above the runway surface. WAAS now provides similar capability, but at many more runway ends, without airport infrastructure requirements.

This is significant because the number of approach procedures based on WAAS has now exceeded twice the number of approach procedures based on its ground-based predecessor, the Category-1 ILS. The number of WAAS procedures will continue to grow. Additionally, safety is improved as more aircraft are provided with vertically-guided approaches to more runway ends. This also improves flight planning options. The way pilots navigate and aviation in general has definitely been transformed.

By virtue of its extensive coverage area, WAAS can provide vertically-guided approach capability at thousands of airports and airstrips where this capability had previously not been available.

It is also a core element in transitioning to the satellite-based air traffic control system of the future. WAAS is designed to improve the accuracy and ensure the integrity of positioning and timing information from GPS satellites.

Although the WAAS was designed for aviation users, it supports a wide variety of non-aviation uses including agriculture, surveying, recreation, and surface transportation—just to name a few.

These stations are located throughout the continental U. WAAS hardware consists of: 38 ground reference stations, 3 master stations, 3 geostationary satellites with navigation transponders onboard, 6 uplink stations, 2 operational control centers, and the WAAS terrestrial communications network. The U. CORS is also being modernized to support real-time users. Additional real-time sites are contributed by a variety of U. The result is the world's largest real-time GPS tracking network, with more than global sites.

The GDGPS network is highly redundant, by design, to provide a unique measure of reliability to the many critical applications that depend on it, such as real-time GPS integrity monitoring, and precise differential corrections. On average, the network is fold redundant for GPS meaning that at any given time each GPS satellite is observed, on average, by 25 ground sites , and is minimally fold redundant.


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