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Ionospheric Delay
The error analysis for the Global Positioning System is important for understanding how GPS works, and for knowing what magnitude of error should be expected. The GPS makes corrections for receiver clock errors and other effects but there are still residual errors which are not corrected. GPS receiver position is computed based on data received from the satellites. Errors depend on geometric dilution of precision and the sources listed in the table below. Overview User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, \ \sigma_ , of about . The standard deviations, \ \sigma_R, for the coarse/acquisition (C/A) and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e., RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
GPS Satellite NASA Art-iif
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. It does not require the user to transmit any data, and operates independently of any telephonic or Internet reception, though these technologies can enhance the usefulness of the GPS positioning information. It provides critical positioning capabilities to military, civil, and commercial users around the world. Although the United States government created, controls and maintains the GPS system, it is freely accessible to anyone with a GPS receiver. The GPS project was started by the U.S. Department of Defense in 1973. The first prototype spacecraft was ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Satellite Based Augmentation System
Augmentation of a global navigation satellite system (GNSS) is a method of improving the navigation system's attributes, such as precision, 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. Satellite-based augmentation system Satellite-based augmentation systems (SBAS) support wide-area or regional augmentation through the use of additional satellite-broadcast messages. Using measurements from the ground stations, correction messages are created and sent to one or more satell ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Clock Drift
Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to drift, causing eventual divergence unless resynchronized. In particular, the drift of crystal-based clocks used in computers requires some synchronization mechanism for any high-speed communication. Computer clock drift can be utilized to build random number generators. These can however be exploited by timing attacks. In non-atomic clocks Everyday clocks such as wristwatches have finite precision. Eventually they require correction to remain accurate. The rate of drift depends on the clock's quality, sometimes the stability of the power source, the ambient temperature, and other subtle environmental variables. Thus the same clock can have different drift rates at different occasions. More advanced clocks and old mechanical cloc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Assisted GPS
Assisted GNSS (A-GNSS) is a GNSS augmentation system that often significantly improves the startup performance—i.e., time-to-first-fix (TTFF)—of a global navigation satellite system (GNSS). A-GNSS works by providing the necessary data to the device via a radio network instead of the slow satellite link, essentially "warming up" the receiver for a fix. When applied to GPS, it is known as assisted GPS or augmented GPS (abbreviated generally as A-GPS and less commonly as aGPS). Other local names include A-GANSS for Galileo and A-Beidou for BeiDou. A-GPS is extensively used with GPS-capable cellular phones, as its development was accelerated by the U.S. FCC's 911 requirement to make cell phone location data available to emergency call dispatchers. Background Every GPS device requires orbital data about the satellites to calculate its position. The data rate of the satellite signal is only 50 bit/s, so downloading orbital information like ephemerides and the almanac directl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ephemeris
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly velocity) over time. Historically, positions were given as printed tables of values, given at regular intervals of date and time. The calculation of these tables was one of the first applications of mechanical computers. Modern ephemerides are often provided in electronic form. However, printed ephemerides are still produced, as they are useful when computational devices are not available. The astronomical position calculated from an ephemeris is often given in the spherical polar coordinate system of right ascension and declination, together with the distance from the origin if applicable. Some of the astronomical phenomena of interest to astronomers are eclipses, apparent retrograde motion/planetary stations, planetary es, sidereal t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Choke Ring Antenna
A choke ring antenna is a particular form of omnidirectional antenna for use at high frequencies (VHF & UHF). It consists of a number of concentric conductive cylinders around a central antenna. Due to its intricate construction, it is often enclosed in a protective cover or radome when placed outside and exposed to the elements. Benefits Choke ring antennas are notable for their ability to reject multipath signals from a source. Since the path that a signal takes from a transmitter to receiver can be used to measure the distance between the two, this makes it highly suited for GPS and radar applications. In a GPS ground-based receiver, a choke ring antenna can provide millimeter precision measurements for use in surveying and geological measurements. The choke ring design was originally developed at the Jet Propulsion Laboratory. - A basic description of choke ring antenna benefits and limitations. See also * Trimble Inc. - one manufacturer of this product. * Patch antenn ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Multipath Interference
In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. When the same signal is received over more than one path, it can create interference and phase shifting of the signal. Destructive interference causes fading; this may cause a radio signal to become too weak in certain areas to be received adequately. For this reason, this effect is also known as multipath interference or multipath distortion. Where the magnitudes of the signals arriving by the various paths have a distribution known as the Rayleigh distribution, this is known as Rayleigh fading. Where one component (often, but not necessarily, a line of sight component) dominates, a Rician distribution provides a more accurate model, and this is known ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Atmospheric Pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 760mm Hg, 29.9212 inchesHg, or 14.696 psi.International Civil Aviation Organization. ''Manual of the ICAO Standard Atmosphere'', Doc 7488-CD, Third Edition, 1993. . The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm. In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius—especially the dense atmospheric layer at low altitudes—the Earth's ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From the planetary surface of the Earth, the average height of the troposphere is in the tropics; in the middle latitudes; and in the high latitudes of the polar regions in winter; thus the average height of the troposphere is . The term ''troposphere'' derives from the Greek words ''tropos'' (rotating) and '' sphaira'' (sphere) indicating that rotational turbulence mixes the layers of air and so determines the structure and the phenomena of the troposphere. The rotational friction of the troposphere against the planetary surface affects the flow of the air, and so forms the planetary boundary layer (PBL) that varies in height from hundreds of meters up to . The measures of the PBL vary according to the latitude, the landform, and th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 g of water per cubic metre of air at , but only 8 g of water per cubic metre of air at . Three primary measurements of humidity are widely employed: absolute, relative, and specific. A ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
GPS Aided Geo Augmented Navigation
The GPS-aided GEO augmented navigation (GAGAN) is an implementation of a regional satellite-based augmentation system (SBAS) by the Government of India. It is a system to improve the accuracy of a GNSS receiver by providing reference signals. The Airports Authority of India (AAI)'s efforts towards implementation of operational SBAS can be viewed as the first step towards introduction of modern communication, navigation and surveillance / air traffic management system over the Indian airspace.GAGAN Update Dr. Arjin Singh, Additional GM, Directorate of Global Navigation System, Airport Authority of India The project has established fifteen Indian reference stations, three Indian navigation land uplink stations ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
