\"The RTC counts milliseconds since the GPS Epoch which is Jan 6, 1980, and increments on each count so that each second in GPS time has a unique number. This is different from Unix time which starts Jan 1, 1970 and has a fixed number of seconds per day so for days that have an extra second in them (leap second) Unix uses the same second twice. This means that over the years an ever increasing number of leap seconds have to be accounted for when converting time between these domains.\"
\n\n\"Ionospheric time delay corrections based on the extended single layer model over low latitude region\"
\nAuthors: Sahithi Karanam, D.Venkata Ratnam, J.R.K. Kumar Dabbakuti - https://doi.org/10.1016/j.geog.2019.02.002
\n\"Ionospheric delay error is considered to be one of the most prominent factors impacting the Global Navigation Satellite Systems (GNSS) positioning and navigation accuracies. Due to dispersive nature and anisotropic of the ionosphere above certain regions, the positioning accuracy is seriously affected when using a precision-limited model.\"
\nhttps://www.sciencedirect.com/science/article/pii/S1674984718300442
\n\"Tropospheric Effects on GNSS\"
\nAuthor: Dr. M. Bakry El-Arini https://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf
\n\"The troposphere contains about 80% of the atmosphere. It is a few kilometers above the Earth’s surface, In this layer, the average temperature decreases with height (e.g., -5ºC to -7ºC/km).\"
\nTropospheric Delay
\n\"Signal received by GNSS satellite is refracted by the atmosphere as travels to the user on or near the Earth’s
\nsurface. The atmospheric refraction causes a delay, depends on: Actual path of the curved ray and Refractive index of the gases along that path.\"
\"For a homogenous (or symmetric) atmosphere around the user antenna, the delay depends only on the vertical profile of the atmosphere and the elevation angle.\"
\nhttps://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf
\n\"What's The Differences Between the 5 GNSS Constellations?\" By Posted by Rob Rutkowski on Jun 5, 2019
\n\"Many people get GNSS and GPS technology confused. A good way to think about the Global Navigation Satellite Systems (GNSS) is as the backbone (or underlying technology) behind GPS. The Global Positioning System (GPS) GPS is a GNSS constellation, but GNSS is not always GPS. GPS one of the 5 GNSS constellations used around the world.\"
\n\"The 5 GNSS constellations include GPS (US), QZSS (Japan), BEIDOU (China), GALILEO (EU), and GLONASS (Russia).\"
\n\"The main reason for all 5 satellite constellations is availability and redundancy. If one system fails, another GNSS constellation can help take over.\"
\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations
\n\"The GPS SV enable/disable record is used to enable or disable a selection of the 32 GPS satellites. By default, the receiver is configured to use all satellites that are in good health. This record is useful for enabling satellites that are not in good health.\"
\nhttps://www.trimble.com/OEM_ReceiverHelp/v5.11/en/ICD_Command64h_AppFile_SVEnableDisable.html
\nSv Time
\n\"SV time. The time of transmission from a particular space vehicle is known as SV time. SV time at the receiver must then be corrected for the errors in the SV clock with respect to GPS time and for periodic relativistic effects. The SV clock error is transmitted in each data frame by a set of polynomial coefficients. The relativistic correction is computed from the SV orbital parameters normally used for SV position determination. The true GPS time of transmission is the result.\"
\nhttps://ilrs.gsfc.nasa.gov/docs/timing/gpsrole.pdf
\nSatellite-based positioning, By R. Knippers.
\n\"The GPS-receiver computes the distances (ranges) to the satellites. It receives GPS-codes and Carrier waves from the satellite. The GPS-receiver measures in fact pseudo distances (pseudo-ranges) to the satellites. To determine a position in a 3 dimensional space it takes in theory 3 distance measurements from 3 satellites.\"
\n\"Accurate positioning requires an extra distance measurement from a fourth satellite to eliminate the receiver clock error.\"
\n\"Distance = (velocity of light) x (travel time)\"
\n\"Pseudo-range = (velocity of light) x (travel time) + (receiver clock error) + (other errors)\"
\n\n\"The user range error (URE) of the GPS signals in space is actually the same for the civilian and military GPS services. However, most of today's civilian devices use only one GPS frequency, while military receivers use two.\"
\n\"Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications.\"
\n\"With augmentation systems, civilian users can actually receive better GPS accuracy than the military.\"
\nhttps://www.gps.gov/systems/gps/performance/accuracy/
Measurement of position from four satellites
\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/05
\n\"Positional accuracy with a single receiver, for civilian use, approximately equals 2 to 5 m horizontally and height accuracy is generally 5 to 10 m, for 95% of the time. The positional accuracy is affected by GPS satellite orbit errors, the atmosphere and receiver clock errors. To give better accuracy the known errors must be accounted for. The GPS satellite orbit errors and other errors introduced into the signal travel time due to it travelling through the atmosphere, cannot be computed by a single receiver in real time. The real-time positional accuracy of a single receiver can be greatly improved by using a more accurate technique known as Differential GPS (dGPS).\"
\nhttp://earthwise.bgs.ac.uk/index.php/OR/15/057_Principles_of_GPS
\nGPS
\n\" GPS is the pioneer in the world of GNSS. It's the oldest GNSS system that began operation in 1978 and was made available for global use in 1994.\"
\n\"GPS operates in a frequency band referred to as the L-Band, a portion of the radio spectrum between 1 and 2 GHz. L-Band was chosen for several reasons, including: Ionospheric delay is more significant at lower frequencies, Simplification of antenna design and minimize the effect that weather has on GPS signal propagation.\"
\nQZSS
\n\"The Quasi-Zenith Satellite System (QZSS) is the regional satellite system from Japan and is sometimes referred to as the \"Japanese GPS\". A great benefit of QZSS is this it's compatible with GPS. This ensures a sufficient number of satellites for stable, high-precision positioning.\"
\nBEIDOU
\n\"BEIDOU is a Chinese satellite navigation system that consists of two separate satellite constellations, BeiDou-1 and BeiDou-2 (and soon-to-be BeiDou-3). Once fully launched and operational, BeiDou-3 will provide an alternative to U.S GPS, GLONASS, or GALILEO. BeiDou-3 is expected to be even more accurate with millimeter-level accuracy (with post-processing).\"
\nGALILEO
\n\"GALILEO is Europe's GNSS system that's compatible with GPS and GLONASS. It started providing service in December 2016.
\nGALILEO's receivers track the satellite constellation's position in what's called the \"GALILEO Reference System\" using satellite technology and triangulation principles. The Galileo system is divided into three main segments: Space, Ground
\nand User\"
GLONASS
\n\"Finally, GLONASS is Russia's version of GPS. Development began in 1976 by the Soviet Union. There are 5 versions of GLONASS including: GLONASS (1982), GLONASS-M (2003), GLONASS-K (2011), GLONASS-K2 (2015), GLONASS-KM (2025 - Currently in research phase).
\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations
\nPoor/lack of Satellite Signal, leading to Loss of Accuracy.
\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/057.
\n\"The great disadvantage of GPS is that as the satellite signal is quite weak, line of sight from the receiver to the satellite is essential. Therefore GPS cannot be used indoors or in areas where a clear view of the sky is not possible such as in forests, next to tall buildings or in deep valleys. Great care should always be taken in positioning GPS receivers and when using twin receivers ensuring each receiver can track the same satellites\"
\n\"The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS.\"
\n\"Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is \"mean sea level,\" which is supposed to roughly approximate the equipotential surface, but obviously can't be directly measured inland.\" By Rob Skelly
\n\"An ellispoid is a mathematical model of the earth that approximates its three dimensional shape. See this definition. Elevation on top of the ellipsoid is 0, but since it's just an approximation one can be above or below the ellipsoid at any given point. \"Elevation above the surface of the ellipsoid\" is the distance between the measurement and the 0 value of the ellipsoid.\"
\n'The Z value in a given coordinate system has to be based on something--a height above a generalized shape of the earth. MSL is one way to do it, but in my experience the majority of cases use ellipsoids as approximate figures. GPS, for example, uses WGS84 as the global coordinate system, and with it is the WGS84 ellipsoid.\" By Wes
\n\nAuthor: By Witold Fraczek, Esri Applications Prototype Lab
\n\"The accuracy of GPS height measurements depends on several factors but the most crucial one is the \"imperfection\" of the earth's shape. Height can be measured in two ways. The GPS uses height (h) above the reference ellipsoid that approximates the earth's surface. The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL. The signed difference between the two heights—the difference between the ellipsoid and geoid—is the geoid height (N).
\nThe figure bellow shows the relationships between the different models and explains the reasons why the two hardly ever match spatially.
\nhttps://www.esri.com/news/arcuser/0703/geoid1of3.html
", "messages": [ { "id": 1310787, "postDate": "2021-05-17T00:55:46.050Z", "content": "\"The RTC counts milliseconds since the GPS Epoch which is Jan 6, 1980, and increments on each count so that each second in GPS time has a unique number. This is different from Unix time which starts Jan 1, 1970 and has a fixed number of seconds per day so for days that have an extra second in them (leap second) Unix uses the same second twice. This means that over the years an ever increasing number of leap seconds have to be accounted for when converting time between these domains.\"
\n\n\"Ionospheric time delay corrections based on the extended single layer model over low latitude region\"
\nAuthors: Sahithi Karanam, D.Venkata Ratnam, J.R.K. Kumar Dabbakuti - https://doi.org/10.1016/j.geog.2019.02.002
\n\"Ionospheric delay error is considered to be one of the most prominent factors impacting the Global Navigation Satellite Systems (GNSS) positioning and navigation accuracies. Due to dispersive nature and anisotropic of the ionosphere above certain regions, the positioning accuracy is seriously affected when using a precision-limited model.\"
\nhttps://www.sciencedirect.com/science/article/pii/S1674984718300442
\n\"Tropospheric Effects on GNSS\"
\nAuthor: Dr. M. Bakry El-Arini https://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf
\n\"The troposphere contains about 80% of the atmosphere. It is a few kilometers above the Earth’s surface, In this layer, the average temperature decreases with height (e.g., -5ºC to -7ºC/km).\"
\nTropospheric Delay
\n\"Signal received by GNSS satellite is refracted by the atmosphere as travels to the user on or near the Earth’s
\nsurface. The atmospheric refraction causes a delay, depends on: Actual path of the curved ray and Refractive index of the gases along that path.\"
\"For a homogenous (or symmetric) atmosphere around the user antenna, the delay depends only on the vertical profile of the atmosphere and the elevation angle.\"
\nhttps://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf
\n\"What's The Differences Between the 5 GNSS Constellations?\" By Posted by Rob Rutkowski on Jun 5, 2019
\n\"Many people get GNSS and GPS technology confused. A good way to think about the Global Navigation Satellite Systems (GNSS) is as the backbone (or underlying technology) behind GPS. The Global Positioning System (GPS) GPS is a GNSS constellation, but GNSS is not always GPS. GPS one of the 5 GNSS constellations used around the world.\"
\n\"The 5 GNSS constellations include GPS (US), QZSS (Japan), BEIDOU (China), GALILEO (EU), and GLONASS (Russia).\"
\n\"The main reason for all 5 satellite constellations is availability and redundancy. If one system fails, another GNSS constellation can help take over.\"
\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations
\n\"The GPS SV enable/disable record is used to enable or disable a selection of the 32 GPS satellites. By default, the receiver is configured to use all satellites that are in good health. This record is useful for enabling satellites that are not in good health.\"
\nhttps://www.trimble.com/OEM_ReceiverHelp/v5.11/en/ICD_Command64h_AppFile_SVEnableDisable.html
\nSv Time
\n\"SV time. The time of transmission from a particular space vehicle is known as SV time. SV time at the receiver must then be corrected for the errors in the SV clock with respect to GPS time and for periodic relativistic effects. The SV clock error is transmitted in each data frame by a set of polynomial coefficients. The relativistic correction is computed from the SV orbital parameters normally used for SV position determination. The true GPS time of transmission is the result.\"
\nhttps://ilrs.gsfc.nasa.gov/docs/timing/gpsrole.pdf
\nSatellite-based positioning, By R. Knippers.
\n\"The GPS-receiver computes the distances (ranges) to the satellites. It receives GPS-codes and Carrier waves from the satellite. The GPS-receiver measures in fact pseudo distances (pseudo-ranges) to the satellites. To determine a position in a 3 dimensional space it takes in theory 3 distance measurements from 3 satellites.\"
\n\"Accurate positioning requires an extra distance measurement from a fourth satellite to eliminate the receiver clock error.\"
\n\"Distance = (velocity of light) x (travel time)\"
\n\"Pseudo-range = (velocity of light) x (travel time) + (receiver clock error) + (other errors)\"
\n\n\"The user range error (URE) of the GPS signals in space is actually the same for the civilian and military GPS services. However, most of today's civilian devices use only one GPS frequency, while military receivers use two.\"
\n\"Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications.\"
\n\"With augmentation systems, civilian users can actually receive better GPS accuracy than the military.\"
\nhttps://www.gps.gov/systems/gps/performance/accuracy/
Measurement of position from four satellites
\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/05
\n\"Positional accuracy with a single receiver, for civilian use, approximately equals 2 to 5 m horizontally and height accuracy is generally 5 to 10 m, for 95% of the time. The positional accuracy is affected by GPS satellite orbit errors, the atmosphere and receiver clock errors. To give better accuracy the known errors must be accounted for. The GPS satellite orbit errors and other errors introduced into the signal travel time due to it travelling through the atmosphere, cannot be computed by a single receiver in real time. The real-time positional accuracy of a single receiver can be greatly improved by using a more accurate technique known as Differential GPS (dGPS).\"
\nhttp://earthwise.bgs.ac.uk/index.php/OR/15/057_Principles_of_GPS
\nGPS
\n\" GPS is the pioneer in the world of GNSS. It's the oldest GNSS system that began operation in 1978 and was made available for global use in 1994.\"
\n\"GPS operates in a frequency band referred to as the L-Band, a portion of the radio spectrum between 1 and 2 GHz. L-Band was chosen for several reasons, including: Ionospheric delay is more significant at lower frequencies, Simplification of antenna design and minimize the effect that weather has on GPS signal propagation.\"
\nQZSS
\n\"The Quasi-Zenith Satellite System (QZSS) is the regional satellite system from Japan and is sometimes referred to as the \"Japanese GPS\". A great benefit of QZSS is this it's compatible with GPS. This ensures a sufficient number of satellites for stable, high-precision positioning.\"
\nBEIDOU
\n\"BEIDOU is a Chinese satellite navigation system that consists of two separate satellite constellations, BeiDou-1 and BeiDou-2 (and soon-to-be BeiDou-3). Once fully launched and operational, BeiDou-3 will provide an alternative to U.S GPS, GLONASS, or GALILEO. BeiDou-3 is expected to be even more accurate with millimeter-level accuracy (with post-processing).\"
\nGALILEO
\n\"GALILEO is Europe's GNSS system that's compatible with GPS and GLONASS. It started providing service in December 2016.
\nGALILEO's receivers track the satellite constellation's position in what's called the \"GALILEO Reference System\" using satellite technology and triangulation principles. The Galileo system is divided into three main segments: Space, Ground
\nand User\"
GLONASS
\n\"Finally, GLONASS is Russia's version of GPS. Development began in 1976 by the Soviet Union. There are 5 versions of GLONASS including: GLONASS (1982), GLONASS-M (2003), GLONASS-K (2011), GLONASS-K2 (2015), GLONASS-KM (2025 - Currently in research phase).
\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations
\nPoor/lack of Satellite Signal, leading to Loss of Accuracy.
\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/057.
\n\"The great disadvantage of GPS is that as the satellite signal is quite weak, line of sight from the receiver to the satellite is essential. Therefore GPS cannot be used indoors or in areas where a clear view of the sky is not possible such as in forests, next to tall buildings or in deep valleys. Great care should always be taken in positioning GPS receivers and when using twin receivers ensuring each receiver can track the same satellites\"
\n\"The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS.\"
\n\"Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is \"mean sea level,\" which is supposed to roughly approximate the equipotential surface, but obviously can't be directly measured inland.\" By Rob Skelly
\n\"An ellispoid is a mathematical model of the earth that approximates its three dimensional shape. See this definition. Elevation on top of the ellipsoid is 0, but since it's just an approximation one can be above or below the ellipsoid at any given point. \"Elevation above the surface of the ellipsoid\" is the distance between the measurement and the 0 value of the ellipsoid.\"
\n'The Z value in a given coordinate system has to be based on something--a height above a generalized shape of the earth. MSL is one way to do it, but in my experience the majority of cases use ellipsoids as approximate figures. GPS, for example, uses WGS84 as the global coordinate system, and with it is the WGS84 ellipsoid.\" By Wes
\n\nAuthor: By Witold Fraczek, Esri Applications Prototype Lab
\n\"The accuracy of GPS height measurements depends on several factors but the most crucial one is the \"imperfection\" of the earth's shape. Height can be measured in two ways. The GPS uses height (h) above the reference ellipsoid that approximates the earth's surface. The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL. The signed difference between the two heights—the difference between the ellipsoid and geoid—is the geoid height (N).
\nThe figure bellow shows the relationships between the different models and explains the reasons why the two hardly ever match spatially.
\n", "rawMarkdown": "# Features in Google Smartphone Decimeter Challenge\n\n# Feature millisSinceGpsEpoch\n\n\"The RTC counts milliseconds since the GPS Epoch which is Jan 6, 1980, and increments on each count so that each second in GPS time has a unique number. This is different from Unix time which starts Jan 1, 1970 and has a fixed number of seconds per day so for days that have an extra second in them (leap second) Unix uses the same second twice. This means that over the years an ever increasing number of leap seconds have to be accounted for when converting time between these domains.\"\n\nhttps://docs.legato.io/15_08/c_rtc.html#:~:text=The%20RTC%20counts%20milliseconds%20since,time%20has%20a%20unique%20number.&text=This%20means%20that%20over%20the,converting%20time%20between%20these%20domains.\n\n# Feature ionoDelayM \n\n\n\"Ionospheric time delay corrections based on the extended single layer model over low latitude region\"\n\nAuthors: Sahithi Karanam, D.Venkata Ratnam, J.R.K. Kumar Dabbakuti - https://doi.org/10.1016/j.geog.2019.02.002\n\n\"Ionospheric delay error is considered to be one of the most prominent factors impacting the Global Navigation Satellite Systems (GNSS) positioning and navigation accuracies. Due to dispersive nature and anisotropic of the ionosphere above certain regions, the positioning accuracy is seriously affected when using a precision-limited model.\"\n\nhttps://www.sciencedirect.com/science/article/pii/S1674984718300442\n\n# Feature tropoDelayM \n\n\"Tropospheric Effects on GNSS\"\n\nAuthor: Dr. M. Bakry El-Arini https://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf\n\n\"The troposphere contains about 80% of the atmosphere. It is a few kilometers above the Earth’s surface, In this layer, the average temperature decreases with height (e.g., -5ºC to -7ºC/km).\"\n\n\nTropospheric Delay\n\n\"Signal received by GNSS satellite is refracted by the atmosphere as travels to the user on or near the Earth’s \nsurface. The atmospheric refraction causes a delay, depends on: Actual path of the curved ray and Refractive index of the gases along that path.\"\n\n\"For a homogenous (or symmetric) atmosphere around the user antenna, the delay depends only on the vertical profile of the atmosphere and the elevation angle.\"\n\nhttps://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf\n\n# Feature constellationType\n\n\"What's The Differences Between the 5 GNSS Constellations?\" By Posted by Rob Rutkowski on Jun 5, 2019\n\n\"Many people get GNSS and GPS technology confused. A good way to think about the Global Navigation Satellite Systems (GNSS) is as the backbone (or underlying technology) behind GPS. The Global Positioning System (GPS) GPS is a GNSS constellation, but GNSS is not always GPS. GPS one of the 5 GNSS constellations used around the world.\"\n\n\"The 5 GNSS constellations include GPS (US), QZSS (Japan), BEIDOU (China), GALILEO (EU), and GLONASS (Russia).\"\n\n\"The main reason for all 5 satellite constellations is availability and redundancy. If one system fails, another GNSS constellation can help take over.\" \n\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations\n\n# Feature receivedSvTimeInGpsNanos \n\n\"The GPS SV enable/disable record is used to enable or disable a selection of the 32 GPS satellites. By default, the receiver is configured to use all satellites that are in good health. This record is useful for enabling satellites that are not in good health.\"\n\nhttps://www.trimble.com/OEM_ReceiverHelp/v5.11/en/ICD_Command64h_AppFile_SVEnableDisable.html\n\nSv Time\n\n\"SV time. The time of transmission from a particular space vehicle is known as SV time. SV time at the receiver must then be corrected for the errors in the SV clock with respect to GPS time and for periodic relativistic effects. The SV clock error is transmitted in each data frame by a set of polynomial coefficients. The relativistic correction is computed from the SV orbital parameters normally used for SV position determination. The true GPS time of transmission is the result.\"\n\nhttps://ilrs.gsfc.nasa.gov/docs/timing/gpsrole.pdf\n\n# Principle of positioning\n\nSatellite-based positioning, By R. Knippers.\n\n\"The GPS-receiver computes the distances (ranges) to the satellites. It receives GPS-codes and Carrier waves from the satellite. The GPS-receiver measures in fact pseudo distances (pseudo-ranges) to the satellites. To determine a position in a 3 dimensional space it takes in theory 3 distance measurements from 3 satellites.\"\n\n\"Accurate positioning requires an extra distance measurement from a fourth satellite to eliminate the receiver clock error.\"\n\n\"Distance = (velocity of light) x (travel time)\"\n\n\"Pseudo-range = (velocity of light) x (travel time) + (receiver clock error) + (other errors)\"\n\nhttps://unstats.un.org/unsd/geoinfo/ungegn/docs/_data_ICAcourses/_HtmlModules/_Documents/D06/documents/D06-04_KnippersPPTeaching.pdf\n\n# Is military GPS more accurate than civilian GPS?\n\n\"The user range error (URE) of the GPS signals in space is actually the same for the civilian and military GPS services. However, most of today's civilian devices use only one GPS frequency, while military receivers use two.\"\n\n\"Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications.\"\n\n\"With augmentation systems, civilian users can actually receive better GPS accuracy than the military.\"\nhttps://www.gps.gov/systems/gps/performance/accuracy/\n\nMeasurement of position from four satellites\n\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/05\n\n\"Positional accuracy with a single receiver, for civilian use, approximately equals 2 to 5 m horizontally and height accuracy is generally 5 to 10 m, for 95% of the time. The positional accuracy is affected by GPS satellite orbit errors, the atmosphere and receiver clock errors. To give better accuracy the known errors must be accounted for. The GPS satellite orbit errors and other errors introduced into the signal travel time due to it travelling through the atmosphere, cannot be computed by a single receiver in real time. The real-time positional accuracy of a single receiver can be greatly improved by using a more accurate technique known as Differential GPS (dGPS).\"\n\nhttp://earthwise.bgs.ac.uk/index.php/OR/15/057_Principles_of_GPS\n\n#CONSTELLATIONS\n \nGPS\n\n\" GPS is the pioneer in the world of GNSS. It's the oldest GNSS system that began operation in 1978 and was made available for global use in 1994.\"\n\n\"GPS operates in a frequency band referred to as the L-Band, a portion of the radio spectrum between 1 and 2 GHz. L-Band was chosen for several reasons, including: Ionospheric delay is more significant at lower frequencies, Simplification of antenna design and minimize the effect that weather has on GPS signal propagation.\"\n\nQZSS\n\n\"The Quasi-Zenith Satellite System (QZSS) is the regional satellite system from Japan and is sometimes referred to as the \"Japanese GPS\". A great benefit of QZSS is this it's compatible with GPS. This ensures a sufficient number of satellites for stable, high-precision positioning.\"\n\nBEIDOU\n\n\"BEIDOU is a Chinese satellite navigation system that consists of two separate satellite constellations, BeiDou-1 and BeiDou-2 (and soon-to-be BeiDou-3). Once fully launched and operational, BeiDou-3 will provide an alternative to U.S GPS, GLONASS, or GALILEO. BeiDou-3 is expected to be even more accurate with millimeter-level accuracy (with post-processing).\"\n\nGALILEO\n\n\"GALILEO is Europe's GNSS system that's compatible with GPS and GLONASS. It started providing service in December 2016.\nGALILEO's receivers track the satellite constellation's position in what's called the \"GALILEO Reference System\" using satellite technology and triangulation principles. The Galileo system is divided into three main segments: Space, Ground\nand User\"\n\nGLONASS\n\n\"Finally, GLONASS is Russia's version of GPS. Development began in 1976 by the Soviet Union. There are 5 versions of GLONASS including: GLONASS (1982), GLONASS-M (2003), GLONASS-K (2011), GLONASS-K2 (2015), GLONASS-KM (2025 - Currently in research phase).\n\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations\n\nPoor/lack of Satellite Signal, leading to Loss of Accuracy.\n\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/057.\n\n\"The great disadvantage of GPS is that as the satellite signal is quite weak, line of sight from the receiver to the satellite is essential. Therefore GPS cannot be used indoors or in areas where a clear view of the sky is not possible such as in forests, next to tall buildings or in deep valleys. Great care should always be taken in positioning GPS receivers and when using twin receivers ensuring each receiver can track the same satellites\"\n\n#Feature heightAboveWgs84EllipsoidM \n\n\"The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS.\"\n\n\"Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is \"mean sea level,\" which is supposed to roughly approximate the equipotential surface, but obviously can't be directly measured inland.\" By Rob Skelly\n\n\"An ellispoid is a mathematical model of the earth that approximates its three dimensional shape. See this definition. Elevation on top of the ellipsoid is 0, but since it's just an approximation one can be above or below the ellipsoid at any given point. \"Elevation above the surface of the ellipsoid\" is the distance between the measurement and the 0 value of the ellipsoid.\"\n\n'The Z value in a given coordinate system has to be based on something--a height above a generalized shape of the earth. MSL is one way to do it, but in my experience the majority of cases use ellipsoids as approximate figures. GPS, for example, uses WGS84 as the global coordinate system, and with it is the WGS84 ellipsoid.\" By Wes\n\nhttps://gis.stackexchange.com/questions/157005/meaning-of-elevation-above-surface-of-ellipsoid#:~:text=The%20elevation%20above%20the%20ellipsoid,d%20get%20from%20a%20GPS.\n\n#Mean Sea Level, GPS, and the Geoid\n\nAuthor: By Witold Fraczek, Esri Applications Prototype Lab\n\n\"The accuracy of GPS height measurements depends on several factors but the most crucial one is the \"imperfection\" of the earth's shape. Height can be measured in two ways. The GPS uses height (h) above the reference ellipsoid that approximates the earth's surface. The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL. The signed difference between the two heights—the difference between the ellipsoid and geoid—is the geoid height (N). \n\n\nThe figure bellow shows the relationships between the different models and explains the reasons why the two hardly ever match spatially.\n\nhttps://www.esri.com/news/arcuser/0703/geoid1of3.html\n\n", "votes": 30 } ], "comments": [], "raw_markdown_by_id": { "1310787": "# Features in Google Smartphone Decimeter Challenge\n\n# Feature millisSinceGpsEpoch\n\n\"The RTC counts milliseconds since the GPS Epoch which is Jan 6, 1980, and increments on each count so that each second in GPS time has a unique number. This is different from Unix time which starts Jan 1, 1970 and has a fixed number of seconds per day so for days that have an extra second in them (leap second) Unix uses the same second twice. This means that over the years an ever increasing number of leap seconds have to be accounted for when converting time between these domains.\"\n\nhttps://docs.legato.io/15_08/c_rtc.html#:~:text=The%20RTC%20counts%20milliseconds%20since,time%20has%20a%20unique%20number.&text=This%20means%20that%20over%20the,converting%20time%20between%20these%20domains.\n\n# Feature ionoDelayM \n\n\n\"Ionospheric time delay corrections based on the extended single layer model over low latitude region\"\n\nAuthors: Sahithi Karanam, D.Venkata Ratnam, J.R.K. Kumar Dabbakuti - https://doi.org/10.1016/j.geog.2019.02.002\n\n\"Ionospheric delay error is considered to be one of the most prominent factors impacting the Global Navigation Satellite Systems (GNSS) positioning and navigation accuracies. Due to dispersive nature and anisotropic of the ionosphere above certain regions, the positioning accuracy is seriously affected when using a precision-limited model.\"\n\nhttps://www.sciencedirect.com/science/article/pii/S1674984718300442\n\n# Feature tropoDelayM \n\n\"Tropospheric Effects on GNSS\"\n\nAuthor: Dr. M. Bakry El-Arini https://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf\n\n\"The troposphere contains about 80% of the atmosphere. It is a few kilometers above the Earth’s surface, In this layer, the average temperature decreases with height (e.g., -5ºC to -7ºC/km).\"\n\n\nTropospheric Delay\n\n\"Signal received by GNSS satellite is refracted by the atmosphere as travels to the user on or near the Earth’s \nsurface. The atmospheric refraction causes a delay, depends on: Actual path of the curved ray and Refractive index of the gases along that path.\"\n\n\"For a homogenous (or symmetric) atmosphere around the user antenna, the delay depends only on the vertical profile of the atmosphere and the elevation angle.\"\n\nhttps://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/Tropospheric%20Effect%20on%20GNSS.pdf\n\n# Feature constellationType\n\n\"What's The Differences Between the 5 GNSS Constellations?\" By Posted by Rob Rutkowski on Jun 5, 2019\n\n\"Many people get GNSS and GPS technology confused. A good way to think about the Global Navigation Satellite Systems (GNSS) is as the backbone (or underlying technology) behind GPS. The Global Positioning System (GPS) GPS is a GNSS constellation, but GNSS is not always GPS. GPS one of the 5 GNSS constellations used around the world.\"\n\n\"The 5 GNSS constellations include GPS (US), QZSS (Japan), BEIDOU (China), GALILEO (EU), and GLONASS (Russia).\"\n\n\"The main reason for all 5 satellite constellations is availability and redundancy. If one system fails, another GNSS constellation can help take over.\" \n\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations\n\n# Feature receivedSvTimeInGpsNanos \n\n\"The GPS SV enable/disable record is used to enable or disable a selection of the 32 GPS satellites. By default, the receiver is configured to use all satellites that are in good health. This record is useful for enabling satellites that are not in good health.\"\n\nhttps://www.trimble.com/OEM_ReceiverHelp/v5.11/en/ICD_Command64h_AppFile_SVEnableDisable.html\n\nSv Time\n\n\"SV time. The time of transmission from a particular space vehicle is known as SV time. SV time at the receiver must then be corrected for the errors in the SV clock with respect to GPS time and for periodic relativistic effects. The SV clock error is transmitted in each data frame by a set of polynomial coefficients. The relativistic correction is computed from the SV orbital parameters normally used for SV position determination. The true GPS time of transmission is the result.\"\n\nhttps://ilrs.gsfc.nasa.gov/docs/timing/gpsrole.pdf\n\n# Principle of positioning\n\nSatellite-based positioning, By R. Knippers.\n\n\"The GPS-receiver computes the distances (ranges) to the satellites. It receives GPS-codes and Carrier waves from the satellite. The GPS-receiver measures in fact pseudo distances (pseudo-ranges) to the satellites. To determine a position in a 3 dimensional space it takes in theory 3 distance measurements from 3 satellites.\"\n\n\"Accurate positioning requires an extra distance measurement from a fourth satellite to eliminate the receiver clock error.\"\n\n\"Distance = (velocity of light) x (travel time)\"\n\n\"Pseudo-range = (velocity of light) x (travel time) + (receiver clock error) + (other errors)\"\n\nhttps://unstats.un.org/unsd/geoinfo/ungegn/docs/_data_ICAcourses/_HtmlModules/_Documents/D06/documents/D06-04_KnippersPPTeaching.pdf\n\n# Is military GPS more accurate than civilian GPS?\n\n\"The user range error (URE) of the GPS signals in space is actually the same for the civilian and military GPS services. However, most of today's civilian devices use only one GPS frequency, while military receivers use two.\"\n\n\"Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications.\"\n\n\"With augmentation systems, civilian users can actually receive better GPS accuracy than the military.\"\nhttps://www.gps.gov/systems/gps/performance/accuracy/\n\nMeasurement of position from four satellites\n\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/05\n\n\"Positional accuracy with a single receiver, for civilian use, approximately equals 2 to 5 m horizontally and height accuracy is generally 5 to 10 m, for 95% of the time. The positional accuracy is affected by GPS satellite orbit errors, the atmosphere and receiver clock errors. To give better accuracy the known errors must be accounted for. The GPS satellite orbit errors and other errors introduced into the signal travel time due to it travelling through the atmosphere, cannot be computed by a single receiver in real time. The real-time positional accuracy of a single receiver can be greatly improved by using a more accurate technique known as Differential GPS (dGPS).\"\n\nhttp://earthwise.bgs.ac.uk/index.php/OR/15/057_Principles_of_GPS\n\n#CONSTELLATIONS\n \nGPS\n\n\" GPS is the pioneer in the world of GNSS. It's the oldest GNSS system that began operation in 1978 and was made available for global use in 1994.\"\n\n\"GPS operates in a frequency band referred to as the L-Band, a portion of the radio spectrum between 1 and 2 GHz. L-Band was chosen for several reasons, including: Ionospheric delay is more significant at lower frequencies, Simplification of antenna design and minimize the effect that weather has on GPS signal propagation.\"\n\nQZSS\n\n\"The Quasi-Zenith Satellite System (QZSS) is the regional satellite system from Japan and is sometimes referred to as the \"Japanese GPS\". A great benefit of QZSS is this it's compatible with GPS. This ensures a sufficient number of satellites for stable, high-precision positioning.\"\n\nBEIDOU\n\n\"BEIDOU is a Chinese satellite navigation system that consists of two separate satellite constellations, BeiDou-1 and BeiDou-2 (and soon-to-be BeiDou-3). Once fully launched and operational, BeiDou-3 will provide an alternative to U.S GPS, GLONASS, or GALILEO. BeiDou-3 is expected to be even more accurate with millimeter-level accuracy (with post-processing).\"\n\nGALILEO\n\n\"GALILEO is Europe's GNSS system that's compatible with GPS and GLONASS. It started providing service in December 2016.\nGALILEO's receivers track the satellite constellation's position in what's called the \"GALILEO Reference System\" using satellite technology and triangulation principles. The Galileo system is divided into three main segments: Space, Ground\nand User\"\n\nGLONASS\n\n\"Finally, GLONASS is Russia's version of GPS. Development began in 1976 by the Soviet Union. There are 5 versions of GLONASS including: GLONASS (1982), GLONASS-M (2003), GLONASS-K (2011), GLONASS-K2 (2015), GLONASS-KM (2025 - Currently in research phase).\n\nhttps://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations\n\nPoor/lack of Satellite Signal, leading to Loss of Accuracy.\n\nJones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/057.\n\n\"The great disadvantage of GPS is that as the satellite signal is quite weak, line of sight from the receiver to the satellite is essential. Therefore GPS cannot be used indoors or in areas where a clear view of the sky is not possible such as in forests, next to tall buildings or in deep valleys. Great care should always be taken in positioning GPS receivers and when using twin receivers ensuring each receiver can track the same satellites\"\n\n#Feature heightAboveWgs84EllipsoidM \n\n\"The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS.\"\n\n\"Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is \"mean sea level,\" which is supposed to roughly approximate the equipotential surface, but obviously can't be directly measured inland.\" By Rob Skelly\n\n\"An ellispoid is a mathematical model of the earth that approximates its three dimensional shape. See this definition. Elevation on top of the ellipsoid is 0, but since it's just an approximation one can be above or below the ellipsoid at any given point. \"Elevation above the surface of the ellipsoid\" is the distance between the measurement and the 0 value of the ellipsoid.\"\n\n'The Z value in a given coordinate system has to be based on something--a height above a generalized shape of the earth. MSL is one way to do it, but in my experience the majority of cases use ellipsoids as approximate figures. GPS, for example, uses WGS84 as the global coordinate system, and with it is the WGS84 ellipsoid.\" By Wes\n\nhttps://gis.stackexchange.com/questions/157005/meaning-of-elevation-above-surface-of-ellipsoid#:~:text=The%20elevation%20above%20the%20ellipsoid,d%20get%20from%20a%20GPS.\n\n#Mean Sea Level, GPS, and the Geoid\n\nAuthor: By Witold Fraczek, Esri Applications Prototype Lab\n\n\"The accuracy of GPS height measurements depends on several factors but the most crucial one is the \"imperfection\" of the earth's shape. Height can be measured in two ways. The GPS uses height (h) above the reference ellipsoid that approximates the earth's surface. The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL. The signed difference between the two heights—the difference between the ellipsoid and geoid—is the geoid height (N). \n\n\nThe figure bellow shows the relationships between the different models and explains the reasons why the two hardly ever match spatially.\n\nhttps://www.esri.com/news/arcuser/0703/geoid1of3.html\n\n" } }