GNSS Introduction and Principles of Operation PDF

Global Positioning Systems (GPS) and Global Navigation Satellite Systems (GNSS) Introduction and Principles of Paul Hinds PhD, Learning Outcomes Introduction GPS Overview GPS Signal How GPS positions are measured Sources of Errors How to improve GPS accu...

Global Positioning Systems (GPS) and Global Navigation Satellite Systems (GNSS) Introduction and Principles of Paul Hinds PhD, Learning Outcomes Introduction GPS Overview GPS Signal How GPS positions are measured Sources of Errors How to improve GPS accuracy Global Navigation Satellite System (GNSS) Introduction During the 1970’s global positioning systems (GPS) emerged out of the space program and relied on signals transmitted from satellites. First paid for and developed by the military. The entire scope of satellite systems used in positioning is now referred to as global navigation satellite systems (GNSS) Provide precise timing and positioning information anywhere on the earth surface. Can operate day or night, rain or shine and do not require a clear line of sight between surveying stations. A departure from convention surveying procedures, which rely on observed angles and distances to determine point positions. Global Positioning Systems (GPS) Overview GPS is a satellite-based navigation system operated by the US government. Consists of a network of a minimum of 24 satellites that orbit the earth. The satellites emit signals (radio waves) that are picked-up on earth by GPS receivers. To determine its location on earth, the receiver requires signals from a minimum of three different satellites but, best to use at least four. Was originally intended for military applications but has been made available for civilian use since the 1990s. GPS is a type of Global Navigation Satellite System (GNSS). GPS Satellite GPS satellites are positioned in precise, circular orbits 18,000 kilometers (11,000 miles) above the Earth. They orbit once every 12 hours. The precise travel time of the signal from these satellites is necessary to determine the distance or range to the satellite GPS Constellation | Time and Navigation (si.edu) Global Positioning Systems (GPS) Overview GPS consists of three segments: space, control, and user  The Space Segment: consists of a minimum of 24 operational satellites in six circular orbits 18,000 km (11,000 miles) above the earth at an inclination angle of 55 degrees with an 11 hour 58 minute period.  Typically, the satellites are spaced in primary orbital slots so that at any time a minimum of 6 satellites will be in view to users anywhere in the world.  Global Positioning Systems (GPS) Overview The Control Segment: consists of a master control station in Colorado Springs. Current Operational Control Segment includes a master control station, 11 command and control antennas, and 16 monitoring sites The monitor stations track the position all GPS satellites in view and collect ranging information from the satellite over time. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes extremely precise satellite orbits and make future predictions of the orbits. The information is then formatted into updated navigation messages for each satellite. The information is then uploaded to the satellite to https://www.gps.gov/multimedia/images/GPS- be used by receivers to predict satellite control-segment-map.pdf positions. Global Positioning Systems (GPS) Overview The User Segment: consists of the receivers, processors, and antennas that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise position, velocity and time. Consist of two categories;  The Standard Position Service (SPS) – use the L1 and L2 broadcast frequency (1575.42 and 1227.60 MHz frequencies respectively). These are pseudo- random codes for timing and contains a navigation message with ephemeris data. - Ephemeris data is used by GPS receivers to estimate location relative to the satellites and as a result position on earth  The Precise Position Service (PPS) – broadcast on both the L1 and L2 frequency but only available to the military.  GPS Signal To independently determine ground position in real time a system of accurate measurement of signal travel time from satellite to receiver is necessary. Carrier adjusted with pseudorandom noise (PRN) codes consisting of unique sequences of binary values (zeros and ones) generated according to a special mathematical algorithm. Satellites transmit two or more different PRN codes L1 frequency is modulated with the precise code (P-code) and also a coarse/acquisition code (C/A code). The P-code, however, is normally encrypted and is available only to authorized users (Military use), approximate 10 times more accurate than the C/A code – makes correction to ionosphere refraction which is the largest error source in positioning.. C/A code allows receivers to acquire satellite as well as allowing receivers to determine their approximate position.  How GPS position is measured Satellite send out a broadcast message with precise time PRN code is broadcast with a starting time indicating the front edge of one chip. Receiver simultaneously generates a duplicate PRN code. Matching the incoming satellite signal with the identical receiver-generated signal derives the time it takes for the signal to travel from satellite to receiver. The signal delay is converted to travel time and the known signal velocity, the distance to the satellite can be computed.  How GPS position is measured Knowing: Satellite position and pseudo random code information (ephemeris or almanac data) Speed of signal 300,000 km/s (speed of light) The position of the receiver can be calculated using trilateration A minimum of 4 line-of-sight satellites are required. Timing of the signals needs to be VERY Distance between the receiver and the satellites = propagation time x radio precise, so an atomic clock is used. wave speed - Search Images (bing.com) Latitude and longitude can be calculated quite precisely (under the right conditions) but elevation will How does GPS work? (youtube.com) always be the weak axis. GPS Error Sources  Instrument Errors i. Clock biases: both receiver and satellite clocks are subject to errors. Can be removed mathematically. GPS position calculations, depend on measuring signal transmission time from satellite to receiver; this, in turn, depends on knowing the time on both ends. NAVSTAR (NAVigation Satellite Timing and Ranging) satellites use atomic clocks. These errors are minimized by calculating clock corrections (at monitoring stations) and transmitting the corrections along with the GPS signal to appropriately GPS receivers. ii. Receiver noise: The electronic of the receiver operate within a specific tolerance. Small variation occur in the generation and processing of the signals that can translate into errors in the pseudorange and carrier-phase observation. Periodic calibration checks and test of receiver electronics should be made  GPS Error Sources  Natural Errors i. Refraction: due to the transit of signals through the atmosphere signals from the satellite are delayed. Upper atmosphere (ionosphere): As GPS signals pass through the upper atmosphere (the ionosphere 50-1500km above the surface), signals are delayed and deflected. By modeling ionosphere characteristics, GPS monitoring stations can calculate and transmit corrections to the satellites, which in turn pass these corrections along to receivers. - Troposphere (lowest part of the atmosphere – 10-12 km in altitude - Stratosphere goes up to 50 km https://gisgeography.com/gps-accuracy-hdop- pdop-gdop-multipath/ GPS Error Sources  Natural Errors ii. Multipath Effects: GPS signals travel from satellites through the atmosphere directly to GPS receivers. GPS receivers must discriminate between signals received directly from satellites and other signals reflected from surrounding objects, such as buildings, trees, and even the ground. - Multipath errors are particularly common in urban or woody environments, especially those with large valleys or mountainous terrain. Ghilani, 2022 https://gisgeography.com/gps-accuracy-hdop-pdop- gdop-multipath/ Dilution of Precision Accuracy of GPS positioning is affected by the arrangement of satellites in the sky. The ideal arrangement (of the minimum four satellites) is one satellite directly overhead, three others equally spaced nearer the horizon (but above the mask angle). GPS coordinates calculated when satellites are clustered close together in the sky suffer from dilution of precision(DOP) GPS receivers report several components of DOP, including Horizontal Dilution of Precision (HDOP) and Vertical Dilution of Precision (VDOP). The combination of these two components of the three-dimensional position is called PDOP - position dilution of precision. A key element of GPS mission planning is to identify the time of day when PDOP is minimized.  GDOP/PDOP – Geometric/Position Dilution of Precision GDOP (geometric dilution of precision) or PDOP (position dilution of precision) describes the error If the satellites are physically close caused by the relative position of the GPS together, then you have poor GDOP. satellites. This lowers the quality of your GPS The more signals a GPS receiver can positioning potentially by meters. “see” (spread apart versus close together), the more precise it can be. GPS Accuracy: HDOP, PDOP, GDOP & Multipath - GI S Geography  DOP Factors Computed through error propagation Numbers which when multiplied by the errors (Table 13.3) give the size of the error expected with no correction to the ranges based on the geometry of the observed constellation of satellites- e.g., if DOP factor is 2, the 2 time the size of error will be the estimated ranges for that time and location. For best possible constellation of satellite, the average value of HDOP is under 3 and under 6 for PDOP (Table 13.4) Source: Ghilani, C.D., 2022 How to Improve GPS Accuracy Two major techniques are: 1. Differential GPS (DGPS):  A relative positioning technique that employ two GPS receivers. The two receivers must collect data from the same satellites simultaneously  The position may be a precise absolute position, or just an approximate position. The relative position between two receivers can achieve sub-meter accuracy.  One receiver occupies a base-station (point whose coordinates are precisely known from previous survey), and the other receiver or receivers (known as rovers) whose positions are unknown.  By placing a receiver on a station with known position, the pseudorange error in the signal can be determined.  With base station and rover relatively close (often less than a kilometer), the pseudorange error at both base station and rover will have approximately the same magnitude.  Positional accuracy is generally in the range of 1-10mm with elevation difference accuracies around 5mm.  DGPS can be done in real time with a radio transmitter, the process is called real-time differential GPS (RTDGPS).  How to Improve GPS Accuracy 2. Satellite-based Augmentation System (SBAS) A Satellite-Based Augmentation System (SBAS) is a regional or global navigation satellite system that enhances the accuracy, integrity, and availability of signals from existing global navigation satellite systems The Wide Area Augmentation System https://gisgeography.com/gps-accuracy-hdop-pdop- gdop-multipath/ (WAAS) has a network of ground tracking This augmented system broadcasts the corrected error in real base stations that collect GPS signals and time along with the GPS signal. This is the principal idea of a determine range errors. satellite-based augmentation system (SBAS) and can provide sub- - Errors transmitted to meter GPS accuracy. geosynchronous satellites that relay correction to rovers. GPS Measurements Static - receivers are motionless on the Earth during the observation Kinematic - the receivers are either in periodic or continuous motion Real Time Kinematic (RTK) – Provide immediate results in the field DGPS - Simultaneous use of two or more code-based receivers that can provide positional accuracy within one meter Global Navigation Satellite System (GNSS) What Does Global Navigation Satellite System Mean? GNSS is a constellation of satellites that provide positioning to many devices through either autonomous (point) positioning or differential (relative) positioning. - Autonomous positioning utilize one GNSS receiver whereas differential positioning utilizes two or more receivers simultaneously tracking the same satellites. Used to determine precise location on the surface of the Earth – through several known satellite positions and measured distances between receiver and satellites, the position of the receiver can be obtained. Provides global coverage. GNSS Constellation of Satellites NAVSTAR, USA (GPS) - Constellation available is 30 + satellites GLONASS, Russia - 24 satellites in the constellation Beidou, China - 30 satellites in the constellation Galileo, European Union - 30 satellites in the constellation GPS Vs GNSS Global Navigation Satellite System The initial motivation for a satellite system was for military applications, but it has now progressed to more extensive civil applications, including the following: Aviation Disaster warning and emergency response Land transportation Maritime Mapping and surveying Monitoring of the environment Precision agriculture Natural resources management Research, such as climate change and ionospheric studies Wireless networking Photographic geocoding Mobile satellite communications Precise time reference Military precision-guided munitions  GPS field log. (Courtesy of Geomatics, Canada) Videos i. GPS Modernization (youtube.com) ii. How GPS Works Today (youtube.com) iii.The Navy Navigation Satellite System (1967) (youtube. com)