GNSS Geomatics (2024/2025) - PDF

GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Geomatics for Urban and Regional Analysis GNSS A few basic notes about GNSS...

GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Geomatics for Urban and Regional Analysis GNSS A few basic notes about GNSS The acronym GNSS (Global Navigation Satellite System) refers to the global navigation satellite system, consisting of: (Global Navigation Satellite System)  US NAVSTAR Global Positioning System (GPS) and its use by surveyors  Russian GLONASS system  Galileo European system, not yet fully operational  Chinese Beidou system  Indian IRNSS system  QZSS from Japan There are many similarities between these systems, in this lecture we will mainly limit ourselves to the presentation of the GPS Source: gssc.esa.int Multi-constellation signal tracking: many channels needed The receiver needs to be able to generate replicas of the codes broadcast by each satellite in order to track them. Because of the radically different codes used by GLObal'naja NAvigacionnaja the different constellations, it is necessary Sputnikovaja Sistema to devote a certain number of channels to specific satellite signals. A single GNSS receiver needs many satellite tracking channels. Many of the channels are generic, that is, they will BeiDou Navigation Satellite track similar signals, such as some of the System (BDS; Chinese: 北斗卫星 导航系统; pinyin: běidǒu wèixīng GPS signals and some of the Beidou dǎoháng xìtǒng) signals. But, some of the channels are set aside to receive specific code signals (such Quasi-Zenith Satellite System as from some of the Galileo satellites). (Japan) The figure shows an example of how NavIC (Navigation with Indian many receiver tracking channels could Constellation), operational name potentially be utilized when tracking just for Indian Regional Navigation Satellite System (IRNSS): 5 individual satellites. Location Based Service: services based on the localization of a person or an object 8 GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 1 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli The GPS system Born for military purposes, run by the DoD, US Department of Defense In constant evolution since 1973 Launch of the first satellite: 1978 Completion of the constellation: 1994 Subsequent developments: – new generation of satellites – new frequencies 10 The GPS satellite The satellites constellation Design of the system: 21 satellites + 3 spare, constellation completed in 1994 6 orbital planes at 55° inclination, on each one 4 satellites orbit 20200 km distance from the Earth 12 hours orbits in any part of the world, 24 hours a day, at least 4 satellites visible each satellite is "seen" from every point of the Earth for at least 5 hours # of GPS visible satellites Control stations (I) N.B. the satellites are not geostationary because, being the system controlled by the US DoD, they must pass at least The orbits of the satellites: Network of control and monitoring stations once a day over US territory ground tracks during a day While a control station can send and receive data from satellites, the single user can only receive. All control stations continuously receive the signals emitted by the satellites. At the control stations there are very precise instruments (e.g. clocks such as those of satellites) GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 2 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Control stations (II) Instrumentation for the user Receiver(s) Main functions of the control stations: Geodesy / Surveying [high precision] Antenna(s) keep the atomic clocks of the satellites synchronized with each other Batteries keep the orbits of the satellites under control (orbit (software for data corrections, calculation of the predicted and precise processing) ephemeris) monitor the status of the satellites (failures, malfunctions) Positioning, navigation [medium-low precision] Differences in respect to traditional Antenna antenna topographical surveys and Pros: receiver integrated – h24 data collection, weather independent – No need of intervisibility between points datalogger – Long distances – Measurement of absolute point positions in a global datum (WGS84) and 3D vectors between points – High reliability Cons: rover – Need of satellite visibility – Elevation referred to the ellipsoid master – Long data acquisition time to achieve the best results – In the early years you needed two receivers to work in relative mode, but today we have a lot of different solutions 18 The GPS signal How the system works? The position of the receiver’s antenna on the ground is determined by measuring the distance from the satellites, used as Two carrier frequencies in band L: reference: their position is known (ephemeris) and is sent to – L1: 1575.42 Mhz, wavelength = 19 cm the receiver – L2: 1227.6 Mhz, wavelength = 24 cm Three modulating codes: two for if you measure the I am on a point distance to only distance measurement and one for one satellite... on this sphere transmitting data: – C/A code, only on L1 (1.023 Mhz, lenght 293 m, 480 Watt) the possibilities are – P code (10.23 Mhz, length 29.3 m) if you measure the distance to two reduced to two points, P1 on L1 (240 Watt) satellites... but one is impossible my position is on (outside the Earth) P2 on L2 (41 Watt) this circumference – NAVDATA: message on both frequencies, 50 Hz Satellites status by measuring the distance to three satellites you can Data for correction of orbits and satellite find the position of the point (receiver antenna) clocks GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 3 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli How the distance to a satellite is calculated? But measuring the distance from three satellites is not enough! Why? By measuring the time it takes for the signal to travel from the satellite to the receiver (with the The satellites have very precise (and expensive) atomic clocks on board speed of light). It is very short (e.g. 0.06 sec if the which are kept (fairly) synchronized with each other by the control satellite is on the receiver's zenith) stations “GPS time” the measurement requires very accurate clocks The receivers have lower quality (but still very stable) clocks that are not synchronized at all with each other and with the GPS time The code that the satellite sends is generated An error of 1 microsecond in the synchronization between satellite- simultaneously also inside the receiver. But the clock and receiver-clock would mean 300 m of error in the distance signal takes time to reach Earth... measurement! in order for the signal coming from the In addition to the three coordinates X, Y, Z (or ϕ, λ, h) also the satellite to be synchronized with the copy that synchronization error between the receiver-clock and the GPS time is is generated in the receiver, the latter must be an unknown factor to be solved t Pseudo-random code delayed 4 unknowns it is necessary to receive and measure the distance from different for each satellite the delay provides the measurement of the at least 4 satellites, not only 3 travel time of the signal the distance is calculated Different errors sources The SA effects on the positioning Atmosphere – Ionosphere (80-500 km), high density of electrically charged particles After SA removal – Troposphere (within 10 km), the main meteorological events take place SA active (after May 2, 2000) there, strong presence of water, high local variability Clocks and orbits of satellites Receivers: instability of the oscillator, internal noise Multipath: multiple paths due to reflection on surfaces close to the antenna. It can be limited or eliminated with suitable receivers and antennas (in the past…) Selective Availability (SA): Introduced voluntarily by the US government, phased out May 2, 2000 – error on the ephemeris transmitted (every hour) – dithering, variation of clock data (every 4-15 min) GPS topographic surveys require the use of two or more receivers Software for static data processing that operate simultaneously (relative measurements) for high accuracy topographical surveys Network scheme and station occupation In surveying, we normally determine the 3D vector between two receivers instead of the absolute coordinates of the points. Differentiating the measures obtained at the two receivers, the systematic errors - common to both - can be deleted. GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 4 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Effects of the differential correction A simple technique for differential correction 7400 senza correzione differenziale con correzione differenziale 7390 7380 7370 7360 N o Reference station r d (master) on a point with (m) 7350 known WGS84 Mobiile station coordinates (rover) 7340 The DX, DY, DZ corrections calculated at the reference station (master), by 7330 comparing its known coordinates with those determined by GPS, are applied to the coordinates determined at the same time by the mobile station (rover). 7320 The correction can be applied in a post-processing session or in real-time if a 4140 4150 4160 4170 4180 4190 4200 4210 4220 radio link is available between master and rover Est (m) Real time transmission of the GPS corrections: two possibilities antennas Setup of a rover station Radio antennas A master station managed antenna GPS autonomously sends the corrections via antenna radio radio. The WGS84 coordinates of the rover master station must be known exactly and two receivers are required. master use of differential correction transmission services through L-band satellites; the corrections are generated by a network of reference stations. A single appropriately prepared receiver is sufficient. You have to pay a subscription, with two options: datalogger optimized performance on a predetermined area: the system provides receiver + the best differential solution once the current coordinates are specified radio modem the corrections always come from a single reference station that can be selected by the user Using one or more reference stations and a suitable network Surveys in kinematic / stop & go mode infrastructure, it is possible today to work in real time with a single receiver only, achieving optimal results 44 GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 5 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli What about the GPS precision? Many GNSS apps on Android or iOS (not for geodetic applications!) It can vary from a few meters to less than 1 cm! The precision in the calculated position depends on: GPStest - measurement in absolute (one receiver only, e.g. for navigation: a few m) or in relative mode (two or more receivers, common errors are eliminated) - type of observation used (code, phase) and type of user (military, civilian) [the C/A code has a precision (rms) of 3 m, the P code is 10 times more accurate, the phase has rms of mm] - hardware / software capabilities of the receiver/s NB to work in relative you need at least two receivers working at the same time! - measurement mode - static (from one hour to several days on a point, there are also permanent stations that are permanently active) - kinematic (moving point) - rapid-static (e.g. 10-30', between points within 15 km) - type of software for data processing GNSS apps on Android or iOS (not for geodetic applications!) GPSdata+ 47 GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 6 G. Bitelli