Guidelines for the use of GNSS in surveying and mapping

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Ellipsoid In geodesy, unless otherwise specified, a mathematical figure formed by revolving an ellipse about its minor axis. Used interchangeably with spheroid. Ephemeris The set of data which describes the position of a celestial object as a function of time. The GNSS ephemeris is used in the processing of GNSS observations. Either the broadcast ephemeris from the satellite navigation message or a precise ephemeris calculated from GNSS tracking stations can be used, depending on application. Epoch A point in time which is the reference for a set of co-ordinates. The measurement interval or data frequency, as in recording observations every 15 seconds. In this example loading data using 30- second epochs means loading every other measurement. ETRF (European terrestrial reference frame) See Reference frame. ETRS (European terrestrial reference system) See Reference system. Fast switching channel A switching channel with a sequence time short enough to recover (through software prediction) the integer part of the carrier beat phase. Fixed integers See Integer bias search. Float solution A baseline solution that does not fix the integer ambiguity values to whole numbers. The values are left as non-integer real numbers giving the baseline a higher RMS. than a fixed baseline. In general float solutions are not acceptable as final baseline measurements. Full wave Term used to differentiate between measurements made with single-squared (codeless) and codetracking receivers. Specifically, a receiver tracking L2 P-code can make measurement using the whole L2 wavelength (24cm): the full wave. Fundamental frequency The fundamental frequency used in GPS is 10.23MHz. The carrier frequencies L1 and L2 are integer multiples of this fundamental frequency. L1=154F=1575.42MHz, L2=120F=1227.60MHz. Galileo The European Commission’s GNSS system. Geocentre The mass centre of the earth. Geodetic datum A mathematical model designed to best fit part or all of the geoid. Conventional datums depended upon an ellipsoid and an initial station on the topographic surface established as the origin of the datum. Such datums were defined by the dimensions of the spheroid, by the geodetic latitude, longitude and the height of geoid above the ellipsoid at the origin, by the two components of the deflection of the vertical at the origin, and by the geodetic azimuth of a line from the origin to some other point. Geocentric datums are designed to give the best possible fit worldwide rather than to depend upon values determined at an initial station. Their origin is the geocentre of the earth (see WGS 84 below). Geoid The particular equipotential surface which most closely approximates to mean sea level in the open oceans and which may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity. Geoidal separation Height of the geoid relative to the ellipsoid. GDOP (Geometric dilution of precision) The relationship between errors in user position and time and in satellite range. GDOP = PDOP + TDOP . See PDOP. GLONASS The Russian GNSS system. GNSS Global Navigation Satellite System (the generic term for satellite navigation systems, including GPS, GLONASS, Galileo and Compass). GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING |67
GPS (Global Positioning System) The United States GNSS system. GPS-ICD-200 The GPS interface control document is a US government document that contains the full technical description of the interface between the satellites and the user. GPS receivers must comply with this specification if they are to receive and process GPS signals properly. GPS week GPS time started at Saturday/Sunday midnight on 6 January 1980 and the weeks are numbered from that date, up to 1024 weeks. The first week ‘rollover’, the start of the next renumbering, occurred on 21 August 1999. Gravitational constant The proportionality constant in Newton’s Law of Gravitation is G = 6.672 x 10 -11 m 3 kg -1 s -2 . Half-wave Measurements made using L2 squared measurements. The squaring process results in only half of the original L2 wavelength being available, a doubling of the original L2 frequency. HDOP (Horizontal dilution of precision) See DOP and PDOP. Height – ellipsoidal The distance above or below the ellipsoid measured along the normal to the ellipsoid at that point. Not the same as elevation above sea level. GNSS receivers output position-fix height as the height above the ITRS89 ellipsoid. HOW (Handover word) The word is the GNSS message that contains time synchronisation information for the transfer from the C/A code to the P-code. IERS International Earth Rotation Service. Initialisation The moment when a rover GNSS receiver in a high precision real-time dynamic system (RTK) solves the integer ambiguity and gains a real-time high precision fixed baseline solution. Integer bias search The biases calculated in the float solution are fixed to integers. A search is then undertaken to find closely related sets of integers. These sets are compared to the initial set by dividing the sum of the squares of residuals of the trial set by that of the original set. A strong data set allows only one set of integers, and produces a ratio factor greater than 3.0. A weak data set may accept several different sets of integers with only small changes in its sum of the squares fit, thus producing a small ratio factor. Integer bias terms The receiver counts the radio waves from the satellite, as they pass the antenna, to a high degree of accuracy. However, it has no information on the number of waves to the satellite at the time it started counting. This unknown number of wavelengths between the satellite and the antenna is the integer bias term. Integrated Doppler A measurement of Doppler shift frequency of phase over time. Ionospheric delay The ionosphere is a non homogeneous (both in space and time) and dispersive medium. A wave propagating through the ionosphere experiences variable delay. Phase delay depends on electron content and affects carrier signals. Group delay depends on dispersion in the ionosphere as well, and affects signal modulation. The phase and group delay are of the same magnitude but opposite sign. ITRF (International terrestrial reference frame) See Reference frame. JPO Joint Program Office for GPS located at the USAF Space Division at El Segundo, California. The JPO consists of the USAF Program Manager and Deputy Program Managers representing the Army, Navy, Marine Corps, Coastguard, National Imagery and Mapping Agency (NIMA) and NATO. Kalman filter A numerical method used to track a time-varying signal in the presence of noise. If the signal can be characterised by some number of parameters that 68 | GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING
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GN 13/2010 RICS Practice Standards,
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Published by the Royal Institution
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Appendices 59 A GNSS verification,
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Preface This second edition of the
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Part 1 - Guidelines 1 The role of G
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applications which are unsuitable f
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Table A: Control survey guidelines
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Table C: Positioning guidelines Gui
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Table D2: Survey methods - dynamic
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Table D3: Survey methods - dynamic
Page 21 and 22: 2.2 Surveyor’s procedures The sur
Page 23 and 24: Table F: Planning Guideline Explana
Page 25 and 26: Table H: Data processing Guideline
Page 27 and 28: Table 3.5: Reporting Guideline Expl
Page 29 and 30: Table K1: Atmospheric refraction -
Page 31 and 32: Table K3: Interference Consideratio
Page 33 and 34: Part2-Technical commentary 4 GNSS s
Page 35 and 36: Although these observables have dif
Page 37 and 38: Historically, single-frequency stat
Page 39 and 40: 4.4 Real-time dynamic surveys With
Page 41 and 42: 5 Operational considerations Sectio
Page 43 and 44: Errors in the troposphere which are
Page 45 and 46: In addition, phase centre variation
Page 47 and 48: 6 Co-ordinate reference frames or s
Page 49 and 50: particular country’s geodetic ref
Page 51 and 52: 6.3.2 Transformations from ITRS89 e
Page 53 and 54: 6.3.5 Co-ordinate transformation is
Page 55 and 56: eceivers are observing at the same
Page 57 and 58: The field procedures adopted should
Page 59 and 60: 7.4.4 System initialisation Operati
Page 61 and 62: aw data. Advanced processing strate
Page 63 and 64: values with their computed precisio
Page 65 and 66: Appendix B: Information on receiver
Page 67 and 68: The following general information s
Page 69 and 70: Appendix E: Sample procedures relat
Page 71: tracking had been maintained. The i
Page 75 and 76: or as the corrections that would be
Page 77 and 78: Simultaneous measurements Measureme
Page 79 and 80: Checklists for GNSS survey specific
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