GPS has many advantages over Traditional Terrestrial Surveying Techniques

Transcript

1 GPS

2 Traditionally GPS has many advantages over Traditional Terrestrial Surveying Techniques These traditional techniques rely on line of sight between the survey instrument and a target If an obstructions exists, it must be traversed around Typically distance measurement is limited to 5 Km Weather can limit operations. eg fog, rain etc

3 Why GPS? Weather Independent Does not require line of sight Gives high Geodetic Accuracy Can be operated day and night Quicker and requires less manpower Economical advantages Common Coordinate System Wide Range of Applications Competitively Priced

4 GPS General Characteristics Developed by the US Department of Defense Provides Accurate Navigation m Worldwide Coverage 24 hour access Common Coordinate System Designed to replace existing navigation systems Accessible by Civil and Military

5 GPS E Il SISTEMA GPS E COMPOSTO DA TRE SEGMENTI Space Segment NAVSTAR NAVSTAR :: NAVigation NAVigation Satellite Satellite Time Time and and Ranging Ranging Satelliti Satelliti con con periodo periodo di di rotazione rotazione di di ore ore Km Km User Segment Receive ReceiveSatellite Signal Signal Control Segment 11 Master Master Station Station 55 Monitoring Monitoring Stations Stations

6 IL SEGMENTO SPAZIALE I satelliti GPS assicurano: Mantenere una scala temporale molto precisa, ciascun satellite è dotato di 4 orologi atomici. Emettere segnali a frequenza molto stabile; segnale frequenza di 10,23 MHz con due onde portanti L1-L2. Ricevere e memorizzare le informazioni provenienti dal sistema di controllo. Effettuare limitate manovre orbitali. Effettuare a bordo calcoli limitati. Ritrasmettere le informazioni a Terra

7 IL SEGMENTO DI CONTROLLO E ASSICURATO DA 5 STAZIONI TERRESTRI: Colorado Springs, Hawaii, Isola dell Ascensione, Isola delldi Diego Garcia, Isola di Kwajalein. Questo segmento è incaricato della manutenzione della totalità del segmento spaziale: Determinare e correggere le orbite dei satelliti. Fornire i parametri di navigazione dei satelliti (effemeridi) Fornire gli almanacchi delle presenze satellitari in base al giorno e l ora per un determinato luogo.

8 IL SEGMENTO UTILIZZATORE Il segmento utilizzatore è costituito dall insieme dei ricevitori di segnali GPS: un antenna per la ricezione e l amplificazione del segnale satellitare; una o più schede elettroniche per l elaborazione dei segnali ricevuti; un dispositivo (di comando) per l elaborazione e la memorizzazione dei dati.

9 Xl X l ll lll Outline Principle : Range lx Xll Vll Vlll Vl lv V

10 Xl X l ll lll Outline Principle : Range lx Xll Vll Vlll Vl lv V

11 Xl X l ll lll Outline Principle : Range lx Xll Vll Vlll Vl lv V

12 Xl X l ll lll Outline Principle : Range Principio della misura della Pseudodistanza lx Xll Vll Vlll Vl lv V Range = Time Taken x Speed of Light

13 LA PSEUDODISTANZA Il ricevitore conosce per definizione il tempo di emissione di ciascun segnale orario satellitare e misura il tempo di ricezione del segnale stesso mediante il suo orologio interno. s? vt?? c? t E chiamata pseudodistanza perché la distanza satellite ricevitore è ottenuta con la precisione della differenza oraria degli orologi. Questa differenza rappresenta la quarta incognita del sistema di equazioni per determinare il punto.

14 Nel caso di un problema statico un punto nello spazio è individuato da tre coordinate. Considerando il segnale che viene emesso da un satellite, questo può essere rappresentato da una sfera in cui in ogni istante t 1 il raggio è descritto dalla distanza che il segnale percorre nello spazio. t 1 In un punto qualsiasi della superfice di questa sfera passano sfere quindi per individuare un punto nello spazio servono 3 sfere che si intersecano quindi 3 equazioni in 3 incognite (tre satelliti)? 2

15 Mentre nel nostro caso abbiamo oggetti in moto rispetto ad orologi diversi... Sono quindi necessari minimo 4 satelliti per risolvere questo sistema? x, y, z t? P, Maggiore è il numero dei canali di un ricevitore meglio esso sarà in grado di effettuare simultaneamente delle misure di pseudodistanze su diversi satelliti.

16 Outline Principle : Position R3 R1 R2 3 Spheres intersect at a point 3 Ranges to resolve for Latitude, Longitude and Height

17 Outline Principle : Position The satellites are like Orbiting Control Stations Ranges (distances) are measured to each satellite using time dependent codes Typically GPS receivers use inexpensive clocks. They are much less accurate than the clocks on board the satellites A radio wave travels at the speed of light (Distance = Velocity x Time) Consider an error in the receiver clock 1/10 second error = 30,000 Km error 1/1,000,000 second error = 300 m error

18 Point Positioning 4 Ranges to resolve for Latitude, Longitude, Height & Time It is similar in principle to a resection problem

19 GPS Signal Structure Each GPS satellite transmits a number of signals The signal comprises two carrier waves (L1 and L2) and two codes (C/A on L1 and P or Y on both L1 and L2) as well as a satellite orbit message Fundamental Fundamental Frequency Frequency MHz MHz 10 x 154 L1 L MHz MHz C/A C/A Code Code MHz MHz P (Y)-Code (Y)-Code MHz MHz x 120 L2 L MHz MHz P (Y)-Code (Y)-Code MHz MHz BPS BPS Satellite Satellite Message Message (Almanac (Almanac& Ephemeris) Ephemeris)

20 Range Determination from Code Observations Pseudoranges (Code) Each satellite sends a unique signal which repeats itself approx. 1 msec Receiver compares self generated signal with received signal From the time difference (dt) a range observation can be determined Receiver clock needs to be synchronized with the satellite clock? T Received Code from Satellite Generated Code from Receiver D = V (? T)

21 Range Determination from Phase Observations Phase Observations Wavelength of the signal is 19 cm on L1 and 24 cm on L2 Receiver compares self-generated phase with received phase Number of wavelengths is not known at the time the receiver is switched on (carrier phase ambiguity) As long as you track the satellite, the change in distance can be observed (the carrier phase ambiguity remains constant)? T D = c? T???? N Received Satellite Phase Generated Phase from Receiver

22 Ricordando che per un onda:?? c? Dove? =lunghezza d onda - c = velocità della luce -? = frequenza del segnale. Per L1?? 8 3? ? 10 9? 19 cm Per L2?? 8 3? ? 10 9? 24 cm

23 Point Positioning Accuracy m A receiver in autonomous mode provides navigation and positioning accuracy of about 10 to 30m

24 Point Positioning Accuracy m A receiver in autonomous mode provides navigation and positioning accuracy of about 10 to 100 m due to the effects of Selective Availability

25 Error Sources Satellite errors Orbit uncertainty Satellite Clock Model Receiver errors Receiver Clock Receiver noise Observation errors Ionospheric Delay Tropspheric Delay Station errors Station Coordinates Multipath

26 User Equivalent Range Errors 400 Metres Satellite Clock Recvr Noise Multipath Tropospheric Ephemeris Ionospheric Recvr Clock

27 How do I Improve my Accuracy? Use Differential GPS

28 Differential Positioning It is possible to determine the position of Rover B in relation to Reference A provided - the coordinates of the Reference Station (A) are known - satellites are tracked simultaneously Differential Positioning eliminates errors in the sat. and receiver clocks minimizes atmospheric delays Accuracy 0.5 cm - 5 m A Baseline Vector B

29 Differential Positioning If using Code only accuracy is in the range of 0.5m - 5 m This is typically referred to as DGPS A Baseline Vector B

30 Differential Positioning If using Phase or Code & Phase accuracy is in the order of 5-10 mm + 1ppm A Baseline Vector B

31 Initial Phase Ambiguity Initial phase Ambiguity must be determined to use carrier phase data as distance measurements over time Time (0) Time (i) Ambiguity Ambiguity Phase Measurement Counted Cycles Phase Measurement

32 Resolving the Ambiguity The effect of resolving the ambiguity is shown below Note that once the ambiguities are resolved, the accuracy of the measurement does not significantly improve with time Accuracy (m) 1.00 Ambiguities Not resolved Ambiguities Resolved Time (mins) Static Rapid Static 0 2 5

33 Dilution of Precision (DOP) A description of purely geometrical contribution to the uncertainty in a position fix It is an indicator as to the geometrical strength of the satellites being tracked at the time of measurement VDOP (Vertical) Includes Height only HDOP (Horizontal) Includes Lat & Lon PDOP (Positional) Includes Lat, Lon & Height GDOP (Geometrical) Includes Lat, Lon, Height & Time Good GDOP

34 Control Segment Master Control Station Responsible for collecting tracking data from the monitoring stations and calculating satellite orbits and clock parameters 5 Monitoring Stations Responsible for measuring pseudorange data. This orbital tracking network is used to determine the broadcast ephemeris and satellite clock modeling Ground Control Stations Responsible for upload of information to the satellites

35 Space Segment 24 Satellites 4 satellites in 6 Orbital Planes inclined at 55 Degrees Km above the Earth 12 Hourly orbits In view for 4-5 hours Designed to last 7.5 years Different Classifications Block 1, 2, 2A, 2R & 2 F 55 Equator

36 Current Status Denver N W 1787m Time : GMT Date : 10/08/97 Window : Cut Off Angle 15 Almanac from : 09/27/97

37 User Segment The most visible segment GPS receivers are found in many locations and applications

38 Esempio di rilievo di un area archeologica mediante GPS in modalità RTK

39 Rilievo posto sulla topografia

40 Ricostruzione 3D di un territorio mediante il rilievo in modo Cinematico

41 GPS Basic Course Basic Geodesy Page 2-1

42 Index - Section 2 GPS Basic Course Definition The Real Earth (The Geoid) The Ellipsoid World Geodetic System (WGS84) Heighting Page 2-2

43 Definition GPS Basic Course Geodesy :- Geo - Earth desy - The study of The study of the earth Geodesy is the science of the measurement and mapping of the earth s surface F.R Helmert (1880) Page 2-3

44 The Real Earth (The Geoid) Equipotential surface that best equates to mean sea level Physical Definition that is a complicated surface Described by an infinite number of parameters Can be sensed by instruments GPS Basic Course N. America Topography Europe S. America Africa Page 2-4

45 GPS Basic Course The Ellipsoid An ellipse is a mathematical figure which is defined by a Semi-Major Axis (a) Semi-Minor Axis (b) It is a simple geometrical surface Cannot be sensed by instruments b a Page 2-5

46 GPS Basic Course The Ellipsoid and Geoid N Topography N. America Europe O 1 S. America Africa Page 2-6

47 GPS Basic Course The Ellipsoid and Geoid Which ellipsoid to choose? N N Topography N. America Europe O 1 O 2 S. America Africa Page 2-7

48 GPS Basic Course The Ellipsoid and Geoid The World Geodetic System WGS 1984 The best mean fit to the Earth N Topography N. America Europe S. America Africa Page 2-8

49 GPS Basic Course World Geodetic System (WGS84) Origin coincides with Earth s center of mass X and Y axis are perpendicular to each other in the equatorial plane Z axis is at right angles to the X,Y plane and coincides with the Earth s rotational axis Z h P?? Y? X Page 2-9

50 GPS Basic Course World Geodetic System (WGS84) The prime orientation (X) is Greenwich Meridian Positionsand Coordinate differences are obtained in the WGS 84 Coordinate System Latitude, Longitude Ellipsoid height Geocentric X,Y,Z coordinates Z? h P? Y? X Page 2-10

51 GPS Basic Course Heighting Heights determined using GPS are referenced to the WGS 84 Ellipsoid Ellipsoid Heights are heights above the ellipsoid P h Topography h = Ellipsoidal height Ellipsoid Page 2-11

52 GPS Basic Course Heighting The Geoid is that equipotential surface (equal gravity) that best equates to Mean Sea Level The geoid undulates dueto theeffects of Topology, geology etc. Orthometric heights are referenced to a Datum which is typically M.S.L M.S.L approximates thegeoid P H h Topography Geoid H = Height above Geoid (~Orthometric Height) Ellipsoid Page 2-12

53 GPS Basic Course Heighting The height difference between ellipsoid and geoid is called the geoidal undulation To obtain orthometric heights, the geoidal undulation must be accounted for P H h Topography N Geoid N = Geoidal Separation Ellipsoid Page 2-13

54 GPS Basic Course Heighting The geoidal undulation may be positive or negative. h = Ellipsoidal height H = Height above Geoid (~Orthometric Height) P H h Topography N = Geoidal Separation N Geoid h = H + N Ellipsoid Page 2-14

55 Lo studio: La conoscenza del Bene Culturale non può prescindere dallo studio e dalla conoscenza del suo rapporto con il territorio che lo contiene. L obiettivo: Utilizzare metodologie integrate per la ricostruzione del Paesaggio Storico. Le metodologie: Acquisizione dati di diversa natura (Storici, Archeologici Geofisici e Geomorfologici) e mutuamente correlati nello spazio utilizzando un Geographical Information System (GIS)

56 PROGETTO INTEGRATO ANALISI DEL TERRITORIO di COLLE del FORNO e SCAVO della NECROPOLI Implementazione di un Sistema Informativo Territoriale per lo studio del sito archeologico di Colle del Forno Dott. Roberto Gabrielli Coordinamento archeologico: Dott.ssa Paola Santoro IAEI Prospezioni Geofisiche: Dott. Salvatore Piro ITABC

57 Area dell abitato di Eretum Area della Necropoli Area interessata dall indagine archeologica.

58 Ricostruzione 3D dell area in esame, abitato + necropoli

59 Foto aerea della necropoli georeferenziata N

60 Tombe rinvenute N

61 Micro avvallamenti ricercati dall analisi morfologica del 3D del terreno Dromos Camera Rappresentazione schematica di una tomba a camera

62 Microrilievo e ricostruzione 3D dell area della necropoli

63 Il modello numerico del terreno vestito con un tessuto sintetico per meglio evidenziare i microavvallamenti

64 Elaborazione numerica vestita mediante una rappresentazione in falsi colori

65 Visione 3D prospettica

66

67 Sovrapposizione dei vari tematismi

68 Possibili presenze di tombe

69 Il saggio effettuato sull area indagata ha messo in luce una fossa sacrificale. Sono stati rinvenuti numerosi scheletri di animali adagiati sopra delle anfore.

70 Ricostruzione 3D della fossa

71