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Lecture 4. Global Navigation Satellite Systems

Lecture 4. Global Navigation Satellite Systems. Contents. Navigation History of GPS GPS Satellite Concept GPS System Segments Principle of Positioning GPS codes and the determination of travel time Accuracies / Error sources. History of Navigation.

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Lecture 4. Global Navigation Satellite Systems

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  1. Lecture 4. Global Navigation Satellite Systems

  2. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Accuracies / Error sources

  3. History of Navigation • Navigation on the Ocean (must be independent from the continents) Latitude and Longitude j – Latitude l – Longitude

  4. History of Navigation • Navigation on the Ocean • Speed and Bearing measurements to keep the course. • Sextant and Chronometer to determine the Latitude and Longitude and update the position

  5. History of Navigation How do we determine the latitude? 90 j = 90 – a - b a j a b

  6. History of Navigation How to determine the Longitude? • 24h = 360o • We set the Chronometer on the Greenwich Meridian at noon to 0h. • We read the time at the local noon on the sea. l = DZeit x 15o

  7. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Accuracies / Error sources

  8. History of GPS • After the 2nd World War it became important to create the global Navigation solution. • In the Space era the satellites could be used as „control points”. • Surveyors used various satellites equiped with reflectors as signals to link control networks (e.g. islands) • After 1960 the „US Navy Navigation Satellite System“ (NNSS) was created, after renamed to TRANSIT

  9. History of GPS • TRANSIT contained 6 satellitesin polar orbitswith the elevation of ca. 1100 km (period ca 107 Min). • The satellites were tracked from ground stations in US, and the orbital parameters were updated twice a day. • The satellites broadcasted the orbital parameters. The position could be computed from the observation of Doppler shifts with the accuracy of 1m. • Disadvantages • Good satellite constellation in every two hours only • Accurate position could be measured by long observations only.

  10. History of GPS • GPS (Global Positioning System) was developed to • determine 3D positions in real time. • determine accurate time Speed Bearing • NAVSTAR GPS (Navigation Satellite Time And Range) from 1973 as prototype • First GPS satellitewas started on June 27, 1977.Orbit elevation ca. 20000 km. Today the term GNSS (Global Navigation Satellite System) is also used. Under this term, the NAVSTAR GPS, GLONASS and GALILEO is also meant.

  11. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Accuracies / Error sources

  12. GPS satellite concept A minimum of 4 satellites must be tracked at the same time to determine position. Basically 3 satellite could be enough, but we have to face some errors. 1 Satellite 2 Satellites

  13. GPS satellite concept Error-free solution Error-free solution Error-free solution 3 Satellites

  14. GPS satellite concept • 24 Satellitesplanned • 12-hour circular orbit in the elevation of 20200 km • 55oinclination • 4 satelliteson each orbital plane (6 Orbital planes – rect ascebsion difference 60°) • At least 3 (now 8) reserve satellites

  15. GPS satellite concept

  16. GPS satellite concept • GPS Carrier phases • L1 = 1575,42 Mhz (C/A Code, P-Code) • L2 = 1227,6 Mhz (P-Code) C/A Kode – „coarse acquisiton code“ (low accuracy) P Kode – „precision code“ (high accuracy)

  17. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Accuracies / Error sources

  18. GPS System Segments • Three segments: • space segment • control segment • user segment

  19. GPS System segments • Space segment: • 24+8 GPS Satellites • Orbits (20183 km, Inclination 55oand 60obetween the orbital planes) • At least 4 Satellites on each orbital plane, where the angular difference is 120o • Period 12 hours • Each satellite has its own ID, which is boradcasted to the user. • The satellites are equiped with atomic clocks.

  20. GPS System segments Ground segment

  21. GPS System segments • Tasks of the ground segment: • Controlling and managing the telemetry and control stations. • Computation of ephemerids (orbit parameters) for each satellite. • Ordering satellite maneuvres. • Computing the data for the almanach • Determine the GPS time (Atomuhr) • Communication link to the satellites

  22. GPS System segments User segment: GPS receivers • track L1 and/or L2 frequencies • track C/A code for at least 4 satellites, and demodulation • Time synchronization (Quartz clocks in the receivers) • Decrypt satellite data from the code observations (orbit, etc.) • receive P(Y) code (US Army) • Compute the pseudo-range to each satellite • Compute the time offset (receiver clock error) • Compute the position.

  23. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Accuracies / Error sources

  24. Principle of Positioning Coordinate system (WGS-84) The coordinates of satellites are given in this system.

  25. Principle of Positioning Equal time circles 2 Satellites 3 Satellites p = cT

  26. Principle of Positioning Accuracies • Depends on the accuracy of distance measurements • Distance measurements – Travel time measurement • 1ms (=) 300m (only atomic clocks could measure with this accuracy) • clock offset – a fourth satellite is also necessary for the determination of positions. (Receiver clock error)

  27. Principle of Positioning Observation equations: p1..p4 Pseudoranges xp, yp, zpCoordinates of the receiver xi, yi, ziCoordinates of the satellite i, i=1..4 DpOffset due to the clock error DTReceiver clock correction eiadditional error

  28. Principle of Positioning The navigation solution provides us a worlwide accurate time system. By determining the receiver clock error, one can restore the GPS time using cheap quartz clocks. Since the GPS time is an atomic time, we can substitute an atomic clock using a GPS receiver.

  29. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Accuracies / Error sources • Differential GPS (DGPS) • Applications in Surveying

  30. GPS codes and their travel time Two frequencies: L1=1575,42 Mhz and L2=1227,60 Mhz Both of them are a multiple of the ground freqency: L0=10,23 Mhz The carrier phases are with PRN-codes („pseudo random noise“) modulated (L1 – C/A u. P, L2 – P). All codes are biploar codes: Positive Amplitude=0 Neg. Alplitude=1

  31. GPS codes and their travel time • What is a PRN code? • It is a statistical distribution of impulses, which are similar to noise. • In reality the sequence of the impulses follow a complicated rule, which is necessary for the decryption. The receiver knows this algorithm. If one can determine the time delay with the accuracy of 1%: C/A Code accuracy of 3 m chiprate 1ms=300m P Code accuracy of 0,3 m chiprate 0,1ms = 30m

  32. GPS codes and their travel time GPS Signal modulation: • The C/A code contains 1023 positive and negative Impulses (bits) with the span of 1 ms each. The whole sequence has the length of 300km (corresponds to 1ms). • A navigation message is also modulated to the carrier freqency with the frequency of 50bit/sec. The length of the message is 1500 bits (30 sec) • These 1500 Bitsare split into 5 sub-frames with 10 words of 30 bits in each subframe. Each subframe is started with two special words: • „Telemetry Word“ (TLM) = Orbit corrections • „Hand Over Word“ (HOW) = GPS time

  33. GPS codes and their travel time Clock corrections are present in the first subframe. In the second and third the orbit parameters and iono corrections are broadcasted.

  34. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Phase observations • Accuracies / Error sources

  35. Phase observations It could be seen that the code observation is too inaccurate for the applications in surveying; A more accurate method is needed for the determination of the satellite-receiver distance. Instead of using the codes, the carrier phases are measured; Since the wavelength of the L1 carrier signal is approx. 19cm, a more accurate distance observation can be achieved (in the order of a few mm)

  36. Phase observations

  37. Contents • Navigation • History of GPS • GPS Satellite Concept • GPS System Segments • Principle of Positioning • GPS codes and the determination of travel time • Phase observations • Accuracies / Error sources

  38. Error sources

  39. Accuracy and error sources • Error sources – signal propagation • The atmosphere has an impact on the signal propagation path. • Troposphere and Ionosphere • Ionosphere • The Ionosphere (70-1000 km elevation) contain electrons and ions, which have an effect on the propagation of electromagnetic signals. • the delays depend on the frequency, intensity of the radiation of the sun (day-night variation, seasonal variation, effect of the solar activity - sunspots);

  40. Accuracy and error sources • Troposphere • Up to 40-70 km (tropospheric refraction) • Meteorological factors (weather, air pressure, temperature, etc.) • Empirical models • overall effect is ca. 2.3m in the vertical direction • Multipath • GPS signals are reflected, and direct and indirect signals are also received. Indirect Direct

  41. Accuracy and error sources • Cycle slips • The continuous signal reception is broken; • Thus the counting of the number of full cycles is erroneous (nl) • Solution: avoid nearby objects that could obscure the satellites

  42. Accuracy and error sources • Error sources – receiver error • Thermal noise • receiver clock corrections • antenna phase center offsets and variations

  43. Accuracy and error sources • Error sources – satellite geometry • Not all of the satellite constellation enables the optimal positioning. • DOP – „Dilution of Precision“ dr accuracy of the pseudorange observation dPOSaccuracy of the positioning PDOP is the reciprocal value of the volume V of a tetraeder of 4 satellites + 1 ground station.

  44. Accuracy and error sources Good Bad

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