航空電子設(shè)備廣漢gpsenglish

1、 Civil Aviation Flight University of China Global Positioning System 全球定位系統(tǒng)全球定位系統(tǒng)Global Positioning System (GPS) Civil Aviation Flight University of China There are three basic parts of the Global Positioning System: the space segment, the control segment, the user segment.(Figure14.1)GPS ElementsSp

2、ace segmentMonitoring stationsControl segment User segmentUpload stationMaster control stationFigure 14.1The Space Segment Civil Aviation Flight University of China Space Segment The space segment consists of 21 operations satellites and three active spares. The complete GPS space system includes 24

3、 satellites, 20,200 km above the earth, take 12 hours each to go around the earth once or one orbit. They are orbit in six different planes and 55 degrees inclination (Figure14.2). GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a s

4、olar eclipse, when theres no solar power. Small rocket boosters on each satellite keep them flying in the correct path. Each satellite contains four atomic clocks. These clocks are accurate to a nanosecond. Basic functions of the satellites are to: Receive and store information transmitted by the co

5、ntrol segment Perform limited data processing by means of onboard microprocessor Maintain a very accurate time base through the use of 4 oscillators,2 cesium clocks , and 2 rubidium clocks Transmit information Execute satellite maneuvering by means of ground-controlled thrustersGPS Elements Civil Av

6、iation Flight University of China GPS ElementsFigure 14.2 Civil Aviation Flight University of China The Control Segment The control segment consists of a master control station(MCS) at Falcon AFB in Colorado Springs, Colorado, and five linked monitoring stations(MS) around the world(Figure14.3). The

7、se stations are located at Hawaii, Ascension Island (South Atlantic), Diego Garcia Island (Indian Ocean), Kwajalein (South Pacific) ,and collocated with MCS at Falcon AFB.GPS ElementsFigure 14.3 Civil Aviation Flight University of China The MCS is the central processing facility for the network and

8、is manned 24 hours per day, 7days per week. It is tasked with tracking, monitoring, and managing the GPS satellite constellation and for updating the NAV-msg. The task of the monitor station is to passively track all GPS satellites in view(up to 11 simultaneously) and collect ranging data from each.

9、 The MSs are very accurate radio receivers located at precisely surveyed locations. Monitor stations do little data processing themselves, rather, send their raw measurements and NAV-msg observations to the master control station. The information is processed by the MCS, where satellite ephemeris an

10、d clock parameters are estimated and predicted. Using this information, the MCS periodically uploads the ephemeris and clock data to each satellite for retransmission in NAV-msg. The updated information is transmitted to the satellites via the ground antennas (Gas), which are also used for transmitt

11、ing and receiving satellites control information . GPS Elements Civil Aviation Flight University of China The User Segment The user segment consists of a variety of military and civilian receiver/processors specifically designed to receiver, decode, and process the GPS satellite ranging codes and na

12、vigation data messages. These include stand-alone units and integrated equipment using GPS in combination with other navigation systems(i.e. Inertial Navigation Systems). Note that the Global Positioning System is designed for two levels of users, the Standard Positioning Service(SPS)and the Precise

13、 Positioning Service(PPS).The DOD(Department of Defense)states very clearly what SPS and PPS are expected to do in the Federal Radio navigation Plan(FRP),which generally equates to the PPS being reserved for military use and SPS for all others . Manufacturers of receiver/processor have designed equi

14、pment to track the GPS satellite radio signals and provide astonishingly accurate(as compared to other current systems)position ,velocity ,and time information. Due to the wide potential for specialized and varied applications of GPS, user equipment can vary significantly in design and function. How

15、ever, on the civilian navigation user market the differences are not greatly pronounced, mainly amounting to calculation, database, and display features. GPS Elements Civil Aviation Flight University of China The Principle of GPS GPS position determination is based on a concept called time of arriva

16、l (TOA) ranging. The interval between the time of transmission and the time of reception is the TOA value. As a example of the TOA ranging concept，assume the transmitter is a foghorn that blows exactly on the minute mark and the receiver is a mariner with a chronometer. The foghorn blows exactly on

17、the minute-precise and known time of transmission. The foghorns sound arrives at the mariners position exactly 10seconds after the minute mark. The mariner then multiplies the TOA value of l0 seconds by the speed of sound (1,088 feet per second at sea level at 32 degrees F), resulting in a range of

18、10,880 feet. If the same mariner could calculate the range from a second foghorn，a position relative to the two foghorns could be determined. Merely draw a circle (TOA-based range circle), centered on the known foghorn positions, with the radius equal to the determined range, for each foghorn. The m

19、ariners location is where the circle intersect (Figure14.4A).Location of Foghorn No.1USERLocation ofFoghorn No.2TOA-basedrange No.1TOA-basedrange No.2A.TOA-based Position DeterminationFigure 14.4 Civil Aviation Flight University of China The Principle of GPS To reduce intersection ambiguity, range m

20、easurements can be made to three foghorns of known location. This will increase the number of intersection point, however, only one intersection point will be common to all the TOAbased range circles；the other intersections will be false(Figure14.4B). The chronometer used for time measurement during

21、 TOA ranging must be very accurate, or precision positioning will not be possible. An error in clock accuracy (called a time-bias error) is, however, correctable. Foghorn No.1USERFoghornNo.2TOA-basedrange No.1TOA-basedrange No.2TOA-basedrange No.3Foghorn No.3 B.Eliminating False Intersection Ambigui

22、ty Figure 14.4 Civil Aviation Flight University of China The Principle of GPSThe effect of a time-bias error is erroneous intersection points (Figure14.4C). If the mariners chronometer was running one second fast in a 10-second period，then the sound of the foghorn that arrived 10 seconds past the mi

23、nute mark would appear to have arrived 11 seconds past the minute mark. This error would cause the mariner to compute an erroneous range of 11，968 feet from that foghorna 1088-foot errorAs the same chronometer would be used to note the TOA from other foghorns，all the TOAbased range observations woul

24、d have range errors of 1088 feet(shown as E in the example). If range observations were made from only two foghorns，the mariner would have an erroneous position fix. However the problem of time-bias errors and ambiguous intersections can be solved. Foghorn No.1USERFoghornNo.2TOA-basedrange No.1+ETOA

25、-basedrange No.2+EEEErroneousposition fixC. Effect of a Chronometer Bias Figure 14.4 Civil Aviation Flight University of China The Principle of GPS Using the same means as before，note there are three dualfoghorn intersections near the mariners true position(Figure14.4D).Distance E between the inters

26、ections of range circles from foghorns one and two，foghorns one and three，and foghorns two and three is strictly a function of the chronometers time bias. By adjusting the range measurements forward or backward until the three dual-foghorn intersections converge at the true position，the chronometers

27、 time bias can be zeroed out. As a result of using three transmitters with certain known facts about each (exact position and time of transmission)，the ranging information can be applied to a map for determining latitude and longitude coordinates. Additionally, the time-bias error es a known and cor

28、rectable quantity. Foghorn No.1USERFoghornNo.2TOA-basedrange No.1TOA-basedrange No.2TOA-basedrange No.3Foghorn No.3D.TOA-based Position and Time DeterminationFigure 14.4 Civil Aviation Flight University of China GPS Ranging Orbiting NAVSTAR satellites are the broadcast beacons (transmitters)at the c

29、enter of TOA-based three-dimensional range spheres. Their signals are sent at the speed of light(186,000 miles per second)and consist of pseudorandom noise(PRN)modulated L-band radio waves. The PRN sequences，C/A(coarse/acquisition)-codes and P(precision)-codes, are predetermined strings of one and z

30、ero data bits generated by an on-board clock that also provides the exact transmit time of the broadcasted signals(precise and known time of transmission). The GPS satellites transmit radio signals via spread spectrum techniques on two frequencies, known as L1 and L2. The L1 channel produces a Carri

31、er Phase signal at 1575.42MHz as well as a C/A and P Code. The L2 channel produces a Carrier Phase signal of 1227.6MHz, but only P Code. These codes are binary data modulated on the carrier signal. The C/A or Coarse/Acquisition Code (also known as the civilian code), is modulated and repeated every

32、millisecond; the P-Code, or Precise Code, is modulated or repeated every seven days.The Principle of GPS Civil Aviation Flight University of China Clock Bias If the GPS receivers clock was synchronized exactly to the on-board satellite clock the TOA values observed by the receiver would be equal to

33、the actual geometric ranges between the satellites and the user divided by the speed of light. However，in GPS it is not practical to adjust the receivers clock to zero the time bias. GPS works with radio signal(traveling at the speed of light)and requires clock accuracy to within a few billionths of

34、 a second. To resolve this time problem，the GPS receivers clock is left free-running，while the data processor in the receiver mathematically determines the amount of adjustment required to zero the clocks time bias. As a result of the processors computations，the receivers observed TOA values are the

35、 actual range from each satellite divided by the speed of light plus the time-bias adjustment. The results are called pseudorange(PR) measurements，because they are similar to measuring the range from the satellites except for the range error of the GPS receivers clock time bias(Figure14.5).The Princ

36、iple of GPS Civil Aviation Flight University of China The following definition is the heart of GPS：A pseudorange measurement is equal to the GPS receivers observed TOA value multiplied by the speed of light，when the observed TOA value includes both the signal propagation delay due to the actual geom

37、etric range and the GPS receivers clock bias.Figure 14.5The Principle of GPSR1R2R3R4EEEEUserspositionPR4PR3PR2PR1NOTE: R = Actual geometric range (from each satellite to user) E = Range error caused by GPS receiver clock bias PR = Pseudorange = R + E = observed TOA value speed of light Civil Aviatio

38、n Flight University of China Position and Time Computations When the GPS receiver begins tracking the PRN sequences from four satellites, and generating TOA values，the receivers data processor takes overBy sampling the TOA values from the GPS receiver for each of four satellitesIt multiplies them by

39、 the speed of light to produce four PR measurements. The four unknown quantities are the users X-position coordinate, Y-position coordinate, Z-position coordinate, plus the time bias (sometimes referred to as the CB or clock bias). As GPS is a three-dimensional positioning system, a fourth TOA-based

40、 range sphere is needed. Movement of the GPS satellites is no consequence, as the NAV-message, which is transmitted from the satellites, contains the information required by data processor to compute the satellites exact position at any point in time(Figure14.6). Data Processor Obtains Pseudorange M

41、easurements (PR1, PR2, PR3, PR4) From Four Satellites Data Processor Performs the Position/Time SolutionThe Principle of GPS Civil Aviation Flight University of China The Principle of GPSPR1PR2PR3PR4 T1 T2 T3 T4PR1 = T1 cPR2 = T2 cPR3 = T3 cPR4 = T4 cc = speed of lightTime-codedsignals transmittedby

42、 each satelliteTime each signalreceived by user Four Ranging Equation:24242424232323232222222221212121)()()()()()()()()()()()()()()()(cCBPRUZUYUXcCBPRUZUYUXcCBPRUZUYUXcCBPRUZUYUXZYXZYXZYXZYXFigure 14.6 Civil Aviation Flight University of China The Receiver of GPS Antenna and the Preamplifier Antenna

43、s used for GPS receivers have broad beam characteristic, thus they do not have to be pointed to the signal source like satellite TV dishes.Antenna and preamplifierRF sectionMicroprocessorControl Display UnitRecording devicesPower supplyFigure 14.7 Civil Aviation Flight University of China The Receiv

44、er of GPS RF(radio frequency) Section and Microprocessor The RF section contains the signal processing electronics in a combination of digital and analog circuits. Different receivers use different techniques to process the signals. Antenna and preamplifierRF sectionMicroprocessorControl Display Uni

45、tRecording devicesPower supplyFigure 14.7 Civil Aviation Flight University of China The Receiver of GPS Control Display Unit The control display unit enables the operator to interact with the microprocessor. It varies greatly for different receivers and applications, ranging from handheld to a video

46、 monitor with full-size keypad.Antenna and preamplifierRF sectionMicroprocessorControl Display UnitRecording devicesPower supplyFigure 14.7 Civil Aviation Flight University of China The Receiver of GPS Recording Devices They are used to record the observations and other useful information extracted

47、from the received signal. Power Supply Receivers need only a reliable low voltage DC power supply.Antenna and preamplifierRF sectionMicroprocessorControl Display UnitRecording devicesPower supplyFigure 14.7 Civil Aviation Flight University of China The Receiver of GPSFigure 14.9Figure 14.8 Civil Avi

48、ation Flight University of China The Errors of GPS There has been a misconception over the past years about the accuracy of GPS. It is true that for many years the US Department of Defense maintained intentional degradation of accuracy called Select Availability (S/A), a system for randomly degradin

49、g the accuracy of the signals being transmitted to civilian GPS receivers. However, the S/A was removed in May 2000.Therefore, the accuracy of GPS should be a discussion based on the type of system (device) and its ability to eliminate error sources and not on the availability of reliable satellite

50、signals. Civil Aviation Flight University of China The Errors of GPS Ionospheric Propagation Effects The ionosphere, which we know is the band of charged particles which lies between 80 and 120 miles above the surface of the earth, affects the propagation speed and thus the travel time of the GPS si

51、gnals thereby degrading the accuracy of the position solution. Ionospheric propagation effects can be offset by the receiver with data received from the satellites. Tropospheric Propagation Effects The lower region of the atmosphere, the troposphere, contains significant amounts of water vapour. The

52、 effect of this is to slow down the satellite signals, thus inducing ranging errors. This tends to degrade position accuracy. However, tropospheric propagation effects are to some extent minimized by appropriate compensation modelling in the receiver. Multi-path Error In a similar manner to the beha

53、viour of signals used by other radio navigation systems, it is possible for some of the satellite signals e.g. the pseudo-random code signals, to reach the receiver antenna after bouncing off the earths surface, as well as directly from the satellite. Thus the receiver can receive signals from diffe

54、rent directions. This can lead to a distortion of the C/A- and P-coded pulses which in turn can induce a ranging error. Civil Aviation Flight University of China The Errors of GPS Ephemeris Error Ephemeris error is the error inherent in the data that defines the satellites current position, which in

55、 turn is transmitted to the receiver. Interference Because GPS signals are relatively weak, harmful interference can cause significant degradation in navigation or complete loss of navigation capability under certain conditions. With more and more extensive use of all bands of the electromagnetic sp

56、ectrum, the potential for interference problems to occur has increased. The trend is likely to continue. Receiver Error This is simply a small ranging error brought about by the difficulty of matching precisely, the receivers emitted digital psuedo-random code with that of the satellites. Civil Avia

57、tion Flight University of China The Errors of GPS Satellite Geometry This means the relative position of the satellites at a specific moment. When satellites are located at wide angles relative to each other, the possible error margin is small (Figure14.10). On the contrary, when satellites are grou

58、ped together or located in a line the geometry will be poor(Figure14.11). The effect of the geometry of the satellites on the position error is called Geometric Dilution of Precision (GDOP). GDOP comprises the components shown below, which can be individually computed but are not independent of each

59、 other:HDOP - Horizontal Dilution of Precision (Latitude, Longitude),describes the effect of satellite geometry on the latitude/longitude errors.VDOP - Vertical Dilution of Precision (Height), describes the effect of satellite geometry on the receiver/processor altitude errors.PDOP - Position Diluti

60、on of Precision (3-D),a combination of HDOP and VDOP Civil Aviation Flight University of China The Errors of GPSFigure 14.10Figure 14.11 Civil Aviation Flight University of China The Differential of GPS As already mentioned, GPS can exhibit variation of accuracy. The adverse effects of these variati