航空电子设备：GPS(English) 全球定位系统

1、Civil Aviation Flight University of China,Global Positioning System,全球定位系统,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 Elements,Figure 1

2、4.1,The 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 satellites, 20,200 km above the earth, take 12 hours each to go around the earth once or one orbit.

3、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 solar eclipse, when theres no solar power. Small rocket boosters on each satellite keep them flying in

4、 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 control segment Perform limited data processing by means of onboard microprocessor Maintain a very accu

5、rate 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 thrusters,GPS Elements,Civil Aviation Flight University of China,GPS Elements,Figure 14.2,Civil Aviation Flight University of China

6、,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). These stations are located at Hawaii, Ascension Island (South Atlantic), Diego Garcia Island (Indian O

7、cean), Kwajalein (South Pacific) ,and collocated with MCS at Falcon AFB,GPS Elements,Figure 14.3,Civil Aviation Flight University of China,The MCS is the central processing facility for the network and is manned 24 hours per day, 7days per week. It is tasked with tracking, monitoring, and managing t

8、he 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. The MSs are very accurate radio receivers located at precisely surveyed locations. Monitor statio

9、ns 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 and clock parameters are estimated and predicted. Using this information, the MCS periodically uploa

10、ds 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 transmitting and receiving satellites control information,GPS Elements,Civil Aviation Flight University of

11、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 navigation data messages. These include stand-alone units and integrated equipment using GPS in combin

12、ation 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 Positioning Service(PPS).The DOD(Department of Defense)states very clearly what SPS and PPS are exp

13、ected 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 equipment to track the GPS satellite radio signals and provide astonishingly accurate(as compared to oth

14、er 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. However, on the civilian navigation user market the differences are not greatly pronounced, mainly amou

15、nting 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 arrival (TOA) ranging. The interval between the time of transmission and the time of reception is the TOA v

16、alue. 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 the minute-precise and known time of transmission. The foghorns sound arrives at the mariners positio

17、n 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 10,880 feet. If the same mariner could calculate the range from a second foghorn，a position relative

18、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 mariners location is where the circle intersect (Figure14.4A,Figure 14.4,Civil Aviation Flight Univers

19、ity of China,The Principle of GPS,To reduce intersection ambiguity, range measurements 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 w

20、ill be false(Figure14.4B). The chronometer used for time measurement during 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,Figure 14.4,Civil Aviation Flight University of China,The Princ

21、iple of GPS,The 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 minute mark would appear to have arrived 11 seconds past the minute

22、 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 would have range errors of 1088 feet(shown as E in the example). If r

23、ange 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,Figure 14.4,Civil Aviation Flight University of China,The Principle of GPS,Using the same means as before，note there

24、are three dualfoghorn intersections near the mariners true position(Figure14.4D).Distance E between the intersections 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

25、 forward or backward until the three dual-foghorn intersections converge at the true position，the chronometers 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

26、a map for determining latitude and longitude coordinates. Additionally, the time-bias error becomes a known and correctable quantity,Figure 14.4,Civil Aviation Flight University of China,GPS Ranging Orbiting NAVSTAR satellites are the broadcast beacons (transmitters)at the center of TOA-based three-

27、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 zero data bits generated b

28、y 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 Carrier Phase signal at 1575.4

29、2MHz 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 millisecond; the P-Code,

30、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 the actual geometric rang

31、es 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 a second. To resolve thi

32、s 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 actual range from each s

33、atellite 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 Principle of GPS,Civil Aviation

34、 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 geometric range and the GPS re

35、ceivers clock bias,Figure 14.5,The Principle of GPS,Civil Aviation 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

36、the GPS receiver for each of four satellitesIt multiplies them by 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

37、GPS is a three-dimensional positioning system, a fourth TOA-based 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 an

38、y point in time(Figure14.6). Data Processor Obtains Pseudorange Measurements (PR1, PR2, PR3, PR4) From Four Satellites Data Processor Performs the Position/Time Solution,The Principle of GPS,Civil Aviation Flight University of China,The Principle of GPS,Four Ranging Equation,Figure 14.6,Civil Aviati

39、on Flight University of China,The Receiver of GPS,Antenna and the Preamplifier Antennas used for GPS receivers have broad beam characteristic, thus they do not have to be pointed to the signal source like satellite TV dishes,Figure 14.7,Civil Aviation Flight University of China,The Receiver of GPS,R

40、F(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,Figure 14.7,Civil Aviation Flight University of China,The Receiver of GPS,Contr

41、ol 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 monitor with full-size keypad,Figure 14.7,Civil Aviation Flight University of China,The Receiver of GPS,Reco

42、rding Devices They are used to record the observations and other useful information extracted from the received signal. Power Supply Receivers need only a reliable low voltage DC power supply,Figure 14.7,Civil Aviation Flight University of China,The Receiver of GPS,Figure 14.9,Figure 14.8,Civil Avia

43、tion 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 degrading

44、 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 s

45、ignals,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 sign

46、als 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 e

47、ffect 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 behavi

48、our 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 differe

49、nt 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 tu

50、rn 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 spect

51、rum, 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 Aviation

52、 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 grouped

53、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 oth

54、er: 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 Dilutio

55、n of Precision (3-D),a combination of HDOP and VDOP,Civil Aviation Flight University of China,The Errors of GPS,Figure 14.10,Figure 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 variat

56、ions may be substantially reduced or eliminated by differential techniques. Further, GPS integrity problems can be addressed within the concept of differential GPS(DGPS) by continuously monitoring the systems integrity and health of individual satellites and making this information a part of the cor

57、rections information scheme. In differential operation, a special GPS receiving facility is precisely located at a fixed point within an area of interest. Received GPS signals are observed in real time and compared with the signals expected to be observed at that fixed point. The differences between

58、 the observed signals and predicted signals are provided to users as differential corrections to increase the precision and performance of those users GPS receivers. (Figure14.12,Civil Aviation Flight University of China,The Differential of GPS,Figure 14.12,Civil Aviation Flight University of China,

59、The Differential of GPS,A DGPS reference station is fixed at a geodetically surveyed position. From this position, the reference station tracks all satellites in view, download ephemeris data from them, and computers corrections based on its real-time measurements and known precise geodetic position. These corrections are then broadcast via radio to GPS users to improve their navigation solution. These are two well-developed methods of handling this: Computing and transmitting a position correction in X-Y-Z coordinates, which is then ap