【導讀】文獻、資料發(fā)表（出版）日期：。山東建筑大學畢業(yè)論文外文文獻及譯文

　　

【正文】 pheric error is the largest error source for a singlefrequency GPS receiver. An important thing to note about navigation data is that each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites. All satellites broadcast at the same two frequencies, GHz (L1 signal) and GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses a code division multiple access (CDMA) spreadspectrum technique where the lowbitrate message data is encoded with a highrate pseudorandom (PRN) sequence that is different for each satellite. The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual message data. The message data is transmitted at 50 bits per second. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code (a socalled Gold code) at million chips per second, and the precise (P) code at million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[36] The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a socalled P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise timeofday to the user. Satellite frequencies L1 ( MHz): Mix of Navigation Message, coarseacquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future satellites. ? L2 ( MHz): P(Y) code, plus the new L2C code on the Block IIRM and newer satellites. ? L3 ( MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other highenergy infrared events. Used to enforce nuclear test ban treaties. ? L4 ( MHz): Being studied for additional ionospheric correction. 山東建筑大學畢業(yè) 論文 外文文獻 及譯文 16 ? L5 ( MHz): Proposed for use as a civilian safetyoflife (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, ? promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2021.[37] Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code. 山東建筑大學畢業(yè) 論文 外文文獻 及譯文 17 Since all of the satellite signals are modulated onto the same L1 carrier frequency, there is a need to separate the signals after demodulation. This is done by assigning each satellite a unique pseudorandom sequence known as a Gold code, and the signals are decoded, after demodulation, using modulo 2 addition of the Gold codes corresponding to satellites n1 through nk, where k is the number of channels in the GPS receiver and n1 through nk are the pseudorandom numbers associated with the satellites. The results of these modulo 2 additions are the 50 bit/s navigation messages from satellites n1 through nk. If the almanac information has previously been acquired, the receiver picks which satellites to listen for by their PRNs. If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite39。s signal, it identifies it by its distinct C/A code pattern. P(Y) code P(Y)碼 山東建筑大學畢業(yè) 論文 外文文獻 及譯文 18 Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.[citation needed] In parison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective. Error sources and analysis Sources of User Equivalent Range Errors (UERE) Source Effect Signal Arrival C/A 177。 3 m Signal Arrival P(Y) 177。 m Ionospheric effects 177。 5 m Ephemeris errors 177。 m Satellite clock errors 177。 2 m Multipath distortion 177。 1 m Tropospheric effects 177。 m 山東建筑大學畢業(yè) 論文 外文文獻 及譯文 19 C/A 177。 6,7 m P(Y) 177。 6,0 m User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, , of about 1 meter. The standard deviations, , for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are puted by taking the square root of the sum of the squares of the individual ponents (. RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS39。ed with the numerical error. Electronics errors are one of several accuracydegrading effects outlined in the table above. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code39。s accuracy. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally.(see summary table near end of Sources of Errors in GPS) Error Diagram Showing Relation of Indicated Receiver Position, Intersection of Sphere Surfaces, and True Receiver Position in Terms of Pseudorange Errors, PDOP, and Numerical Errors The term user equivalent range error (UERE) refers to