IS-GPS-800D (797938), страница 3
Текст из файла (страница 3)
Urbanke, “Efficient Encoding of Low-Density Parity-Check Codes,” IEEETransactions on Information Theory, Vol. 47, N0. 2, February 2001.[2] J. Betz, “Binary Offset Carrier Modulations for Radionavigation,” Journal of the Institute ofNavigation, vol.
48, pp. 227-246, 2001IERS Technical Note 36 International Earth Rotation and Reference System Technical Note 363 SIGNAL REQUIREMENTSThe requirements specified in this section define the requisite characteristics of the SS/USinterface for the GPS L1C signal.3.1 Signal StructureThe GPS SV typically transmits multiple distinct signals modulated on the L1 RF carrier. Thesignals include C/A, P(Y), M, and L1C which are modulated on the carrier frequency.
The L1Csignal defined in this IS consists of two main components; one denoted L1CP to represent a pilotsignal, without any data message, that is spread by a ranging code, and L1CD that is spread by aranging code and modulated by a data message. The L1CP is also modulated by an SV uniqueoverlay code, L1CO.The L1CP and L1CD components are transmitted using ranging codes defined in Section 3.2.2.The SVs shall transmit intentionally "incorrect" versions of the respective ranging codes asneeded to protect users from receiving and utilizing anomalous signals.
These "incorrect" codes2IS-GPS-800D24 Sep 2013are termed non-standard L1CP (NSCP) and non-standard L1CD (NSCD). Non-standard codes arenot for utilization by the users and, therefore, are not defined in this document.The data message on L1CD, denoted DL1C(t), includes SV ephemerides, system time, system timeoffsets, SV clock behavior, status messages, and other data messages. The message structure anddata encoding techniques are defined in Section 3.2.3.The L1CD signal is modulated on the L1 RF carrier using a Binary Offset Carrier (BOC) (1, 1)modulation technique. The L1CP signal is modulated on the L1 RF carrier using a TimeMultiplexed BOC (TMBOC) modulation technique. The TMBOC technique utilized by L1CPsignal uses a combination of BOC (1, 1) and BOC (6, 1) modulation as described in Section 3.3.3.2Signal Definition3.2.1 Signal CharacteristicsThe following specifies the characteristics and quality of the L1C signal.3.2.1.1 Frequency PlanThe carrier frequency for the L1C signal shall be coherently derived from a frequency sourcecommon with other signals within the SV.The nominal frequency of this source as it appears to an observer on the ground is 10.23 MHz.The SV carrier frequency and clock rates, as they would appear to an observer located in the SV,are offset to compensate for relativistic effects.The clock rates are offset by ∆ f/f = -4.4647E-10, which is equivalent to a change in the L1Ccode chipping rate of 1.023 MHz by a ∆ f = -4.5674E-4 Hz.
This results in an offset L1C-codechipping rate of 1.02299999954326 MHz. The nominal carrier frequency (f0) - as it appears toan observer on the ground - shall be 1575.42 MHz. The requirements specified in this IS shallpertain to the signal contained within a 30.69 MHz bandwidth centered about the L1 nominalfrequency. The L1C signal shall utilize a modulation technique of BOC (fs, 1) which specifies asubcarrier frequency of fs × 1.023 MHz and a spreading code chipping rate of 1 × 1.023 MHz =1.023 MHz.3.2.1.2 Signal PolarizationThe transmitted signal shall be Right-Hand Circularly Polarized (RHCP).For an angular range of ±13.8 degrees from nadir, the L1 ellipticity shall be no worse than 1.8dB.3.2.1.3 Carrier Phase NoiseThe phase noise spectral density of the unmodulated carrier shall be such that an approximationto the third order Jaffe-Rechtin phase lock loop, closed-loop transfer function H(f) such that |1 -3IS-GPS-800D24 Sep 2013H(f)|2 = f6/(fn6 + f6) where fn = 3BL/(5Π), which has a 10 Hz one-sided loop noise bandwidth,shall be able to track the carrier to an accuracy of 0.035 radians rms.3.2.1.4 Spurious TransmissionsIn-band spurious transmissions, from the SV, shall be at or below -40 dBc over the bandspecified in 3.2.1.1.
In-band spurious transmissions are defined as transmissions within the bandspecified in 3.2.1.1 which are not expressly components of the L1 signal.3.2.1.5 Correlation LossCorrelation loss is defined as the difference between the SV power received in the bandwidthdefined in 3.2.1.1 and the signal power recovered in an ideal correlation receiver of the samebandwidth using an exact replica of the waveform within an ideal sharp-cutoff filter bandwidthcentered at L1, whose bandwidth corresponds to that specified in 3.2.1.1 and whose phase islinear over that bandwidth. The correlation loss apportionment due to SV modulation andfiltering imperfections shall be 0.2 dB maximum.3.2.1.6 Signal Component Phasing3.2.1.6.1 Phase RelationshipCarriers of the two L1C components defined in Section 3.1 shall be in the same phase within±100 milliradians.
Carriers of the two L1C components shall be in the same phase (within ±100milliradians) as the P(Y)-code carrier. See IS-GPS-200 for phase relationships to other L1signals.3.2.1.6.2 Phase ContinuityWhile a satellite is broadcasting standard L1CP code and standard L1CD code signals with datawhich indicates L1C signal health is OK, there will not be any commanded operation causing anintentional phase discontinuity. This does not apply to phase discontinuities caused by signalmodulation.
Phase discontinuities are subject to the requirements of 3.2.1.6.1.3.2.1.7 Signal Characteristics3.2.1.7.1 Signal CoherenceAll transmitted signals for a particular SV shall be coherently derived from the same on-boardfrequency standard. On the L1 carrier, the chip transitions of the two modulating signals, L1CDand L1CP, shall be such that the average time difference between them (i.e. L1CD/L1CP), andbetween each and the transitions of L1P(Y) (i.e. L1CD/L1P(Y), L1CP/L1P(Y)), do not exceed 10nanoseconds.The variable time difference shall not exceed 1 nanosecond (95% probability), when includingconsideration of the temperature and antenna effect changes during a vehicle orbital revolution.4IS-GPS-800D24 Sep 2013Corrections for the bias components of the time difference are provided to the US in the CNAV2 message using parameters designated as ISCs (reference paragraph 3.5.3.9.1).3.2.1.7.2 Signal DistortionThe duration of the “+1 polarity” portions of the BOC (1, 1) code shall equal the duration of the“-1 polarity” portions of the BOC (1, 1) code within 5 nanoseconds as measured at the zerocrossing point.The duration of the “+1 polarity” portions of the BOC (6, 1) code shall equal the duration of the“-1 polarity” portions of the BOC (6, 1) code within 5 nanoseconds as measured at the zerocrossing point.3.2.1.8 Equipment Group DelayEquipment group delay is defined as the delay between the signal radiated output of a specificSV (measured at the antenna phase center as observed from the signal’s zero crossings) and theoutput of that SV's on-board frequency source; the delay consists of a bias term and anuncertainty.
The bias term is of no concern to the US since it is included in the clock correctionparameters relayed in the navigation data, and is therefore accounted for by the usercomputations of system time. The uncertainty (variation) of this delay, as well as the groupdelay differential, between the reference signal and the signals of L1C, are defined in thefollowing subsections.3.2.1.8.1 Group Delay UncertaintyThe effective uncertainty of the group delay shall not exceed 1.5 nanoseconds (95% probability).3.2.1.8.2 Group Delay DifferentialNot applicable. See Sections 3.2.1.7.1 (Signal Coherence) and 3.5.3.9.1 (Inter-Signal GroupDelay Differential Correction).3.2.1.8.3 Space Service Volume (SSV) Group Delay DifferentialThe group delay differential for the radiated L1 signal with respect to the Earth Coverage signalfor users of the Space Service Volume are provided in http://www.igs.org/products/ssv3.2.1.9 Signal Power LevelsThe SV shall provide an L1C signal strength at End-of-Life (EOL), worst-case, in order to meetthe minimum effective received signal levels specified in Table 3.2-1.
Any combining operationdone by the SV and associated loss is compensated by an increase in SV transmitted power andthus transparent to the user segment. For terrestrial users, the minimum effective received signalpower is measured at the output of a 3 dBi linearly polarized user receiving antenna (located nearground) at worst normal orientation, when the SV elevation angle is higher than 5 degrees andassuming 0.5 dB atmospheric loss.
For orbital users, the minimum effective received signalpower is measured at the output of a 0 dBi ideal right-hand circularly polarized (i.e. 0 dB axial5IS-GPS-800D24 Sep 2013ratio) user receiving antenna (in geosynchronous orbit) at 23.5 degrees off nadir and using 0 dBatmospheric loss. The received signal levels are observed within the in-band allocation definedin Para. 3.2.1.1The SV shall provide signals with the following characteristic: the off-axis relative power(referenced to peak transmitted power) shall not decrease by more than 2 dB from the Edge-ofEarth (EOE) to nadir, nor more than 10 dB from EOE to 20 degrees off nadir, and no more than19.5 dB from EOE to 23.5 degrees off nadir; the power drop off between EOE and ±23.5 degreesoff nadir shall be in a monotonically decreasing fashion.Higher received signal levels than those shown in Table 3.2-1 can be caused by such factors asSV temperature-induced transmitter power variations, voltage variations and power amplifiervariations, and due to variability in link atmospheric path loss.
The terrestrial user’s maximumreceived signal power level resulting from these factors is not expected to exceed -154 dBW totalfor the composite L1C signal. For purposes of establishing user receiver dynamic range forreceiver design and test, the maximum received signal power level is not expected to exceed 150 dBW total for the composite L1C signal.Table 3.2-1. Received Minimum RF Signal StrengthL1CTerrestrialOrbital- 157 dBW- 182.5 dBW*TerrestrialOrbitalL1CP- 158.25 dBW- 183.75 dBW*L1CD- 163 dBW- 188.5 dBW**Over 99.5% of the solid angle inside a cone with a 23.5 degree half-angle with its apex at the SV and measuredfrom 0 degrees at the center of the Earth.3.2.2 PRN Code CharacteristicsThe characteristics of the L1CP-, L1CD-, and the L1CO-codes are defined below in terms of theirstructure and the basic method used for generating them. Figures 3.2-1 and 3.2-2 depictsimplified block diagrams of the scheme for generating the L1C PRN codes.3.2.2.1 L1C CodesThe PRN ranging codes L1CPi(t) and L1CDi(t) for PRN signal number i are independent, timesynchronized, and 10 milliseconds in length at a chipping rate of 1.023 Mbps, for total length of10230 chips.
In addition, there is an overlay modulation code L1COi(t) for PRN signal number iwhich also is independent, time synchronized, and 18 seconds in length at a rate of 100 bps, fortotal length of 1800 bits. The overlay code, L1COi(t), is modulo-2 added to L1CPi(t). (SeeFigure 3.3-2 for timing relationship.)6IS-GPS-800D24 Sep 2013Assignment of these code segments by PRN signal number is given in Table 3.2-2 and Table 3.23.
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