On Generalized Signal Waveforms for Satellite Navigation (797942), страница 14
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Plans to re-establish Full Operation Capability of GLONASS31Global Navigation Satellite SystemsThese plans to re-establish FOC for GLONASS are supported by cooperation works betweenthe governments of India and Russia. According to them, Russia will launch twoGLONASS-M satellites on Indian Geostationary Satellite Launch Vehicle (GSLV) rockets.Additionally, during the December 2005 summit between the Indian Prime Minister and theRussian President, it was agreed that India would share the development costs of theGLONASS-K series and launch them from India.As mentioned above, as of May 9th, 2008, the real number of satellites in operation (14) islower than the planned 18, making the real fulfilment of the GLONASS program objectivesvery difficult.
Indeed, although Russia added six GLONASS-M satellites to the constellationduring 2007 as planned, it was forced to decommission other five satellites since thebeginning of 2008 due to the short design life of the first generation spacecraft. The nextfigure shows the availability of GLONASS over the earth on May 9th, 2008 as provided by[GLONASS Centre].Figure 2.19. Availability of GLONASS May 9th, 2008 (Minimum Elevation Angle 5°)For more information on the current status of all the GLONASS satellites, we refer to[GLONASS Centre] and [GLONASS Constellation Status].
Equally important as the numberof satellites in sight is the power level of the received RF signals from GLONASS satellites.When measured at the output of a 3 dBi linearly polarized antenna the value is assured to benot lower than -161 dBW for the L1 band at an elevation of 5° or more. For the modernizedGLONASS-M satellites transmitting also in the L2 band, the power level under the sameconditions can not be lower than -167 dBW for this band.In the same manner as with the minimum received power, similar figures can be derived forthe maximum received power of GLONASS.
According to this, the maximum received powerlevel will not be more than –155.2 dBW in the specific configuration, as described in[GLONASS ICD, 2002].32Global Navigation Satellite Systems2.5.2 GLONASS Signal PlanAs it was mentioned at the beginning of this chapter, GLONASS, unlike the other GNSSsystems, makes use of a different DSSS technique [G.W.
Hein et al., 2006c] based onFrequency Division Multiple Access (FDMA) to transmit its ranging signals.GLONASS uses FDMA in both the L1 and L2 sub-bands. According to this scheme, eachsatellite transmits navigation signals on its own carrier frequency, so that two GLONASSsatellites may transmit navigation signals on the same carrier frequency if they are located inantipodal slots of a single orbital plane [GLONASS ICD, 2002]. Indeed the actualconstellation is taking advantage of this property since 2005 when the higher frequencychannels had to be turned off to fulfil the CCIR Recommendation 769. We can clearly see thisif we have a look at the satellites assigned to each of the GLONASS planes as shown in thefollowing figure with status as of May 2008.
As is clear to see, antipodal satellites aretransmitting at the same frequency.Figure 2.20. Antipodal Assignment of GLONASS Satellites. The parameter i(k) indicatesthat the satellite in almanac slot i transmits on frequency number kThe red slots indicate that the satellite is in maintenance. Blue means correct operation.Moreover, two different types of signals [GLONASS ICD, 2002] are transmitted byGLONASS satellites: Standard Precision (SP) and High Precision (HP) in both the L1 and L2bands. The GLONASS standard accuracy signal, also known as C/A Code, has a clock rate of0.511 MHz and is designed for use by civil users worldwide while the high accuracy signal(P Code) has a clock rate of 5.11 MHz and is modulated by a special code which is onlyavailable to users authorized by the Ministry of Defence. Since GLONASS-M, both L1 andL2 provide users with the standard accuracy code C/A.
Moreover, the modernized GLONASSwill also transmit FDMA signals on the L3 band and CDMA signals in L1 and L5 as we willsee in chapter 2.5.3.The nominal values of the FDMA L1, L2 and L3 carrier frequencies are defined as:f k L1 = f 0 L1 + k Δf L1f k L 2 = f 0 L 2 + k Δf L 2(2.3)f k L 3 = f 0 L 3 + k Δf L 333Global Navigation Satellite Systemswhere:••k represents the frequency channel,f 0 L1 = 1602 MHz for the GLONASS L1 band,••Δf L1 = 562.5 kHz frequency separation between GLONASS carriers in the L1 band,f 0 L 2 = 1246 MHz for the GLONASS L2 band,••Δf L1 = 437.5 kHz frequency separation between GLONASS carriers in the L2 band,f 0 L 3 = 1201 MHz for the GLONASS L3 band, and•Δf L 3 = 437.5 kHz frequency separation between GLONASS carriers in the L3 band.As we can see, the GLONASS L2 carrier reference signal is 7/9 of the L1 carrier referenceand the GLONASS L3 carrier reference is 3/4 of the L1 carrier reference.
Moreover, it mustbe noted that until 2005 the GLONASS satellites used the frequency channels k = 0,...,12without any restrictions and the channel numbers k = 0 and 13 for technical purposes.Since then GLONASS is only using the frequency channels k = -7,...,+6 and all the satelliteslaunched beyond that year will use filters, limiting out-of-band emissions to the harmfulinterference limit contained in CCIR-ITU Recommendation 769 for the 1610.6 – 1613.8 MHzand 1660 – 1670 MHz Radio-Astronomy bands. It is interesting to note that although thelimitation to use the higher frequency channels does only affect the L1 band, since theparameter k determines the channel in both the L1 and L2 bands, the upper frequencies of L2corresponding to channels +7 to +13 were automatically sacrificed.To have a clearer insight into how the spectra of the GLONASS signals look like, we studynext all the bands in detail.GLONASS L1 BandThe transmitted navigation signal is in both services of L1 a bipolar phase-shift key (BPSK)waveform with clock rates of 0.511 and 5.11 MHz for the standard and accuracy signalsrespectively.
The L1 signal is modulated by the Modulo-2 addition of the pseudo random(PR) ranging code, the digital data of the navigation message and an auxiliary meandersequence. All above-mentioned frequencies are generated coherently using a single onboardtime/frequency oscillator standard [GLONASS ICD, 2002]. For the case of the standardaccuracy signals (C/A), the PR ranging code is a sequence with length the maximum of a shiftregister (m-sequence) and a period of 1 millisecond with bit rate of 511 kbps. The navigationmessage is sent at 50 bps and the auxiliary meander sequence at 100 Hz.Moreover, it is important to note that the GLONASS FDMA L1 band does not exactlycoincide with the GPS and Galileo L1 band. In fact, the GLONASS L1 band ranges from1592.9525 MHz to 1610.485 MHz when only the 14 channels k = -7...+6 are employed.
In the34Global Navigation Satellite Systemsnext figures, each of the channels was filtered to only transmit the main lobe of the BPSKsignal and the PSD was normalized to have unit power within the corresponding transmissionbandwidth.The PSDs of the GLONASS signals are shown in the following figure:Figure 2.21. Spectra of GLONASS signals in L1Once again, in order to have a clearer picture of how overcrowded the RNSS bands arebecoming as more and more countries claim their rights to have their own GNSS, Figure 2.22shows all the systems described so far in the E1/L1 band.Figure 2.22. Spectra of GPS, Galileo and GLONASS Signals in E1/L1It is important to note that the GPS L1C pilot and data signals are shown in quadrature in thefigure although according to [GPS ICD-800, 2006] the final phasing is still open. To finalizesome details on the technical characteristics of the GLONASS L1 signals are presented next:35Global Navigation Satellite SystemsTable 2.10.
GLONASS L1 signal technical characteristicsGNSS SystemGLONASSGLONASSService NameC/A CodeP CodeCentre Frequency(1598.0625-1605.375) MHz ± 0.511 MHzFrequency BandL1L1Access TechniqueFDMAFDMASpreading modulationBPSK(0.511)BPSK(5.11)Sub-carrier frequency--Code frequency0.511 MHz5.11 MHzSignal ComponentDataDataPrimary PRN Code length511N/ACode FamilyM-sequencesN/AMeander sequence100 HzN/AData rate50 bpsN/AMinimum Received Power[dBW]-161 dBWN/AElevation5°N/AIt is important to note that unlike for the case of GPS and Galileo in the previous chapters, thefrequencies do not have to be multiplied by the factor 1.023.2.5.2.1GLONASS L2 BandThe transmitted navigation signal is, as also in L1, a bipolar phase-shift key (BPSK)waveform with similar clock rates as in the L1 band. The L2 signal is modulated by theModulo-2 addition of the PR ranging code and the auxiliary meander sequence.
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