Advanced global navigation satellite system receiver design (797918), страница 6
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However, the current design is basedon an aging chipset that has no flexibility to adapt to the signal specificationsproposed for GPS modernisation and Galileo. In order to continue the evolution ofSSTL’s SGR family of receivers and benefit from the potential of the future signals, anew high-performance flexible receiver architecture is required.
Therefore, a primaryindustrial and commercial aim of this project was to remove the dependency of theSGR range on an aging and potentially obsolete chipset.Many scientific space missions require accuracy greater than the typical 10 metreaccuracy obtained through single frequency GNSS. The next generation of GNSSwill make dual frequency signals available to civil users enabling virtual eliminationof residual errors due to the ionosphere and increased navigation accuracy. GNSS25Background, motivation and goalsambiguity resolution used in attitude determination and many terrestrial applicationscan benefit from multiple frequency receivers [Teunissen et al 2002].
Therefore, thesecond hardware goal was to develop a low-cost receiver architecture capable ofprocessing multiple frequency bands.The combination of future GPS and Galileo constellations will make over 50 GNSSsignals available to the space user. Currently SGR receivers have a maximumcapacity of 24 receiving channels. A receiver with greater channel capacity andcapability to process a range of GNSS signals would significantly benefit a number ofspace applications. GNSS reflectometry requires reflected signals from more thanone satellite to determine wind direction [Komjathy et al 2001] and therefore benefitsfrom more satellites in the sky. Also, more receiver tracking channels enables greaterocean coverage and may allow receivers to perform real-time formations of the delayDoppler map, a valuable tool in GNSS reflectometry.
The reliability of GNSSattitude determination could also be greatly increased through greater receiver channelcapacity, devoting extra channels to each antenna (currently limited to 6 channels perantenna). Hence, the third hardware goal of this research was to provide the abilityfor the receivers to increase the number of tracking channels available to the user.Preferably this should be a flexible choice depending on the application.Both the modernised GPS and Galileo systems will provide pilot tones on some oftheir signals. The pilot tones permit long integration times enabling acquisition andtracking of very weak GNSS signals.
This is particularly useful for GEO receiverswhere navigational positioning is only made possible by receiving the very weaksignals from GNSS satellite antenna side-lobes [Ebinuma et al 2004]. Therefore, anadvanced receiver architecture capable of receiving modernised GPS and Galileosignals may benefit GEO receivers greatly and provide robust PVT information forGEO users. Also, GEO GNSS receivers are currently very expensive and hence userswould benefit from low-cost receiver architecture suitable for the high-radiationenvironments experienced in HEO and GEO.The above examples illustrate the need for new advanced GNSS receiver architecturefor both commercial and scientific space applications.
To summarise, the areas of26Background, motivation and goalsspace receiver architecture to which this research project was intended to contribute towere as follows. Removing the dependency of the SSTL SGR range on an aging and potentiallyobsolete chipset. Low-cost receiver architecture capable of processing multiple frequencybands. Receiver architecture with greater and more flexible channel capacity. Receiver architecture capable of receiving GPS modernised and Galileosignals.
Low-cost receiver architecture suitable for severe radiation environments.Although multi-faceted, these advances in receiver technology generally embody thegoals SSTL desires from this research. However, the design of receiver architecturefor the next generation of GNSS results in a significant number of challenges boththeoretical and practical from which academic value can be derived.The new signals include a host of new complex modulation, multiplexing and codingschemes designed to cover higher bandwidths and deliver higher precision than evertransmitted before see [ESA and GJU 2006] and [GPS SIS 2007]. New approaches toreceiver design and operation are required in order to take full advantage of theproposed signal specifications. Research broad enough to encompass the entireimpact of the new GNSS is beyond the scope of this research project.
Hence, anumber of key areas of investigation were identified, which were of specific interestto SSTL and SSC. From this the academic goals of this research were formed and canbe summarised as follows. Search, acquisition and tracking of Binary Offset Carrier (BOC) modulatedsignals – providing a comparison of the proposed receiver techniques,identifying flaws and determining where improvements can be made.
The effect of complex multiplexing schemes on transmitter and receiverarchitectures and operation – analysing how these multiplexing schemes canbe implemented and what approaches can be adopted in the receiver.27Background, motivation and goals Modulation and high bandwidth signals – evaluating the potential benefits andproblems associated with BOC modulated and high-bandwidth signals.283Signal characteristicsThis chapter details the signal structure of existing and future signals to be transmittedin the L-band GNSS frequency allocations.
It is important to fully understand allpossible future GNSS signal structures in order to evaluate their impact on GNSSreceivers and formulate receiver solutions. Firstly, a review of the current or heritageGPS signal structure is given with signal representations, modulation and codingtechniques. Secondly, the proposed changes to the GPS signal structure by themodernisation programme are detailed. Finally, the current Galileo signal structure,its specifications and properties are described in detail.3.1Heritage GPS signal characteristicsGPS is a spread spectrum system, where the spread signal occupies a bandwidth,much greater than the rate of the data being transmitted [Sklar 1988].
Thisredundancy of bandwidth serves to suppress detrimental effects of interfering signalsand reduces the peak transmitted signal power levels to effectively hide the signal inbackground noise. The spreading technique denoted, Direct Sequence-SpreadSpectrum (DS-SS), refers to technique where a carrier wave is modulated by a datasignal overlaid with a high frequency Pseudo-Random Noise (PRN) spreading signal.A conceptual diagram of a spread spectrum system is shown in Figure 3-1, themodulation onto the carrier is omitted for simplicity.
The narrow bandwidth, Bd ofthe data signal d(t) is spread by a PRN code spreading signal, a(t) of significantlyhigher bandwidth, BS. The transmitted signal then passes through a channel, whichapplies additive noise, n(t) and interfering signals, i(t). A synchronised replicaspreading code signal multiplied onto the received signal will then result in recoveryof the desired signal with some error from thermal noise (spectral density N0) andinterference.29Signal characteristicsTransmitterData,PRN code,d(t)a(t)Spectrum of data Spectrum of spreadspectrum signalChannelReceiverAdditive noise +interfering signals,Synchronisedreplica code,n(t) + i(t)a(t)Spectrum ofreceived signalSpectrum of despread signalSignal d(t)BdBSInterferenceN0N0Figure 3-1, Simplified conceptual spread spectrum systemThe innovation of the GPS is to use the PRN code sequence as a ranging signal.
Incombination with its associated data signal this allows the path difference fromtransmitter to receiver to be recovered. The GPS satellite signals share the samecarrier frequency and are separable in a receiver only because each respectivetransmission employs a unique PRN spreading code. Each effective bit of the PRNcode sequence is called a ‘chip’. Despite the fact that GPS is a code modulation of acontinuous wave (CW) carrier, the navigational or ranging signal is effectively asequence of periodic pulses, with a periodicity equal to the code length and pulsewidth of one chip.
The range to each differently located GPS satellite is measured bythe timing of these pulses and comparing the relative time delay of the pulses enablesthree-dimensional navigation.Heritage GPS uses Binary Phase Shift Keyed (BPSK) modulation [GPS ICD 2007],where ideally the carrier phase changes instantaneously by 180˚, depending on thedata modulated spreading sequence. A PSK modulated signal can be written asS PSK (t ) = 2 P × a(t ) × cos(ωt ) × d (t )3–1where d (t ), a (t ) ∈ (−1,+1) .30Signal characteristicsP is the signal power, d(t) is the bi-phase data signal, a(t) is the bi-phase PRN codespreading signal and ω is the carrier frequency.
The designation BPSK-R( f c ) hasbeen adopted to define this type of modulation, where f c is the chipping rate and is aninteger multiple of 1.023MHz.E2-L1-E1E5A / L5E5BL2E6L2L1L1SAR11641176.45 11891207.14GPSBands1214 12151227.6 1237 1242GalileoBands1251 1260127813001544155915651575.421587 1591 15981609GLONASSBandsFigure 3-2, L-Band GNSS frequency allocationsFigure 3-2 shows the L-band frequency allocations for current and future GNSSsystems. Currently, GPS consists of between 24 and 32 satellites broadcasting threenavigational signals in the L-band; one signal available for civil use transmitted on theL1 carrier frequency and two military signals transmitted on the L1 and L2 carriers.The Heritage GPS signal structure is depicted in Figure 3-3.
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