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Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
Why Simulate? What is a GNSS Simulator? Why should you use one for testing?
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Why Simulate? What is a GNSS Simulator? Why should you use one for testing?

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As applications for GNSS (Global Navigation Satellite System) positioning continue to proliferate, taking a simulation-based approach to receiver testing is more important than …

As applications for GNSS (Global Navigation Satellite System) positioning continue to proliferate, taking a simulation-based approach to receiver testing is more important than ever.

Discover:
- The many pitfalls of ‘live sky’ testing
- The importance of repeatability
- Why comprehensive testing needs controllable parameters
- The methodology of GNSS simulation

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  • 1. Why Simulate?What is a GNSS Simulator? Why should you use one for testing?
  • 2. IntroductionThe number of applications using Global Navigation Satellite System(GNSS) technology is continually increasing. So too is the diversity ofthese applications. Many applications are pushing the requirementsof GNSS technology further than ever before. In some cases it isnecessary to augment GNSS technology with other systems to meetthe performance requirements of a particular application.For GNSS to succeed as a state-of-the-art technology, the design ofthe various parts of the system - in particular GNSS receivers - mustbe of a high standard that ensures reliable performance.SPIRENT eBook Page 2
  • 3. To enable this, it is important that the product development processis based on proper testing from concept to production. This ebookdiscusses this testing, and why RF simulation - which has been thestandard method for over 20 years – should continue to be thechosen method. It also explains the concept of simulation and givesexamples of different simulators for different test applications.This ebook sets out to provide designers,developers, integrators and testers of GNSSreceivers or systems an overview of the technicalbenefits offered by GNSS simulation and explainthe cost benefits to procurement managers,project managers and financial managers whoneed to construct business cases for projectsinvolving GNSS technologies.SPIRENT eBook
  • 4. What is a GNSS RF simulator?Global Navigation Satellite System (GNSS) is a general term fora system that provides navigation and other services to usersworldwide. Each GNSS employs a constellation of satellites, whichbroadcasts signals that are processed by GNSS receivers to determinelocation, speed, and time for users anywhere in the world. Examplesof GNSS in operation or being developed include GPS, GLONASS,Galileo and Compass.SPIRENT eBook Page 4
  • 5. A GNSS simulator provides an effective and efficient means to testGNSS receivers and the systems that rely on them. A GNSS simulatoremulates the environment of a GNSS receiver on a dynamic platformby modelling vehicle and satellite motion, signal characteristics,atmospheric and other effects, causing the receiverto actually navigate according to theparameters of the test scenario. A GNSSreceiver will process the simulated signalsin exactly the same way as it would thosefrom actual GNSS satellites.SPIRENT eBook
  • 6. A GNSS simulator provides a superior alternative for testing, comparedto using actual GNSS signals in a live environment. Unlike live testing,testing with simulators provides full control of the simulated satellitesignals and the simulated environmental conditions. With a GNSSsimulator, testers can easily generate and run many different testscenarios for different kinds of tests, with complete control over:• Date, time, and location. Simulators generate GNSS constellation signals for any location and time. Scenarios for any location around the world or in space, with different times in the past, present, or future, can all be tested without leaving the laboratory.• Vehicle motion. Simulators model the motion of the vehicles containing GNSS receivers, sucas aircraft, ships, spacecraft or land vehicles. Scenarios with vehicle dynamics, for different routesSPIRENT eBook Page 6
  • 7. and trajectories anywhere in the world, can all be tested without actually moving the equipment being tested.• nvironmental conditions. Simulators model effects that impact E GNSS receiver performance, such as atmospheric conditions, obscuration, multipath reflections, antenna characteristics, and interference signals. Various combinations and levels of these effects can all be tested in the same controlled laboratory environment.• Signal errors and inaccuracies. Simulators provide control over the content and characteristics of the GNSS constellation signals. Tests can be run to determine how equipment would perform if various GNSS constellation signal errors occurred.SPIRENT eBook Page 7
  • 8. Why simulate?If we consider a fully-operational GNSS, such as GPS, it is very easyto assume that to test a receiver you would simply connect it to asuitable antenna, put the antenna out of the nearest window, oron the roof of a vehicle or building and check that the receiver canlocate, track and navigate on the GNSS signals received. To someextent, this assumption would be acceptable. This method – whichwill herein be referred to as ‘Live Sky’ - would indeed verify thatthe receiver’s fundamental RF and processing circuits are basicallyworking. However, we are interested in testing, not simply checkingfor operation.SPIRENT eBook Page 8
  • 9. Therefore, Live Sky should never be relied upon for anything morethan a simple operational check, and should certainly not be reliedupon for any testing during a product’s conception – design –development – production and integration life cycle. In fact, in manycases it has been shown that it is also not to be relied upon for in-the-field testing following repair.We will now look in some detail at the reasons behindthese facts.SPIRENT eBook Page 9
  • 10. KnowledgeAt the time of a Live Sky test, there are several unknowns.The unknowns include:Satellite clock errors – over time, these errors should be accountedfor in the navigation message and corrections broadcast, but becausethis message is updated infrequently, it is possible for aclock error to exist, which is not being corrected for.This will result in an incorrect pseudorangemeasurement, and hence an error in thereceiver’s computed position. Repeat testswill see a different clock behaviour, andwill therefore differ in their results.SPIRENT eBook Page 10
  • 11. Using a satellite simulator, there are no errors on the satellite clocks,unless you wish there to be, and then they are precisely known andcan be applied at known times.Also, when the test is repeated, any clock errors defined will beidentical to the previous test.Satellite orbit errors – The position of each satellite as declared in thenavigation message is different to its exact physical position in orbit.This is due to several orbital errors that are caused in part by thegravitational effects of the Sun, Moon and Earth, which serve to addperturbations to the satellites position. Even though the nature of theperturbations is relatively steady and predictable (as are the satelliteorbits) the orbital corrections broadcast in the navigation messagewill not be completely accurate, again, due to the inherent error in theprediction and estimation techniques, plus infrequent updating of theSPIRENT eBook Page 11
  • 12. information. With a simulator, it is possible to either remove all orbitalerrors and use a ‘perfect’ constellation, or allow fully quantifiableerrors to exist in a controlled manner.Navigation data errors – As with any data transmission system, errorsoccur in the data as a result of the modulation, demodulation andtransmission processes. There is robustness built in as, for examplewith the GPS system, the last 6 bits of each word of the navigationmessage are parity bits, and are used for bit error detection. However,errors can still occur, and these will not be accounted for. With asimulator, it is not possible for navigation data errors to occur, unlessthey are deliberately applied.SPIRENT eBook Page 12
  • 13. Atmospheric errors – This is a complex subject. The GNSS signalshave to pass through the layers of the atmosphere, which in its twomain parts comprises the Ionosphere and the Troposphere. Freeelectrons in the ionosphere (70 to 1000km above the earth’s surface)cause the modulation of a GNSS signal to be delayed in proportion tothe electron density (its speed of propagation through the ionosphereis referred to as the group velocity). The same condition causes theRF carrier phase to be advanced by the same amount. (Its speedof propagation through the ionosphere is referred to as the phasevelocity)This dispersive effect of the ionosphere varies according to latitude.It is relatively stable in the temperate regions, but can fluctuatesignificantly in the equatorial or polar regions.SPIRENT eBook Page 13
  • 14. The troposphere (0 to 40km) also affects GNSS signals. Variationsin pressure, temperature and humidity combine to delay the signal.Unlike the ionosphere, the troposphere is a non-dispersive medium,so it delays both the carrier and code equally. The troposphere isdivided into two components; wet and dry. The dry componentcontributes to 90% of the delay, but can be predicted very accurately.The wet component is more difficult to predict due to large variationin atmospheric distribution (or more simply, the weather!) Thechanges to the signal caused by the atmosphere directly contributeto range measurement errors, which cause the receiver to compute anincorrect position.SPIRENT eBook Page 14
  • 15. The effects of the atmosphere on GNSS signals are modelled, andthese models are used by the receiver, together with correctionparameters in the navigation data to partially correct atmosphericerrors. Dual frequency receivers can go a long way to eliminatingthe errors, but single frequency receivers (which most commercialones are) can only use the models available. These models are onlypartly successful in removing atmospheric errors, so there is always aresidual error due to the atmosphere.With a simulator, it is possible to completely disable the atmosphere,thereby removing the errors. Alternatively, errors can be applied to aknown model and are therefore fully accounted for.SPIRENT eBook Page 15
  • 16. Multipath – GNSS signals are line-of-sight, and can be regarded in thesame way as rays of light. If a signal ray falls upon an RF-reflectivesurface at an angle less than the critical angle of internal reflection,it will be reflected, with some attenuation. Therefore, it is possiblefor a receiver to not only receive the direct line-of-sight ray,but also the reflected version. The receiver has no way ofknowing which one of the two is the true LOS signal,so it uses both, and inherits the delay errorpresent on the reflected signal. This is anillustration of a simple single reflected ray. Inreality, multipath is much more complex, butthe net effect is still an error in the receiver’sposition estimate.SPIRENT eBook Page 16
  • 17. With a simulator, it is possible to eliminate multipath completely, orto apply multipath tosignals using various multipath models. In thisway, multipath can be applied in a known, controlled manner enablingits effects on receiver performance to be accurately analyzed, andthe appropriate design alterations or mitigations to be applied. WithLive Sky, it is impossible to quantify the multipath conditions presentat any one time and therefore impossible to analyse and improve areceiver’s performance in its presence.SPIRENT eBook Page 17
  • 18. Interference – GNSS signals are very weak when they reach thereceivers antenna, due to the fact that they have travelled a long wayfrom the satellites. This makes them vulnerable to interference fromexternal sources. Interference can be deliberate (known as jamming or spoofing) or unintentional. The vulnerability of GNSS to interference has been well documented and the discussion is beyond the scope of this document. Interference not only introduces errors in a receiver’s position computation, but can stop it navigating altogether. The problem this causes if interference is present (and cannot be stopped) during a Live Sky test is obvious.SPIRENT eBook Page 18
  • 19. With a simulator, thankfully, no such interference exists by default, butif required, it is possible to simulate it in a controlled and repeatablemanner.Interference which changes as a function of the proximity of its sourceto the receiver can be applied using an interference simulation systemsuch as Spirent’s GSS7765.SPIRENT eBook Page 19
  • 20. RepeatabilityWhen you perform testing on a GNSS receiver, and it highlightsweaknesses in the design, thnormal process is to make changes tothe design with a view to improving it. To confirm if improvementshave been made, you need to repeat the same tests exactly. If LiveSky is being used, it will be impossible to ensure subsequent testsare subjecting the receiver to the same conditions as the original test.SPIRENT eBook Page 20
  • 21. The most obvious difference is the fact that time has progressed, andthe constellation visible to the receiver will be completely different.These are factors that by themselves will ensure the test conditionscannot repeat. The other characteristics that will not remain fixed areatmospheric influences and satellite performance.Therefore, Live Sky is unsuitable as a method for testing with a viewto making design improvements.SPIRENT eBook Page 21
  • 22. With a constellation simulator, every time a test scenario is run, thesignals produced are identical. The scenario will start at the sametime on the same date, and the satellite positions will be identical –even down to the relative phase offsets between the different signals.In thisway you can guarantee that the receiver is being stimulatedwith the exact same signals every time the test is run. Only this waycan you fully determine any improvement (or otherwise) the designalterations have made.SPIRENT eBook Page 22
  • 23. Proper measurement of physical design changes is not the onlyreason for performing completely repeatable testing. If the resultsof testing are required as input to a verification or certificationprocess, they must be reliable and un-ambiguous. For example, iftwo companies are building receivers for a certain critical or safetyof life application, and they have to be certified to an internationaltest standard, then the test conditions must be identical to avoidone company having an advantage over the other. The test methodsused in test standards should always be designed to reduce themeasurement uncertainty as far as possible to preserve the integrityof the tests.SPIRENT eBook Page 23
  • 24. ControllabilityWith any comprehensive testing, finite and accurate control of the testconditions is essential. Fine-tuning of a design or system parametercan often demand very small, closely-controlled manipulation of thetest conditions. With a Live Sky test method, there is little that youhave control of. With the exception of the physical location ofthe test antenna, there is in fact nothing else that youhave any control over. You cannot wind backtime, disable the atmosphere, adjust thesatellite signals, errors, data, orbits - allof which are parameters you need tohave complete control over.SPIRENT eBook Page 24
  • 25. AccuracyA GNSS RF Constellation Simulator is a precisionpiece of test equipment and if properlymaintained, its performance is accuratelyspecified and controlled. The fidelityof a simulator’s signalsis much better than thesignals from a real GNSSsystem, which not onlyallows advanced testing ofa receiver’s true ‘laboratory’performance, but means thatsignal noise contributionsdue to the simulator are wellbelow the level of thermalSPIRENT eBook Page 25
  • 26. noise and therefore will not contribute any noise errors to the test.Two parameters closely related to accuracy are quality and reliability.The precision engineering employed in the simulator’s design andconstruction, and the quality control processes governing thesedisciplines ensure that the equipment gives reliable service for manyyears.SPIRENT eBook Page 26
  • 27. CapabilityIn practically all known applications where vehicle motion is present,a simulator will far exceed the dynamics required to simulate thatmotion. It is therefore possible to test a receiver well beyond theboundaries of its intended operational environment – somethingnot possible by any other means. This allows the true maximumperformance of the receiver to be characterised and accurately de-rated for the intended application.SPIRENT eBook Page 27
  • 28. Commercial viabilityNo project survives without a sound business case. Those responsiblefor managing projects andsetting budgets will have to take this intoaccount. It is often wrongly assumed that simulation only savesmoney over real field trials for applications involving high-dynamicson sophisticated platforms. For example, it is very obvious thatthere is no way a space-grade receiver can be flown in orbit purelyin order to test how well it works, but what is often not so obviousis the fact that simulation can prove to be more cost effective formuch less sophisticated applications. A leading European automotivemanufacturer calculated that the total cost of performing a real drivetest is in the order of £5k per day.SPIRENT eBook Page 28
  • 29. Notwithstanding the technical issues with real-world tests alreadydiscussed, the financial cost-benefits alone are enough todemonstrate the viability of simulation. A few months of drive testingwill pay for a simulator and in many cases makes its choice over realfield trials academic.SPIRENT eBook Page 29
  • 30. The methodology of simulationSo far, we have discussed the reasons for selecting simulation asthe preferred GNSS test method. In this section we will look at themethodology of simulation, examples of different simulators and howthey may be used for testing in different applications.To re-cap we remember that an RF Constellation Simulator reproducesthe environment of a GNSS receiver on a dynamic platform bymodelling vehicle and satellite motion, signal characteristics,atmospheric and other effects, causing the receiver to navigate on thesimulator’s RF signal, according to the parameters of the test scenario.What a simulator is not is a magic box which reproduces the realworld in its entirety. However, far from being a limitation, this is animportant benefit. In the same way that an RF design engineer wouldSPIRENT eBook Page 30
  • 31. not use a random noise generator when he really needs a controlledand quantified test signal, a GNSS receiver tester would not use arandom real-world signal-reproducing device when he really needs acontrollable and repeatable simulated GNSS test signal.SPIRENT eBook Page 31
  • 32. A receiver’s performance will vary depending on the severity ofthe errors and effects applied to the RF signal. Figure 2 shows arepresentation of the signal flow through a typical simulator with thevarious effects being added, until the final RF output, from whichthe complex resultant RF signal is output to the receiver under test.This principle applies to all simulators, with the number of effectsdepending on the capability of the simulator and its intendedapplication.Figure 1: Simulationsignal flowSPIRENT eBook Page 32
  • 33. Figure 2 shows a typical set-up in more detail. A Spirent GSS6700simulator is pictured. Position Velocity and Time data (typically inNMEA 0183 format) from the receiver can be fed back to the simulatorcontrol software and compared with the simulated ‘truth’ data. Thiswill give a very accurate measure of the receiver’s performance againstthe known characteristics of the simulator’s signal.Figure 2: Simulation test set-upSPIRENT eBook Page 33
  • 34. Performing a simulator testSetting up and running a receiver test using a simulator is relativelystraight forward. It can be summarised in two stages:• Definition • Run-timeThe Definition stage is where the required test parameters are set-upusing the simulator control software. At this stage you need to:• Understand the application for the receiver to be tested, and the operating environment• Determine the tests you need to perform• Define the test scenario with the appropriate effects• Understand how to connect the receiver to the simulator in order to maintain the appropriate RF conditionsSPIRENT eBook Page 34
  • 35. The Run-time stage is where the scenario is running and the simulatorhardware is producing the requisite RF signal.At this stage you need to:• Observe the receiver under test and manipulate the simulator as appropriate• Analyse the receiver performance. This can be undertaken either in real-time or by post-test analysis of recorded data. Access to the simulation data (the data used to create the test signal) can be gained in various ways from data-streaming to logging to a file. This data can then be used to compare the receiver’s performance with the ‘truth’ simulation data.SPIRENT eBook Page 35
  • 36. Spirent’s Multi-GNSS simulation platformsSpirent offers a wide range of test systems and capabilities to meetyour Multi-GNSS test needs. Our Multi-GNSS systems are designedwith future development in mind and are expandable to addresstomorrow’s test requirements as well as todays. Whether you areundertaking RD performance testing, integrating devices into yourproduct line, verifying performance or assessing manufacture of Multi-GNSS devices, Spirent has a Multi-GNSS test system available todayto match your needs.SPIRENT eBook Page 36
  • 37. The GSS8000 Multi-GNSS Constellation Simulator; Up to threeRF carriers, selected from a range of constellations and signals(GPS, Galileo, GLONASS and Quazi Zenith Satellite System), can beaccommodated in a single signal generator chassis. This enablesmultiple signals from a single constellation or hybrid systems withsignals from multiple constellations to be tested. The architecturesupports future Compass signals.SPIRENT eBook Page 37
  • 38. The GSS6700 Multi-GNSS Simulation System offers up to 36 channelsof combined GPS/SBAS, GLONASS and Galileo L1 signals from a singlechassis, 12 channels for each constellation. The GSS6700 is availablewith one, two or three constellations enabled. Different softwarecapabilities and flexibility are available to suit different test needs. Forexisting Spirent STR4500 or GSS6560 customers who today test GPS/SBAS L1 only, the GSS6700 offers the ability to simulate not only GPS/SBAS but also GLONASS and Galileo.SPIRENT eBook Page 38
  • 39. The GSS6300 Multi-GNSS Signal Generator is designed specificallyfor production test applications where a single channel is requiredfor controlled GNSS testing. The GSS6300 can generate a single,combined GPS/SBAS, GLONASS and Galileo signal to enable testingof GPS only or Multi-GNSS devices in a production environment. Forexisting Spirent GSS6100 customers, the GSS6300 has an identicalcapability, form factor and interfaces when specified in GPS/SBASconfiguration. In addition, the GSS6300 offers the benefit of on-site(even in-rack) upgradability to add GLONASS and Galileo generationcapability concurrently with GPS/SBAS.SPIRENT eBook Page 39
  • 40. Spirent’s Multi-GNSS simulation platforms Spirent GSS8000 Spirent GSS6700 Spirent GSS6300 Multi-GNSS Constellation Multi-GNSS Constellation Multi-GNSS Signal Simulator system generatorFor more information on GNSS applications, GNSS receiver testing andthe benefits that GNSS simulation can offer you,visit www.spirent.com/positioningSPIRENT eBook Page 40
  • 41. We hope you found this Why Simulate? E-Book of interest.We are continually adding new content to our websiteon a regular basis. Bookmark this link:www.spirent.com/positioningVisit the Spirent GNSS Blog, there are currentlymore than 90 posts with 2 to 3 new posts addedper week. Catch up on what’s new:www.spirent.com/Blog/PositioningNeed more information?gnss-solutions@spirent.comShare?Facebook LinkedIn Twitter Technorati Google +1 Digg Delicious Reddit Stumbleupon MC
  • 42. Spirent Spirent Federal Systems Got a smartphone? +44 1803 546325 +1 714 692 6565 Scan the QRglobalsales@spirent.com info@spirentfederal.com Code for morewww.spirent.com/positioning www.spirentfederal.com information

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