Choosing a GNSS simulator?


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Choosing the right testing tools (and methods) during the R&D, verification and production of a GNSS-enabled chipset or device isn’t always easy. This guide offers simple, practical advice.

- What GNSS radio frequency (RF) simulators are, and how they work
- Why radio-frequency simulation is a vital testing method
- Why a record and playback approach is more effective than using live GNSS signals
- The benefits of simulating multiple satellite constellations

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Choosing a GNSS simulator?

  1. 1. Choosing aGNSS simulator?Getting it right. Why all simulators are not the same
  2. 2. Getting it RightProduct designers, manufacturers and system integrators involvedin GPS development have used GPS simulators for R&D, integration,verification and production testing for years. But with theproliferation of GPS applications coupled with the commercialisationof GLONASS, the development of future Global Navigation SatelliteSystems (GNSS) and the integration of these systems, GPS and GNSStesting has become more mainstream.If this is new to you, don’t worry, wecan help you as all GNSS simulatorsand test approaches are not thesame. You need to understand whatthese devices can do and how theycan benefit your particular needs.SPIRENT eBook Page 2
  3. 3. The case for RF simulationThe case for using a GNSS simulator when designing any form oflocation aware equipment is a compelling one. The ability to controltest conditions, to simulate new satellite systems long before theygo “live” and the ability to precisely repeat tests time and time againis a key application of simulation that cannot be achieved throughusing live satellites for testing.There is however, a case wherecapturing the richness of live skysignals to playback later in the labcomplements simulation as a sourceof GNSS signals for test. But moreof that later.SPIRENT eBook
  4. 4. What is a simulator?An RF Constellation Simulator reproduces the environment of aGNSS receiver on a dynamic platform by modelling vehicle andsatellite motion, signal characteristics, atmospheric and othereffects, causing the receiver to actually navigate according tothe parameters of the test scenario.GSS6300 Multi-GNSS GSS6700 Multi-GNSS GSS7735 Multi-ChannelSignal Generator Constellation Simulator GPS L1/L2 production test simulatorSPIRENT eBook Page 4
  5. 5. However, during the past decade, a number of alternative deviceshave emerged that claim to offer a complementary or alternative toGNSS simulation in a number of different ways.The mix now includes (in no particular order):• Live sky• Pseudolites• RF record and playback systems (RPS)• Single-channel RF simulators• Multichannel RF constellation simulatorsSo where would we use these devices andhow do I decide what’s right for me? Well thatall depends on your test requirements as allapproaches bring with them a mix of pros and cons.SPIRENT eBook Page 5
  6. 6. The tests requiredLaboratory testing of any GNSS receiver design will require a range ofstandard tests in order to exercise the full functionality of the receiverunder both “normal” and “special” operating conditions. These tests, *which might be carried out individually or combined, will comprise:• Time to first fix• Acquisition sensitivity• Tracking sensitivity• Reacquisition time• Static navigation accuracy• Dynamic navigation accuracy• Radio frequency interferenceSo how does each solution stack up against requirements?* More information on GNSS Receiver testing can be found by downloading the eBook ‘Simulation Versus Real World Testing’SPIRENT eBook Page 6
  7. 7. Live skyThe first obstacle that will be encountered in integrating GNSSreceiver testing into a production test setup is pretty obvious.As such tests are performed at the end of the productionline, they are inevitably performed indoors.And regardless of whether the equipmentis designed to work indoors or outdoors,the roof and walls of the building willintroduce variables into the test that willnegate its effectiveness.So-called “live-sky” testing is thereforeimpossible without relaying theGNSS signals from outdoors to theproduction tester.SPIRENT eBook
  8. 8. It is a relatively simple exercise to capture live GNSS signalsand re-radiate them within the production test environment.However, this comes with its own set of shortcomings.First, radiating any signal in such an environment might haveunforeseen consequences on other tests that are performed onthe product; and conversely, other RF signals and noise withinthe production test area may well impact on the integrity of theGNSS signals.SPIRENT eBook Page 8
  9. 9. More importantly, though, the inherently dynamic nature of GNSSsignals means that while each unit may well be tested in the samephysical location (i.e. in the production test fixture), the relativepositions of the GNSS satellites and the signal pertubations anderrors due to propagation conditions will be different for every unittested. And, not surprisingly, this makes direct comparison betweenresults unreliable at best.SPIRENT eBook Page 9
  10. 10. Real-world, live-sky testing does have some advantages’re testing real signals and at the very basic level,it’s inexpensive but it has some significant drawbacks which,in practice, preclude controlled testing.Key points:• Testing using real signals• Can be inexpensive• end user or test site cannot have any control An over the GNSS signal being transmitted• The signals seen incident to the GPS receiver antenna are constantly changing as the GPS system constantly changes (precesses)SPIRENT eBook Page 10
  11. 11. • There are occasional signal errors, often unknown to the receiver at the time• Atmospheric conditions change significantly and have a significant impact on single frequency systems• Testing at multiple geographic locations proves to be expensiveSPIRENT eBook Page 11
  12. 12. PseudoliteA pseudolite (or pseudo satellite) is simply a device used toemulate the role of a satellite, and as such can be thought ofas a simulator. This class of devices includes transmitters usedto augment indoor coverage as well as those used to simulatefuture satellite constellations before launch.SPIRENT eBook Page 12
  13. 13. Key points:• Pseudolites are not for testing• They can be used to augment GNSS, but major issues are: · Synchronisation · odified receiver needed to cope with high power signals M and stationary transmitters• Mostly used for professional applications · .g. mining or use in E container portsSPIRENT eBook Page 13
  14. 14. RF record and playback system (RPS)A GNSS RF RPS does exactly as the name suggests. In simple terms,the whole RF spectrum in a specified bandwidth is downconverted,digitised and stored as samples on suitable storage media.For playback, the samples are converted back to an analogue signal,up-converted and output at the original RF frequency.All RF energy in the specified bandwidth is recorded. In addition tothe GNSS signals interference is also recorded. This may be frommany sources such as other RF communications, broadbandinterference from electrical systems (vehicle ignition etc.), and evenradiation from solar activity. Noise is also inevitably added by therecording and playback process, but with appropriate low-noise gainat the start of the process this can be minimised.SPIRENT eBook Page 14
  15. 15. In terms of the recorded GNSS signals, these will be as they arriveincident upon the recorder’s antenna so they will include the effects ofthe propagation environment and the satellite and observers’ motion,this includes multipath in its full richness, fades, signal loss and carrierphase/signal group delay due to atmospheric influences.Key points:• RPS gives you the power and flexibility of performing one field An test and repeating it many times in the lab.• Records all signals in band• Record real world environments and effects• Can be a cost effective solutionSPIRENT eBook Page 15
  16. 16. • RPS is complementary to GNSS simulation and is important for the verification part of a complete GNSS test plan, where precise knowledge of the test signal is not important, but precise repeatability of the signal ‘richness’ of the real world is• You can’t test future signals e.g. Galileo• Results can be difficult to interpret• RPS has limited control• Ideal for testing in complex environments, where recreation of signal and fading effects in a simulator is a major challengeSPIRENT eBook
  17. 17. Single channel RF simulatorsOn the face of it, a simple single channel RF simulator would appearto be suitable for GNSS receiver testing. However, many of therange of tests required (see page 2) are not possible. Single channelunits are widely used for production testing where “stimulation”of one receiver channel as a Go/NoGo or sensitivity test is enoughto determine the circuitry works. For RD testing a single channel can help with simulating the receiver RF and enabling single channel tracking and sensitivity measurements. However, there is more to testing a GNSS receiver than simply producing a signal at the correct frequency. Page 17
  18. 18. Key points:• Without the ability to simulate a full satellite constellation a receiver cannot be fully tested for its primary function• future receiver tests will include more than just GPS L1 As C/A code, a single channel RF simulator should have the capability to test GLONASS L1 and Galileo L1 either out of the box or through a field upgrade• power level accuracy and stability are important to RF ensure valid resultsSPIRENT eBook Page 18
  19. 19. Multichannel RF constellationsimulatorsThere is, however, one solution that is capable of handling allof the tests necessary for designing, developing and integratingGNSS receivers in the laboratory.The multichannel GNSS constellation simulator is capableof running all seven standard tests of GNSS receiverperformance, together with their individual variations.What’s more, today’s simulators can be used to producesignals from satellite constellations that do not yet existin the real world, allowing developers to create multi-GNSS designs that will work reliably with GPS,GLONASS, Galileo and Compass systems.SPIRENT eBook
  20. 20. This type of equipment is widely used for RD, integration,validation, production and post sales testing.Key points:• multichannel GNSS constellation simulator should provide A the capability to recreate the GNSS RF signal in such way that it should be fully representative of the real signal expected by a receiver• should allow a user to control the simulation environment It and all parameters of the signal should be known to the user• multichannel GNSS constellation simulator should enable the A provision of complete repeatability in the signal creation• Results are easily interpreted, quantifiable and certifiable• You’re not testing real world signalsSPIRENT eBook Page 20
  21. 21. To complement simulation testing at the verification stage, theexpense of field trials can be dramatically reduced by bringing thereal-world back to the Lab with a Record Playback System (RPS). Page 21
  22. 22. ConclusionWhile the majority of the simulation options offer some of theattributes required for testing GNSS receivers, many of theseinstruments also have their shortcomings but as mentionedpreviously, it depends on your test application.The key point to note is that the markets we operate in arechanging fast. The GPS system is being modernized, GLONASSis almost at full strength, Galileo and Compass signals will becoming online over the next few years. Augmentation systemssuch as EGNOS, WAAS and the soon to be GAGAN and QZSSsystems add to the mix of signals and technologies available tothose developing GNSS.SPIRENT eBook Page 22
  23. 23. Any system you are looking to purchase must be able to supportthese signals now or have a defined roadmap of when that supportwill be available.At Spirent we advocate a progressive approach to testing,incorporating simulation, Record Playback with final confirmationusing real world signals:SPIRENT eBook Page 23
  24. 24. Progressive testing approaches Simulation Record Live Drive Testing Performance evaluation Playback Final confirmation Threat modes Environment capture Failure modes Performance tuning Safety cases Known problem areas Statistical results analysis OptimisationSPIRENT eBook Page 24
  25. 25. The Spirent GNSS and Wi-Fi SolutionsSpirent is the industry leader for GNSS simulator products.Digital RF signal generation techniques patented by Spirent haveled to the development of high-fidelity, world-leading GNSS RFSimulation Systems.Spirent offers several different models of GNSS simulators thatsupport a variety of different applications and cover the full spectrumof civilian and military GNSS testing needs.Spirent products range from basic single-channel simulators,suitable for simple production testing, through multi-channel,multi-constellation simulators, suitable for the mostdemanding research and engineering applications.SPIRENT eBook Page 25
  26. 26. For more comprehensive testing, Spirent also offers products thatsimulate additional system elements simultaneously with the GNSSconstellation signals, such as inertial sensors, various automotivesensors, Assisted GPS (A-GPS) + Assisted GLONASS (A-GLONASS)data, SBAS and GBAS augmentation system signals, interferencesignals, GNSS Record Playback and Wi-Fi Positioning.Spirent GSS8000 Spirent GSS6700 Spirent GSS6300 Spirent GSS6400 Spirent GSS5700Multi-GNSS Constellation Multi-GNSS Constellation Multi-GNSS Signal Record Playback Wi-Fi Access PointSimulator system generator System simulatorSPIRENT eBook Page 26
  27. 27. If you found this Spirent Choosing a GNSS simulator E-Book ofinterest, we think you’ll definitely like the Spirent ApplicationNote: Multi-GNSS benefits, challenges and test considerations.Download it here.We are continually adding new content to our websiteon a regular basis. Bookmark this the Spirent GNSS Blog, there are currentlymore than 90 posts with 2 to 3 new posts addedper week. Catch up on what’s more information?gnss-solutions@spirent.comShare?Facebook LinkedIn Twitter Technorati Google Buzz Digg Delicious Reddit Stumbleupon MCD00169 Issue 1-02
  28. 28. Spirent Spirent Federal Systems Got a smartphone?+44 1803 546325 +1 714 692 6565 Scan the Code for information
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