If you are developing any form of device which relies on satellite signals such as GPS, then you will need to ensure it can perform to the demands of the job. But how do you ensure it’s ready to meet the demands of the real world before the device is ready for market? You need to carry out suitable testing using a GPS simulator.

Laboratory testing becomes of utmost importance due to the lack of the real live-sky signals, especially with satellites which are yet to be launched.

Satellite tracking will be the cornerstone to the success of your GPS enabled device. Failure to test this aspect correctly can kill your device once it goes to market.

GNSS/GPS Simulation: General Principles

GNSS stands for Global Navigation Satellite System, and is the standard generic term for satellite navigation systems that provide global coverage. This term includes GPS, GLONASS, Galileo, Beidou and other regional satellite navigational systems.

IconSince the early days of GNSS, there have essentially been two major alternatives available to those wishing to test a navigation system: field test and laboratory simulation. Today, best practice indicates that most testing is done under controlled, repeatable conditions in a secure laboratory. This enables both nominal and adversarial conditions testing, including testing to the limits of both real and theoretical performance. It also allows development of receivers for GNSS systems that are currently unavailable or lacking a full constellation.

Real-world, live-sky testing has significant drawbacks which, in practice, preclude controlled testing. A summary of the advantages of testing with GNSS simulators, compared to live testing with actual GNSS constellations, is shown in the table below.

Live Testing with Actual
GNSS Constellations

No control over constellation signals
Limited control over environmental conditions
Not repeatable; conditions are always changing
Unintended interference from FM, radar, etc.
Unwanted signal multipath and obscuration
No way to test with GNSS constellation errors
Expensive field testing and vehicle trials
Limited to signals available in GNSS constellations
Competitors can monitor field testing

Laboratory Testing with
GNSS Simulators

Complete control over constellation signals
Complete control over environmental conditions
Fully repeatable
No unintended interference signals
No unwanted signal effects
Easily test scenarios with GNSS constellation errors
Cost-effective testing in laboratory
Testing of present and future GNSS signals
Testing conducted in secure laboratory

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Performing a GPS simulator test

Setting up and running a receiver test using a simulator is relatively straight forward. It can be summarised in two stages:

Definition: The Definition stage is where the required test parameters are set-up using 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 conditions*

Run-time: The Run-time stage is where the scenario is running and the simulator hardware 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.

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What is a GPS RF simulator?

Satellite dishA GPS simulator provides an effective and efficient means to test GPS receivers and the systems that rely on them. A GPS simulator emulates the environment of a GPS receiver on a dynamic platform by modelling vehicle and satellite motion, signal characteristics, atmospheric and other effects, causing the receiver to actually navigate according to the parameters of the test scenario. A GPS receiver will process the simulated signals in exactly the same way as it would those from actual GPS satellites.

Our GPS simulators provide a superior alternative for testing, compared to using actual GPS signals in a live environment. Unlike live testing, testing with simulators provides full control of the simulated satellite signals and the simulated environmental conditions. With a GPS simulator, testers can easily generate and run many different test scenarios for different kinds of tests, with complete control over:

  • Date, time, and location. Simulators generate GPS 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 GPS receivers, such as aircraft, ships, spacecraft or land vehicles. Scenarios with vehicle dynamics, for different routes and trajectories anywhere in the world, can all be tested without actually moving the equipment being tested.
  • Environmental conditions. Simulators model effects that impact GPS 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 GPS constellation signals. Tests can be run to determine how equipment would perform if various GPS constellation signal errors occurred.

Why not download your free eBook on Testing GPS with a simulator.

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Why use GPS simulators?

Cell Phone AntennaWhen you consider a fully-operational GNSS, such as GPS, it is very easy to assume that to test a receiver you would simply connect it to a suitable antenna, put the antenna out of the nearest window, or on the roof of a vehicle or building and check that the receiver can locate, track and navigate on the GNSS signals received. To some extent, this assumption would be acceptable. This method – which will herein be referred to as ‘Live Sky’ - would indeed verify that the receiver’s fundamental RF and processing circuits are basically working.

However, we are interested in testing, not simply checking for operation. Therefore, Live Sky should never be relied upon for anything more than a simple operational check to confirm successful operation in the presence of real world impairments, and should certainly not be relied upon for any testing during a product’s conception – design – development – production and integration life cycle. There are however times when testing real world signals is the easiest way to confirm performance in the presence of real world impairments or real world operational challenges. A Record & Playback system complements the capability of a GNSS simulator, enabling the full ‘richness’ of the real world environment to be captured and played back in the lab.

We will now look in some detail at the reasons behind these facts.

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The problem with Live Sky Testing


At the time of a Live Sky test, there are several unknowns. The unknowns include:


Satellite clock errorsicon

Over time, these errors should be accounted for in the navigation message and corrections broadcast, but because this message is updated infrequently, it is possible for a clock error to exist, which is not being corrected for.

Simulator benefits: Using a satellite simulator, there are no errors on the satellite clocks, unless you wish there to be, and then they are precisely known and can be applied at known times.


Satellite orbit errorsIcon

The position of each satellite as declared in the navigation message is different to its exact physical position in orbit. This is due to several orbital errors that are caused in part by the gravitational effects of the Sun, Moon and Earth, which serve to add perturbations to the satellites position.

Simulator benefits: With a simulator, it is possible to either remove all orbital errors and use a ‘perfect’ constellation, or allow fully quantifiable errors to exist in a controlled manner.


Navigation data errorsicon

As with any data transmission system, errors occur in the data as a result of the modulation, demodulation and transmission processes. There is robustness built in as, for example with the GPS system, the last 6 bits of each word of the navigation message are parity bits, and are used for bit error detection. However, errors can still occur, and these will not be accounted for.

Simulator benefits: With a simulator, it is not possible for navigation data errors to occur, unless they are deliberately applied.


Atmospheric errorsicon

The GPS signals have to pass through the layers of the atmosphere, which in its two main parts comprises the Ionosphere and the Troposphere. Free electrons in the ionosphere (70 to 1000km above the earth’s surface) cause the modulation of a GPS signal to be delayed in proportion to the electron density (its speed of propagation through the ionosphere is referred to as the group velocity). The same condition causes the RF carrier phase to be advanced by the same amount. (Its speed of propagation through the ionosphere is referred to as the phase velocity)

Simulator benefits: With a simulator, it is possible to completely disable the atmosphere, thereby removing the errors. Alternatively, errors can be applied to a known model, and are therefore fully accounted for.


MultipathIcon

GPS signals are line-of-sight, and can be regarded in the same way as rays of light. If a signal ray falls upon an RF-reflective surface at an angle less than the critical angle of internal reflection, it will be reflected, with some attenuation. Therefore, it is possible for a receiver to not only receive the direct line-of-sight ray, but also the reflected version. The receiver has no way of knowing which one of the two is the true LOS signal, so it uses both, and inherits the delay error present on the reflected signal.

Simulator benefits: With a simulator, it is possible to eliminate multipath completely, or to apply multipath to signals using various multipath models. In this way, multipath can be applied in a known, controlled manner enabling its effects on receiver performance to be accurately analyzed, and the appropriate design alterations or mitigations to be applied. With Live Sky, it is impossible to quantify the multipath conditions present at any one time, and therefore impossible to analyze and improve a receiver’s performance in its presence.


Interferenceicon

GPS signals are very weak when they reach the receivers antenna, due to the fact that they have travelled a long way from the satellites. This makes them vulnerable to interference from external sources. Interference can be deliberate (known as jamming or spoofing) or unintentional. The vulnerability of GPS to interference has been well documented and the discussion is beyond the scope of this page.

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.

Simulator benefits: With a simulator, thankfully, no such interference exists by default, but if required, it is possible to simulate it in a controlled and repeatable manner. Interference which changes as a function of the proximity of its source to the receiver can be applied using an interference simulation system such as Spirent’s GSS7765.

Download your free eBook - Effects Of Interference On GNSS Signals.


RepeatabilityIcon

When you perform testing on a GPS receiver, and it highlights weaknesses in the design, the normal process is to make changes to the design with a view to improving it. To confirm if improvements have been made, you need to repeat the same tests exactly. If Live Sky is being used, it will be impossible to ensure subsequent tests are subjecting the receiver to the same conditions as the original test.

The most obvious difference is the fact that time has progressed, and the constellation visible to the receiver will be completely different. These are factors that by themselves will ensure the test conditions cannot repeat. The other characteristics that will not remain fixed are atmospheric influences and satellite performance.

Therefore, Live Sky is unsuitable as a method for testing with a view to making design improvements.

Simulator benefits: With a constellation simulator, every time a test scenario is run, the signals produced are identical. The scenario will start at the same time on the same date, and the satellite positions will be identical – even down to the relative phase offsets between the different signals. In this way you can guarantee that the receiver is being stimulated with the exact same signals every time the test is run.


ControllabilityIcon

With any comprehensive testing, finite and accurate control of the test conditions is essential. Fine-tuning of a design or system parameter can often demand very small, closely-controlled manipulation of the test conditions.

Simulator benefits: With a Live Sky test method, there is little that you have control of. With the exception of the physical location of the test antenna, there is in fact nothing else that you have any control over. You cannot wind back time, disable the atmosphere, adjust the satellite signals, errors, data, orbits – all of which are parameters you need to have complete control over.


AccuracyIcon

A GPS RF Constellation Simulator is a precision piece of test equipment and if properly maintained, its performance is accurately specified and controlled. The fidelity of a simulator’s signals is much better than the signals from a real GPS system, which not only allows advanced testing of a receiver’s true ‘laboratory’ performance, but means that signal noise contributions due to the simulator are well below the level of thermal noise, and therefore will not contribute any noise errors to the test.

Simulator benefits: Two parameters closely related to accuracy are quality and reliability. The precision engineering employed in the simulator’s design and construction, and the quality control processes governing these disciplines ensure that the equipment gives reliable service for many years.


Record & Playback Systems do have a role to playIcon

Thorough evaluation of receiver performance requires that the impact of these various sources of previously described impairments is assessed. An emerging technique for performing this testing is by recording the RF signal for subsequent playback in the lab

Simulator benefits: Simulation allows absolute control of the test environment where individual sources of impairment can be added or removed at will. Simulation also allows the evaluation of signals not yet available from space or extremes of vehicle motion which may be expensive or difficult to trial. Indeed the generation of synthetic signals derived from mathematical models represents the ultimate in control.


Commercial viabilityIcon

No project survives without a sound business case. Those responsible for managing projects and setting budgets will have to take this into account. It is often wrongly assumed that simulation only saves money over real field trials for applications involving high-dynamics on sophisticated platforms. For example, it is very obvious that there is no way a space-grade receiver can be flown in orbit purely in order to test how well it works, but what is often not so obvious is the fact that simulation can prove to be more cost effective for much less sophisticated applications. A few months of drive testing will pay for a simulator and in many cases makes its choice over real field trials academic.

Simulator benefits: A leading European automotive manufacturer calculated that the total cost of performing a real drive test is in the order of £5k per day. Notwithstanding the technical issues with real-world tests already discussed, the financial cost-benefits alone are enough to demonstrate the viability of simulation.

Why live signals could damage your brand

The methodology of GPS simulation

An RF Constellation Simulator reproduces the environment of a GPS receiver on a dynamic platform by modelling vehicle and satellite motion, signal characteristics, atmospheric and other effects, causing the receiver to navigate on the simulator’s RF signal, according to the parameters of the test scenario. What a simulator is not is a magic box which reproduces the real world in its entirety. However, far from being a limitation, this is an important benefit. In the same way that an RF design engineer would not use a random noise generator when he really needs a controlled and quantified test signal, a GPS receiver tester would not use a random real-world signal-reproducing device when he really needs a controllable and repeatable simulated GPS test signal.

A receiver’s performance will vary depending on the severity of the errors and effects applied to the RF signal. Figure 1 shows a representation of the signal flow through a typical simulator, with the various effects being added, until the final RF output, from which the complex resultant RF signal is output to the receiver under test. This principle applies to all simulators, with the number of effects depending on the capability of the simulator, and its intended application.

GNSS RF Constellation Simulator

Figure 2 shows a typical set-up in more detail. A Spirent GSS7000 simulator is pictured. Position Velocity and Time data (typically in NMEA 0183 format) from the receiver can be fed back to the simulator control software and compared with the simulated ‘truth’ data. This will give a very accurate measure of the receiver’s performance against the known characteristics of the simulator’s signal.

Simulation test set-up

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Suitable products for GPS receiver testing

GSS7000 Multi-GNSS, Multi-Frequency Simulator Series

GSS7000

A flexible platform for GNSS testing, the GSS7000 supports any combination of GPS/SBAS/QZSS, GLONASS, Galileo and BeiDou-2 signals and offers up to 256 channels across 4 frequency bands

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GSS9000 Multi-Constellation Simulator

GSS9000

High-performance multi-frequency RF simulator for R&D testing, offering all signals and codes for GPS, GLONASS, Galileo, BeiDou-2 QZSS, and SBAS


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Testing GPS with a Simulator

Discover more about the different methods that can be used to test GPS-enabled devices.

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Guide to GNSS testing

Learn how to construct a GNSS test plan with this comprehensive eBook.

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What is GPS Receiver Testing

Get an introduction to the fundamentals of testing GPS receivers.

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