Free eBook: Testing Multipath Performance of GNSS Receivers
Multipath effects can compromise the accuracy of GNSS receivers by creating errored signal data that leads to incorrect calculation of positioning, velocity and time.
Download a free eBook to discover how RF simulation can help to mitigate multipath effects during the GNSS receiver design phase. Topics include:
- The impact of multipath effects on GNSS receiver accuracy
- Why rigorous testing of multipath performance is essential for all GNSS receivers
- The benefits of using RF simulation to model and mitigate multipath issues
- Recommended testing approaches for different multipath effects
Simply enter a few details opposite to receive your free eBook—and happy reading!
About Spirent
Spirent has been the global leader in GNSS testing for near 30 years. Spirent delivers navigation and positioning test equipment and services to governmental agencies, major manufacturers, integrators, test facilities and space agencies worldwide.
TESTING MULTIPATH PERFORMANCE of GNSS Receivers
How multipath simulation can be used to evaluate the effects of multipath on the performance of GNSS receivers
The multipath phenomenon
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Should GNSS receivers be tested
in the real world to guarantee
their proper operation?
Multipath effects cannot be
ignored as they can compromise
the performance and
accuracy of any
receiver design
2
But how do you ensure rigorous testing
of all the types of
multipath effects ?
Only through controlled
simulation.
Put simply, multipath is the
propagation phenomenon that results
in any specific radio signal reaching
the receiving antenna by two or
more different paths. It is caused by
reflection and diffraction of the
original signal by any number of
physical phenomena, ranging from
the built environment to geological
features, trees, the ground and even
water.
All of these factors, individually (and in combination) can cause
multipath propagation.
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Types of multipath
Because of the huge diversity of structures and media
involved, multipath phenomena can take a wide range
of forms. However, there are two basic types:
Constructive interference occurs when the
reflected signal arrives in-phase with the original (line
of sight) signal.
Destructive interference occurs when the signals
are out of phase.
Constructive interference will amplify the signal and
increase the resulting pseudorange, whereas
destructive interference will cause a signal fade and
reduce the resulting pseudorange.
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Why is this a problem for GNSS receivers?
In a global navigation satellite system receiver, multipath effects can cause a
stationary receiver's output to imply that it is randomly jumping from point to
point or creeping.
This is because the receiver uses the measured transmission time of the signal
to determine its pseudo-range, and multipath effects will cause that time to
vary. In-phase multipath will generally cause an over-reading of pseudo-range,
and out-of-phase multipath will lead to an under-reading of pseudo-range.
When the unit is moving, the jumping or creeping will often be less pronounced,
but it can still degrade the displayed accuracy to a point where the performance
will be unacceptable.
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Multipath mitigation
There are a number of multipath mitigation techniques used by receiver
manufacturers, ranging from various mathematical correlation and filtering
techniques to different antenna designs.
Multipath limiting antennas can be particularly useful when dealing with signals
from satellites that are low on the horizon: the reflections causing multipath
effects in these instances can be extremely complex, depending on the nature
of the ground level surface.
Mathematical techniques include a-posteriori multipath estimation, which
provides a measure of the multipath error by using the fourth replica of the
pseudorandom number code.
However...
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No multipath mitigation measures can be taken
without first establishing the scale of the problem for
any specific GNSS receiver design...
And that entails rigorous testing
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Testing times
While it is true that GNSS receivers are expected to operate under real-world
conditions, the practicalities of testing how a receiver deals
with multipath performance in the real world is very
difficult. The test setup would be unwieldy as it would be necessary to isolate
and quantify all the signals reaching the receiver as well as assessing its own
response to the signals. And the necessity to test the receiver with a
representative sample of different multipath effects would make the process
both long-winded and open to a myriad of uncertainty.
However, the use of a suitably equipped GNSS simulator in a laboratory
environment provides both the flexibility to test different scenarios and
the repeatability to assure the quality of the test results.
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Simulating multipath effects
When choosing and setting up a GNSS simulator for multipath testing there are
two important factors to consider.
1: The simulator must have a sufficient number of output channels to provide adequate
coverage of multipath effects, which in some cases will mean one channel for the line-of-
sight signal and several additional multipath signal channels for each satellite.
Fortunately, multipath effects are most pronounced in restricted environments such as
urban canyons where fewer satellites are visible to the receiver. Hence 12 channels will
generally be sufficient.
2: Software control is essential. With a large number of measurements to be made and
absolute repeatability essential, suitable test software is not a luxury. Spirent offers
nine different types of multipath models or ways of applying multipath, therefore the
ability to automate test procedures is an important consideration in reducing time to
market.
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Fixed offset multipath
The fixed-offset multipath is the most basic test setup, under which the
simulator produces a simple “echo” signal with a constant range and
power offsets from the line-of-sight signal. Multiple echo signals can be
produced up to the maximum channel count of the simulator, and the line-of-
sight signal can be removed to simulate conditions where the main signal is
lost.
The simplicity of the fixed-offset multipath test is in that it does not include the
effects of movement of either the satellites or the receiver, relative to any
physical obstruction and the pseudorange changes due to satellite and
vehicle relative motion are the same for line-of-site and multipath signals.
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Ground reflection multipath
The ground reflection multipath test is particularly useful in modelling the
multipath effects of satellites that are low on the horizon. It is also particularly
pertinent for receivers that are intended for marine applications as the sea is a
particularly effective reflecting medium for GNSS signals.
The only major requirement for the test is that the antenna of the
receiver is positioned above the ground to receive the reflected signal.
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Doppler offset multipath
The Doppler offset multipath test involves a more complex test scenario that
was originally devised for the 3GPP TS25.171 mobile phone standard. In
addition to the range and power offsets specified in the fixed-offset test, the
user also specifies a Doppler frequency offset, which will cause the delay
between the multipath and the line-of-sight signal to vary over time.
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Reflection pattern multipath
Reflection pattern multipath testing is particularly useful as it allows the user to
model the electrical properties of a simulated antenna for the test, rather than
use the receiver's antenna itself. This then removes any variations there
may be in the reception pattern of the receiver antenna and tests the receiver
itself. It is ideally applicable to systems where the host vehicle itself is a major
source of the multipath signals.
It allows you to specify different multipath delays for different signal
arrival angles. There will be greater variation in multipath delay when the
relative movement of the antenna with respect to the arriving satellite signals
increases.
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Legendre multipath
This model is ideal for modelling multipath signals in a relatively static
environment, Legendre multipath testing uses a fifth-order Legendre polynomial
to create the relative amplitude and delay of the imposed signals. And while the
reflected signals are not necessarily representative of the relative geometry of
the satellites and the receiver, the technique is highly reproducible, and can be
used to create particularly complex interference patterns.
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Polynomial multipath
The polynomial multipath scenario is similar to the Legendre case, but the
polynomial coefficients are less rigorously defined. The downside of this is that
the multipath profile does not have fixed boundaries like the
Legendre polynomial, and so the same offsets cannot necessarily be applied to
tests of differing durations.
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Sinusoidal multipath
The sinusoidal multipath overcomes the limitation of the standard polynomial
multipath by producing a time-varying multipath signal that is well
bounded and easily defined.
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Land mobile multipath (LMM)
Developed specifically to simulate the signal environment experienced by
portable devices such as mobile phones, the LMM model is considerably
more comprehensive than any of the other multipath test scenarios. And,
consequently, software control is essential.
Land mobile multipath simulation software allows the test engineer to define
the signal conditions from a menu of predefined environments for each test.
These provide statistical models that produce: direct line-of-sight signals with
Rician fading (as in destructive multipath); reflections with Raleigh fading (as in
constructive multipath), power decay and exponential decay; and deep fading
of echoes, providing a Doppler offset. The Spirent simulator software comes
with a variety of pre-programmed routines that simulate specific
environments such as urban canyons, flyovers and trees.
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Fader multipath
The final test scenario is also different in nature to all the others in that it
overcomes the requirement to have a dedicated hardware channel in the
simulator for each signal. Fader multipath testing allows the user to produce up
to four multipath signals from each simulator channel, with each
different sub-channel separated digitally in the simulator hardware. This
approach allows highly flexible modelling in which the level, delay and
phase of each sub-channel can be independently defined.
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Case proven
With end users of GNSS receivers becoming ever more accustomed to pinpoint
location accuracy in every possible environment, multipath effects cannot be
ignored, as they have the potential to compromise the performance and
accuracy of any receiver or system design. Multipath mitigation is
essential, and this must entail testing the design to ensure that the
mitigation techniques chosen are effective for the receiver's normal
usage scenario.
And...
Because real-world testing can only (by definition) produce
complex, largely unquantifiable and unrepeatable multipath
environments, a GNSS simulator offers the only
practical option for multipath testing that
is both accurate and repeatable in the
controlled environment of the test laboratory.
Spirent GSS8000 Multi-GNSS Constellation Simulator
Spirent GSS6700 Multi-GNSS Constellation system
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Document Number: MCD00136AAA Issue 1-00
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