Free eBook: Simulation versus Real World Testing
When designing and developing most GNSS-enabled devices, knowing how they will perform in different locations and environments is vital.
But the scope of ‘real world’ testing is severely limited—not only by time and cost considerations, but by a lack of repeatability and control.
Download a free eBook and discover how GNSS simulation solves the problems of ‘real world’ testing, helping you to:
- Reduce time-to-market with faster testing of new devices
- Eliminate external variables and conduct tests impractical in the real world
- Get a clearer vision of where performance can be enhanced
Simply enter a few details opposite to receive your free eBook—and happy reading!
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.
REAL WORLD TESTING
How to undertake controlled testing of your
GNSS receiver design
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That would be wrong and
Should global navigation
satellite system GNSS
receivers only be tested
using real world signals
to guarantee their proper
Simulation, by definition, reproduces the signals and effects a
receiver sees in the real world, but under the controlled
conditions of the laboratory, making GNSS receiver testing a far
more consistent and reproducible exercise.
Importantly, the signals from the simulator are exact known
quantities … right down to the bit level. And, by providing the
ability to test the different performance parameters of the
receiver individually (or in concert), the simulated environment
will provide much clearer insight into the true performance of
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The Nine Key Tests
There are nine key tests that together determine the performance
of any GNSS receiver:
1. Cold-start time to first fix
2. Warm-start time to first fix
3. Hot-start time to first fix
4. Acquisition sensitivity
5. Tracking sensitivity
6. Reacquisition time
7. Static navigation accuracy
8. Dynamic navigation accuracy
9. Radio frequency interference
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Some tests may not be as critical as others, depending on the
intended application of the receiver. But between them, these
nine tests cover all the important aspects of receiver
performance that will impact the end-user experience.
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Test 1: Cold-start time to first fix (TTFF)
This is one of the great tests of a GNSS receiver because it will be the first thing
that a user notices. Time to first fix is always an important metric, and the
“cold-start” version is just that — the receiver is starting from scratch, with no
memory of any previous reading. The time is unknown, the current almanac and
ephemeris are unknown, and (obviously) the current position is unknown.
It is also a test that is far better performed with a simulator,
because, the sure way of measuring this quantity is to run a series of tests on
each receiver and take an average time, with each test based on a completely
new location — several thousands of kilometres from the previous one.
Try running that test using real-world satellite signals!
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Test 2: Warm-start time to first fix
The second test is similar to the first, but the difference is important. For the
so-called “warm start”, the time and almanac are retained within the receiver's
memory. However, the ephemeris data are either unknown or out
of date and the position is within 100km of the last fix.
And while single measurements can be performed equally as well in the real
world, the added control of using a simulator in the laboratory (and absence of
outside influences) allows the test to be performed with total certainty.
Using a simulator, it’s also readily possible to take multiple measurements and
average the results. And when you have altered your design or set-up, you can
quantify the improvement by re-running exactly the same tests with exactly the
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Test 3: Hot-start time to first fix
Although the hot-start TTFF is the least arduous of the time to first fix
measurements for the receiver, in many ways it’s probably the most important,
as this will be the performance that the end user will experience most often.
In this scenario, the receiver has full data on time, the almanac and the
ephemeris, and the position is within 100km of the last fix. All that is required is
for the receiver to collect the full navigation message from the simulator. As
with other TTFF tests, and due to the importance of the measurement, it is
advisable to run the test several times with different satellite geometries
to calculate an average value for the TTFF.
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Test 4: Acquisition sensitivity
The sensitivity of any GNSS receiver is key to its performance, and acquisition
sensitivity is the first for these important measurements as it defines the
minimum received power level at which the receiver can obtain a fix.
This is another test where the simulator is an essential tool. It is
only through the ability to control the power output from the simulator (on
individual satellites, or all at once) that an accurate measure of acquisition
sensitivity can be obtained.
Ideally, the simulator should be capable of very fine power control to within
0.1dB to obtain the most accurate possible measure of acquisition sensitivity.
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Test 5: Tracking sensitivity
As with acquisition sensitivity, the fine control of power levels is
essential in determining tracking sensitivity — the minimum power level
at which the receiver can maintain lock. Crucially, it is tracking sensitivity
measurements that will highlight the errors inherent in the design of the
receiver's PLL-based tracking loops. These include phase error, dynamic stress
error and thermal noise.
The test itself is relatively easy: with the receiver locked on to the simulator's
output, simply lower the simulator power output until the lock is lost. Multiple
repeats of the test with different satellite geometries will ensure
that an accurate average measure is recorded.
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Test 6: Reacquisition time
Reacquisition time is a particularly important measurement for vehicle-based
receivers, which will inevitably lose satellite signals when travelling through
tunnels or even under bridges. For example, the end user will not be impressed
if the receiver misses a turn instruction because it has not reacquired the signal
after passing such an obstruction.
Again, the simulator allows total control over the test, reducing the
signals from each satellite by at least 60dB to ensure that the receiver loses
complete lock, raising it again to normal power and measuring the time taken to
reacquire the lock.
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Test 7: Static navigation accuracy
This is in many ways the most difficult test to predict, as there are so many
different factors — both internal and external — that can affect a receiver's
performance. So this is another case where the controlled environment
of the laboratory is essential to remove external variables such as the
effects of the ionosphere and troposphere – or indeed include them, but in a
A useful tip here is to simulate a static position of 0 degrees latitude, 0 degrees
longitude and 0 metres elevation, as it will make it easy to observe the
receiver's divergence from the simulated position.
Again, multiple measurements should be taken to allow for different satellite
positions and factors such as receiver thermal performance. The resulting
metric is typically quoted as a statistical average of the many performance
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Test 8: Dynamic navigation accuracy
Particularly important for vehicle-mounted receivers, dynamic navigation
accuracy involves taking a series of measurements while the receiver is moving
in one, two or three axes. While such measurements could theoretically be
taken reproducibly on a test track, the simulator again has a trick up its sleeve
that inevitably leads to improved measurement accuracy.
The simulator control software has the ability to simulate the relative motion of
the receiver and satellites. And with a high dynamic performance simulator, this
means that virtually all types of vehicle motion profiles can be
simulated, with high fidelity even with the most extreme maneouvers.
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Test 9: Radio frequency interference
Because GNSS receivers are such sensitive instruments, it is almost inevitable
that they are susceptible to radio frequency interference — most of which will
be accidentally generated. However, there are also instances where a jamming
signal might be deliberately broadcast in order to lock out a navigation system.
There are many commercial interference simulators on the market that can be
used to obtain a measure of a receiver's susceptibility to any given frequency of
RFI. However, by using a coherent interference source that is directly coupled
to the GNSS simulator and dynamically controlled by the same system software,
far greater insight into a receiver's performance can be obtained,
allowing designers to take appropriate filtering measures to improve their
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The use of a multichannel RF constellation simulator in
testing the performance of a GNSS receiver has many
Indeed, many of the parameters that are key to the performance of a receiver
simply cannot be reliably tested in the real world - in some cases
due to simple practicality, in others because of external variables that render
test results unreliable.
It is only by rigorous simulator-based testing, under the controlled conditions of
the test laboratory, that the nine key performance indicators of any GNSS
receiver can be determined with absolute certainty and full repeatability.
Armed with these results, GNSS receiver developers can perfect their designs,
leading to improved products that exceed users' expectations.
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Spirent GNSS Simulators
Spirent is the industry leader for GNSS simulator products. Spirent offers several different models of GNSS
simulators that support a variety of different applications and cover the full spectrum of 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 most demanding research and engineering
applications. For more comprehensive testing, Spirent also offers products that simulate additional system
elements simultaneously with the GNSS constellation signals, such as inertial sensors, various automotive sensors,
Assisted GPS (A-GPS) + Assisted GLONASS (A-GLONASS) data, SBAS and GBAS augmentation system signals,
interference signals and Wi-Fi Positioning.
Spirent’s Multi-GNSS simulation platforms
Spirent GSS8000 Multi-GNSS
Spirent GSS6700 Multi-GNSS
Spirent GSS6300 Multi-GNSS Signal
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If you found this article of interest
You can find more GNSS related technical articles, white papers and eBooks at the
Spirent Positioning website.
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your business, please contact Spirent to discuss your particular situation and
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