Application Note: Multi-GNSS for Technology Developers
The days of GPS having the world’s skies to itself are gone. With the GLONASS satellite constellation reaching full operational capability, and Europe and China’s constellations in development, the time of Multi-GNSS has arrived—bringing with it new challenges and opportunities for developers of location-aware technology.
Download the Application Note to discover:
- Why Multi-GNSS enables improved continuity, availability, integrity and accuracy
- Why simulation is the only option for testing the performance of Multi-GNSS receivers
- How to use a Spirent GNSS simulator to test interoperability and compatibility issues
Simply enter a few details opposite to receive the Application Note—and happy reading!
About Spirent
Spirent has been the global leader in GNSS testing for 25 years. Spirent delivers navigation and positioning test equipment and services to governmental agencies, major manufacturers, integrators, test facilities and space agencies worldwide.
Application note DAN016 Issue 1-01
Multi-GNSS
Benefits, challenges and test considerations
for Technology Developers
Spirent Communications PLC
Paignton, Devon, TQ4 7QR, England
Web: http://www.spirent.com/positioning
Tel: +44 1803 546300
Fax: +44 1803 546301
Copyright
© 2009 Spirent. All Rights Reserved.
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accordance with relevant national laws. All other registered trademarks or trademarks
are the property of their respective owners.
The information contained in this document is subject to change without notice and
does not represent a commitment on the part of Spirent. The information in this
document is believed to be accurate and reliable; however, Spirent assumes no
responsibility or liability for any errors or inaccuracies that may appear in the
document.
Table of Contents
SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • i
Scope .................................................................................................. 2
Introduction ........................................................................................... 3
Why Multi-gnss? ....................................................................................... 5
Availability and continuity .......................................................................... 5
Integrity ............................................................................................... 7
Accuracy ............................................................................................... 7
Multi-GNSS challenges ............................................................................... 8
Interoperability and compatibility ................................................................ 8
Understanding the need for RF simulation in testing .......................................... 10
Whose constellation is it anyway? ................................................................ 10
What are the most important characteristics of proper test methodologies? .............. 10
Using a Spirent GNSS simulator to test interoperability and compatibility issues ......... 12
Signal content control .............................................................................. 12
Timing control ....................................................................................... 13
Navigation data modification ..................................................................... 14
Other considerations for Multi-GNSS testing. ................................................... 16
Assessing the true benefit ......................................................................... 16
Spot the difference ................................................................................. 16
Chip Integration and cost-reduction trade-offs ................................................. 17
Testing the limits of the chip ..................................................................... 18
Be sensitive to the environment .................................................................. 19
A classic receiver test, but just as critical for a Multi-GNSS receiver. ...................... 19
Jamming and interference. ........................................................................ 20
Test the effects ..................................................................................... 20
The sky‟s the limit .................................................................................. 20
Test solutions ........................................................................................ 21
GSS8000 Multi-GNSS Constellation Simulator ................................................... 21
GSS6700 Multi-GNSS Constellation Simulator ................................................... 22
GSS6300 Multi-GNSS Constellation Simulator ................................................... 23
For more information ............................................................................... 24
Multi-GNSS: Benefits, challenges and test considerations for Technology Developers
2 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
SCOPE
This Application Note discusses the benefits and challenges of Multiple Global
Navigation Satellite Systems (Multi-GNSS) and presents the test solutions available
today which are critical to the proper technology development of receiver systems
that will use Multi-GNSS signals and services.
It is written for designers and developers of GNSS receivers and GNSS enabled devices
and equipment, and for project and procurement managers responsible for sourcing
test equipment.
Multi-GNSS: Benefits, Challenges and test considerations for Technology Developers
SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 3
INTRODUCTION
For 31 years, since the first navigation satellite made it into orbit and began transmitting
signals, we have become used to – and even taken for granted – the ability to perform
autonomous, 3-D satellite-based positioning virtually anywhere on (and in the vicinity of) the
Earth. The variety of applications that have employed this technology has increased beyond
anything envisaged in the early days of satellite navigation, and continues to grow. For a
large proportion of those 31 years the American NAVigation System with Time And Ranging-
Global Positioning System (NAVSTAR-GPS) was the sole, fully-operational system. GPS was
originally created for military navigation requirements, but the discovery of the ability to
navigate from the signal originally intended only for approximate acquisition of the „main‟
ranging signal, led to the first commercial use of GPS, and much later, in the year 2000, the
removal of the deliberate degradation of this signal kick-started the rapid increase of
commercial GPS usage.
The Americans were not alone however, as the then Soviet Union developed its
GLObal'naya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) system. GLONASS nearly made
it, and was completed in 1995, but it subsequently fell into rapid disrepair, such that it could
not be considered as having Full Operational Capability (FOC).
As the global use of GPS increased, and with the viability of using GLONASS in question,
strategic and political questions about the increasing reliance on a system owned, operated
and maintained solely by the USA began to be asked. This situation led to the beginning of
development of other GNSS systems. In 1998 The European Council called on the European
Commission to explore the possibility of developing a common system with the United States.
Discussions were held to clarify possible options. As the Americans were not prepared, for
military reasons, to envisage joint ownership or an effective role for Europe in controlling the
GPS system, cooperation would be possible;
“Either in the existing GPS system controlled by the United States; Or in the development of a
GNSS based on two navigation systems using complementary interoperable satellites: GPS and
Galileo”. The commission selected the latter option, and Galileo was born.
More recently China has started developing its own system called Beidou (Compass) and there
are other, regional systems in development.
In recent years, many believed that the introduction of Galileo, together with the existing use
of GPS would bring the commercial world into the Multi-GNSS era. As it turns out, the two
systems to do that will now be GPS and GLONASS, which after much investment is now rapidly
approaching Full Operational Capability, and it is in fact GPS + GLONASS that is currently
being used as a true GNSS system of systems. Today, leading manufacturers are following the
GPS + GLONASS path. Spirent has extended its multi-GNSS test systems to meet Commercial
Market, R&D, Integration, Verification and production test needs.
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It is also worth mentioning that the term Multi-GNSS may also be applied to additional signals
within the same system in so far as they contribute to an improvement in navigation
performance. For example, most civilian users of GPS, have relied on the single C/A-code
signal on L1. Soon, they will have access to L1C, L2C, which officially supports civilian dual-
frequency receivers, and L5, which is a robust signal allowing enhanced atmospheric error
mitigation, certainly more than is possible with a single-frequency receiver.
Finally, we must not forget augmentation systems. The Wide Area Augmentation System
(WAAS) was instigated by the US Federal Aviation Administration to improve the accuracy of
GPS so that aircraft could rely on it more for all phases of flight. The International Civil
Aviation Organization (ICAO) calls this type of system a Satellite Based Augmentation
System (SBAS). Europe and Asia are developing their own SBASs, the European Geostationary
Navigation Overlay Service (EGNOS) which will be operational by October 2009, and the
Japanese Multi-functional Satellite Augmentation System (MSAS). All three are designed to
the same specification (RTCA/DO-229D) and are fully interoperable. Other regional systems
include the Quasi-Zenith Satellite System (QZSS) in the Pacific region, the Indian GPS Aided
Geo Augmented Navigation (GAGAN) and the Indian Regional Navigation Satellite System
(IRNSS). Commercial systems include StarFire and OmniSTAR. Ground-based localised systems
also exist, such as the Local Area Augmentation System (LAAS) for siding aircraft navigation
on final approach and landing, and Differential-GPS systems based on networks of ground
monitoring stations.
Inertial Navigation Systems (INS) also play an important part in the augmentation of GNSS,
and are already widely employed in airborne and automotive applications. Assisted-GNSS is
also a critical feature in improving time to fix and accuracy, particularly in the use of GNSS in
mobile phone systems where navigational assistance data valid for a mobile handset‟s specific
location is sent to the handset via the backhaul telephone network in advance of de-
modulation of such data from the GNSS satellites by the handset‟s on-board GNSS chip-set.
The discussion of INS and A-GNSS is however outside the scope of this paper, which
concentrates on Multi-GNSS in relation to stand-alone receiver performance improvement.
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WHY MULTI-GNSS?
The strategic and political reasons for the development of „other‟ GNSS systems were not the
only ones. The technical benefits were of course a significant contributor. The fundamental
requirements of any navigation system are still as summarised in Figure 1 and they also show
the benefits brought by Multi-GNSS navigation.
FIGURE 1: FUNDAMENTAL GNSS PERFORMANCE MEASURES
It is clear that more signals can bring improvements to all four of these areas, what needs
further understanding is how and why, and what are the situations where there is no
improvement, or even a degradation.
AVAILABILITY AND CONTINUITY
This is clearly where major benefits can be gained with Multi-GNSS. In the toughest operating
environments, such as deep urban canyons, the availability of signals for use by standalone
satellite navigation applications can be severely limited due to the simple fact that over a
given period of time, less satellite signals can be seen in this environment than can be seen in
an open-sky environment over the same time period. In many cases there are just not enough
signals visible for a sufficient amount of time for a receiver to navigate.
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Adding ranging signals increases the availability and the likelihood of being able to see enough
signals, continuously for successful navigation. In this situation, providing the additional
signals are useable, there is only gain, even if the signal is of a lower accuracy than the
others, because it moves the situation from no navigation possible to navigation possible. This
principle is illustrated to good effect below. Figure 2 shows a sky plot of visible GPS
satellites. If a 45 degree elevation mask is used (represented by the inner circle on the sky
plots) it can be seen that there are only 3 visible satellites – not enough for a complete time-
solved 3-D position fix. Adding Glonass (G15) puts the critical 4th satellite in view as shown in
Figure 3. Adding Galileo (A14, 16 & 20) further improves the situation, as shown in Figure 4
FIGURE 2: GPS ONLY
FIGURE 3: GPS & GLONASS
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FIGURE 4: GPS, GLONASS & GALILEO
INTEGRITY
The original GPS system lacked the level of in-built integrity monitoring demanded by today‟s
safety-critical applications. Autonomous systems were introduced in an attempt to solve this
problem, most notably RAIM (Receiver Autonomous Integrity Monitoring) which is an algorithm
within a receiver intended to detect when a satellite‟s normally predictable behaviour
becomes un-predictable. New GNSS systems are designed with much more built-in integrity,
especially on signals which carry specific critical services such as Galileo‟s safety-of-life
service on E5b. There is also inter-GNSS integrity capabilities where a receiver compares fixes
between different systems and issues an alert if the difference is greater than a certain
threshold.
ACCURACY
Stand-alone single-system GNSS navigation (i.e. GPS only) with no augmentation is suitable for
many applications, but there are applications which demand greater accuracy. The
development of Augmentations systems, whether for specific users, such as WAAS for
aviation, or more general multi-application use such as Quazi-Zenith in the pacific region is
well documented. These developments enable the use of GNSS in applications where
otherwise it would not be accurate enough. The addition of GNSS satellites can sometimes
bring an improvement, not least because the more satellites used in a navigation computation
the less dilution of precision. However, it may make accuracy worse unless the extra
satellites offer the same performance. A lot of this depends on interoperability, which is
discussed later. Newer designs of satellite and navigation signals which are more robust,
coupled with improved models for atmospheric correction will also contribute to an overall
improvement in accuracy. In the future a stand-alone receiver may well be using GPS +
Glonass + Galileo + Compass + Quazi-Zenith or another regional system + SBAS + improved
models and corrections. This is a very different and much more complex environment than
the time honoured L1 C/A code only situation that still dominates today.
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MULTI-GNSS CHALLENGES
There are many challenges facing Multi-GNSS receiver developers, integrators and
manufacturers. The most fundamental challenge is the need to design from scratch, adapt or
re-design devices to operate using new GNSS systems. A full understanding of each system‟s
Interface Control document (ICD), which describes the system architecture and operation is
essential, so is a test tool which precisely implements each ICD in order to generate authentic
emulated signals.
INTEROPERABILITY AND COMPATIBILITY
Most challenges regarding Multi-GNSS relate to the topic of interoperability - how much
improvement can be obtained by using different GNSS‟s together over just a single system? –
and compatibility - the similarity between system operation. The greater the interoperability
and compatibility are the simpler and cheaper the receiver technology. Ironically, the two
systems likely to be the first to be used together are also the least interoperable and
compatible, namely GPS and Glonass. This is due to the lack of cooperation in earlier years as
the two systems were being developed. The rest of the GNSS‟s are likely to be fairly
interoperable.
The primary concerns regarding interoperability and compatibility are listed below (GPS,
Glonass and Galileo are assumed):
Signal modulation – GPS and all of the other GNSS‟s employ Code Division Multiple Access
(CDMA) modulation techniques. Glonass is the exception as it uses Frequency DMA (FDMA) for
channelisation, although „K-type‟ satellites – due for launch in 2010 - will transmit CDMA
signals. Receivers using GPS and Glonass will have more complex RF circuitry, as they need to
operate over multiple frequencies for Glonass. Recent development is chip technology now
make mass-market CDMA + FDMA receivers a reality, and these are being introduced for
applications such as Personal Navigation Devices (PND‟s) and Mobile Telephones.
Navigation data messages – each GNSS has its own Signal-in-Space Interface Control
Document (ICD) which details the format of the messages contained in the navigation data. A
GNSS receiver must not only have the processing capability to de-modulate and correlate the
RF signals, but also to process completely separate navigation data messages for each system.
Inter-system time offsets – Each GNSS operates on different time references. GPS uses a
continuous, atomic timescale. GPS Time (GPST) which equaled UTC:(UNSO) when it started
back in 1984, however, UTC is a varying timescale (to align with the rotation of the earth), so
it is no longer the same as GPST. Galileo intends to use TAI, although there is some question
over this approach as TAI is a scientific time reference and not recommended for broadcast. 1
1 Panel discussion on GNSS Interoperability, 36th annual PTTI meeting, Torino, Italy
Multi-GNSS: Benefits, Challenges and test considerations for Technology Developers
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Reference frames – Each GNSS computes position with reference to different reference
frames (GPS uses WGS-84, Galileo uses the GTRF and Glonass uses PZ-90). While the latest
releases of these frames align more closely to the International Earth Rotation and Reference
Systems Service’s Terrestrial Reference Frame (ITRF), there are still differences. Any receiver
using two GNSS‟s must understand and apply any offsets between reference frames and
between the ITRF in order to compute position properly. 2
2 Panel discussion on GNSS Interoperability, 36th annual PTTI meeting, Torino, Italy
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UNDERSTANDING THE NEED FOR RF EMULATION IN TESTING
Before discussing the use of an RF Constellation Simulator as a test tool for addressing Multi-
GNSS challenges, it is important to understand why it is the best option.
This section looks briefly at the key important points which show why RF emulation is the
preferred methodology.
The popular name for this type of equipment is a „simulator‟. In reality it is more accurately
described as an emulator. However, being more widely recognised, the convention of
„simulator‟ is used here.
WHOSE CONSTELLATION IS IT ANYWAY?
Where no constellation currently exists:
A lack of real signals prevents testing
Evaluation is limited to software modeling only
There is no confidence in the system performance of a receiver
There is no way of conducting dynamic testing
Even if a fully operational real constellation exists in space:
You have no control over it, so you cannot alter it in an way in order to perform
certain tests
There are many unknown errors
It is always changing, and is never repeatable
o Atmospheric conditions change
o Satellite orbits change
These issues rule out methods such as live-sky or record and replay testing for all but a quick
sanity-check test
WHAT ARE THE MOST IMPORTANT CHARACTERISTICS OF PROPER TEST
METHODOLOGIES?
Any product test, at any stage of a product‟s life cycle must have the following
characteristics to make it meaningful, robust, accountable and reliable:
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Precision emulation of signals to a given ICD
Lacking ambiguity
A High degree of control
Highly accurate test signals
Inherent and exact repeatability
Great flexibility
Allows stressing of a product/application in a controlled environment
Allows introduction of errors in a quantifiable way
RF constellation simulation is the only test methodology that has all these characteristics, and
in the context of GNSS receiver development, a simulator is effectively your GNSS under your
control for your specific test requirements.
Figure 5 illustrates the test process involving the simulator and device under test
FIGURE 5: SIMULATOR TEST SET-UP
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12 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
USING A SPIRENT GNSS SIMULATOR TO TEST INTEROPERABILITY AND
COMPATIBILITY ISSUES
Spirent‟s GSS6300, GSS6700 and GSS8000 series of GNSS RF constellation Simulators are
essential tools for testing a GNSS device‟s ability to cope with interoperability issues. Not only
are the proven advantages of RF emulation over other methods of testing applicable, but
more fundamentally, the present absence of signals in space for all new GNSS systems except
for GPS and Glonass (FDMA) make this approach impossible in any case. In some ways this is
good, as it ensures proper test methods are employed, and saves time that may otherwise be
spent gathering meaningless results using the instantaneously unknown and un-controllable
„live‟ signals.
Each of the individual GNSS systems with their appropriate constellation, orbits, signals,
timing and navigation data are produced by the simulator in accordance with the relevant
Interface Control Document (ICD) and combined to present a signal to the receiver-under-
test‟s antenna, just as it would see for real. Practically any parameter can be manipulated,
allowing signals and data messages to be changed, inter-system timing to be adjusted and
transformation matrices between reference frames to be adjusted.
Full flexibility and control of test definition and execution is afforded by Spirent‟s SimGEN
software, which controls the GSS6700 and GSS8000 series simulators. Some examples of the
capabilities SimGEN has for creating scenarios to test GNSS interoperability issues are shown
below. The full complexity of scenarios is also carried down in an embedded way to Spirent‟s
SimREPLAY software, which runs the GSS6700 simulators.
SIGNAL CONTENT CONTROL
SimGEN allows the user to control signal content parameters for each simulated GNSS. Figure
6 shows the Signal Control windows of SimGEN for Glonass and Galileo. Signal parameters can
be adjusted on a per-satellite basis, and/or applied to all satellites. This flexibility enables
the user to test receivers with different carrier, code and data combinations, and to assess
possible signal interference issues.
Multi-GNSS: Benefits, Challenges and test considerations for Technology Developers
SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 13
FIGURE 6: SIGNAL CONTROL WINDOWS
TIMING CONTROL
SimGEN gives full control of inter-system timing offsets. Figure 7 shows both the Galileo and
Glonass system time settings. It is possible to set the time offset to UTC (leap seconds), and
apply a rate of change to this as the scenario runs. For Galileo, the time offset to GPS
(declared in the GGTO message in the navigation data) can be set, as well as a divergence
between the simulated (RF) signal timing and that declared in the navigation data. As
happens with a real system as time advances after a navigation data upload.
FIGURE 7: SYSTEM TIME OFFSET WINDOWS
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14 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
These features provide great flexibility in defining tests to ensure receivers are correctly
applying the necessary offsets and timing, which are critical to accurate navigation.
Figure 8 shows the options available for setting different matrices to transform the Glonass
reference frame into the GPS WGS-84 frame. Different transform types are available, as well
as the ability to adjust the individual matrix parameters via a Generic Helmert
transformation. The Galileo reference frame is treated the same as GPS by the simulator, as
they both align very closely to each other.
FIGURE 8: GLONASS REFERENCE FRAME TRANSFORMATION MATRIX
NAVIGATION DATA MODIFICATION
An important capability for the simulator is the ability to modify and apply errors to the
navigation messages of each GNSS. SimGEN has comprehensive editors allowing manipulation
of any bit of the navigation message, as well as adjustment of specific messages and their
scheduling, and the control of integrity and health messages. It is also possible to simulate
the effects of the age of the navigation data, such as diverging ephemeris and degradation of
clock correction parameters. Figure 9 shows a typical editor used for modifying navigation
data:
FIGURE 9: NAVIGATION DATA MODIFICATION EDITOR
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SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 15
Figure 10 shows a typical editor for applying errors to navigation data
FIGURE 10: NAVIGATION DATA ERROR EDITOR
Figure 11 shows the entry page for defining the content and timing of navigation data
uploads, which is another method for scheduling changes to the navigation message.
FIGURE 11: NAVIGATION DATA UPLOAD CONTENT
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16 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
OTHER CONSIDERATIONS FOR MULTI-GNSS TESTING.
ASSESSING THE TRUE BENEFIT
As we discussed earlier, the fundamental requirements of any navigation system have not
changed with the arrival of the multi-GNSS age, in fact the added complexity of multi-GNSS
being used simultaneously increases the amount of testing required, particularly when it
comes to assessing the improvement (or otherwise) of adding additional signals.
Spot the difference
An RF constellation simulator is the perfect tool for performing comparative tests. Complete
repeatability is a powerful feature that only a simulator can offer. It is possible to run and re-
run tests with one GNSS, then add another, and another, and measure the change in
performance, while keeping all other test parameters the same. It is also very easy to add or
remove signals from a particular GNSS, this can even be done in real-time. Figure 12 shows
the power control window for a simulation containing Galileo, Glonass and GPS signals. As can
be seen, the Glonass signals have been turned off. This is achieved in just two mouse clicks.
In the same way, all satellites for another GNSS could be disabled or enabled, and individual
satellites could be enabled or disabled. Power level control in 0.1dB steps per channel is also
very easy to apply. There are also several alternative methods available, such as pre-scripted
commands to be actioned at specific times into a scenario. This finite control is echoed
throughout SimGEN, making it extremely powerful and controllable.
FIGURE 12: PER-SATELLITE POWER LEVEL DISPLAY
Figure 13 shows an alternative method. A single command is listed in the User Actions file to
turn on all Glonass signals at a pre-determined time.
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SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 17
FIGURE 13: USER ACTION FILE EDITOR
CHIP INTEGRATION AND COST-REDUCTION TRADE-OFFS
Developing receivers that integrate multi-GNSS technology is not a new concept. GPS +
Glonass receivers for example have been readily available for over ten years. For this reason
many of the challenges are well understood, and there are many papers discussing them.
What is new is the transition of multi-GNSS designs from „high-end‟ receiver platforms into
chipsets suitable for the mass-market. The common constraints of cost, size, power
consumption and processing power all play their part in raising the bar when it comes to
achieving the desired result.
Certain trade-offs will inevitably have to be made, most likely in the resources devoted to
tracking and integrated processing of multi-GNSS signals. While expensive receivers such as
those used for geodesy applications will be sophisticated enough to employ code-phase or RTK
type techniques using best multi-carrier Ambiguity resolution algorithms, and true multi-
frequency tracking, not all devices can afford be this capable. RF performance is often
compromised because multi-GNSS capability is needed but with increase in overall cost
prohibited.
A common problem, especially in GNSS-enabled phones, is use of one common clock oscillator
(TCXO) for both the telecommunications operation and the GNSS operation. This is not an
ideal solution by any means, as variations to the TCXO frequency are undesired from the GNSS
receiver‟s point of view.3 Unfortunately, the oscillators are often the most expensive single
items in the design, and economics often forces designers down the fateful single-clock route.
They then have to spend time developing techniques that advise the GNSS receiver of clock
changes in sufficient time for them to be corrected for in the PVT calculation. In addition, a
very close watch on phase noise is required, as this directly affects code and carrier loop
tracking performance by inducing jitter. The quality of the TXCO will also determine its
sensitivity to dynamic stress (G-sensitivity), which is yet another contributor to tracking error.
3 GNSS on the GO: Sensitivity and performance in receiver design. Lomer, Fulga & Gammel SIGE, Inside GNSS article, spring 2008
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18 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
Much of the performance analysis and manipulating of a design to accommodate these design
trade-offs can be thoroughly examined using a simulator.
Testing the limits of the chip
With an RF constellation simulator it is possible to stimulate a receiver with a ‟full‟ sky of
GNSS signals – something not possible in the ‟real‟ world today. This will push the tracking and
processing capability of the chip-set to its limits. It is even possible, through manipulation of
the satellite positions or the vehicle height within the simulator to place more satellites in
view of the receiver antenna than may ever be seen for real – thus testing the theoretical
performance of the receiver. Accurate design de-rating figures can then be obtained.
Figure 14 shows a crowded SimGEN sky plot. The simulator will be generating complete
signals (sometimes with several different codes) for each satellite, making a complex
composite signal for a receiver to de-modulate and process.
FIGURE 14: MULTI-GNSS SKY PLOT
The simulator is able to introduce the effects of dynamic stress. This will be absent from a
receiver being tested on a simulator because it is not moving physically. The known G-
sensitivity of the receiver‟s clock oscillator can be added to the test scenario to compensate
for this. Figure 15 shows the entry window in SimGEN.
FIGURE 15: G-SENSITIVITY PARAMETERS
A combination of effects can be built into a scenario, and detailed measurement of C/No
performance can then be made. Errors can then be very easily removed, one at a time, to
identify the main causes of performance deterioration. Focus can then be put on areas of the
design where changes will have the most benefit.
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SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 19
BE SENSITIVE TO THE ENVIRONMENT
Good sensitivity is still the most important consideration for any receiver. With Multi-GNSS,
there will be more signals entering a receiver‟s front end. This will lead to an increase in
noise, which will affect the receiver by increasing the bit error rate and code / carrier loop
jitter. Also, in devices such as mobile phones, the dominant source of interference will be
from the telecommunications radio part of the device. A typical handset will transmit over a
watt (>+30dBm) in the designated band, but due to the form of the device, interference from
the telecommunications radio signals can easily interfere in the GNSS band and overpower the
relatively weak GNSS signals, directly impacting the GNSS receiver‟s sensitivity. Good RF
isolation is essential.
Another consideration is that of the Low Noise Amplifier (LNA). Due to cost constraints it is
most common for a device to use one LNA to cover both the telecommunications signals and
GNSS signals. The large dynamic range in power between the two services means the LNA
must be very linear, whilst not consuming too much supply current, in order to avoid gain
compression, which may well result from high-level telecommunications signals, but is
certainly not desired for the GNSS signals.
A classic receiver test, but just as critical for a Multi-GNSS receiver.
A simple to execute, yet fundamental test is the sensitivity test. SimGEN offers several easy
methods for precisely controlling the satellite power so that the point where a receiver looses
lock (its minimum sensitivity level) can be determined. Equally, the point of re-acquisition
can also be determined. Spirent has produced a separate Application Note, DAN003,
discussing these and other fundamental characterization tests.
Figure 16 shows SimGEN‟s real-time power control window, with sliders allowing precise
adjustment of the satellite powers. An alternative method is to use a User action file to
adjust the powers in a timed way.
FIGURE 16: PER-SATELLITE POWER CONTROL
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JAMMING AND INTERFERENCE.
One of the benefits of Multi-GNSS operation is the increased resilience to interference
brought about by multi-frequency operation. The vulnerability of the GPS C/A code on L1 is
well documented (see reference {x}) A receiver my still suffer similar vulnerability when using
Multi-GNSS signals only at L1, but using L1, E5, L2 or other combinations and more robust
codes will improve its capability, as it is less likely that an interference source will obliterate
all signals on all frequencies. On the downside, GNSS signals that are not being used (M-code,
Galileo PRS for example) are still present at the receiver‟s antenna, and are effectively
interference, as they do not convey any information required by the receiver.
Test the effects
A simulator can be used readily to test the effects of interference. Some of Spirent‟s models
are available with an interference option, allowing a range of interfering signals to be added
to the GNSS signal. CW, AM, FM, pulsed and Noise signals with full control over each one‟s
settings can be generated. The effects of noise in a dynamic environment can also be tested,
as the characteristics of the interfering signal are adjusted dynamically in harmony with the
simulated vehicle‟s position in relation to the interference source. The presence of un-used
GNSS signals can also be simulated with M-Noise and PRS-Noise functions, enabled via a single
check-box in SimGEN. Figure 17 shows M-Noise enabled in SimGEN.
FIGURE 17: CODE AND CARRIER CONTROL WINDOW
THE SKY’S THE LIMIT
A statement that is not only true for the future of GNSS, but also true of the test capabilities
or a Spirent RF constellation simulator. This Application Note has introduced some of the key
topics relating to Multi-GNSS, and shown just some snap shots of how a simulator is the right
choice for testing the effects of these issues on a GNSS receiver. The business of designing,
manufacturing, integrating and selling GNSS technology is driven more and more by tough
commercial requirements, especially in a world starting to tentatively emerge from economic
recession. The business cases for such products need to stand up to testing like never before.
No one can afford to make mistakes at any point along the way, not least when it comes to
product testing. Investment in RF simulation as a test and verification methodology helps
keep projects safely away from the cliff edge on the right path. How do you test your GNSS?
Multi-GNSS: Benefits, Challenges and test considerations for Technology Developers
SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 21
TEST SOLUTIONS
Below is a summary of Spirent‟s current simulator products
GSS8000 MULTI-GNSS CONSTELLATION SIMULATOR
The GSS8000 series has been designed to meet all the demanding requirements of research
and development teams involved in satellite navigation and positioning systems. Due to its
modular design, the GSS8000 can be readily adapted to the requirements of different
applications. Up to 3 RF carriers, selected from a range of constellations and signals, can be
accommodated in a single signal generator chassis.
A GSS8000 system comprises a controller computer running Spirent‟s powerful simulation
software SimGEN, and a signal generator configured to meet specific test needs. Multiple
chassis can be combined to form an integrated, coherent signal generator if more signals or
outputs are required. An extensive range of system extensions allows users to tailor their
system to their specific needs, today and in the future.
Standard capabilities enabled through SimGEN include simulation of atmospheric effects,
multipath reflections, terrain obscuration, antenna reception gain and phase patterns,
differential corrections, trajectory generation for land, air, sea and space vehicles and
comprehensive error generation.
An easy to use graphical user interface (GUI) allows modification of a wide range of variables
from pre-set defaults, enabling the user to focus their time on the areas of test important to
them. Complete scenarios are readily shared between systems, supporting collaborative
activities and speeding up the R&D cycle.
Multi-GNSS: Benefits, challenges and test considerations for Technology Developers
22 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
GSS6700 MULTI-GNSS CONSTELLATION SIMULATOR
The GSS6700 supports any combination of GPS/SBAS, GLONASS and Galileo L1 signals and
provides accurate, repeatable combined multi-GNSS signals. The GSS6700 can be configured
with up to 12 channels of one constellation only or with multiple constellations, for example:
GPS only
GLONASS only
Galileo only
GPS and GLONASS
GPS and Galileo
GPS, GLONASS and Galileo
Up to 36 channels are supported with 12 channels of simulation per constellation.
The GSS6700 is available with a range of software capability to suit differing test
requirements.
SimGEN is Spirent‟s fully flexible simulator software suite and would typically be required in
technology development and R&D applications. SimGEN offers a complete and flexible
scenario generation capability including control of the constellations, propagation, terrain
obscuration, antenna patterns, multipath, vehicle trajectory and a range of error models.
For environments where repeat testing using the same test cases is needed, for example
verification testing, Spirent‟s SimREPLAY software is available. SimREPLAY supports operation
of the simulator replaying pre-defined scenarios and is ideal for repeat comparative
measurements. Scenarios for use with SimREPLAY can be obtained from a number of sources
including an online scenario generation tool available at no additional charge to customers
under warranty or with a support contract.
SimREPLAYplus allows users to generate scenarios locally with features comparable to the
online tool available to supported SimREPLAY users. In addition, SimREPLAYplus enables the
user to define vehicle motion remotely or using a file in the required format.
Multi-GNSS: Benefits, Challenges and test considerations for Technology Developers
SPIRENT APPLICATION NOTE DAN016 ISSUE 1-00 • 23
GSS6300 MULTI-GNSS CONSTELLATION SIMULATOR
The GSS6300 Multi-GNSS Signal Generator has been designed specifically for high volume
production test applications for devices that use commercial GPS/SBAS, GLONASS and/or
Galileo receivers. Controlled testing is vital in ensuring correct assembly and verification of
expected performance parameters in GPS only and Multi-GNSS manufacturing environments.
Spirent‟s GSS6300 provides a robust single channel testing solution with the minimum of
operator intervention.
The GSS6300 can be configured with one channel of a specific constellation or with multiple
constellations, for example:
GPS only
GLONASS only
Galileo only
GPS and GLONASS
GPS and Galileo
GPS, GLONASS and Galileo
For GPS L1 C/A code test applications, the GSS6300 features, interfaces and specification are
the same as the Spirent GSS6100 product that is widely specified for manufacturing test
applications.
Generation of signals from the various constellations is enabled by licence key. If all licence
keys are present the GSS6300 can generate a single GPS signal, a single GLONASS signal and a
single Galileo signal concurrently.
The GSS6300 is designed for easy integration into production test environments. A
comprehensive remote control interface enables control of the unit via standard interfaces,
including IEEE488 and USB.
For operation in the laboratory, for example as a single channel precision GNSS signal
generator for RF design work, a GUI-driven software utility, SimCHAN, is supplied.
Multi-GNSS: Benefits, challenges and test considerations for Technology Developers
24 • SPIRENT APPLICATION NOTE DAN016 ISSUE 1-0
FOR MORE INFORMATION
Please visit our website: http://www.spirent.com/positioning and do not hesitate to contact
your nearest Spirent representative for more detailed information. To find the appropriate
contact details please visit the „Contact Us‟ page on the website and select your location and
application.
If you found this article of interest
www.spirent.com/testmore
Visit the Spirent GNSS blog
www.spirent.com/Blog/Positioning.aspx
Need more information?
mailto:gnss-solutions@spirent.com
Spirent Communications
Aspen Way,
Paignton, Devon,
TQ4 7QR England
Tel: +44 1803 546325
globalsales@spirent.com
www.spirent.com
Spirent Federal Systems Inc.
22345 La Palma Avenue
Suite 105, Yorba Linda
CA 92887 USA
Tel: 1 714 692 6565
info@spirentfederal.com
www.spirentfederal.com
