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The Challenge
As the adoption of location based services becomes more main-
stream, developers are looking for increased accuracy and availability
from positioning solutions. For example services that target
marketing messages need to know which display the user lingered at,
not merely which store they are in, and ideally they would know in
which direction the user is facing!

To provide ubiquitous positioning that reaches indoors and positively
identifies location and orientation in a multi-story building requires
blending the best of multiple sources of information, in other words,
a hybrid solution.

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Ideally such a marriage of technologies allows the strengths of one to
counter the weaknesses of the other in a truly complementary way.
For example the high availability of local low cost inertial sensors
can offset GNSS which may not be available within buildings; whilst
the drift over time inherent in inertial systems can be managed using
regular high accuracy fixes from GNSS, when available.
And the sensors may not be those traditionally associated
with positioning. For example a tourist guide
application describing the scene presented to the
user answering the question “What am I looking
at now ?” may use the users smartphone camera
and cloud computing to overlay captions on the
display with links to appropriate informative
webpages in an augmented reality format.
SPIRENT eBook

By intelligently combining data from multiple sources the designer
aims to retain the best of each and to mitigate their weaknesses.
Truly a case of the whole exceeding the sum of the parts…if it works!

The peculiar test challenge presented is that development of such a
system cannot be done one sensor at a time. The effective blending
of position information from disparate sources is predicated on an
understanding of their strengths and weaknesses, this will require
careful development and importantly, a test environment which
provides total control over multiple, coherent sources to facilitate
iterative improvement.

This e-book aims to introduce the technical challenges and
some of the ideas that Spirent has to provide the
development tools required.

Existing Technology
GNSS alone
“Global Navigation Satellite Systems (GNSS) enable you
to locate your position anywhere on the earth – or around
it – to within millimetres”. Now, you have to admit that is
an amazing statement, and in some cases it is perfectly
true, but there are limits to this incredible technologies
capability. Navigating, autonomously, using signals from
satellites orbiting some 12500km away is an almost
unbelievable technological achievement and has been the
solution to a myriad of application needs, where knowing
‘where’ and ‘when’ you are is important. And, the list of
applications for GNSS navigation is growing all the time.


On the face of it, GNSS would appear to be the panacea for all
navigation and timing requirements. Relatively low cost, low power,
compact devices enable you to locate yourself and know the current
time with pinpoint precision. GNSS is brilliant but is it sufficient to
meet all positioning needs on its own.

While there is absolutely no doubt that GNSS is a must-have
technology for the “where” and “when” needs of today’s applications,
it is not without its limitations and trade-offs. Going back to the
“within millimetres” statement, closer inspection reveals that to
achieve this level of accuracy takes some very sophisticated and
relatively expensive GNSS receiver technology. This is out of
the question for many of the applications that
need accurate timing and navigation.


So what are we left with? Accuracy of a metre? A few metres?
Hundreds of metres? The answer is yes to all of these, because the
performance of GNSS is so variable and depends on so many factors
that it is impossible to state exactly how it will perform in a generic
way. We can however consolidate performance into 4 measures:-

• Availability - The percentage of time a Position, Navigation and
Time (PNT) solution is available
• Continuity – Performance without non-scheduled interruptions
• Integrity – The level of trust in the information provided
• Accuracy – The degree to which PNT data conforms to the
actual position


There are many factors that alter how a GNSS receiver performs
against these measures, so let’s restrict ourselves to the key issues
for consumer applications.

Availability is perhaps the most significant measure to consider given
the kinds of environments consumer devices have to operate in. The
urban scene is undoubtedly a hostile one for a GNSS receiver. Tall
buildings obscure the line of sight navigation signals, and in some
cases - if the obscuration is combined with poor satellite
geometry (not enough satellites in view) – there
won’t be enough signals available to compute a
PNT solution. A minimum of 4 satellites in view
is required.

SPIRENT eBook

Go indoors, and things will probably get
much worse. The extremely weak
(c.-130dBm) signals arriving at
the Earth’s surface which are not
blocked by a building are further
attenuated by its fabric. Highly
sensitive receivers are required,
but these have to spend
longer and take more power
to extract the weaker signals
from the noise.

SPIRENT eBook

Accuracy is another measure which is significantly affected. The
satellite signals, even if they are not obscured, may be reflected, and
take multiple paths to the receiver antenna, much longer than the
direct path, causing false ‘copies’ of the signals to arrive, late, at the
receiver corrupting the PNT calculations. Even worse is the case where
there is no line of sight signal, and the multi-path version is the only
one the receiver sees. Because the receiver uses the satellite’s signals
as ‘radio tape-measures’ any distortion in the signals (e.g. time delay,
phase error) will degrade the PNT solution accuracy. In the case of
weak signals, a high sensitivity receiver may help, but they are more
susceptible to multipath signals.


So, in practice, the continuously-available millimetric accuracy
heralded at the start is rarely achieved, indeed, accuracy of several
metres can be a challenge. Therefore it is clear that GNSS alone is not
enough. We need to consider how to optimise its benefits and how
to bring the four measures of performance up to acceptable levels
for our application. This can only be achieved by augmenting GNSS.
Adding another system or multiple systems alongside GNSS to help it,
correct it or substitute for it is the practical way forward. Collectively,
this approach is known as Hybrid Positioning, the big opportunity
for performance improvement arising from the complementary nature
of the strengths and weaknesses of the various technologies. In
combination the technologies can perform better (more accurate,
higher availability etc) than any of the technologies independently.
Team work rather than competition is the way forward it seems!


The need for test.

With so many potential issues, testing of GNSS navigation systems is
essential. Spirent has been the leader in this field for over 25 years,
and has solutions for testing receivers on all existing and planned
GNSS systems, as well as new solutions for Hybrid Positioning.


Options: Sensors
A modern smartphone contains an array of sensors incorporated for a
number of purposes but all potentially available for positioning. MEMs
gyros and accelerometers, barometric sensors and magnetometers all
have a direct application in estimating position.

The inertial sensors can be used in the estimation of position by
sensing the movement of the body from a known initial position.
In this way GNSS technology, which provides accurate positions but
not all the time, and inertial technology which allows a position
estimate to be advanced in the absence of any external signals, form
a perfect partnership. The addition of magnetometers to provide
heading and barometric sensors to provide height completes a very
useful set of sensors.

Options: WiFi

As we have seen, the properties of radio wave propagation mean that
they lend themselves to being used for navigation – after all, that’s
the foundational principle of GNSS.


So, with that in mind, what is to stop other radio-based techniques
being used for the same purpose? Well, actually, in some cases
nothing much. One such technique is Wi-Fi positioning. You have – in
most urban areas – many Wireless Access Points transmitting radio
signals with identifying properties. You can therefore determine your
position by looking for a ‘fingerprint’ of Access Point visibility in a
large database of such devices. Combining this with a measure of
the power level received versus the database’s record of the access
point’s transmit power to determine range, will enable
you to trilaterate from multiple Access Points.

This technique is gaining traction in the marketplace,
with both database technology suppliers and phone
handset/chipset manufacturers offering or looking Spirent GSS5700
Wi-Fi Access Point
to offer solutions. Positioning Simulator


Spirent is the only test equipment provider to offer a Wi-Fi test
solution. The GSS5700 Wi-Fi Access Point Positioning Simulator
consists of a ‘smart’ signal generator using a core application
running on an internal CPU with touch-screen, remote MMI control
via Ethernet. Access Point Control includes the ability to specify/
change, MAC address, SSID, TX channel power level/dynamic path
loss and simulated ‘visibility’. Supporting up to 24 Access Points in
one chassis, and being able to operate with Spirent’s GSS6700 GNSS
simulators to give coherent GNSS/Wi-Fi test signals, makes
the GSS5700 Wi-Fi Access Point Positioning Simulator an
essential test solution for this positioning technology.


Options: Cellular
What is Cellular Positioning?

Cellular positioning utilizes the cellular network
infrastructure to compute position and obtain a fast
and coarse location estimate. Compared to A-GPS/A-
GLONASS, cellular network positioning is generally
faster (~5 seconds), but it is less accurate (50 meters
to several kilometres). It is quite often used as
“fallback” technology when GPS, GLONASS and Wi-Fi
signals are unavailable.


Cell Identification (Cell-ID), the simplest location technology, locates
a user based on the cell they are using, though the accuracy of
this technique varies widely. Cell-ID positioning establishes the cell
to which a user equipment (UE) is currently connected and uses
that cell location as a rough estimate of the UE’ s current position.
Ecell-ID positioning extends the Cell-ID positioning technique with
supplementary information that narrows down the location within the
cell where the UE is located. This supplementary information includes
Timing advance and Neighbour cell measurements.

Cellular network triangulation, either signal strength-based or time-
based, can be used to improve accuracy and time-based methods
such as Time Difference of Arrival are common.


Cellular positioning methods currently in use comprise: Cell identity
(Cell-ID) positioning; Enhanced cell identity (Ecell-ID) positioning;
Assisted Global Positioning System (A-GPS) positioning; Advanced
Forward Link Trilateration (AFLT) in CDMA networks, Enhanced
Observed Time Difference (E-OTD) in WCDMA networks and Uplink
time difference of arrival (UTDOA) positioning. Observed downlink
time difference of arrival (OTDOA) positioning technique is currently
under discussion for LTE networks.


Why Cellular Network Positioning?
Pros:
• Cellular Networks cover majority of populated areas
• ellular technology is built into all mobile phones and increasing
C
number of other mobile devices (like connected PNDs)
• Works indoors and in urban environments
• Very fast response time
• omplementary to GNSS and
C
other Location Technologies
Cons:
• ➢ Relatively poor accuracy


Even though all these techniques have some limitations, it is possible
to use some as stand-alone or in conjunction with GNSS and other
technologies to overcome the individual limitations. Considering the
global scale and spread of cellular networks, cellular positioning
would form an integral part of any hybrid strategy.


Other Sensors
The complications faced by engineers aiming to meet the ever
increasing requirements of emerging location based services (LBS)
on accuracy and availability of positioning solutions mean looking
for different and sometimes novel sources of positioning information.
Use of cell-id techniques, Wi-Fi and other sensors such as low cost
MEMs inertial are becoming more common and present unique
test challenges. The GSS6400 rises to the challenge by enabling
the user to record two channels of serial digital data alongside the
digitized GNSS spectra. Signals direct from sensors or via a bus such
as CANbus are captured and buried within the GNSS data ensuring
coherent playback. An alternative use of this facility is to record a
timing signal such as a GPS-derived 1pps which allows subsequent
synchronization of external systems such as rate tables etc.


Comprehensive, End-to-End Location Technology Testing
Spirent’s 8100 Location Technology is the evolution of the solution
known for years as the “Spirent ULTS”. It offers the most advanced,
comprehensive environment available for testing A-GPS/GLONASS-
enabled GSM, UMTS and LTE devices.

Location testing can be performed for standalone or hybrid location
technology using A-GPS and A-GLONASS with Wi-Fi, Cell ID and ECID.
This fully automated, flexible and configurable solution offers Control
and User Plane implementations, including complete SUPL support.

Spirent plays a major role in the development of test requirements
and specifications for location technologies within leading standards
organizations.


Blending
– a conclusion of sorts

An unfortunate but inescapable conclusion is that
whilst each technology has its strengths none are on
their own sufficient to satisfy the accuracy, availability,
integrity and cost requirements of the burgeoning
location based services market. A combination of
complementary technologies would appear to hold the
answer but blending the approaches to get the best
of each is a challenging technical task. Spirent aims
to support this endeavour by drawing on its extensive
experience and wide range of tools to provide bespoke
test solutions for hybrid positioning developers.

We hope you found this Hybrid Positioning E-Book of interest.
We are continually adding new content to our website
on a regular basis. Bookmark this link:
www.spirent.com/positioning

Visit the Spirent GNSS Blog, there are currently
more than 90 posts with 2 to 3 new posts added
per week. Catch up on what’s new:
www.spirent.com/Blog/Positioning

Need more information?
gnss-solutions@spirent.com

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Hybrid Positioning

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