Free eBook: Common GNSS System Errors and How to Test Them
Every global navigation satellite system, from GPS to GLONASS and Galileo, has inherent system errors that designers and developers of GNSS receivers must understand and account for.
Download a free eBook to learn about the common system errors inherent in GNSS systems today, and how you can simulate their effects in the lab for testing and risk mitigation. You’ll learn:
- Common errors in the space segment, control segment and user segment of GNSS systems
- How to test your receiver’s ability to handle and mitigate the effects of GNSS system errors
- Why RF simulation is the only viable approach to robust and accurate GNSS error testing
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.
Common GNSS system
errors and how to test them
Errors inherent in GNSS systems
With increasing numbers of manufacturers adding GNSS receivers to
their products, it is important to appreciate that no global navigation
satellite system (GNSS) is perfect. Indeed, there are several system
errors inherent in each GNSS. And receivers must compensate for
these errors in order to provide reliable outputs.
Failure to address these system errors nullen designing and
manufacturing GNSS receivers and products integrating GNSS receivers
nullll ultimately lead to poornullerforming and unreliable end products.
Page 2SPIRENT eBook
Fortunately, the manullrity of these GNSS system errors are nullell
understood, and their effects can be recreated in the controlled
environment of the test laboratory using a radio frenullency
GNSS simulator.
Page 3SPIRENT eBook
What comprises a GNSS system?
nullch global navigation satellite system comprises three distinct
components, each of nullich has its onull associated errorsnull
nulle space segment comprises the constellation of satellites.
nulle control segment is made up from at least one master control
station and its associated monitoring and data uplinnullstations.
nulle user segment comprises the systemnulls GNSS receivers and other
systems that use the GNSS signals.
Page 4SPIRENT eBook
Page 5SPIRENT eBook
Space segment errors
nullere are tnull types of problems that can occur in the space segment
of any GNSSnullthose caused by the positioning of the satellites
themselves and those caused by errors in the clocnull carried by
each satellite.
nullven the smallest satellite clocnullerrors can cause huge inaccuracies in
navigation. And nullile the clocnull carried by each satellite in any GNSS
constellation are highly accurate atomic devices, even these can drift
over time.
Page 6SPIRENT eBook
nullo a lesser enullent, the enullct positioning of each satellite nullthin
a constellation can have an effect on the value of its navigation
message. nullis is because satellite orbits are perturbed by any
number of factors, including the gravitational innullences of the
Sun and nulloon, the elliptical nature of the geometry of the nullrth
and solar radiation pressure.
Page 7SPIRENT eBook
Control segment errors
All control segment errors are manifested in the contents of the
navigation message transmitted by each satellite. nullere are three
basic typesnullephemeris prediction errors, navigation data errors and
ionospheric prediction errors.
nullphemeris prediction errors nullll prevent a receiver from tracnullng any
given satellite nullsimply because it is not nullere the receiver calculates
it should be at any given time.
nullrrors in navigation data can easily occur in the tnullcenullaily upload of
navigation messages from the control segment to each satellite in the
space segment.
For more information, donullload the Spirent nullboonull null nullesting multipath performance of GNSS receiversnull
Page 8SPIRENT eBook
And because the contents of the navigation data message are so
crucial to the operation of any GNSS receiver, the consenullences
of even a bitnullevel error can be severe.
Ionospheric prediction errors also originate in the control sector,
and depend on the enullct ionospheric model that is embedded in
the navigation message. If the control centre chooses the nullong
coefnullients to apply, the ionospheric delay relayed to a receiver
can be as much as nullnullout.
Page 9SPIRENT eBook
User segment errors
nullost errors in the user segment stem from faults in the receiver itself
and are those commonly tested in the design and characterisation
of GNSS receivers. nullese errors include incorrect compensation for
ionospheric and tropospheric delay, as nullell as fundamental design
nullnull such as noise nullthin the design and factors such as interchannel
bias, multipath mitigationnulland integration issues.
nullor more information on multipath mitigation, donullload the Spirent nullboonullnull nullesting multipath performance of GNSS receiversnull
Page 10SPIRENT eBook
nullne other source of user segment errors are the users themselvesnull
Not surprisingly, incorrect operation of a GNSS receiver
nullll lead to incorrect results. nullis emphasises
the importance of a nullell designed
user interface so that operation
is at best intuitive, or at least
instructions are available for the
operation of the device.
SPIRENT eBook
Finding nullnullt nullth GNSS receinullrs
nullearly, the only nully to ensure that any GNSS receiver can cope
nullth any form of system error is to repeatedly test the device to
ensure that it can respond to the error and continue to produce
reliable results.
nullonullever, this is one form of GNSS receiver testing that can never
be performed nullth livenullnull signals from realnullorld satellites.
nulle nullole point of the problem is that these system errors are
anomalies. nulley cannot be predicted, and so they cannot be
nullcapturednullfrom the real nullrldnull
nullonullload the Spirent nullboonullnull Simulation versus nulleal World nullestingnull
Page 12SPIRENT eBook
nulle only practical solution is to simulate these system errors using a
GNSS null simulator .
Page 13SPIRENT eBook
Simnullating space segment errors
nullsing a suitable GNSS null simulator under softnullre control, users can
recreate all manner of space segment errors, includingnull
· satellite clock errors
· intentional satellite clock noise
(also known as selective availability)
· orbital perturbations
· and anomalies in satellite geometry
In each case, the tests can be
performed nullth complete connullence
that the condition has been simulated
and also nullth complete repeatability.
SPIRENT eBook
Simnullating control segment errors
Similarly, a full set of control segment error scenarios can be recreated
using the same GNSS null simulator. nullese includenull
· ephemeris prediction errors
· all sorts of navigation data errors
· and ionospheric prediction errors.
A nullde variety of these tests are available
as readynullrogrammed softnullre routines,
ensuring complete accuracy and
repeatability.
SPIRENT eBook
Simnullating nuller segment errors
nullost user segment errors are covered by the standard test scenarios
used in GNSS receiver characterisation. nullese includenull
· tropospheric delay errors
· multipath effects
· and a wide range of receiver errors.
In each case, the tests are available as
readynullonullun softnullre routines that
enable the GNSS null simulator to
precisely model the error in nullestion.
SPIRENT eBook
And, as the tests are completely repeatable, GNSS receiver designers
can use them time and time again to ensure that their design
improvements do cater for all these nullonull sources of error.
Page 17SPIRENT eBook
nullnullng nullth errors
No global navigation satellite system is perfect. And it is of paramount
importance that designers and manufacturers of GNSS receivers
understand all the sources of error inherent in such systems
and their linullly impact on both accuracy and
reliability.
It is only by simulating the effects
of these errors that companies can
design and manufacture GNSS
receivers capable of providing the
levels of accuracy and reliability
that users have come to enullect
from todaynulls electronic devices.
SPIRENT eBook
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