common gnss systems errors and how to test them · errors inherent in gnss systems with increasing...
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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 when designing and manufacturing GNSS receivers and products integrating GNSS receivers will ultimately lead to poor-performing and unreliable end products.
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Fortunately, the majority of these GNSS system errors are well understood, and their effects can be recreated in the controlled environment of the test laboratory using a radio frequency GNSS simulator.
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What comprises a GNSS system?
Each global navigation satellite system comprises three distinct components, each of which has its own associated errors:
The space segment comprises the constellation of satellites.The control segment is made up from at least one master control station and its associated monitoring and data uplink stations.
The user segment comprises the system’s GNSS receivers and other systems that use the GNSS signals.
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Space segment errors
There are two types of problems that can occur in the space segment of any GNSS: those caused by the positioning of the satellites themselves and those caused by errors in the clocks carried by each satellite.
Even the smallest satellite clock errors can cause huge inaccuracies in navigation. And while the clocks carried by each satellite in any GNSS constellation are highly accurate atomic devices, even these can drift over time.
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To a lesser extent, the exact positioning of each satellite within a constellation can have an effect on the value of its navigation message. This is because satellite orbits are perturbed by any number of factors, including the gravitational influences of the Sun and Moon, the elliptical nature of the geometry of the Earth and solar radiation pressure.
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Control segment errors
All control segment errors are manifested in the contents of the navigation message transmitted by each satellite. There are three basic types: ephemeris prediction errors, navigation data errors and ionospheric prediction errors.
Ephemeris prediction errors will prevent a receiver from tracking any given satellite – simply because it is not where the receiver calculates it should be at any given time.
Errors in navigation data can easily occur in the twice-daily upload of
navigation messages from the control segment to each satellite in
the space segment.
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And because the contents of the navigation data message are so crucial to the operation of any GNSS receiver, the consequences of even a bit-level error can be severe.
Ionospheric prediction errors also originate in the control sector, and depend on the exact ionospheric model that is embedded in the navigation message. If the control centre chooses the wrong coefficients to apply, the ionospheric delay relayed to a receiver can be as much as 50% out.
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User segment errors
Most 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. These errors include incorrect compensation for ionospheric and tropospheric delay, as well as fundamental design flaws such as noise within the design and factors such as interchannel bias, multipath mitigation and integration issues
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One other source of user segment errors are the users themselves! Not surprisingly, incorrect operation of a GNSS receiver will lead to incorrect results. This emphasises the importance of a well designed user interface so that operation is at best intuitive, or at least instructions are available for the operation of the device.
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Finding fault with GNSS receivers
Clearly, the only way to ensure that any GNSS receiver can cope with 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.
However, this is one form of GNSS receiver testing that can never be performed with live-sky signals from real-world satellites. The whole point of the problem is that these system errors are anomalies. They cannot be predicted, and so they cannot be “captured” from the real world.
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The only practical solution is to simulate these system errors using a GNSS RF simulator.
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Simulating space segment errors
Using a suitable GNSS RF simulator under software control, users can recreate all manner of space segment errors, including:
· 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 with complete confidence that the condition has been simulated and also with complete repeatability.SPIRENT eBook
Simulating control segment errors
Similarly, a full set of control segment error scenarios can be recreated using the same GNSS RF simulator. These include:
· ephemeris prediction errors· all sorts of navigation data errors· and ionospheric prediction errors.
A wide variety of these tests are available as ready-programmed software routines, ensuring complete accuracy and repeatability.
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Simulating user segment errors
Most user segment errors are covered by the standard test scenarios used in GNSS receiver characterisation. These include:
· tropospheric delay errors· multipath effects· and a wide range of receiver errors.
In each case, the tests are available as ready-to-run software routines that enable the GNSS RF simulator to precisely model the error in question.
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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 known sources of error.
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Living with 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 likely 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 expect from today’s electronic devices.SPIRENT eBook
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