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Andy Walker

Andy Walker is marketing manager at Spirent Communications’ Positioning Technology division in Paignton, UK. He has more than 20 years experience in technical marketing, sales and business development management roles in the T&M industry.

After studying electrical and electronic engineering in Glasgow he moved into marketing and has a CIM post graduate diploma in marketing and a Masters in Marketing from Plymouth University Business School. He is a Chartered Marketer and enjoys running and kayaking.

Recent Posts

The Uses of Automated Testing Software: Integrating a GNSS Receiver

For manufacturers of location-aware devices, the decision on where to place the GPS/GNSS receiver within the overall product design is a critical one. “You’re holding it wrong” Many people will remember the problematic Apple iPhone 4 design that resulted in dropped calls if the handset was held “the wrong way”. Earlier this year it was Samsung’s turn, as issues with the location of the GPS antenna in its Galaxy S III smartphone prompted the company to issu...

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Limitations of Live Sky Testing in a Production Environment

Should manufacturers use live satellite signals to test location-aware devices? One challenge for manufacturers of location-aware devices is to ensure that every single unit leaving the factory will perform exactly as it should. A malfunctioning product will be found out as soon as it gives users an inaccurate position or delivers navigation instructions that are patently wrong. And thanks to the power of social media, news of a poorly-functioning product can go viral, causing reputational a...

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GNSS receivers move from precision agriculture to the backyard

One of the most economically successful applications of GNSS has been in precision agriculture: a concept that was developed in the USA during the 1980s. Since that time techniques have improved and have seen more widespread adoption, improving both yields and productivity. Although GNSS receivers are not the only technology involved, they do play a key role providing positional feedback to enable agricultural machinery to operate in exactly the right spot and to cover large areas with the gre...

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Simulation equals simplification

One of the most important benefits of testing a GNSS receiver using an RF simulator rather than using either live or recorded signals from real-world satellites is the ability to simplify the test in order to observe a specific response from the receiver. The formula for calculating the estimated position error of a GNSS receiver is a complex one that takes into account many outside factors. When using a simulator, the majority of these outside variables can be eliminated from the equatio...

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Why is reacquisition time important?

One of the critical performance parameters for almost any GNSS receiver is reacquisition time–or how long it takes the receiver to make sense of the available satellite signals after an obstruction has been cleared. Applications in which reacquisition time is particularly important include automotive sat-nav devices and the railway industry. In both these cases, the vehicle housing the receiver is likely to experience periodic interruptions from overhead obstructions, such as tunnel...

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What are the benefits of narrow correlator spacing?

Narrow correlator spacing is a key technique for multipath mitigation in GNSS receivers. Whereas earlier GPS receivers used a 1.0 chip spacing in their delay lock loops (DLLs), the reduction of this spacing offers a number of benefits, reducing tracking errors in the presence of both general noise and multipath phenomena. The reduction in noise is achieved with narrower spacing because the noise components of the early and late signals characterised by multipath interference are correlated and...

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What is the SCDM?

The SCDM is the System for Differential Correction and Monitoring, which is a key part of a Russian-operated satellite based augmentation system (SBAS) for both GLONASS and GPS. The SCDM uses a ground network of monitoring stations on Russian territory as well as some overseas stations. The stated object for the SCDM is to provide metre-level accuracy for GLONASS users, with target levels of 1-1.5m for horizontal accuracy and 2-3m for vertical accuracy. The correction and integrity data will...

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The importance of repeatability in GNSS receiver testing

In any testing regime at any stage in the lifecycle of a GNSS receiver repeatability of tests is essential. Regardless of whether you are designing a receiver from scratch, comparing different receiver designs, developing new applications, integrating receivers into end equipment or production testing finished kit, if you are not able to run exactly the same test (or test sequence) time and time again then the results of the tests will not be conclusive. In development work, for example, the a...

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How does a GNSS account for relativity?

Among the inevitable problems with any global navigation satellite system, relativistic effects such as time dilation, gravitational frequency shift and eccentricity effects all have to be accounted in order for the system to function correctly. As explained by the theory of relativity, the clocks on each satellite will run fractionally faster than those on Earth because of their constant movement and height relative to the Earth-centred nonrotating reference frame. Gravitational frequency s...

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GPS modernisation and the L1C signal

Another of the major improvements that is scheduled in the modernisation of the Global Positioning System (GPS) is the L1C signal. This is a freely available civilian-use signal that will be broadcast on the same 1575.42MHz frequency as the L1 signal broadcast be the existing GPS constellation. As it is broadcast on the same frequency as the existing L1 signal, older receivers will still be able to receive the C/A signal from the new L1C signal. The L1C will be available with first Block III G...

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Vehicle models in GNSS receiver testing

One of the most important aspects of testing the reliability and performance of any GNSS receiver is testing the dynamic performance of the system in its intended application. And while this may seem like a prime case for taking the receiver on the road (or rail, sea, air etc.) for “live-sky” testing, all the usual limitations of this methodology will apply: the tests are inherently random and non-repeatable, and the costs associated with taking staff and equipment out of the la...

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GNSS receivers on course for sporting precision

One of the fastest-growing sectors for the use of GNSS receivers is the sporting goods industry, where data from such devices are used for a wide range of activities, from analysing the workrate of footballers as they move around the pitch to keeping track of competitors in all manner of races. One of the first sports to embrace the technology was golf, where GNSS receivers are used to plot the position of a player's ball in relation to a map of the hole that is being played. The map display w...

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What is MEO?

MEO, or medium Earth orbit, is the region of space surrounding the Earth that is commonly used by navigation satellites. Also referred to as intermediate circular orbit (or ICO), MEO is found between the low Earth orbital altitude of 2000km and the geostationary orbital altitude of 35,786km. The MEO region is becoming increasingly congested, with the GPS constellation orbiting at an altitude of 20,200km, the GLONASS constellation at an altitude of 19,100km, the Galileo constellation at an alti...

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The increasing influence of GPS timing

The old saying “time is money” has never been more true than today, with almost every aspect of modern living governed to some extent by precision timing. The world's financial markets require split-second timing to seal every transaction. Wireless communications networks need to be precisely synchronised to enable optimum use of the radio spectrum. Similarly, power companies need precision time synchronisation throughout their distribution networks for optimal power efficiency and f...

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What is carrier phase tracking?

Carrier phase tracking is a highly accurate means of positioning used in GNSS surveying applications. The product of the period of the carrier frequency times the speed of light gives the wavelength, which is about 1900mm for the GPS L1 carrier. With a 1% of wavelength accuracy in detecting the leading edge, this component of pseudorange error can be as low as 2mm, compared with 3000mm for the C/A code and 300mm for GPS P code. However, in order to employ carrier phase tracking and make use ...

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Why test GNSS receivers for EMC?

Although there is no specific product standard for EMC in GNSS receivers, testing for immunity to electromagnetic interference has to be an essential part of the product development process. Any receiver designed for low-level signals will inevitably be susceptible to electromagnetic interference. And GNSS receivers inevitably are required to operate reliably in areas where they will be subject to any number of sources of interference. These range from natural phenomena such as solar radi...

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What is the Land Mobile Multipath model?

Developed specifically to meet the demands of the mobile phone industry, the Land Mobile Multipath (LMM) model provides a pragmatic approach to assessing the multipath performance of location aware mobile handsets. Rather than taking the analytical approach to multipath performance testing typically used for other GNSS receivers, the LMM model provides a database of ready-written test scenarios and allows the test engineer to define the signal conditions from this database. This enables t...

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Simulating roadside buildings with SimGEN Vertical Planes Feature

One of the major considerations in designing a GNSS receiver for automotive applications is to ensure that the performance is not unduly affected by roadside buildings. Such structures can both obscure satellite signals from the receiver and reflect them, leading to potentially complex multipath effects. Clearly, it is impractical for a receiver designer to take his or her latest project “on the road” to test out its performance in the presence of every possible size, shape and combina...

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High-precision GNSS market set for growth

With the latest report from market research guru ABI Research predicting that the high-precision GNSS sector is set to double in size between 2011 and 2016, there will be strong temptation for new companies to enter this potentially lucrative part of the market. Already consumer chipset manufacturers are looking to exploit the potential of the forthcoming open L2C and L5 signals, and with China accounting for around half of the market in 2010, new manufacturers from that locality are s...

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What is PRN code?

Pseudorandom noise (PRN) codes are an important element of code division multiple access (CDMA) based satellite navigation systems. Each satellite within a GNSS constellation has a unique PRN code that it transmits as part of the C/A navigation message. This code allows any receiver to identify exactly which satellite(s) it is receiving. The PRN codes act as spreading codes in the spread-spectrum communications system, and must be carefully chosen to minimise interference between each ...

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Compass on track for global coverage

While the Russian GLONASS constellation is slowly approaching full operational status, developers of future multi-GNSS systems also need to focus on the likely timeline for availability of the Chinese Beidou-2 or Compass system. The successful launch of the latest satellite for the Beidou-2 constellation means that nine such satellites have been placed into orbit since 2007. Further launches are planned for this year and next. And the Chinese authorities reckon to be on course to provi...

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Why test GNSS receivers for EMC?

Although there is no specific product standard for EMC in GNSS receivers, testing for immunity to electromagnetic interference has to be an essential part of the product development process. Any receiver designed for low-level signals will inevitably be susceptible to electromagnetic interference. And GNSS receivers inevitably are required to operate reliably in areas where they will be subject to any number of sources of interference. These range from natural phenomena such as solar ra...

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What is CNAV?

CNAV is the name for the civilian navigation message that will be carried by the modernized GPS system. And while the CNAV message will carry similar data to the existing NAV message, its structure will be completely different, with a packetised format that will increase message bandwidth to allow for greater information density and pave the way for future system expansion. To this end, the system is designed to support 63 satellites, compared with 32 for the L1 NAV message. Each packe...

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GNSS Chipset Expansion Must be Underpinned by Quality

With a recent report predicting that consumer and precision GNSS receiver IC shipments will reach two billion units by 2016, it is important to remember that quantity must be underscored by quality. The report, by ABI Research, highlights the importance of new vertical markets, such as tablets, cameras and fitness equipment, as well as the growth of LBS (location-based services) and LBA (location-based advertising) applications, all of which will contribute to a quadrupling of the market wit...

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How will GPS/GLONASS Chipset Integration Help the Consumer?

The integration of GPS and GLONASS systems within the same receiver offers many advantages to both the manufacturer and the end user. And now that this integration is happening at the chipset level, several further advantages can be added to the list. On the basic technology level, the combination of GPS with GLONASS will provide greatly improved performance for the end user. The ability for the receiver to access signals from both constellations will provide greater availability, impr...

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Do you Need a Screened Enclosure to Test a GNSS Receiver?

The traditional logic that all RF testing should be carried out in a screened enclosure has much to recommend it. But do you always need to go to this level of protection when testing a GNSS receiver with a simulator? The short answer is no, because the preferred method of testing is to remove the antenna from the equation and connect the simulator directly to the receiver. However, there will be occasions where the antenna cannot be removed. And there will also be certain acceptance t...

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GNSS Record & Playback Complements Testing With Simulators

All Global Navigation Satellite Systems (GNSS) work by measuring the transmission-time delay from a satellite to the receiver. With a clear view of the sky and an unobstructed path to multiple satellites a modern GNSS receiver is able to calculate its position rapidly and accurately. However atmospheric propagation effects can alter the speed of the signals to an indeterminate degree, signals can be obscured by buildings or reflected off surfaces such as the sea, all of which will compromise the...

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Spirent STR4760 Currently Testing Navigator Space Qualified Receiver Used on Hubble Space Mission

We very rarely talk about specific products in this Blog, we prefer to discuss applications and trends within the GNSS community but for this particular entry we thought we’d make an exception. The Navigator is a space-qualified GPS receiver, being developed by a team led by Dr. Carl Adams at the NASA Goddard Space Flight Center (GSFC), designed to highly elliptical and geosynchronous orbits. It has flown as part of a Remote Navigation Sensor experiment on the Hubble Space Telescope Se...

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What is Binary Offset Carrier Modulation?

Binary offset carrier modulation (or BOCM) is a split-spectrum modulation scheme used by the Galileo navigation satellite system. BOCM is a square subcarrier modulation. A signal is multiplied by a rectangular subcarrier of a frequency that is equal to or higher than the CDMA rate. Following this subcarrier multiplication, the spectrum of the signal is divided into two parts. The purpose for using BOC modulation in Galileo is to reduce interference with BPSK-modulated signals such as C...

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What is Wi-Fi Positioning?

Although satellite-based navigation systems are the preferred means for establishing location in “open” terrain, they do suffer shortcomings in areas where the satellites are obscured from the receiver, particularly by man-made structures such as buildings. In short, the performance of GNSS receivers cannot be guaranteed indoors or in densely populated “urban canyon” environments. Wi-Fi positioning is a technique that has been developed to overcome these limitations and augment GNSS-based po...

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What is GAGAN?

The GPS Aided Geo Augmented Navigation or GPS and Geo Augmented Navigation system (GAGAN) is a space-based augmentation system (SBAS) funded by the Indian government. Interestingly, Gagan is not just an acronym: it is also a Sanskrit word for the sky. The GAGAN space transponder is due to be launched on 19th May 2011 on an Ariane-5 launcher from the European Space Agency (ESA) spaceport in Kourou, French Guiana. This will be the second attempt to get GAGAN into orbit. The first satelli...

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Record and Playback Systems in Multi-GNSS Receiver Testing

Record and playback systems (RPSs) such as the new Spirent GSS6400 can be used at many stages of the GNSS receiver design cycle and for many different tasks. However, one application that may not seem immediately obvious is in testing the ability of multi-GNSS receivers to work with both existing and future satellite systems. Clearly, the RPS cannot be used to playback signals from non-existent satellites. But used in conjunction with a simulator, the RPS can play a key role in...

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What is CNSS?

CNSS is the Compass Navigation Satellite System, which will eventually comprise up to 30 medium-earth-orbit satellites and five geosynchronous satellites to provide true global coverage. This Chinese system is distinct from that country's existing Beidou I satellite system, which has been operating since 2003 but provides only domestic coverage using three geosynchronous satellites. Like other systems, the CNSS will provide two levels of service. The free service for civilian users will offer p...

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Record and playback systems in multi-GNSS receiver testing

Record and playback systems (RPSs) such as the new Spirent GSS6400 can be used at many stages of the GNSS receiver design cycle and for many different tasks. However, one application that may not seem immediately obvious is in testing the ability of multi-GNSS receivers to work with both existing and future satellite systems. Clearly, the RPS cannot be used to playback signals from non-existent satellites. But used in conjunction with a simulator, the RPS can play a key role in inves...

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GPS Modernization: What is M-Code?

An important part of the current GPS modernization program, M-code is the name given to a new signal that is designed to improve both the security and anti-jamming properties of military navigation using GPS. Importantly, the M-code is designed to be autonomous, and so users will be able to calculate their positions using only the M-code signal (unlike the existing military P(Y) code, which also requires use of the C/A code). Radically, in addition to the normal wide-angle broadcast, t...

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What is a GPS almanac?

In the wider world, an almanac is an annual publication dedicated to information such as weather forecasts, tide tables, lunar cycles etc. A typical almanac will contain tabular information covering a particular field or fields, and will be arranged according to the calendar. However, in the world of satellite navigation systems, the almanac is a regularly updated digital schedule of satellite orbital parameters for use by GNSS receivers. The almanac for any g...

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What is Dilution of Precision in GNSS Receivers?

Although a GNSS receiver requires only four satellite signals to provide a complete positional fix in three dimensions, the accuracy of this fix depends to some degree on the exact positions of the four satellites relative to the receiver. If the four signals acquired come from satellites spread throughout the sky relative to the receiver, then the fix should be highly accurate. But if all four are observed in close proximity to each other within a single quadrant, then the fix will be less ...

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What is RAIM?

RAIM stands for receiver autonomous integrity monitoring, a technology used in GNSS receivers to assess the integrity of the GNSS signals that are being received at any given time. It is particularly applicable to receivers intended for safety-critical applications, and in particular in aviation applications. The RAIM concept makes use of redundant satellite signals – i.e., any that are available above and beyond those needed to produce a position fix. If the pseudorange data in any of...

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What is WGS-84?

The World Geodetic System provides a standard co-ordinate frame for the Earth, a standard spheroidal reference surface (or ellipsoid) for raw altitude data, and a gravitational equipotential surface (or geoid) that defines the nominal sea level. WGS-84 is the most recent version of the system, which was originated in 1984 and revised in 2004. Earlier schemes included WGS-72, WGS-66 and WGS-60. WGS-84 is the reference coordinate system used by GPS. The origin of the WGS-84 co-ordinate s...

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What is a Terrain Obscuration Model?

Particularly relevant for ground-vehicle-based GNSS receivers, terrain obscuration is the phenomenon of temporary and intermittent masking of GNSS signals when manoeuvring at low altitude in mountainous terrain. This can be a critical limiting factor on the performance of GNSS receivers in certain applications, and so the ability to simulate such effects is an important tool in improving the reliability and performance of GNSS receiver designs. Spirent SimGEN™ software contains a compr...

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Time Discrepancies in Multi-GNSS Receivers

One of the major issues in testing GNSS receivers designed for use with multiple satellite systems is that the different systems do not necessarily share the exact same time-bases. And while the differences may be tiny, time is such a critical quantity in satellite navigation that even microsecond differences can create large accuracy errors. The problem is a legacy of the GPS system, which has used its own time-base (GPS time) since it began in the early 1980s, rather than the globall...

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GPS Modernization and the L2C Signal

L2C is the name given to one of the new signals to be broadcast from the satellites in the modernised GPS constellation. This new signal is intended for civilian use (hence the “C”), and will be broadcast on the L2 frequency at 1227.6MHz by all satellites from block IIR-M onwards. The L2C signal is one of the key means by which the modernized GPS will offer improved accuracy and availability for civilian applications based on dual-frequency receivers. Not only is the signal intended to...

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Japanese Earthquake Highlights Importance of GNSS Timing

One of the more surprising consequences from the March 2011 earthquake in Japan was that the forces unleashed shifted the earth's mass sufficiently to accelerate its rotation, shortening each day by no less than 1.8 microseconds, according to calculations by NASA. And while such effects are clearly imperceptible to humans, their cumulative effect on GNSS timekeeping would cause significant inaccuracies that would be unacceptable to any GNSS application. Each GNSS will deal with the tim...

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GPS Modernization With Improved Civilian Signals

In addition to the new signals to be broadcast under the GPS modernization project, there are to be two significant changes to the existing civilian signals, both designed to improve the performance of GPS receivers. The first is an additional data-free pilot signal and the second is the addition of forward error correction (FEC) encoding to the navigation message. The new data-free signal will be broadcast alongside the normal data signal, acting as an easy-to-acquire pilot signal. On...

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When Will Galileo be Ready?

The official opening on 20th December 2010 of the Fucino Galileo Control Centre, 130km east of Rome, has brought the Galileo global navigation satellite system one step closer to fruition. However, a continuing shortage of funding for the project suggests that while Galileo will be available by 2014, the service will initially be limited. Earlier in 2010, the European Commission confirmed that funding was available to launch four in-orbit validation (IOV) satellites by 2014, with the f...

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Motion Simulation with GNSS Scenario Software

For most GNSS receivers, static navigation accuracy is only part of the story. The ubiquitous automotive satnav system is the most common case where dynamic accuracy is essential. And users will only be satisfied with the performance of their receivers if turn-by-turn navigation instructions are both accurate and timely. So while you may be familiar with the concept of testing the performance of a GNSS receiver in the lab with a GNSS simulator, how can this be extended to take into acc...

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Testing GPS in the Rail Network: What’s the deal with using GPS in trains?

There’s a simple, compelling, theory around using GPS in trains: by using GPS for position information, coupled with train communication systems, the need for track-side signalling goes away. The massive infrastructure cost associated with signalling goes away as everything needed is on-board each train. The reality surrounding this vision, however, is that rail infrastructure has to be heavily regulated and standards-driven due to the safety critical nature of the industry. The ...

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Threat of import duties to increase appeal of GLONASS

The Russian Federation has come up with further “encouragement” for manufacturers to support the GLONASS satellite navigation system with the threat of import duties to be levied on any mobile handsets without GLONASS functionality. Deputy Prime Minister Sergei Ivanov has revealed that the duty will be “about 25%”, will be introduced by 2012 and will be levied on handsets imported to the Russian Federation with GPS-only positioning technology. Three more GLONASS -M satellites are schedu...

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How do you test an IVNS?

The integrated in-vehicle navigation system (IVNS) is becoming an increasingly popular feature in many automotive applications, ranging from relatively straightforward driver information systems right through to sophisticated tracking and fleet management systems. Such systems, which combine a GNSS receiver with one or more dead reckoning (DR) sensors, provide superior positional accuracy over GNSS alone, and are particularly useful in areas prone to signal obscuration or complex multipath e...

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JAXA Chooses Spirent’s GSS8000 to benchmark their QZSS receiver performance

In a recent post I explained the concept behind QZSS (What is QZSS) – Since the publication of that post and to further help in the development of the Quazi-Zenith Satellite System (QZSS) programme, the Japanese Aerospace Exploration Agency (JAXA) has selected Spirent’s GSS8000 Multi-GNSS Constellation Simulator to verify QZSS receivers. The highly elliptical orbits of QZSS allow satellites to dwell at high elevations, improving coverage in urban canyons and providing additional overhead ran...

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What is QZSS?

The Quasi-Zenith Satellite System (QZSS) is a GPS augmentation system that aims to greatly improve GNSS accuracy over Japan and the rest of East Asia. The first satellite in the system, dubbed Michibiki, was successfully launched from the Tanegashima space centre on 11th September and reached its quasi-zenith orbit on 27th September 2010. QZSS aims to enhance GPS services both by improving the availability of GPS signals and by performance enhancement to increase the accuracy and relia...

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What is a Pseudolite?

A pseudolite (or pseudo-satellite) is any device that performs a specific task that would otherwise require a satellite. So, for example, a dedicated transmitter might be deployed to extend the reach of a GNSS to areas where the satellite signals are either blocked or jammed, and there has been considerable success with deployment of pseudolites to extend GPS coverage indoors. These are relatively simple devices that only transmit the coarse acquisition code. Pseudolites are also used ...

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Why Does a GNSS Simulator Need a GUI?

There is no doubt that testing GNSS receivers is a complex business. Today's GNSS simulators are sophisticated instruments with increasing levels of features and complexity. Yet time-to-market pressure on GNSS receiver designers and manufacturers dictate that these simulators must be easy to understand and intuitive to operate. The key to this ease of use is in the graphical user interface (or GUI). And properly designed, the GUI will help simulator users access important information q...

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What is Pseudorange?

The pseudorange is an approximation of the distance between a satellite and a GNSS receiver. A GNSS receiver will attempt to measure the ranges of (at least) four satellites as well as their positions when their positional data were transmitted. With the satellites' orbital parameters supplied in the almanac within the message, each position can be calculated for any point in time. The pseudoranges of each satellite are obtained by multiplying the time taken for each signal to re...

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What is Record & Playback

All Global Navigation Satellite Systems (GNSS) work by measuring the transmission-time delay from a satellite to the receiver. With a clear view of the sky and an unobstructed path to multiple satellites a modern GNSS receiver is able to calculate its position rapidly and accurately. However atmospheric propagation effects can alter the speed of the signals to an indeterminate degree, signals can be obscured by buildings or reflected off surfaces such as the sea, all of which will compromise...

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The Importance of Static Navigation Accuracy in a GNSS Receiver

Read on and get your Free copy of the Spirent eBook explaining how to undertake controlled testing of your GNSS receiver design. Of all the performance metrics for a GNSS receiver, navigation accuracy is probably the most important. More to the point, pinpoint static navigation accuracy is essential to the end user. And if he or she doesn't get the exact location fix expected when the receiver is powered up for the first time, there will be little confidence in the overall performance ...

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What is the Local Area Augmentation System?

The Local Area Augmentation System (LAAS) is a ground-based augmentation system (GBAS) specified by the US Federal Aviation Authority for use at airports to augment the accuracy of GNSS-based navigation. The LAAS focuses its service on the airport area (around 30-50km radius) for precision approach, departure procedures and terminal area operations. It broadcasts its correction message over a VHF radio datalink from a ground-based transmitter. The FAA plans to replace legacy navi...

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Are you Involved in GPS/GNSS Production Test?

In many production environments, a single channel GPS signal simulator is the preferred way to ensure that each device meets defined parameters. The Spirent 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 manufactu...

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GNSS Simulation: The General Principles

The core requirement of any GNSS receiver test, whether for development, integration or production purposes, is for a controlled, repeatable signal. For many tests, the signal control includes flexibility over test case, or scenario, conditions that enable performance testing at nominal and extreme or error-state conditions. Real-world, live-sky testing has significant drawbacks which, in practice, preclude controlled testing. These drawbacks of live-sky testing include: ...

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What is Ionospheric Scintillation?

Ionospheric scintillation is the term given to irregularities in the ionosphere caused by so-called “space weather”. Key sources of ionospheric scintillation include solar winds and magnetic storms. Historically, the level of scintillation has been seen to follow the 11-year solar cycle and peak at the time of maximum sunspot activity. Therefore the next peak is due in 2012. Scintillation occurs most frequently at tropical latitudes at night. It occurs less frequently at high latitudes...

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What is a GNSS Test Scenario?

Although the word “scenario” has been much overused (and misused) in common language, it has particular relevance in terms of testing GNSS receivers. The word can be correctly defined as “an outline or model of an expected or supposed sequence of events”, and this provides an insight to the importance of a test scenario in assessing and/or comparing the performance of one or more GNSS receivers. A GNSS simulator under software control can be used to generate test scenarios of varying d...

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What are MEMS Sensors?

One of the simplest and most popular ways of improving the accuracy of navigation systems is to combine GNSS-based navigation with inertial navigation using MEMS sensors. But what are they? MEMS stands for micro-electromechanical systems, and MEMS sensors comprise a class of devices that are micromachined from bulk silicon. As they are made from silicon, it’s possible to produce devices such as accelerometers that combine both a sensing element and the associated signal conditioning ci...

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What is the Klobuchar Model?

The ionosphere is the single largest error source in point positioning after the application of precise GNSS orbit and clock products, and there are a number of mathematical models that have been proposed to mitigate its effects. Of these, the model developed by John A Klobuchar is used by the GPS system and broadcast by every satellite. The Klobuchar model is something of a compromise between computational complexity and accuracy. However, it is reckoned to be capable of correcting up...

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Multipath mitigation in Marine GNSS Receivers

On the face of it, the marine environment might appear relatively benign for GNSS receivers. After all, there is virtually no chance of signal obscuration from buildings or trees (although the odd cliff might come into play), so logically, the design of a marine GNSS receiver should be a piece of cake. However, such devices are particularly prey to multipath interference – both from the surface of the sea and from the superstructure of the vessel itself. And left untreated multipath ef...

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Questions Facing Modern GNSS Developers

In the past, designers of satellite-based navigation systems have been mostly restricted to using a single system and service, the GPS C/A code. The current revolution in the industry is giving rise to not only new GNSS systems, some GNSS systems, notably GPS and Galileo, have multiple services available to the commercial GNSS designer. The question of which system or blend of systems a GNSS designer should consider, along with the potential for multiple services is thus an entirely ne...

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The use of Multiple Sensors in Improving Satellite Navigation Performance

Today most commercial receivers are GPS L1 C/A code only. However, for many applications, the single frequency GPS performance is inadequate and many technology developers are turning to other sensors to compliment GPS. Today we have many forms and functions, depending on the application and hence needs. Integrated in-vehicle navigation systems often compliment the GPS position with dead reckoning navigation information from wheel rotation sensors. Often these are the same sensors used...

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Could GPS Technology Help Reduce Vehicle Emissions

We are generally being encouraged through government sponsored advertising to spend less time in our cars or not to rev our car engines too much. By doing so we can help to save fuel, the world’s resources and, by implication, do our bit towards reducing climate change. The European Union has a long-standing target for car manufacturers to reduce average CO2 emissions for their vehicles to 120g/km by 2012. A longer term target is 80g/km by 2020. The view of the industry has been that this is...

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Modelling Atmospheric Effects on GNSS Reception

No matter how well any GNSS receiver might be designed, there are a number of outside influences that can have major effects on the performance of the receiver in the real world. In particular, atmospheric conditions can significantly degrade performance in some designs, and unless suitable compensatory measures are taken companies can find themselves selling equipment that can only operate accurately under “ideal” conditions. Ionospheric Scintillation is one such phenomenon that is very lik...

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GLONASS set for Automotive Boost

With global coverage of the GLONASS constellation now scheduled before the end of 2010, Russian Prime Minister Vladimir Putin has announced plans for a major expansion in its use on the nation's roads. In announcing the deadline for total coverage, Mr Putin revealed that all new vehicles sold in the Russian market from 2012 will be required to include GLONASS receivers. GLONASS tracking devices are already routinely installed in commercial and emergency services vehicles throug...

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Will all Receivers Eventually be Multi-GNSS?

That was one of the questions put to John Pottle Marketing Director, Spirent Communications, Positioning Group by Coordinates magazine. You can find his answer and the Spirent view on many other interesting questions regarding future GNSS trends and applications at the following link. http://mycoordinates.org/eventually-all-receivers-will-be-multi-gnss/

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What you Need to Know About Multi-GNSS Testing

As new GNSS systems appear, it becomes more apparent that you need a Multi-GNSS test solution. Here at Spirent we’ve been advocating Multi-GNSS for quite some time but now someone else has taken up the mantle. If you haven’t already done so, check out this comprehensive article in Inside GNSS magazine. Three receiver designers and researchers explain how they view Multi-GNSS simulators as an essential tool throughout the entire receiver development cycle: from research, development, de...

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Multipath Mitigation in Marine GNSS Receivers

On the face of it, the marine environment might appear relatively benign for GNSS receivers. After all, there is virtually no chance of signal obscuration from buildings or trees (although the odd cliff might come into play), so logically, the design of a marine GNSS receiver should be a piece of cake. However, such devices are particularly prey to multipath interference–both from the surface of the sea and from the superstructure of the vessel itself. And left untreated multipath effe...

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What is EGNOS?

Hosted by the European Space Agency, the European Commission and Eurocontrol, EGNOS is the European Geostationary Navigation Overlay Service, and is the first pan-European satellite navigation system. EGNOS comprises just three satellites, and acts as an enhancement to the US-based GPS system for safety critical applications in aviation and marine environments. The EGNOS Open Service has been up and running since 1st October 2009. This provides freely available positioning data through...

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What is an Ephemeris?

An ephemeris is quite simply a table giving the coordinates of a celestial body at specific times during a given period. The word comes from the same Greek root as “ephemeral”, which strictly means short-lived, but has come to mean inconsequential. However, in terms of GNSS systems, the ephemeris is certainly not inconsequential. Each GNSS satellite includes ephemeris data in the signal it transmits. This comprises a set of parameters that can be used to accurately calculate the locati...

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GPS GLONASS - are you ready?

GPS/GLONASS - You may not be working on it, but many of your competitors are Work continues apace on the GLONASS constellation, with the commencement of a headquarters building that will also house an office of the United Nations IT and satellite navigation agency. And the head of the Russian Space Agency, Anatoli Perminov, took the opportunity of laying the foundation stone of the new building to confirm that the GLONASS constellation would achieve full global coverage before January 2011. He ...

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Why are we Talking About Multi-GNSS

Today, navigation and positioning technology is no longer just about GPS L1 C/A code. GPS is being modernized, the GLONASS constellation is nearly complete, new systems including QZSS, IRNSS, Galileo and Compass are on the way. Multi-GNSS offers significant opportunities and challenges to GNSS technology, system and application developers. Spirent multi-GNSS simulation systems are now being purchased by customers developing commercial systems and most chipset manufacturer...

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US Initiative Expands Appeal of Multi-GNSS Systems

A new US national space policy document unveiled recently by President Obama marks a major change of direction on the relationship between the country's GPS system and other GNSS systems around the world. And the change can only accelerate the development and interoperability of systems such as GLONASS, Compass and Galileo. Whereas US policy as affirmed in a December 2004 national security directive was focused on maintaining the country's lead in GNSS on a unilateral basis, the new in...

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China Edges Towards Global Navigation Coverage

The latest contender in the global navigation sweepstakes has moved a little closer with the launch of the fourth satellite in China's second-generation Beidou constellation during the first week of June 2010. Beidou (which means Big Dipper) will cover all of China and neighbouring lands by 2012, and will then be expanded to provide global coverage through a constellation of 35 Compass satellites by 2020. Compass will differ from other GNSS systems in that five of the intended 35 satel...

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Could GPS Technology Help Reduce Vehicle Emissions?

Many world governments have a long-standing target for car manufacturers to reduce average CO2 emissions for their vehicles. The European Union target is 120g/km by 2012 and longer term to 80g/km by 2020. The view of the industry has been that this is very challenging and unlikely to be met via conventional approaches alone. Indeed, the 2012 target already represents a slip from an original 2005 target date. As most manufacturers will not be able to meet the 2012 target, further slippages (p...

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Working With the Strengths and Weaknesses of Satellite Navigation Systems

GPS specifically, and GNSS more generally, works fantastically well in its native mode of operation with an open view of the sky. High vehicle speeds, even in an aircraft manoeuvring at several times the speed of sound, are well within the capabilities of the GPS system. To use more specific language, the accuracy and continuity of positioning information is very high in open sky conditions. Back down to earth, a person walking with their GPS on the edge of the street in a typical town ...

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GPS Time and Leap Seconds

Time is an important component of any satellite navigation system, and it is essential that any receiver attached to the system has a clock that is fully up to date. The current GPS system uses its own timescale, which is closely linked to (but not completely in sync with) Co-ordinated Universal Time (or UTC). And to allow GPS receivers to give users the precise time according to UTC, the precise value of the current offset between the two clocks is broadcast by the satellite system. Wh...

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Galileo to Bring Additional Services

Despite continuing delays in its introduction, when the new European Union funded Galileo constellation goes live in 2014 it will provide a number of novel services. Designers of next-generation Multi-GNSS systems need to factor in these new capabilities in order to keep their equipment ahead of the competition. Importantly, Galileo is designed provide more precise location data from that provided by GPS or GLONASS, and will be accurate down to the one-meter range. The data will also i...

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Safety Critical Navigation on the Rails

To those engineers more familiar with automotive or marine navigation systems, the concept of using GNSS receivers for navigation on railways might seem a case of “overkill”. After all, there are only so many places a train can go, and these are firmly bounded by two steel rails. However, the exact knowledge of the position of any train on any rail system allows the rail operator to both improve service and increase traffic density by reducing the headways associated with fixed line-side sig...

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GPS Modernization and the L5 Signal

One of the most significant additions among the raft of changes that are being made to the GPS system is the addition of a second safety-of-life signal for civilian use. This new L5 signal is centred at 1176.45MHz in the worldwide Aeronautical Radio-navigation Services band, and will be broadcast at roughly twice the power of the existing L1 and L2C signals. It also features wider bandwidth and longer spreading codes, and will be particularly useful for enabling aircraft to make precision la...

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Making the Connection in GNSS Testing

One question that regularly crops up in discussions about GNSS receiver testing concerns exactly how the signals get from the GNSS simulator to the device under test. Is it better to radiate the simulator signal to the receiver's antenna, or should you couple them directly? The short answer to this is that a direct connection from the simulator to the receiver's antenna port will always provide the most controlled test environment with no risk of outside influence. The connection is us...

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NMEA Data Explained

The navigation industry often refers to NMEA data. But what is it? And why is it so important for the GNSS receiver industry? The NMEA is the US National Marine Electronics Association, which acts, among other things, as a standards body for the industry. And one of its most important standards is NMEA 0183, which defines electrical and data specifications for serial communications between all manner of marine electronic devices. These include everything from echo sounders, sonars and ...

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Running Interference in GPS Receivers

It goes without saying that any RF device as sensitive as a GPS receiver will be inherently vulnerable to interference. Clearly, care needs to be taken at both the design and integration stages to minimise interference effects. But what interference sources need to be considered? And how do you know if your receiver can deal with them? Most potential sources of interference are obvious and predictable: The effects of fixed-frequency transmitters for TV, radio and the like can easily be modelle...

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GNSS Receiver Integration: Not Just the sum of the Parts

The ability to integrate a GNSS receiver into an end product offers new possibilities for manufacturers in a wide range of both consumer and industrial markets. However, designers of such products need to be aware that even the most highly integrated GNSS receiver module is not a “fit and forget” component. As with any radio frequency system, there are design rules that must be followed, and even then the interaction of the receiver with the other functions of the product can create some sur...

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GLONASS Constellation Nears Readiness

The Russian Deputy Prime Minister Sergei Ivanov has confirmed that the country's GLONASS system will have 100% global availability before the end of 2010. The news follows the launch of three new satellites during March 2010, bringing the GLONASS constellation up to 19 operational satellites of the 24 required for full service. GLONASS, or Global'naya Navigatsionnaya Sputnikovaya Sistema (literally Global Navigation Satellite System) had fallen into severe disrepair after the f...

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GNSS Receiver and Location-Based Services

Location-Based Services put Pressure on GNSS Receiver Performance The addition of GPS receivers to today's smartphones, netbooks and other internet enabled devices is allowing mobile operators and other service providers to exploit a growing market for location-based services. These can range from social networks offering “find a friend” applications, to location based marketing and advertising. And you can be sure that developers will come up with many more new applications for lo...

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The Importance of Time to First Fix

Time to first fix is a crucial performance parameter for any satellite navigation system because it is the first and most easily appreciated evidence that the end user will have of the quality of the receiver. When you consider that this applies equally to potential users trying out receivers in the shop and to users maintaining satisfaction with the systems they have bought, then a few seconds here and there can make the difference between a happy customer and one that buys your competitor'...

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Multi-GNSS: The Future of Navigation

If you're a GNSS technology, system or application developer involved in the design and implementation of a GNSS project today, you need to take into account the full range of satellite systems and signals that will be available in the near future and understand the challenges and opportunities you face. Using satellites from more than one system brings special challenges and design choices for receiver design and evaluation. But what exactly is the timescale before these new systems are operati...

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What is a GNSS Simulator?

We sometimes get carried away into thinking everyone must know what a GNSS simulator is but in reality the proliferation of GPS / GNSS applications into many aspects of technology in such a short time span means that some people have had very little experience with GPS / GNSS technologies. So for those non-experts out there, I’d like to help. A GNSS simulator is a signal generator that provides an effective and efficient means of testing GNSS receivers and the systems that rely on the...

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