Spirent Positioning Blog

US Initiative Expands Appeal of Multi-GNSS Systems

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Fri, 30 Jul 2010 13:00:00 GMT

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 initiative reflects a more open attitude and calls for the USA to “engage with foreign GNSS providers to encourage compatibility and interoperability, promote transparency in civil service provision, and enable market access for US industry”. It also allows that “foreign positioning, navigation and timing services may be used to augment and strengthen the resiliency of GPS”.

The news will be a welcome shot in the arm for manufacturers that have already moved towards Multi-GNSS systems in advance of the provision of services. These manufacturers have been able to test their designs in advance of service provision using Spirent's Multi-GNSS simulators, which are capable of simulating all published GNSS signals.

The new policy document can be found at http://www.whitehouse.gov/the-press-office/fact-sheet-national-space-policy.

China Edges Towards Global Navigation Coverage

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Wed, 28 Jul 2010 11:00:00 GMT

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 satellites will be in geostationary orbit, while the other 30 will have medium earth orbits similar to the GPS, GLONASS and Galileo constellations.

Although very little has been officially announced about the signals to be transmitted by the new system, the launch of the first Compass satellite in 2007 did enable independent researchers to build a Compass receiver. However, the lack of official data means that no commercial work is likely in the near future.

The Spirent GSS8000 Series of Multi-GNSS constellation simulators have been designed to be compatible with the Compass system. And as soon as the Chinese authorities release the Compass ICD, Spirent will be looking to make a solution available. This will enable users to design Multi-GNSS receivers that will have true global appeal, offering compatibility with GPS, GLONASS and Galileo in addition to the Compass system.

Could GPS Technology Help Reduce Vehicle Emissions?

Adam.Reese@spirent.com (Adam Reese) — Wed, 21 Jul 2010 12:00:00 GMT

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 (possibly to 2015) and concessions (exclusions for heavier vehicles) are already on the table.

So what has all this got to do with GPS? The answer is that GPS is one technology that might be able to help reduce vehicle emissions. How might this work? Let’s assume that your vehicle is a hybrid, running on batteries part of the time to help meet the 120g/km and certainly the 80g/km target. The more the batteries are used, the lower the emissions. Hybrid car systems have limits set on the depth of discharge that the battery systems can be taken to before the engine kicks in. Often these limits include quite large margins.

Imagine for a minute, though, that your car also has a GPS system. This means that the vehicle can benefit from knowing not only where it is, but also from where it is going. Specifically, if the car systems know what’s coming up ahead, this information particularly related to inclines, could be used to optimize the performance of the hybrid system. In a simplistic example, if the car is going up hill, the batteries can be used more if the vehicle system knows that a down gradient is coming where a predictable level of recharging will be possible. It doesn’t take much of a leap to wonder, could this process be maximized by taking the decisions out of the hands of the driver? Automatic transmission coupled to knowledge of the road coming up (from the GPS system) could provide an answer. By knowing the nature of the road ahead, for example corner radius, duration and gradient, the transmission system could be optimised for economy.

Initially, at least, the driver would need an option to override such a system and optimise for sporty performance, fast response etc. There has been talk of “vehicle trains” for some time, whereby on higher classes of road the vehicle systems would take over and maintain optimum speed and distance from other vehicles. The barriers to these systems becoming a reality are rapidly being removed.

From a satellite navigation perspective, two key elements required are position accuracy to the lane level and high integrity, or trustworthiness, of the data. Lane level accuracy ideally requires dual frequency satellite navigation capability. This enables the atmospheric ambiguity to be backed out of the position calculation, providing sub metre accuracy.

Integrity is more challenging, particularly when autonomous vehicles travelling at high speed and in close proximity are concerned. This is likely to be the limiting factor in vehicle trains becoming a reality. In practice, a variety of approaches will be necessary to ensure the safety guarantees that will be expected. These will include augmentation systems like proximity radar and inertial sensors. Also likely, are roadside re-calibration systems that act as reference stations for the mobile satellite navigation systems in the moving vehicles.

Spirent has wide experience of helping companies in this sector understand their test issues and the best approach to testing. Spirent can help, contact gnss-solutions@spirent.com for more information.

Working With the Strengths and Weaknesses of Satellite Navigation Systems

Adam.Reese@spirent.com (Adam Reese) — Mon, 19 Jul 2010 14:35:00 GMT

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 or city could well have a very different experience. First the continuity of service could be affected by the receiver losing its lock on the visible GPS satellites. This could be due to the satellites being blocked behind buildings. Or the receiver may be located inside a building or shopping centre where the received satellite power is too low. Or the user could unwittingly point the antenna at the ground rather than the sky. In fact, these and a whole range of related effects can cause major difficulties for GPS receivers.

Secondly, even if the user is experiencing good continuity, the accuracy of the solution can be affected by multipath signals being seen and interpreted as ‘good’ by the GPS receiver. This will ‘trick’ the receiver, which will read the effective distance from the satellite to the user as being the bounced signal length, rather than the direct signal length.

Errors from a few metres to several hundred metres are quite common from multipath effects. As well as continuity and accuracy, the ability to trust the position being given, deserves careful consideration. There are multiple factors that affect integrity. Some are common, such as local interference from TV or microwave stations and, particularly near the equator, sun spot activity (sun spot activity is something we will devote more time to at a later date). Others are relatively rare such as satellite clock or transmission errors. When these do occur, however, they can cause major position errors, up to several kilometres in extreme cases.

The importance of these effects depends on what the positioning information is being used for. There are a great many systems that rely on GPS for commercial purposes. Examples are road tolling, congestion charging, tracking and logistics. In these cases, GPS alone may well be sufficient for positioning, but safeguards need to be designed to ensure proper use and system accuracy. Whatever the application, the system design team should be very clear, up front, what the accuracy, continuity and integrity requirements of the system are. In a vehicle navigation system the ultimate responsibility for safety lies with the driver, who should ignore incorrect instructions that might compromise safety. In this case, the priority of the design team may be low cost of manufacture above performance. At the other extreme, safety critical systems such as aircraft landing or rail signalling require high and guaranteed integrity, accuracy and continuity. Equally important are trade-off decisions between time, cost and quality at the project level, system level and user terminal level. Only with these defined and agreed can a proper test plan be developed. Often, such a test plan will require a mix of ‘live’ and ‘laboratory’ testing.

Often GPS alone is not sufficient and needs to be complemented, or augmented, by additional sensors or systems. A common test approach is to capture real data from the field and then recreate elements of that field data in a controlled laboratory environment. At its simplest, a navigation system developer can drive a route in the real world and capture ‘NMEA’ data from the receiver being used. This data can then be used to create a trajectory in the lab test system, also to vary factors such as satellite power levels and satellite visibility.

By using progressive test cases and approaches, the designer can design-in the performance they require. Equally important, the design team can understand the limitations of their system and ensure that processes and operational use cases take account of these to provide the appropriate level of service.

In summary, everyone working with GNSS technology development or using GNSS systems in a professional capacity should be aware of the inherent strengths and weaknesses of the GPS and other GNSS systems. At the receiver and system design level, many of the problems can be overcome by a logical and progressive test approach linked to the design objectives. The team that ignores inherent weaknesses and does not strive to take account of them is in for a miserable time indeed.

Download the Spirent eBook; Testing Multipath Performance of GNSS Receivers

GPS Time and Leap Seconds

Adam.Reese@spirent.com (Adam Reese) — Wed, 14 Jul 2010 12:00:00 GMT

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.

While UTC is maintained centrally using super-precise atomic clocks, it does have to be adjusted occasionally to keep in sync with the earth's changing rotation and to reflect mean solar time. And just as our calendar is periodically adjusted by the addition of a day each leap year, UTC is periodically adjusted by the addition (or subtraction) of a leap second. These events will usually take place on the last day of June or December. But they are relatively rare, amounting to approximately 0.6 seconds per year.

These leap second events are virtually imperceptible as far as telling the time goes, but are broadcast when they occur to enable clocks around the globe to stay synchronized with UTC. But for satellite navigation systems they are essential information. And so the leap second event is broadcast to each receiver as part of the navigation data message.

Clearly, a receiver's response to the arrival of such a message is critical. And that is why Spirent's GNSS simulators offer facilities for testing the response of receivers to leap second events. More information on Spirent’s GPS and GNSS test solutions can be found on our website www.spirent.com/positioning

Galileo to Bring Additional Services

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Thu, 08 Jul 2010 11:30:00 GMT

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 include accurate altitude measurements, and improved coverage services at high latitudes. Crucial to this, each of the 27 live satellites in the constellation will broadcast no fewer then ten different navigation signals, enabling a degree of service differentiation not yet seen from any other satellite system.

Galileo will offer five main services when fully operational. The standard “free-to-air” Open Service, the high-integrity Public Regulated Service and the Search and Rescue Service are scheduled to be fully operational by 2014. At this point, trials will also begin on the remaining two services: a value-added centimeter-accurate Commercial Service and an open Safety Of Life Navigation Service for applications where guaranteed accuracy is essential.

Clearly, without the necessary satellites in place—and indeed even when they are—GNSS receiver designers need a proper means of testing their products, and this comes in the form of the Spirent family of Multi-GNSS simulators. These acclaimed test tools already have the certified capability of simulating Galileo navigation signals, both alone and in combination with GPS and Glonass signals.

With early adopters likely to pay a premium for the ability to access the new Galileo services, manufacturers cannot afford to be late with their support for the Galileo system. And that means simulating the signals now well in advance of the 2014 launch.

Safety Critical Navigation on the Rails

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Tue, 06 Jul 2010 11:00:00 GMT

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 signaling without compromising safety, making it key to maximising the efficiency of the network.

Unfortunately, railway networks are pretty hostile environments for GNSS receivers. Trains spend significant periods of time in deep cuttings and tunnels, obscuring signals or creating complex multipath effects, and the electrically noisy nature of the power systems involved adds further complications. So while GNSS tracking is desirable for rail operators, it is by no means easy to deploy reliably. And as safety is paramount in any public transportation system, the integrity and reliability of the systems is essential.

Although live-sky testing of GNSS systems on railway networks would appear to be obvious way of testing the effectiveness of the system, it is both unreliable and expensive. While any live-sky test scenario may be “real”, it will not be repeatable and will not be able to test the system under specific conditions that might affect its performance. What's more, the costs of obtaining a train path on a section of a busy rail network to test a GNSS system, and the cost of operating the train would be unacceptable.

GNSS simulation offers the solution, with the ability to test the system under all possible conditions. Spirent's SimGEN simulation software can be used to create test scenarios that recreate railway-specific conditions, giving rail operators complete confidence in the integrity and reliability of their GNSS systems. Further details can be found in our Application Note DAN011 on “Testing GNSS for railway applications at http://www.spirent.com/Solutions-Directory/GSS8000.aspx.

E-Call 112 and how it Affects GNSS

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Tue, 01 Jun 2010 13:00:00 GMT

When the worst happens, seconds count. E-call, the European Commission's telematics project, is expected to save 2,500 lives annually in the EU by saving time in getting the emergency services to the right place as promptly as possible. An e-call can be initiated manually by vehicle occupants or automatically by the vehicle itself. Once communications is established with the emergency services (Public Safety Answering Point or PSAP), a data stream known as the Minimum Set of Data or MSD can be sent from the scene directly to the emergency services.

As you’d expect the MSD contains position information and so for our purposes in the area of positioning technology we need to establish that the vehicle knows its position at all times even if inverted in a ditch!

Testing to meet standards requires a degree of control and repeatability not readily available from field trials. If running such a test program you may reasonably expect a substantial reduction in uncertainty, test time and spend by incorporating a simulator. Using features such as antenna gain and phase masks, lever arms and the ability to switch between antenna patterns ( to simulate an inverted car?) allows the rapid assessment of performance which may simply not be practicable using live-sky and real vehicles.

For further information check out the following links:

eCall--Calls that 'dial' 112

eSafety Support

www.spirent.com/positioning

How to Reduce the Challenge of GPS Integration

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Fri, 21 May 2010 12:00:00 GMT

Have you been tasked with the integration of GPS (or GLONASS) chips into a product or system? If so you will be facing a number of challenges.

Selecting which chipset or module to use
Ensuring that the tiny RF signals are getting through
Evaluating the performance
Designing for test and manufacturability whilst providing rapid time to market.

Your first step should be to consider what characteristics you need in your receiver… they are NOT all the same! For example a receiver designed for high accuracy may not be optimised for a rapid start-up. Look at the manufacturers’ datasheets and you will find that they are often difficult to compare using slightly different parameter definitions. Testing the units yourself may be the only way to get a truly accurate and directly comparative set of results and repeated testing under identical test conditions is a job for a GNSS constellation simulator.

Having identified the receiver you wish to use, the next challenge is obtaining the same performance you observed on the evaluation board, but in your circuit. Once again, GNSS simulation provides a controllable, repeatable test. Any issues that arise during this stage can often be resolved by sharing the test scenario with your chosen vendor. Scenarios are portable between Spirent machines so provided your vendor has a Spirent simulator (and most do) problem resolution can be significantly fast.

Finally, you need to define a baseline performance that can be used for manufacturing and regression testing. Once again the control and repeatability aspects of the GNSS simulator are powerful allies in this task.

In summary, if you follow these 3 steps you'll sleep more peacefully:

Choose your vendor after consideration of the cost and performance trade-offs offered. Perform an evaluation yourself. Confirm they use a GNSS simulator so that you can share troublesome scenarios.
Develop your circuit and layout with consideration for RF losses and interference sources. Confirm operation using your GNSS simulator.
Define a baseline performance that should be obtainable in production testing. A controlled, repeatable test using a GNSS simulator allows rapid evaluation of changes in the GPS (e.g., firmware) or surrounding circuitry (e.g., WiFi, Bluetooth, board layout etc).

Testing GNSS Receivers in a Production Environment

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Wed, 19 May 2010 13:00:00 GMT

Manufacturers of consumer products routinely perform functional testing on all production output, and it would appear that adding some form of location testing to these production test routines would be sufficient to verify the reliability of the GNSS receiver within the end product. However, it is all too easy to adopt the attitude that the simplest of tests will suffice – particularly when the duration of each test can have a significant impact on productivity.

Although it may be the case that all the other functions of the end product can be assessed with a relatively straightforward go/no-go test, taking such an attitude with a GNSS receiver is fraught with danger. The end user will expect the product to perform adequately under a wide variety of conditions. This means that the receiver will need to be tested not just for an “ideal” situation, but also for adequate performance in the presence of multipath interference, all manner of jamming signals and less than ideal atmospheric conditions.

Key test challenges in a Production line environment

The first obstacle that will be encountered in integrating GNSS receiver testing into a production test setup is pretty obvious. As such tests are performed at the end of the production line, they are inevitably performed indoors. And regardless of whether the equipment is designed to work indoors or outdoors, the roof and walls of the building will introduce variables into the test that will negate its effectiveness. So-called “live-sky” testing is therefore impossible without relaying the GNSS signals from outdoors to the production tester.

It is a relatively simple exercise to capture live GNSS signals and re-radiate them within the production test environment. However, this comes with its own set of shortcomings.

First, radiating any signal in such an environment might have unforeseen consequences on other tests that are performed on the product; and conversely, other RF signals and noise within the production test area may well impact on the integrity of the GNSS signals.

More importantly, though, the inherently dynamic nature of GNSS signals means that while each unit may well be tested in the same physical location (i.e. in the production tester fixture), the relative positions of the GNSS satellites will be different for every unit tested. And, not surprisingly, this makes direct comparison between results unreliable at best.

GNSS tests within a Production line environment

In order to fully assess the performance of a GNSS receiver embedded in any piece of equipment, it is important to work out exactly what response is required. There will be varying degrees of performance required, depending on the end application, but the requirement will be for a combination of navigational accuracy and sensitivity under a wide range of operational conditions. There will also be a requirement for the equipment to work with not just the existing Global Positioning System, but also with the forthcoming enhanced GPS, GLONASS and Galileo systems at the very least.

Some systems may have no direct output. Or, more to the point, the output may only be in the form of an alarm or trigger that is supposed to be produced with proximity to certain co-ordinates. This does not however mean that the performance demands on the receiver are any less arduous. It would however, dictate the pass/fail criteria for the production test.

GNSS test solutions for a Production line environment

Given the inherent variability of any type of live-sky testing, it is logical to seek a more precise and repeatable stimulus against which the performance of an embedded GNSS receiver can be assessed. And this can be supplied in the form of a GNSS simulator.

A multichannel multi-GNSS simulator under software control can produce all the necessary signals required to test the relevant performance criteria of any embedded GNSS receiver in any location-enabled device. Most importantly, it can do so consistently and repeatably for every unit to be tested, ensuring that manufacturing output is 100% fit for purpose.
The tests typically performed on any navigation device are inherently complex, covering the full range of performance criteria from navigational accuracy and sensitivity to acquisition time and immunity to interference. These tests have been designed to ensure the performance of dedicated GNSS receivers, and have been proved over successive generations of personal navigation devices.

Fortunately, once the desired performance of the design has been characterised, the production tests for the end product can be refined into a considerably smaller set of acceptance criteria that can be performed in a relatively short time (as little as 5 minutes).

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.

Certification of GNSS Devices

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Tue, 18 May 2010 13:00:00 GMT

Traditionally, civilian use of GPS was seen as free and to a large extent “at your own risk”. The typical performance one might expect was stated in the relevant Interface Control Documents (ICD’s) but no guarantee of service was given. The reason for this was the historical remit of GPS as a system to satisfy US military requirements, the civilian use of the coarse acquisition (C/A) ranging code being essentially a by-product of its primary use, which was to provide classified receivers a ‘first step’ towards fast acquisition of the precise ‘P(Y)’ code.

With the increasing number of GNSS services that will offer certain guarantees of service scheduled to be available over the coming years, there is an increasing requirement for certification of these services and the GNSS devices that use them.

To facilitate these requirements, new performance standards are being written which are dictating the tests to be performed.

Test integrity is essential

All proper testing must have good integrity – that’s a given, however, it is especially so with certification, verification and type approval testing, particularly when the device and / or application is safety critical. This is why using quantifiable, traceable and accurate test methods are important. Accredited test laboratories use precision test equipment which is calibrated to standards traceable to National references for all certification and type approval testing. The same must therefore apply to GNSS testing. This is why using calibrated precision RF constellation simulators is the only way to perform this kind of testing. The inherent uncertainty and variability of using other methods (such as real GNSS signals) rules them out completely.

Spirent’s pedigree in producing high-fidelity, precision test equipment is well proven over more than 25 years. Spirent has supplied equipment into critical test programmes across the world. These equipments have often been put through rigorous certification and validation programmes to ensure they are themselves fit for certification testing. The most recent examples of this are;

The certification by the European Space Agency and Galileo Supervisory Authority of the world’s first Galileo RF constellation simulator—Spirent’s GSS7800 for the Ground Receiver Chain (GRC) and Test User Segment (TUS) receiver developments in the IOV phase of the Galileo programme.
The certification of Spirent’s GLONASS implementation by the Russian authorities

So, to ensure your testing is up to standard, make sure you select test equipment from Spirent, the world’s leading and most proven simulator provider.

GPS Modernization and the L5 Signal

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Mon, 10 May 2010 13:00:00 GMT

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 landings in high multipath environments as well as reducing errors due to the ionosphere.

The first of the new constellation of satellites capable of broadcasting the L5 signal, the GPS IIR-20(M), was launched from Cape Canaveral Air Force Station, Florida, on 23rd March 2010, and began broadcasting the L5 signal on 10th April 2010. But while this satellite will serve as ample proof of concept, the full L5 signal coverage will not be available until the GPS modernisation is complete, which is unlikely before 2013.

However, designers of equipment can prepare for GPS modernisation by simulating all the new signals today using Spirent's range of GNSS simulators. Not only do these instruments cover the new L5 signal, as defined in the new ICD-GPS-705 standard, they also cover all the other enhanced signals that will be provided by the new modernised GPS constellation. Beyond GPS, they also cover all signals for the GLONASS and Galileo GNSS’s as well as the regional and local augmentation systems.

Making the Connection in GNSS Testing

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Fri, 07 May 2010 12:45:00 GMT

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 usually performed using a simple coaxial cable that acts as a 50 ohm transmission line, or it may require the addition of a low-noise amplifier in cases where the receiver is designed to use an active antenna.

In either case, making the direct connection – always using high-quality cables and components – will enable all tests to be made under controlled conditions.

Of course, some types of location-enabled equipment have no external antenna port, and in many cases the antenna is entirely hidden from view. So here practicality dictates that a radiated signal must be used. This does, however, leave the test setup open to all manner of outside interference, and this can make results unreliable. So for full confidence in the results of the test we recommend putting the equipment under test inside an RF screened enclosure together with the radiating antenna from the simulator.

NMEA Data Explained

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Wed, 28 Apr 2010 12:00:00 GMT

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 anemometers to gyrocompasses, autopilots and (importantly) GNSS receivers.

NMEA data comes in the form of “sentences” that are unique to each piece of equipment, but which can be read by all other equipment adhering to the standard. Each sentence begins with a dollar sign and ends with a carriage return, and can comprise no more than 80 characters of ASCII text. A number of standard sentences are defined, each identified by a prefix such as the “GP” used for GNSS (GPS) receivers.

The standard also allows hardware manufacturers to define their own proprietary sentences. All proprietary sentences begin with the letter P and are followed by three letters that identify the manufacturer controlling that sentence. For example a proprietary Garmin sentence would start with “PGRM”.

While these NMEA sentences are designed to be used for communication between navigational devices, they can also be used in the test laboratory. For example, the output recorded from a GNSS receiver in the field can be used to program motion profiles into a GNSS simulator to test other units.

Running Interference in GNSS Receivers

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Tue, 27 Apr 2010 12:00:00 GMT

It goes without saying that any RF device as sensitive as a GNSS 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 modelled and accounted for. Indeed, one advantage of working with multi-GNSS receivers is that some are multiple-frequency devices, and therefore inherently more resistant to interference on any specific frequency.

However, there are two sources of interference that you ignore at your peril: the first of these is internally generated interference, and this is particularly relevant in devices such as location-enabled mobile handsets. The typical handset has a transmitter capable of transmitting more than 1W in very close proximity to the GNSS receiver (and two may even share some signal-path components). Careful design partitioning is essential to prevent interference.

The second is more subtle, but will increase over time, and concerns the increasing number of constellations and signals coming on stream. So say, for example, you have designed your multi-GNSS receiver to work with both GPS and GLONASS, and then the Galileo system comes on-line, then the receiver will view the uncorrelated signals from Galileo satellites as interference. At a simpler level, a GPS-only receiver will view both GLONASS and Galileo as interference. In a similar way, all civilian receivers need to work in the presence of classified signals such as GPS Y and M codes and Galileo PRS.

Fortunately, even if you are still working with single-GNSS receivers then you can test your designs for interference from other systems using a Spirent Constellation simulator equipped with the interference option. And this can even be used to simulate unwanted GNSS signals.

GNSS Receiver Integration: Not Just the sum of the Parts

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Fri, 16 Apr 2010 12:00:00 GMT

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 surprising results.

Although radio frequency design is not the “black art” it was once considered, the design rules are inevitably more complex than for digital design. Even a slight failure to follow the rules can lead to poor performance, and as far as the end user is concerned “close but no cigar” is not acceptable. Worse still, once a design has been characterised and accepted, seemingly small variations in manufacturing tolerance, or a purchasing manager saving a few pennies by substituting a cheaper component can throw the performance right out the window.

The only guarantee of continued product quality and reliable performance is 100% functional testing of the finished product. And that will mean integrating a GNSS simulator in the production test setup. Fortunately, Spirent's production test simulators have been developed with the demands of manufacturing in mind.

GLONASS Constellation Nears Readiness

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Thu, 15 Apr 2010 12:00:00 GMT

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 fall of the USSR, at one point with only eight satellites operational. Two further launches are planned during 2010, bringing the total number of working satellites to 24, with three in reserve.

The Russian Institute of Space Device Engineering has revealed that it is close to completing a co-ordination plan that will see eight different CDMA signals on four frequencies. The first of these will be the existing GLONASS L3 frequency, with an open signal centred at 1201.743MHz and an encrypted signal at 1208.088MHz. The additional CDMA signals will be introduced at the new L1, L2 and L5 GLONASS frequencies.

Several manufacturers have begun production programmes for GPS plus GLONASS Multi-GNSS receiver chips and modules. OEMs intending to integrate these devices can test them ahead of the final launches using Spirent Constellation Simulators which can be configured for GPS, GLONASS and Galileo capability.

Location-Based Services put Pressure on GNSS Receiver Performance

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Wed, 14 Apr 2010 12:30:00 GMT

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 location data as the market matures.

Regardless of the nature of the service, all location-based services put GNSS receiver performance at a premium. After all, there is no point in giving a user a mobile coupon for a business they passed just ten seconds ago, and telling them they are standing within 10 metres of it when they can clearly see they are 100 metres away, this is hardly likely to endear them to any service. While assisted GPS can play a role in the equation, operator-independent location-based services rely purely on the performance of the GNSS receiver in the user's equipment.

The growth of such services means that it is now even more important to ensure that your GNSS receiver design delivers the best possible performance under all operating conditions. And the only way of verifying this performance is by simulating all those possible conditions using a GNSS simulator.

The Importance of Time to First Fix

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Tue, 13 Apr 2010 12:00:00 GMT

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's product.

But how can you be sure that your receivers TTFF is optimized for best performance? Clearly, you test it. And you can even run the same tests on your competitor’s products. But (and here's the rub) be sure that you are running exactly the same tests on each piece of equipment. That means having the receivers in the same position, the satellites in the same position, identical atmospheric conditions etc – otherwise comparisons are meaningless.

So while the performance you are trying to assess will be felt by the user in the real world, there are so many factors that impact on receiver performance that it is close to impossible to make meaningful comparisons using real-world testing.

Using Spirent's constellation simulators under controlled laboratory conditions is the best way to ensure that products are tested with a level playing field. What's more, the ability to run a near infinite number of test scenarios reproducible under software control on multiple receivers will provide ample evidence that your products will outperform the competition under all manner of conditions.

Multi-GNSS: The Future of Navigation

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Wed, 03 Mar 2010 13:00:00 GMT

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 operational?

The first of these systems is GPS itself, which is currently being modernised with extra ground stations and new satellites, and will supply additional navigation signals for both civilian and military users.

The target date for completion is around the middle of this decade, but with major incentives available for contractors, who knows?

Before that, though, we should see the restoration of the full Russian GLONASS system. Originally a jewel in the Soviet crown, the system fell into disrepair with the fall of the USSR. However, with substantial help from India, Russia has committed to get the system back up and running during 2010.

The European Galileo system has been something of a political football, but in 2007 the European Union took over the project from the private consortium that had instigated it, and committed to complete the system by 2013. However, subsequent EU communications now talk of a 2014 start date.

Finally, the Chinese government has committed to expanding its own local Beidou system into a global network dubbed Compass. No date has been set for completion.

Anyone involved in the design and applications of these systems can't afford to wait until the signals go live before starting to develop a solution, if you do, you'll lose your market to some quicker and sharper player. Fortunately, Spirent's multi-GNSS test systems already support modernised GPS, GLONASS and Galileo and are available today!

How a GNSS Simulator can Help Test the Multipath Performance of GNSS Receivers

Leslie.Gollehon@spirent.com (Leslie Gollehon) — Thu, 25 Feb 2010 13:00:00 GMT

Like any form of radio receiver, a global navigation satellite system receiver will be subject to interference from multipath effects arising from the reflection and refraction of its intended satellite signals by both natural and man-made artifacts. However, unlike some radio systems in which a small degree of interference may be tolerable to the end user, multipath interference will have an unacceptable effect on a GNSS receiver, making the output both unstable and inaccurate.

There are several multipath mitigation techniques available to the GNSS receiver designer, but in order both to assess the initial requirement and to measure the effectiveness of the measures taken, extensive testing is required. However, such is the diversity of multipath effects and the factors creating them, it is simply impractical to reproduce any form of meaningful tests using real-world GNSS signals. Therefore, the use of a GNSS simulator in the laboratory offers the only practical solution.

Crucially, a simulator system with suitable software will be able to simulate all the various multipath effects both singly and in combination, enabling designers to assess the true performance of their designs and the effectiveness of their multipath mitigation strategies.

A good GNSS simulator will not only offer the pre-defined models already discussed, but will also give the user complete control of the signals being generated. This control will allow intricate manipulation of the signals at digital baseband, which in turn will allow any effect to be implemented.

Remember, the most important thing is to ensure that you have complete knowledge of your test signals at all times. The moment you stimulate your receiver with an unquantified signal is the moment you introduce unwanted uncertainty into your tests.

With more and more users coming to rely on the accuracy of their global navigation satellite system receivers and increasing numbers of location-based services leveraging this accuracy, it is clear that interference caused by multipath effects cannot be allowed to compromise the accuracy of any GNSS receiver design. Ever-more sophisticated and effective multipath mitigation techniques are becoming available, but it is only by controlled, analytical and statistical testing at all stages of the design process that these techniques can be applied and proven.

Real-world testing can never hope to repeatably replicate all the potential multipath effects that may occur in a limited test timescale, nor can it be expected to provide even a representative sample. And so the use of laboratory-based simulation offers the most reliable, accurate and repeatable test regime to ensure the performance of any GNSS receiver design.

However, not all simulators are born equal, and not all simulation software is made equal. Only by choosing high-quality proven multichannel hardware with software that provides maximum coverage of multipath effects and models can the GNSS receiver designer be sure that his or her design will provide optimum performance under all conditions. And that is the route to end-user satisfaction.

If you want more information on Spirent’s range of GNSS simulators contact globalsales@spirent.com or check out our website at www.spirent.com.

Why are we Talking About Multi GNSS?

Adam.Reese@spirent.com (Adam Reese) — Tue, 16 Feb 2010 16:20:00 GMT

A few years ago the Sat Nav system in your car was considered a luxury now almost every PDA, mobile phone and PC has built-in GPS technology. However, navigation and positioning technology is no longer just about GPS L1 C/A code. The GPS constellation is being modernized, the GLONASS constellation is nearly complete with 19 satellites transmitting as you read, new systems including the Japanese QZSS, the European Galileo and the Chinese Compass constellations are on the way.

GPS, the backbone of our current satellite navigation systems for the past 10 years will be only one of four Global Navigation Satellite Systems (GNSS) and four Satellite Based Augmentation Systems (SBAS) which will be available by the middle of this decade. There are already over 60 GNSS and SBAS satellites in operation (including 32 from GPS) and more than 130 are planned.

This multi-GNSS environment offers opportunities to improve performance to meet increasing user demands. In particular, end user availability is potentially improved by using more than one constellation. Benefits to end users can also include improved integrity, continuity and accuracy, depending on the situation and priorities of the application. However this multi-GNSS environment also offers significant opportunities and challenges to GNSS technology, system and application developers’ to meet these increasing user demands e.g.

How can I test future signals which do not yet exist in space such as L5 and L2C on GPS
How can I test GNSS constellations which are partially deployed or not yet deployed (e.g. Galileo)

Well, there is a solution. GPS / GNSS simulators generate the same kinds of signals transmitted by GPS / GNSS satellites, thus GPS / GNSS receivers process the simulated signals in exactly the same way as signals from actual satellites. GPS / GNSS simulators are the most powerful test method for GNSS receivers and applications and have three core attributes which are more difficult (or impossible) to achieve using live-sky test methods, these are repeatability, control and scalability:

A simulator can repeat exactly the scenario time-after-time enabling test results to be accurately compared during the development process. Further, repeatable test scenarios can be created to include failure conditions which may only occur sporadically or randomly in the real-world and are difficult to capture with live-sky methods
Simulators give full control over all the performance characteristics for GNSS. This means that single scenario can be created which tests a GNSS receiver or application in ways which might require several live-sky sessions to emulate. The control of GNSS parameters offered by a simulator also means clearly defined, repeatable and rigorous test standards can be established and documented
Simulators offer a scalable test method

Only simulators can test for GNSS constellations which are partially deployed (e.g. Galileo) and future signals such as L5 and L2C on GPS
Simulators are the best solution for establishing test standards across dispersed development centers
Simulators generate data for multiple scenarios and locations compared to live-sky, which are single-scenario single-location solutions
Simulator test scenarios can be quickly added or modified through software whereas changing test conditions for live-sky requires new data to be collected

GNSS simulators are the test method of choice for developers and test engineers. They offer a rigorous, repeatable and cost effective means of exploring, benchmarking and testing GNSS receivers, applications and systems. No other method can offer the same flexibility in generating new test scenarios or the ability to incorporate both current and future GNSS and frequencies.

If you’re not working on multi-GNSS projects right now, you probably will be soon. Find out more on how Spirent’s multi-GNSS product portfolio could help you today.

Why use a GNSS Simulator?

Adam.Reese@spirent.com (Adam Reese) — Thu, 11 Feb 2010 05:00:00 GMT

Why would I use a GPS / GNSS simulator, if I want to do any testing I just stick my antenna out of the window, attach it to my receiver and away I go. Well that’s all good and well but for those requiring a rigorous GPS test environment, live sky testing has some serious limitations including a lack of repeatability, control and scalability. Not to mention the fact that you can’t test future signals in space e.g. GPS L2C and L5, partially deployed constellations or constellations that don’t yet exist.

Testing with GPS / GNSS simulators is the widely-accepted best practice for validating the performance of GNSS receivers and systems in many different scenarios and operating conditions in a controlled laboratory environment. Simulators are used extensively in academia and industry, in virtually all GNSS receiver manufacturing and major system integration, and in many different application fields, including navigation, positioning, telecommunications, aviation, automotive, and space, for both civilian and military applications. Using simulators facilitates several stages of research and product development, including requirements analysis, design and development, integration, production, maintenance, and support.

GNSS simulators provide many benefits, including

Control. Simulators allow complete control over all aspects of test scenarios, including GNSS constellation signals and environmental conditions.
Flexibility. Users can easily define different scenarios for different testing needs.
Completeness. Equipment can be tested under different operating conditions, ranging from nominal to extreme, including conditions that are impractical or impossible to produce in live testing.
Repeatability. Test scenarios are the same every time they are executed.
Reliability. Because all test conditions are controlled, test results are reliable, and equipment performance can be evaluated against known truth data.
Cost. Tests are conducted in the laboratory, without extra expenses for field tests and test vehicles.
Efficiency. Many different tests can be completed in the same laboratory test bed, without reconfiguring or relocating equipment. New test scenarios can be created and executed quickly.
Realism. The performance of GNSS receivers and systems are tested using the actual hardware. Simulators with real-time control capabilities support advanced hardware-in-the-loop (HWIL) testing.
Future. Simulators provide effective means of testing new and future GNSS capabilities that are not yet supported by actual constellations, such as the GPS L2C and L5 signals and the Galileo system.

A summary of the advantages of testing with GNSS simulators, compared to live testing with actual GNSS constellations, is shown in the table below.

Live Testing with Actual GNSS Constellations	Laboratory Testing with GNSS Simulators
No control over constellation signals	Complete control over constellation signals
Limited control over environmental conditions	Complete control over environmental conditions
Not repeatable; conditions are always changing	Fully repeatable
Unintended interference from FM, radar, etc.	No unintended interference signals
Unwanted signal multipath and obscuration	No unwanted signal effects
No way to test with GNSS constellation errors	Easily test scenarios with GNSS constellation errors
Expensive field testing and vehicle trials	Cost-effective testing in laboratory
Limited to signals available in GNSS constellations	Testing of present and future GNSS signals
Competitors can monitor field testing	Testing conducted in secure laboratory

If ensuring final product quality, whilst also meeting tight project timescales is important to you then you require the capability to simulate realistic, repeatable and controlled GNSS signals that our single-channel and multi-channel test platforms offer.

If you want more information on how we can help, contact globalsales@spirent.com

What is a GNSS simulator?

Adam.Reese@spirent.com (Adam Reese) — Tue, 09 Feb 2010 05:00:00 GMT

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 them. A GNSS simulator provides control over the signals generated by GNSS constellations and over the global test environment all within a box, so that testing can be conducted in controlled laboratory conditions. GNSS simulators generate the same kinds of signals transmitted by GNSS satellites, thus GNSS receivers process the simulated signals in exactly the same way as signals from actual satellites.

A GNSS simulator provides a superior alternative for testing compared to using actual GNSS signals in a live environment. Unlike live testing, testing with simulators provides full control of the simulated satellite signals and the simulated environmental conditions. With a GNSS simulator, users can easily generate and run many different scenarios for diverse kinds of tests, with complete control over

Date, time, and location. Simulators generate GNSS constellation signals for any location and time. Scenarios for any location around the world or in space, with different times in the past, present, or future, can all be tested without leaving the laboratory.
Vehicle motion. Simulators model the motion of the vehicles containing GNSS receivers, such as aircraft, ships, or automobiles. Scenarios involving vehicle dynamics for different routes and trajectories anywhere in the world can all be tested without moving the equipment being tested.
Environmental conditions. Simulators model effects that impact GNSS receiver performance, such as atmospheric conditions, obscurations, multipath reflections, antenna characteristics, and interference signals. Various combinations and levels of these effects can all be tested in the same controlled laboratory environment.
Signal errors and inaccuracies. Simulators provide control over the content and characteristics of GNSS constellation signals. Tests can be run to determine how equipment would perform if various GNSS constellation signal errors occurred.

Hopefully that brief overview helped but if you want more information on how Spirent’s range of GNSS simulators can help you, contact globalsales@spirent.com.