Editor's Choice

Navigating a way through GNSS

16 May 2018 Editor's Choice Telecoms, Datacoms, Wireless, IoT

Using a global navigational satellite system (GNSS) to enable tracking and location-based services can deliver big benefits, in terms of efficiency and productivity, in a wide range of industrial applications. But when it comes to choosing a module to implement GNSS functions, things can get confusing.

There are several GNSS systems to choose from, so which one is right for you? Should you connect to more than one? Will the solution be accurate and responsive? What about power consumption – will using GNSS drain the battery? And which is the better choice – a module with an antenna built in or one that uses an external antenna?

Which GNSS setup to use

GNSS is the generic term for any collection of satellites used to determine location. At present, there are four independent GNSS setups in operation. Three of them – GPS (USA), GLONASS (Russia) and BeiDou (China) – are run by military agencies and made available for civilian use through government consent. The fourth, Galileo (EU), is the only civilian-run GNSS and is designed expressly for commercial use.

In the past, the only GNSS setup available worldwide was GPS. Any GNSS receiver or module had to rely on US-managed GPS satellites to function. This has changed in recent years, as the GLONASS, BeiDou and Galileo setups have grown and expanded.

Each new setup brings its own unique attributes to the mix. For example, the Russian GLONASS setup is known for providing better coverage at higher latitudes, and the EU’s Galileo system, when fully operational, is expected to deliver best-in-class accuracy of just around 1 metre. The Chinese BeiDou setup supports a text messaging feature that can provide location to a third party. The messaging service has already proven useful, helping Chinese rescue teams working in earthquake-stricken areas by providing guidance during local power outages.

Having four major satellite navigation systems available worldwide is a big step forward for GNSS. By the year 2020, when all four systems are expected to be fully operational, there will be close to 120 total positioning satellites in orbit. All these new satellite systems can either complement GPS or work independently.

This has given rise to multi-GNSS receivers and modules, which can obtain location by simultaneously using signals broadcasted from two or more of the major setups. Having simultaneous access to satellites from more than one GNSS setup does two things – it makes the readings more precise and it reduces the time to first fix (TTFF), which refers to how long it takes the module to acquire a signal and get a fix on its location. A shorter TTFF means less waiting before a reading is available to the module.

Multi-GNSS operation has shown that the biggest improvements in accuracy and responsiveness come not from switching from one system to another but from using more than one system at the same time. The general rule for GNSS is that more satellites is better, and multi-GNSS gives you more by letting you combine signals from satellites associated with different constellations.

While GPS-only modules are still a popular option, most of today’s module makers offer products that combine GPS with at least one other GNSS format in a single package.

Is multi-GNSS operation worth it?

Multi-GNSS modules allow simultaneous positioning from at least two satellite navigation systems, resulting in much better positioning accuracy compared to standard GPS-only modules.

Most multi-GNSS modules today can support all four major satellite navigation systems on the same hardware but typically only two to three systems can be enabled simultaneously. You can change from one configuration to another via a simple firmware change or command (though you may need to change the GNSS antenna).

Multi-GNSS technology has come a long way. The first implementations, from six or seven years ago, were expensive and unacceptably power hungry. Widespread adoption of multi-GNSS formats, in smartphones and tablets, has driven down chipset costs, and the difference in power consumption has become negligible. For many industrial applications, the minor increases in cost and power consumption are worth it, given the far superior positioning accuracy provided by today’s multi-GNSS engines.

The added flexibility provided by multi-GNSS modules also offers a competitive advantage over GPS-only modules, in terms of their positioning performance and cost over the long term. With a multi-GNSS module onboard, new products require fewer design iterations and a single product can cover different markets. The design can also expand to support new regions, as needed, without having to swap out the module.

Today’s Multi-GNSS modules typically support GPS along with another format. The most popular combination is a dual system that combines GPS and GLONASS, since the pairing provides excellent coverage worldwide. Dual systems that combine GPS with BeiDou are mostly used in Asia Pacific regions, while dual systems that combine GPS and Galileo are more in demand in the EU.

Relatively new on the market are triple systems, which combine GPS and GLONASS, plus either BeiDou or Galileo. One drawback, however, is power consumption, since receivers that use three systems simultaneously tend to consume more power than their single or dual counterparts.

The future of GNSS is quad systems where GNSS modules will support connection to all four setups simultaneously. Having GPS, GLONASS, BeiDou and Galileo available in a single module will create a new era in GNSS, with readings that are even more accurate and even faster to achieve.

Quad systems will save developers from having to deal with different SKUs of GNSS modules for different regions, since one module will work everywhere. Quad systems will perform the functions of today’s single, dual and triple modules, since a given GNSS setup can be turned off in firmware. The biggest challenge with quad systems will be to keep power consumption low.

Quad systems are expected to be in widespread circulation by 2020. Even if a current design doesn’t use or need all four GNSS setups, it can still be a good idea to choose a module that has a roadmap for quad compatibility, for added flexibility as designs evolve.

How to boost accuracy

Accuracy is one of the most important aspects of GNSS operation, because location information needs to be as precise as possible. As mentioned above, the number of satellites involved in a given calculation has a direct impact on accuracy, but there are other things to think about, too.

Clear views

At the development stage, consider how the end device will be used and minimise any obstructions, such as a metal case, that might prevent direct line of sight with satellites.

Orientation and positioning of the end device matter, too. For example, a GNSS device on the roof of a car works better than one on the dashboard, and both work better than a device sitting on the back seat.

The best operating environment is open sky, which means avoiding trees, tall buildings, overpasses, and anything else that prevents a direct view of everything above the horizon. Using a Multi-GNSS format increases the chances of connectivity in challenging environments, since there are more satellites available at once.

Clean layout

GNSS modules are, in general, very sensitive to interference, so it’s a good idea to keep them isolated in the product layout. Position the module so it will be pointed toward the sky, and place it away from the edges and corners of the PCB. Keep it away from tall and metallic components, use a larger ground plane to get better performance, and place other radios and high-speed lines as far away as possible.

Modules that support anti-jamming tend to perform better in the presence of other radios, such as Wi-Fi, GSM/GPRS, 3G/4G, Bluetooth and so on.

Satellite-based augmentation system (SBAS)

SBAS, which was originally developed for civil aviation, uses error correction to make GNSS data more precise. There are two parts to the setup: ground reference stations and geo-stationary satellites.

The ground reference stations receive signals from GPS/GLONASS/BeiDou satellites, perform error correction on the data using a number of factors, and then upload the corrected data to geo-stationary satellites in orbit. The geo-stationary satellites then transmit the corrected data, in a standard GNSS format, to any SBAS-enabled GNSS receiver.

This process of scrubbing satellite data can improve accuracy by several metres. SBAS-enabled modules typically store the SBAS algorithms in Flash memory and will automatically connect to an SBAS geostationary satellite whenever one is available.

How to reduce wait times for GNSS data

A satellite in orbit follows a fairly predictable path, but there are variations that can make it difficult to pinpoint. Providing the module with data that helps calculate the satellite’s exact location reduces TTFF and makes the module more responsive. Techniques that shorten TTFF essentially pre-load the module with the data needed to find satellites, so there’s less time-consuming searching to be done before acquiring a signal.

There are three categories of TTFF, reflecting different startup conditions in the module. With a hot start, the receiver has been in standby mode and already has the current time and position data stored in memory, so the TTFF is short. With a warm start, the receiver has been off for a longer period. It has recent positioning, and can find new connections, but not as quickly as from a hot start. With a cold start, the receiver has no previous data to work with. This is the longest TTFF, since the receiver is basically starting from scratch searching for satellites.

An external antenna tends to enhance signal reception and thereby improve TTFF. Reducing interference, with a good layout as described above, can help too. There are two other features to look for:

Assisted GPS (A-GPS)

A-GPS is an alternate way of providing satellite coordinates, so modules don’t have to depend solely on the satellites themselves for location information. With A-GPS, triangulation data is provided by a network, using Wi-Fi, cellular, or a direct network connection. A-GPS data is made available by the GNSS module provider or a third party, and can be downloaded periodically. The information is typically good for 7, 14 or 30 days.

Using A-GPS is one of the best ways to increase responsiveness – it can lower TTFF by up to 50% in cold-start conditions – but because A-GPS data is provided by an A-GPS server, over a network connection, the device has to be equipped with network connectivity. Modules with embedded Flash can store the data internally, while ROM-based modules need the host to store it.

Embedded Assist System (EASY)

Similar to A-GPS, EASY gives modules a way to predict where satellites are likely to be. The EASY algorithm runs on the module and is used to calculate satellite orbit data for an improved TTFF. Since the EASY algorithm resides on the module, there’s no extra circuitry required, and there’s no need for a network connection to download data.

The EASY algorithm calculates orbits over a shorter period than A-GPS – up to three days with EASY instead of up to a month with A-GPS – but using EASY can still have a big impact on TTFF. For example, a cold start TTFF can go from 30 to 15 seconds, and a warm start can go from 15 to 5 seconds. For this reason, features like EASY are increasingly common in GNSS modules today.

Whether to consider dead reckoning

Dead reckoning is a way of calculating current position based on the direction and distance travelled from a known point. It is used in conjunction with GNSS systems to provide navigation when satellites are temporarily unavailable, such as when a car passes through a tunnel or a thickly forested area.

Some module providers offer solutions that already include the necessary gyro sensors and accelerometers for dead reckoning, but the extra componentry is costly. Dead reckoning is a standard feature in the automotive segment, but higher costs have, to this point, prohibited its use in industrial applications. That is changing, though, and more GNSS applications are likely to include dead reckoning as implementation costs continue to drop.

How to minimise GNSS power consumption

GNSS functions are notoriously power hungry, and that can pose a challenge in battery-powered systems. Supporting more than one GNSS format increases the power requirements, too, so using dual, triple, or quad system modules places extra pressure on the power budget.

For a long time, there was a trade-off between accuracy and power consumption – greater precision came at the cost of using more energy. But things have improved in recent years. Specific power consumption will still vary, depending on the GNSS format used (GPS and GLONASS typically require the least amount of energy), but many of today’s GNSS modules offer a difference of only 1 or 2 mA between single and dual operation.

Look for features that minimise power consumption, such as a switched-mode power supply (SMPS). Also, look for components that increase sensitivity, including low-noise amplifiers (LNAs) and surface acoustic wave (SAW) filters, real-time clock (RTC) crystals and temperature-compensated crystal oscillators (TCXOs). These components may, in and of themselves, increase power consumption, but they reduce TTFF, which in turn lowers overall power consumption.

Also, look for a module that can operate in different power modes. For example, a standby mode will typically reduce power consumption to less than 1 mA when positioning and tracking functions aren’t active, and some modules support a periodic mode, which lets the module switch between full and standby power automatically, based on set intervals.

This makes it possible to tailor energy use according to the specific operating environment. Some module suppliers also offer intelligent algorithms that provide an even higher level of autonomous operation and extend battery life even further.

Some of the latest GNSS modules offer special low-power modes, which reduce the average power consumption to less than 10 mA, even when using multi-GNSS operation. However, using these low-power modes can reduce accuracy, so they may not be right for every application.

Using a module with or without an onboard antenna

GNSS modules that already have an antenna onboard tend to be easier to design in, since there are fewer components to deal with. You don’t need special expertise in radio frequency (RF) techniques to finish the design, and the overall cost may be lower as a result. But the design may use a larger footprint, since the integrated antenna increases the size of the GNSS module.

GNSS modules that work with an external antenna tend to have a smaller footprint and have a lower price point because they integrate fewer components. The design can be optimised at a deeper level, since antenna placement and configuration can have a sizable impact on performance. On the other hand, having to work with the antenna and fine-tune the design makes the development process more complex, and can add cost.

Module performance is, in part, a reflection of the placement and orientation of the module, and of the overall RF design. A poor layout or an inferior RF design can deteriorate performance and can cause the system to fail CE/FCC certification. Using a module with an onboard antenna can make things easier, since it saves the design team from having to format the necessary connection and matching between the GNSS module and the antenna.

For more information contact Gyula Wendler, Arrow Altech Distribution, +27 (0)11 923 9709, gwendler@arrow.altech.co.za, www.arrow.altech.co.za


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