High-Precision Receiver Design: More than Accuracy

September 2, 2015  - By

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Anticipating New, Different Application and User Needs

Users in emerging applications may have different requirements from traditional high-precision users. New users increasingly look to the technology not solely for position, but to navigate them through the environment, often autonomously or semi-autonomously. Tracking all of the new multi-GNSS signals, and then using the large number of inputs in the positioning engine, drives the amount of processing power and memory required onboard the receiver. These in turn drive the cost, size and power consumption of the receiver in exactly the opposite direction from the expectations of customers.

By Jason Hamilton

In considering the future of high-precision satellite navigation, we need to consider what users of the technology are trying to accomplish, and which growing and emerging applications will drive adoption of GNSS technology in the future. These applications will drive growth in our industry if we can correctly anticipate their future needs.

Traditional applications of high-precision GNSS are well understood, but what these customers have demanded from GNSS can be at odds with what users in emerging applications require. Survey and mapping users were early adopters of high-precision GNSS and remain large user segments. Surveying with GNSS requires the very best accuracy that GNSS can achieve. Every centimetre of accuracy matters. Power and size are important product attributes to survey manufacturers. Mapping customers increasingly are asking for not just position, but orientation of a camera or other sensors.

Once accuracy challenges were well in hand, the topic of availability came into play. It was no longer good enough to have an accurate position in open-sky situations. Applications demanded continuous positions that were accurate in more and more corner cases and challenging environments.

In addition to using GNSS to measure location in an environment, new applications are increasingly looking to the technology to navigate them through the environment — often autonomously, or semi-autonomously. For these users, whether operating on a farm, in a mine, on the ground, or in the air, position accuracy is only part of the requirement. Solution accuracy of course matters, but other receiver attributes such as real-time quality control and solution integrity monitoring, are equally or more important.

Multi-constellation, multi-frequency GNSS provides tremendous opportunity and also presents significant challenges for receiver manufacturers. Constellation and frequency support has previously been a differentiator among high-precision GNSS providers, and among product generations. The relative stability of the satellite constellation definition means that the signals broadcast from space will be relatively predictable for some time into the future, and as such, GNSS products are increasingly supporting “all in view,” the ability to track everything that is broadcast.

The benefits of more satellites, more frequencies (and resulting frequency combinations) and modern signal structures have been well publicized. As new and modernized GNSS constellations come on line, they will deliver more robust positioning in increasingly challenging environments such as urban centers, open-pit mines and under tree cover. We will be able to account for atmospheric effects more accurately, which will help during times of high ionospheric activity and extend the length of RTK baselines. Users have a great deal to look forward to from their next-generation receivers.

All of these improvements necessitate pretty dramatic changes in receiver design. Tracking four global constellations and numerous regional SBAS systems increases the complexity of tracking and positioning firmware and algorithms. Tracking multiple frequencies and signal types on each of these constellations drives the receiver channel count up substantially. The days of the 12-channel receiver are gone. Channels, typically implemented within the manufacturers’ custom chips, drive application-specific integrated circuit (ASIC) complexity, which drives cost, power consumption and physical size. Some of this can be mitigated through the use of smaller process geometries, embedded processors and peripherals, and RF chip integration; however, there are down-stream effects to all of these signals as well.

Challenges

Once your receiver has enough ASIC channels to track all-in-view, you need to do something with all that data. The receiver’s tracking sub-system generates code (pseudorange), carrier-phase and Doppler measurements for every signal on each satellite. With four global and multiple regional constellations and up to four frequencies on each satellite, that amounts to a great deal of data. These measurements are what we turn into position, through a range of different positioning algorithms from code positioning to real-time kinematic (RTK) to precise point positioning (PPP). Tracking all of these signals, and then using the large number of inputs in the positioning engine, drives the amount of processing power and memory required onboard the receiver. These in turn drive the cost, size and power consumption of the receiver in exactly the opposite direction from the expectations of customers.

Bandwidth. Communications bandwidth is also a future challenge. Positioning methods, such as RTK, that transmit base-station observations for each GNSS signal to field rover receivers, will require much more bandwidth in the all-in-view future. PPP, which provides a state-space correction of the underlying GNSS error sources, is a promising alternative to RTK that scales better with more satellites than RTK and provides performance that is good enough for many applications.

Utilizing the multiple frequencies available from modern constellations also presents challenges to receiver designers. RF designers are faced with the opposing challenges of making GNSS receivers and antennas smaller, lighter and lower cost, while also supporting more GNSS broadcast frequencies and mitigating against increasing amounts of interference in the L-band RF spectrum from non-GNSS uses. Robust RF design makes the difference between a system that works most of the time, and a system that works reliably all of the time.

Expectations

If we now come back to the expectations of end users, the challenges are clear. Most customers actually don’t care about all-in-view tracking, how many satellites are tracked, or about what the receiver is up to behind the scenes. Users will judge their GNSS receiver on whether or not they are receiving a position that meets the requirements of their application. Are they meeting their targets for accuracy, availability, latency, data rate, and does the receiver fit from a size, power consumption, regulatory and cost perspective? After a certain level, more observations do not make the solution more accurate or more robust. Manufacturers need to carefully manage the tradeoffs in their systems on behalf of users to produce the best quality position possible, while still meeting the customer expectations on all the other receiver attributes.

Sensor Fusion. Demands of new applications drive GNSS providers to consider more than just position. Most vehicle control applications require orientation information as well as highly accurate position. Multiple-antenna GNSS heading systems are becoming smaller than ever. Inertial measurement device technology is also evolving quickly. Miniature micro-electro-mechanical systems (MEMS) inertial sensors can now deliver performance that only a few years ago was exclusive to large, heavy, bulky systems. The integration of GNSS and inertial technologies has been well adopted in highly demanding applications like aerial and ground mapping. As the size, weight and cost of the technology continues to shrink, sensor fusion in many forms will become the standard for all machine control and autonomous vehicle applications.

Safety. This is a key consideration for system designers working on remotely or optionally piloted and autonomous systems. Position and orientation accuracy is important, but so, too, is assuring that the solution is right and can be trusted. The accuracy of the solution needs to be characterized in real time so that control systems can react as necessary to protect users on and around the vehicle. Often in these applications, accuracy can be traded off against the robustness and reliability of the solution. This presents new ways of thinking for firmware and algorithm developers who have focused for so long on solution accuracy.

Support. Lastly, let’s not forget having reliable supply of high-quality product, and expert customer service to back it up. As high-precision GNSS attracts new users in a range of new industries, they are less often geodesists or geomatics engineers. The products absolutely need to be easy to use correctly, backed up by complete and accurate product documentation and supported by world-class application engineers.


Jason Hamilton is vice president of marketing at NovAtel Inc. Since joining the company, he has held a number of research, development and product management roles. Jason holds a Bachelor of Science degree in geomatics engineering from the University of Calgary and an MBA from Royal Roads University.

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