GNSS Simulators Serve as Industry Bellwether
April 15, 2009 By: Rob Lorimer GPS WorldProfessional OEM Newsletter, April 2009
Let me start this month’s article with an apology to the teams at JAVAD GNSS and OmniSTAR. The first edition of March’s Professional OEM editorial (subsequently corrected online) at first characterized state-of-the-art features for commercial off-the-shelf OEM products as having 72 channels, whereas it should of course have stated 216 channels, as JAVAD GNSS offers. I also omitted, in that first version, mention of OmniSTAR’s HP and XP products in the continuous and autonomous navigation section.
Continuing development of state-of-the art receivers (such as JAVAD’s 216-channel TRIUMPH products) pushes the envelope in terms of GNSS engineering and helps drive development in other parts of the industry. We find a good example of this in GNSS simulators.
The ability to test components, receivers, products, and applications constitutes a critical part of the research and development process. As the demand for GNSS products has increased, more receiver manufacturers and application developers have entered the market, and sales of GPS and GNSS simulators have increased proportionally. In this respect simulators are a bellwether for the GNSS industry, both mass market and professional.
The term GPS simulator is widely used, encompassing something as simple as a piece of software outputting an NMEA position and velocity string (for example Sailsoft’s GPSSIMUL package). However, for professional OEM the term more usually refers to a device which provides signals to the RF section of a GPS receiver, that is, as if the signals were coming from an antenna. In turn a GNSS simulator provides signal characteristics for both existing systems, such as GPS or GLONASS, and systems in deployment, such as Galileo and QZSS.
There are two techniques in common usage. The first records and replays real-world RF signals from navigation satellites. The second uses software simulators combined with signal generators to replicate navigation satellite RF signals.
The record and replay technique is found in a number of products including LabSat from the UK company Racelogic. The system first records raw GPS signals from an antenna to a hard disk. When the logged data is replayed, the receiver recreates the movement of the antenna and also experiences any multipath, atmospheric, or other effects encountered during the logging process. This record-and-replay technique can also be applied to inertial measurement units (IMU) with the IMU and GPS data synchronized during replay.
By contrast, the CAST Navigation CAST-2000 is an example of a GPS constellation simulator which generates up to 16 GPS SVs of C/A, P code, and M-noise on L1 and L2. In this case movement, multipath, and atmospheric conditions are user-configured and generated as part of the simulation.
Hybrid systems also exist, for example the Navsys Advanced GPS Hybrid Simulator (AGHS). In this case digital data is recorded from either real-world GPS field trials or MATLAB generated signal simulation. The recorded signals can either be played back as an RF signal into GPS receivers under test, or can be directly played back as a digital signal into a compatible digital GPS receiver.
One important differentiator for GPS and GNSS software simulators is their ability to consistently create signal errors and events (for example, receiver-autonomous integrity monitoring, or RAIM), which may only occur sporadically or randomly in the real world. A second important differentiator for GNSS simulators is their ability to generate signals for new frequencies (such as L2C, L5) before they are launched, or create a full constellation simulation (such as Galileo) before one actually exists.
Examples of high-end GNSS simulators include Spirent’s GSS8000 series and iFen’s NavX-NCS. To give an indication of the power of these types of products, the GSS8000 series includes simulation of atmospheric effects, multipath reflections, terrain obscuration, antenna reception gain and phase patterns, differential corrections, trajectory generation for land, air, sea and space vehicles, and comprehensive error generation for GPS (L1, L2, L5), Galileo (E1, E5ab, E6), GLONASS (L1, L2), and SBAS (L1, L5).
The Spirent GSS8000
The overall market for simulators, record-replay and hybrid products, broadly divides into two groups.
The first user group resides in universities, research labs, and corporate R&D units, where we find many of the early adopters and innovators who have helped drive simulator development. This group covers the full gamut of GNSS devices and systems, with the number of simulators sold correlating to the number of companies and engineers working on GNSS R&D; consequently, volumes have steadily tracked the overall development of the GNSS marketplace. As might be expected, scientific, defense, and professional GPS communities dominate the high-end simulator market.
Whereas the first group has been around since the advent of GPS simulators, the second has really exploded onto the scene in the last five years. These are the users incorporating GPS simulators for volume production testing. Usage of simulators in this segment is a function of manufacturing volumes, so simulators sales have soared as consumer GPS production lines have come on-stream. The booming demand for L1 GPS simulators (and I use here the broad definition) started with PNDs and has continued with the rise of GPS smartphones. Although GNSS simulator companies are reticent to discuss numbers, they do confirm that the production testing segment has rapidly emerged as a significant market for them.
L1 GPS simulators are used for both integrity testing (is the antenna talking to the receiver?) and functional testing (is the receiver computing a correct position?). With smartphone OEMs wanting test regimes as short as 1 second, many GPS simulator manufacturers have a steep learning curve to meet the demands of high-volume production. Those with a background in production testing (for example, Spirent) probably have a head start, but they in turn must compete with some large and experienced incumbents such as Agilent and Rodhe & Schwarz.
In many respects the GPS simulator market has split along professional/volume lines similar to the GPS marketplace. However, it is interesting to note that while all the longstanding GPS manufacturers have clearly divided into volume (exemplified by Garmin) or professional (such as Trimble), at least some of those in the GNSS simulator business are able to keep a foot in both camps.
As I mentioned at the beginning of the article, GPS and GNSS simulator sales are a good bellwether for the entire GNSS industry. So if you want to get some insight into how things are faring in these troubled times, try chatting with that friendly simulator salesperson at the next GPS conference you attend.
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