The Evolution of Spirent GNSS Simulation

Spirent’s simulation systems have changed significantly from their technology beginnings, which can be traced back to World War II radars. The company and its technology have evolved to keep pace with today’s growing population of GNSS constellations and to meet the challenges that receiver manufacturers and users encounter in an ever-complex integrated GNSS environment.

In the early days of GPS when there were only enough satellites for a fix at odd times of the day or night, these nighttime expeditions were the only form of testing that we could get our hands on. Then as the constellation grew, we were delighted when eventually you could do open sky testing whenever you needed. It never even occurred to us that more exhaustive, more complex testing would become essential as time progressed.

If you walked into any GNSS manufacturer’s testing facility nowadays, the ubiquitous test rack at the heart of most test validation systems might well include a Spirent simulator of some vintage. I recall when we were bringing up receivers in engineering, one of our concerns was how the heck could we afford another one of these beasts for the guys down in production? After we already broke the bank when we managed to convince management that we couldn’t live without a Spirent, we were wondering who we’d push to the front of the line to tell the boss that we had to buy yet another one for the guys on the production line. At one time before a cut-down single channel box became available, we shared our simulator with production who operated the system remotely and a coax run provided RF onto the production floor. We still did open sky testing in R&D, but the complex validation scenarios would have been impossible for the team without our Spirent simulation system.

Recently I got to wondering where Spirent had come from and how come they had become one of the leading players in GNSS simulation. I did recall that they were UK based, that there were a number of name changes and that at one stage they also had receiver capability. So I got talking with John Pottle who’s always been my marketing window into Spirent, and Peter Boulton who’s been my principle technical contact. I was interested in Spirent’s background, their engineering capability, how they got where they are now and where they plan to go in the future.

Its not surprising that Spirent’s roots go way back in England to the period of the second world war. England developed radar as an early warning system that helped win the air combat Battle of Britain. Following the extensive blitz bombing of London, the UK government subsequently re-located the radar technology team well out of harm’s way to the distant and more secure southern tip of England, and that technology team formed the core of a high-tech group based in Paignton, Devon which eventually evolved to focus on GNSS simulation.

Southern England – Paignton base for Spirent.

It’s a nice area to live in, with fewer people, smaller towns and a very pleasant climate. So the technology guys and their families hung around and the government facility became Standard Telephones (STC) and Cables Defence Systems. Focusing in those days on travelling wave guides, cathode ray tubes, and radar amplifiers and the like, this business grew to include solid-state amplifiers, satellite communications and repeaters for fiber-optic networks. This all needed test equipment and a test division grew up to service STC’s technology groups.

As GPS came on line, the UK Government Royal Aircraft Establishment (RAE) needed GPS simulation capability to verify GPS system performance, and STC came up with a test system equipped with 6 dual-frequency satellite signal sources with additional jamming sources and a range of military data interfaces.  The computer operating system was VMS running on a Digital Microvax2 platform, the software was written in DEC Fortran and the DOS-like user interface had textual menus with a graphics terminal for X-Y plots. Just like we had racks of equipment for the original single channel GPS receivers, GPS simulation systems started in the same way.

RAE GPS Simulation System 1987.

In parallel STC was also working on a contract to develop a military GPS receiver, and several of the GPS ASICS used in that receiver found their way into the simulator. Simultaneously, the RAE contract was extended to include provision of full SA-A/S capability, which was delivered in 1988. This classified system was used to formally evaluate the Rockwell-Collins 3A receiver SA-A/S implementation – at the time this test system was the only one available capable of emulating all the features of SA-A/S.

As it became clear in1988 that GPS would have a wider commercial market, STC began to invest in simulation systems for commercial receiver manufacturers.

With dual frequency and up to 10 satellite channels, the STR2740 was still quite large as it was based on the floor standing Microvax2. Porting the software to a desktop VMS workstation gave us the more familiar STR2760 that was first displayed at the ION-GPS-1991 convention in Albuquerque. This initial unit was actually purchased from the ION display show floor and STC had to hustle to quickly make more!

Then ownership passed to Northern Telecom in Canada, who was initially interested in STC’s fibre-optic communications technology and products. After a few years, Northern Telecom changed its name to Nortel – so then we all started talking about ‘Nortel simulators’. The next phase of internal development re-tuned the technology and the resulting 1997 STR4760 simulator boasted double the channel capacity and enabled the inclusion of GLONASS and SBAS capability.

STR4760 Simulator 1997.

In the same timeframe, development of a Controlled Radiation Pattern Antenna (CRPA) was underway in Paignton, but this didn’t quite fit with a business focus on testing, so the CRPA line was sold to Cossor, which was subsequently merged with Raytheon — and the well-known GAS-1 mil-spec CRPA was the outcome. The GPS receiver technology went along with the CRPA to Cossor and ultimately on to Raytheon.

In 1997 the Nortel name also disappeared as Bowthorpe in UK became the new owners and the group became known as ‘Global Simulation Systems’ and we then had “GSS” simulators for a period, but by 2000 the parent company changed its name to Spirent, and that name seems to have stuck.

When SA was switched off in 2000, the potential for commercial GPS became apparent to the Spirent team and this fired up investment in a brand new range of products for the commercial GPS L1 C/A code marketplace – units can often be found in use for single channel production testing, whilst other multi-channel simulators are in use for commercial, pre-production, R&D and verification.

Full L2C, L5 and M-code GPS modernisation was introduced in 2004 while retaining essential systems and scenarios backward compatibility. Spirent’s approach has been to endeavour to get to market early with new signal capability for early adopters.

Support for all Galileo signals and services arrived in 2006 and the GSS8000 series in 2008 added a wide range of additional signal generation capabilities as well as GLONASS L1/L2 and QZSS.

GSS8000 Series Simulator 2008.

SimGEN has been the Microsoft Windows user interface provided by Spirent since around 2002.

SimGEN interfaces to external receivers, and enables external vehicle trajectory input via various interfaces. High speed remote control is also possible and logging/displaying/plotting is also available for report generation and results analysis.

So today, Spirent has accumulated a significant range of simulation capabilities:

• Galileo RF constellation simulators for all frequencies & services
• GPS L1 C/A and P/Y, L2C, L5, M-Code, M-Noise, L1C
• GPS SBAS (MSAS, WAAS, EGNOS, Gagan)
• GLONASS L1/L2
• QZSS L1 C/A, SAIF, L1c, L2c and L5 signals
• R&D systems for the IRNSS regional system program
• Automotive sensor simulation
• SimGEN emulation of Aircraft Landing Augmentation System (GBAS)
• SimINERTIAL adds stimulation of test Inputs for several types of inertial sensors.
• Equipment for both GNSS manufacturing and field testing

With around 25 in-house engineers and a number of outside consultants, the technical team is not huge. But with 27 years of accumulated experience in GNSS simulation, and a large ‘vault’ of key technologies, Spirent is well positioned for the challenges that the world’s multiple, evolving GNSS constellations are presenting to manufacturers.

So what’s next for the Spirent simulator business? Well the Chinese COMPASS constellation is coming on fast, so even though there is still no complete, usable public ICD available, Spirent has adopted the same approach used when release of the Galileo ICD was restricted by ESA – Spirent supplies a COMPASS simulator which has the ‘real’ modulation and frequencies, but the customer inputs the navigation messages.

Spirent is also getting some traction from users who want simulation systems to model specific applications – like car motion sensors to simulate the inputs of in-vehicle navigation system, or full ground segment monitoring and fully integrated message generation for GBAS aircraft landing systems or simulation designed for testing of integrated GPS/Inertial systems.

The days of relying on GNSS alone for navigation and positioning may be fast disappearing, so its likely that things will get even more complex. While there may be some significant questions, such as which combination of GNSS frequencies/signals/constellations to choose from to optimise performance for a particular application, the focus for developers is getting much broader than GNSS or even multi-GNSS alone. Or you could say that the problem has shifted from proving GPS receiver performance alone, to proving, and improving systems and applications performance to meet increasingly demanding end-user needs.

For example, in defence applications where integrity and resilience are key focus areas, inertial navigation is used to complement GNSS, and adaptive antenna technology helps to overcome intentional interference threats. In commercial markets, getting good accuracy everywhere has led to hybrid approaches that include cellular and Wi-Fi positioning and augmentation from MEMS inertial sensors.

Spirent’s product road maps appear to reflect this shift in customer needs. This year we should expect to see Spirent GNSS/inertial test capability for commercial inertial sensors, and also manufacturing and functional testing of consumer devices that include not only GNSS but also Wi-Fi, Bluetooth and other emerging technologies such as near-field communications (NFC) contactless technologies.

So a varied range of GNSS simulation capabilities which match up to the challenges which users face in the real world — and with over 800 simulations systems supplied world-wide, Spirent is surely setting the pace for the evolving GNSS & systems simulation marketplace.

Tony Murfin
GNSS Aerospace