SDX 17.10 is released with GPS L5 support, timing improvements and more

Fall is finally here and to celebrate we have released a new version of SDX. Release 17.10 of SDX adds support for GPS L5, timing improvements to synchronize your GNSS simulation with your test environment, and enhanced features for BeiDou GNSS simulation. Read on to learn more about this exciting new release of SDX, the software-defined GNSS simulator with sky-high performance and unmatched flexibility.

SDX support for GPS L5

If you already know everything there is to know about L5, you can safely skip to the release details. Otherwise, we recommend that you read on.

A bit of history about GPS

GPS has been in development since the early 1970s, following early experiment from the US Navy with positioning using Doppler from Satellites. The US Department of Defense (DoD) wanted a robust positioning system and developed GPS as we know it during the 70s throughout the 90s.

From their early beginnings, GPS satellites were designed to broadcast multiple signals. Even the first GPS satellites were broadcasting two signals:

  • C/A (Coarse/Acquisition) on L1 frequency. This was called the Standard Positioning Service, or SPS for short.
  • P on L1 and L2 frequency. This signal was restricted for use by the US military, and was and thus encrypted when modulated with the W-code to create the P(Y) encrypted signal. This became the Precise Positioning Service, or PPS.

Since the GPS beginnings, three signals were broadcast on two GNSS bands

The Standard Service, however, was initially degraded by a measure called “Selective Availability” (or SA for short) for national security reasons. With positioning errors ranging in the dozens of meters, accurate positioning and navigation using the GPS public signal was impossible.

Although GPS was initially intended for military usage only, the KAL Flight 007 incident in the 1980s is credited with triggering a chain of events that led US political leaders to consider evolving GPS into a dual-use (i.e., military and civilian) positioning system. In the 1980s and 1990s, plans to improve the civilian availability of GPS were made, but Selective Availability remained in function. In 1995, after many years in development, GPS FOC (Full Operational Capability) was declared by the US Air Force, and soon after, an official GPS modernization plan was approved by the US Congress. Finally, on May 2, 2000, “Selective Availability” on L1 CA was discontinued, which allowed widespread civilian use of GPS to begin. Consumer devices using the GPS L1 CA signal were being commercialized and quickly adopted by the public. Meanwhile, modernization was slowly taking place.

Modernizing GPS

Over the years, and even before FOC was achieved, GPS satellites were improved. Nowadays, various satellite models are currently in operation, while others are planned for launch.

Various GPS satellites models were placed into orbit, and new models are planned to launch soon. The new public signal L5 is being broadcast by GPS Block IIF and the upcoming GPS III satellites.

The 1998 GPS modernization plan is still being implemented and includes the addition of multiple new civilian signals dedicated to improving the reliability and accuracy of GPS for the general public. Three public signals are part of this modernization (L1C, L2C, and L5) along with the implementation of an improved military signal, M-code. In 2004, the first modernized GPS satellite (Block IIR-M) was launched, and it soon began transmitting a second civilian signal (L2C).

What is GPS L5?

GPS L5 is a new civilian signal, part of the GPS modernization plan. GPS L5 was created with safety-of-life transportation in mind, and to bring high-performance applications to the public. It features various improvements over the “legacy” CA signal, mainly:

  • a data-less acquisition aid in the form of an additional “pilot carrier” signal that is easier to acquire by a GNSS receiver: this signal is acquired first, and once successful, the receiver can better “hook” on to the data signal.
  • a forward error correction (FEC) coding included in the NAV message. Since the message transmission rate is slow, interruptions in the transmission usually mean that the receiver must download the whole message again from the beginning, which can lead to delays. The FEC mitigates those interruptions and enables faster message downloads.

GPS L5 is a signal located in the Aeronautical Radionavigation Services band; its name derives from the U.S. designation for its frequency, 1176.45 MHz. When combined with another signal on another frequency (such as L1 CA or the planned L1C signal), it enables ionospheric correction, leading to improved accuracy of the navigation solution. This advantage, originally restricted to users of the P(Y)-Code, will now be available to the public as we see L5 adoption in the future.

GPS L5 status

At the time of writing of this article, GPS L5 is pre-operational, and broadcasting from 12 GPS satellites. At the rate it’s being implemented, it is expected that this new signal will be fully available in 2024.

SDX and GPS L5

Although GPS L5 is not yet ready for primetime, GNSS receiver manufacturers are actively developing chipsets that will use the new civilian signals. In some fields, such as surveying, the transition from dual-frequency usage (i.e.,using semi-codeless acquisition of P-Code) to using the new civilian signals is strongly encouraged. For other fields, such as aviation and transportation, GPS L5 will see rapid adoption, bringing the multiple advantages of a modernized signal being broadcast over the lower GNSS band. Now is the time to begin testing and designing GNSS devices using GPS L5 and SDX.

Timing improvements

For some applications, ensuring the precision of the GNSS simulation start time is crucial. To offer the most flexibility and control over timing, we’ve improved the way SDX handles synchronization and start times, and we’ve propagated the changes into both existing and new API commands. These improvements are particularly useful when integrating SDX with an HIL setup.

The SDX documentation now features clear procedures depicting multiple use cases, from simple configurations up to HIL systems with an external PPS source. Provided with detailed API command examples, this new release of SDX ensures that users get that timing perfect each and every time they run a simulation.

The SDX documentation has been updated with timing procedures and examples

Other SDX improvements

Three other features made the cut for release 17.10. We added:

  • Message modifications for BeiDou GNSS simulation
  • Downlink data logging for BeiDou GNSS simulation
  • Support for sine-phase and cosine-phase BOC for users of Advanced Jammers


As you can see, we have been busy both improving the underlying capabilities of SDX as well as new features to make SDX the best GNSS simulation system ever. Even though the end of 2017 is within view, we are not finished delivering enhancements to SDX this year, so keep an eye out for our next update.