GPS week rollover — are you ready?

In the GPS L1 C/A navigation message, the week number is contained in a 10-bit binary number. This limits the maximum week number to a total of 1024, effectively making 1023 the maximum week after it loops back to 0. Hence, the event named “week rollover” occurs every 19.7 years. Modernized signals solve, or reduce significantly the occurrence of week rollover by using more bits, but many receivers rely solely on the original L1C/A signal to determine the current time.

The first ever GPS week started on January 6, 1980. The weeks counter already rolled over on August 21, 1999, and is scheduled to reach its limit again on April 6, 2019, roughly 10 months from now.

A week rollover is a great example of a difficult, if not impossible, event to test without a GNSS simulator. Testing with the live sky, in this case, is definitely not applicable. Waiting for the event to occur with fingers crossed is not a great idea either.

So, what are the implications?

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Revamping the GNSS Frequency Spectrum Reference Sheet

In 2015, we created a GNSS signals overview document and we called it the GNSS Spectrum Cheat Sheet. It was meant to offer a quick, general view of all GNSS signals aligned on their respective bands, and it proved to be a reference we would use a lot in the Skydel offices and around the lab.

After a bit more than 2 years, the sheet was due for an update. First, the GNSS landscape is evolving with new and growing satellites constellations, and the document was missing a few signals. We also gathered internal and external comments about the sheet, and we had some things noted that we would have liked to see in a new version. We went back to the drawing board, we did some research, improved what was working on the previous version and removed what was not.

And, as we were going, we worked on making something that would look good and that we’d like to print, hand out and share.

So here’s the new and improved GNSS Spectrum cheat sheet! It features all GNSS signals, color-coded and aligned on their central frequencies, and most importantly, every signal is depicted on the same scale, making everything nicely proportional.

The whole sheet uses vector graphics, meaning it can be resized without losing integrity. Since the GNSS spectrum is getting pretty crowded, it’s better to print the sheet on tabloid (11” x 17”) paper format. It can then be folded into letter format.

We even printed it as a poster in our office!

Download the new sheet

You will find a high-quality downloadable PDF version in the resources section of our site. If you have any comment, or spot an error, please let us know! We hope you will enjoy and use it as much as we do.

Closing out a productive year with SDX release 17.12

The year 2017 will soon be coming to an end and looking back at the many improvements we’ve built into our GNSS simulator, we can safely say, “mission accomplished”. Yet there are still a few days remaining to roll out one final “17” release of SDX… so here it is!

So what’s in store for users of SDX 17.12?

Support for 3D antenna patterns

Engineers working with GNSS-enabled technology are often obligated to implement a broad range of antenna patterns, from simple to complex ones. Examples that come to mind are when designing optimal antenna form factors, testing anti-jamming capabilities, or creating equipment for specific environmental applications.

With release 17.12, all SDX users benefit from a 3D upgrade to gain antenna patterns, as well as—new feature!—custom 3D phase offset antenna patterns. These new options replace the previous ones found in the vehicle’s antenna settings.

The new graph UI for 3D Antenna Patterns

A 3D gain or offset antenna pattern is expressed with a graph displaying either the gain or the phase offset, through the 360 degrees azimuth and -90 to 90 degrees elevation, thus creating a full 3D sphere mapping around the antenna.

The CSV import-export options bring customization control over the 3D antenna patterns as well. CSV files are composed of a two-dimensional matrix of values; SDX interpolates between specified azimuth/elevation points thus covering the whole 3D space. The matrix can be of any size; SDX automatically detects the number of rows and columns while importing your data, thus offering convenient flexibility.

The CSV Import/Export function for 3D antenna patterns in SDX 17.12

This enables quick and easy setups, while providing fine-grained control to users requiring large mappings that depict complex patterns.

Master → Slave Configuration Broadcast

One of the most powerful features of SDX is its synchronization capabilities. Along with the timing improvements that were part of the last release, we continue to further refine this feature set in this latest version of SDX. As a result, it is now possible to push a SDX scenario configuration from the master instance to all slave instances. This unique feature brings obvious workflow improvements to any simulation setup.

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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).

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SDX Release 17.8 is now out

Adds advanced interference simulation, Gaussian noise, and more

Freshly out from our coding lab, here’s the latest update to Skydel SDX.

While it’s often a quiet time for many, our team of engineers were busy at work during the “lazy days of summer” producing a brand new release of SDX with a host of new features, including a big one that we have been dropping hints about for a while.

Advanced Jammers Simulation

The “piece de resistance” of this new release is the addition of advanced interference capabilities to SDX.

Why is this a breakthrough? The conventional way of testing GNSS interference uses a hodgepodge of hardware and cables in all directions, is complicated to set up, and is still somewhat limited in functionality. We’ve covered this before if you want the details.

On the other hand, SDX already uses GPU-accelerated computing to create GNSS signals. The next logical step was to add interference generation. That’s what Advanced Jamming is about: a unique way to leverage the power of the GPU/SDR combo to create an unheard of way to simulate interferences.

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