Getting Ahead and Staying Ahead – Testing with the latest GNSS signals
It’s not that long ago that, with GLONASS not in a functional state, there was only one signal available to civilian GNSS users and developers – GPS L1 C/A. Even this lone civilian signal was restricted to +/- 100m accuracy for most of the 1990s due to Selective Availability. Today we have two fully operational global systems; a further two nearing full operational capability; and a number of regional augmentation systems. With each of these systems broadcasting a range of signals on a range of frequencies, there are numerous options for developers to pursue.
The benefits of having a broad range of signals available are multiple. From protecting against local interference and spoofing, to providing more availability due to a greater number of ranging signals in the sky, to mitigating against the effects of a constellation error, using multiple constellations offers protection. When a combination includes the latest signals, this can also translate as improved performance.
What do we mean by “the latest signals”?
The obvious interpretation is the newly broadcast/planned signals – such as BeiDou B1C and B2A and QZSS L6. However, what we are actually referring to is new ICDs, and these could just as easily be for GPS L1 as for a new frequency on a developing system. An Interface Control Document, or ICD for short, is a document detailing the signal structure and navigation data of a given signal. This can be used to define a future GNSS signal, or to redefine an existing signal. For instance, GPS L1 C/A is currently on its eighth ICD.
So, why do we need the latest ICDs?
There are several different ways to look at this question…
The first is to consider that some new ICDs represent newly available (or soon to be available) signals. Adding the capability to use these to your existing signals provides a traditional benefit: the more signals, the greater the availability there is and so the more continuity of operation you can achieve. With more satellites, your system will be less likely to experience positioning degradation, and having additional systems offers a level of security in the case of a GNSS control segment error.
Secondly, where these new signals are on new frequencies your solution can become more robust and more accurate. Multi-frequency can offer resistance to interference, as well as enable techniques that entirely circumvent any delay caused in the atmosphere.
Perhaps most importantly, if you aren’t testing with simulated signals to the latest ICD, then you aren’t testing with the real signals. Changes to the signal structure are all intended to be backwards compatible, but to assume this is to take a significant risk. In addition, configuring your product based on a previous ICD means you miss out on all the benefits provided by the latest. Assuming that your competitors are taking advantage of the improvements in stability and accuracy, you’d be ceding ground to them on performance.
At Spirent, our dedicated development team keep us up-to-date with all the latest releases, improvements, and technological diversifications. We’ve made a commitment to get to work on implementing new ICDs and changes to existing ones as soon as they are released, and over the last year or so this has been quite an undertaking! We’ve successfully implemented the latest ICDs for:
B1C and B2A
BeiDou BDS phase-3 B3I
QZSS L1 C/A and L1S
NavIC (IRNSS) LE
This enables our customers to stay ahead of the market, giving them the greatest opportunities for success.