How GLONASS helps fill in gaps in GPS coverage for a user with a combined GPS/GLONASS receiver.
When the Russian GLONASS system lost three new satellites in a recent launch failure, did this long-standing navigation system lose some momentum? The reports out of Russia had indicated renewed enthusiasm and commitment to put a revived, complete constellation back in its rightful GNSS place. Launch of an L3 CDMA demonstrator satellite is apparently forecast for late February, and work on the Russian SBAS ground network (known as SDCM) also appears to be proceeding. We can only hope that Russia does not back away from its strong push to not only restore one of the world’s primary satellite navigation systems to its former glory, but to also “modernize” the signals it broadcasts.
But why do commercial users care about GLONASS? Everyone is already enjoying an enhanced and growing GPS signal in space with loads of satellites and the promise of a rejuvenated GPS constellation with even more signals and features. Well, actually GLONASS has been around for a long time and it has become a system that a lot of users rely on, especially those in the high-accuracy real-time kinematic (RTK) community.
GLONASS may not itself directly enhance accuracy, but the availability of ~21 healthy satellites certainly helps fill in gaps in GPS coverage for a user with a combined GPS/GLONASS receiver. Those extra space vehicles (SVs) when added into an RTK solution can make the difference between accurate, reliable RTK and no RTK or only intermittent RTK.
Back in the early days of a complete, reliable GLONASS constellation, at least one manufacturer embraced and integrated these signals with GPS and was able to demonstrate the best RTK it had ever seen. Other manufacturers followed, and soon high-precision GPS/GLONASS receivers became the norm. As the GLONASS constellation decayed over the next several years, the advantages of dual-constellation receivers fell away somewhat, and GPS-only RTK receivers became more common. Then, almost 10 years ago, as the news of an impending GLONASS revitalization filtered into users and manufacturers, at first in Europe and then throughout the rest of the world, a new generation of dual-function GPS/GLONASS receivers was developed and fielded.
In the process, manufacturers outside Russia dusted off their old GLONASS tracking algorithms, and it became clear that some inside knowledge might be needed. Over time, it became clear that some GLONASS satellites had their own distinct personality, each of which had to be accommodated in a more complex tracking, position, and RTK solution. Allowances were made, and the ultimate goal of more robust, more reliable RTK using both GPS and GLONASS was eventually achieved.
Robust, reliable RTK mean that survey missions can be completed without intermittent or total loss of high-accuracy measurements. Crews operating in difficult locations could avoid the expense of making repeated measurements or avoid the drastic solution of a subsequent complete re-survey. Dual-function, dual-frequency receivers became cost-effective for these users and those involved in machine control, construction, high-precision mapping, precise shipping-container location, and for many other high-value, difficult operations requiring RTK.
But what happens to users with GPS/GLONASS receivers when Galileo, Compass, IRNSS, QZSS and other regional, more capable SBAS systems around the world all come on line? The promise of huge numbers of additional usable signals has yet to be realized, but provided that momentum is maintained with these developing systems, the signal “landscape” will change considerably in the future. And manufacturers are already anticipating those changes — multiple constellation receivers are already available in the market — but RTK is only possible with signals that exist, so GPS/GLONASS RTK is still king.
It may be reasonable to assume that GLONASS users in countries that are politically aligned or who are trading partners with Russia will hang onto these signals. India and China may have their own systems in the future, but users in both countries already appear to subscribe to the GLONASS club, and politics may play a role in maintaining this user base. Of course, GLONASS use is now mandated in Russia. Surprisingly, European and even North American users may also currently form a significant part of the existing GLONASS user group. As Galileo comes on line, the situation in Europe and elsewhere might well change.
And when GPS L5 comes on line some time in the future, manufacturers will be able to dump their complex P-codeless tracking techniques. Low-cost RTK will become a real possibility, and the advantages of lower-cost receivers may begin to outweigh the cost-benefits of complex multi-constellation receivers for some niche users. But with Galileo E5 and IRNSS L5 on the same (well almost) frequency, its likely that multi-constellation receivers will fall in price with volume and will ultimately become commonplace.
And if you already have all those GLONASS gates in your ASIC and GLONASS tracking algorithms implemented in your future powerful processor, its possible that GLONASS will still form a basic element of precision navigation for most future GNSS users.
Now, should the promised GLONASS innovations keep pace over time with all these other investments in GNSS system growth — and a complete GLONASS constellation with CDMA signals allows manufacturers to reduce receiver complexity and still hang onto GLONASS capability — then it’s a good guess that GLONASS will remain with us all for a long time as a basic system which we will all depend on.