Figure 1. GLONASS high inclined space complex. (Image: Institute of Navigation Technology JSC)
The digital transformation of the global economy requires precise time synchronization and valid object position information. Global navigation satellite systems (GNSS) are the most accurate tool for such tasks.
This year will be 40th anniversary of the launch of the first GLONASS satellite, and we see that the quality of navigation services is driven by the characteristics of today’s satellite navigation signals.
The first fourth-generation Glonass-K2 #13L satellite is scheduled for launch in 2022. It will broadcast a full ensemble of navigation signals — both Frequency Division Multiple Access (FDMA) signals in the L1 and L2 bands and Code Division Multiple Access (CDMA) signals in the L1, L2 and L3 bands. This long-awaited launch will cap a 10-year effort and begin to provide a new platform by broadcasting all the CDMA signals through a single antenna array on the satellite’s geometric axis.
The FDMA antenna array is displaced by 0.9 m from this axis, but this arrangement is done on only two satellites. The next Glonass-K2 satellites, which will be launched beginning in 2024, will have a single antenna array for all navigation signals.
The final second-generation Glonass-M satellite, scheduled to be placed in orbit next year, will provide services by open FDMA signals in the traditional bands at 1.6 GHz and 1.25 GHz. This satellite will be the seventh Glonass-M vehicle able to broadcast GLONASS L3 CDMA signals. There are only two Glonass-K satellites broadcasting this signal now, but more satellites with such a signal will be activated by the end of testing of the GLONASS modernized ground control facility.
We expect the number of satellites able to provide this service to increase by two per year as we replace Glonass-M satellites with Glonass-K and Glonass-K2 satellites.
As of this writing, 15 satellites (62% of the constellation) are working past their guaranteed life times, limiting our ability to increase the system’s accuracy. For the last decade, the signal-in-space range error (SISRE) was 1.4 m, despite the fact that newly launched satellites provide a SISRE of about 0.8 m.
Glonass-K satellites will be launched to maintain the orbit constellation within the next three years, and the accuracy of their signals will be the same or even better. These satellites have a single antenna array for all three bands and could broadcast either FDMA or CDMA signals.
In 2022, the constellation orbits will increase to six satellites in three planes, as we aim to increase the navigation service accuracy and availability (FIGURE 1). See TABLE 1 for satellite orbit parameters. This constellation will make it possible to increase navigation accuracy in the Eastern Hemisphere by about 25% through decreasing the value of the geometric factor.
Table 1. Augmented orbit constellation parameters.
Additionally, this will greatly improve the availability of the GLONASS navigation service in difficult conditions, such as locations where current users can only receive navigation signals from satellites at least 25° above the horizon. New constellation satellites will be based on the Glonass-K platform, which has passed in-orbit qualification and proved it can provide SISRE at 0.3 m. The preliminary design proved that satellites on this platform could provide an in-orbit SISRE below 0.4 m with standard cesium on-board clocks. This signal-in-space accuracy level with valid ionospheric and tropospheric model data from the navigation signal will allow users to receive a position determination error below 2 m in the plane. Navigation services from these satellites will be provided by the CDMA signal in three frequency bands.
The new satellite will weigh about 1,000 kg and be launched into orbit from both Russian spaceports (northern Plesetsk and eastern Vostochny) by the highly reliable Soyuz-2 rocket. The first launch is scheduled for 2026.
One of GLONASS’ important tasks is to transmit the UTC (SU) national time scale to consumers. Over the past few years, significant results have been achieved in this area.
- A complex of high-precision measuring instruments to compare the national coordinate timescale UTC (SU) with the GLONASS timescale was put into operation. These instruments include a transported quantum clock that provides timescale storage with an uncertainty of no more than 1 ns at an observation interval of one day, and with a transportation time of no more than 12 hours. It also provides duplex comparisons of timescales, comparing objects with the permissible uncertainty of ±1.5 ns.
- Timescale storage complexes of secondary and working standards of time and frequency VET1-5 (Irkutsk), VET 1–19 (Novosibirsk), VET 1–7 (Khabarovsk) and RET1-1 (Petropavlovsk-Kamchatsky) were modernized and put into operation, providing an overall uncertainty of 3 · 10-15 and with a maximum permissible shift of the timescale of the complex relative to the national timescale UTC (SU) of ± 10 ns.
- An optical ground-based frequency reference on cold strontium atoms was developed with an uncertainty of reproduction of the frequency unit and time of no more than 1 · 10-17 .
- A keeper of time and frequency units was developed on the basis of a “fountain” of rubidium atoms having a frequency instability of no more than 2 · 10-16 for equipping the standards of time and frequency units and subsequent transmission of more accurate time-frequency information to precision ground and onboard equipment and GLONASS systems.
- A developmental prototype of the national timescale storage complex of the Russian Federation was developed on the basis of a new generation of hydrogen keepers.
The application of the newly developed technical equipment made it possible to significantly reduce the maximum displacement of the national timescale relative to the International Coordinated Time Scale (UTC), which in 2020 was less than ± 3 ns (FIGURE 2). The UTC (SU) timescale ranks among the best national implementations of UTC, according to the International Bureau of Weights and Measures (BIMP).
Figure 2. Displacement of national timescales relative to Universal Coordinated Time (UTC). (Image: VNIIFTRI FSUE)
Many important events are coming for GLONASS users in 2022. They will improve the user characteristics and lay the foundation for further development of the system.
Sergey Karutin is general designer of the Russian GNSS program GLONASS.
Nicolay Testoedov is CEO of JSC Information Satellite Systems Reshetnev (ISS JSC), a Russian satellite manufacturing company.
Sergey Donchenko is general director of the Federal State Unitary Enterprise, Russian metrological institute of technical physics and radio engineering, VNIIFTRI FSUE.