Directions 2020: GLONASS focuses on users

Yury Urlichich, First Deputy Director General, Roscosmos. (Photo: Roscosmos)

By Yury Urlichich, First Deputy Director General of ROSCOMOS State Space Corporation
Sergey Karutin, Designer General of GLONASS
Nikolay Testoedov, Director General, Information Satellite Systems

Roscosmos keeps concentrating on user needs as it did in previous years. Growing digitalization is driving a high demand for high-accuracy navigation services. Space information technologies support user needs by modern digital services, including increasing accuracy of position and velocity determination. Because of this, it is of vital importance for us to ensure that GLONASS provides continuous services and stable performance.

Figure 1. Mature Glonass-M satellites show improved cesium frequency standards performance in terms of daily stability. (Image: Roscosmos)

Performance Standard & ICD

This year, we finished drafting the GLONASS Open Service Performance Standard (GLONASS OS PS; the Russian language version is available). In 2020, the new version of the GLONASS Interface Control Document (ICD) also will be publicly available.

GLONASS OS PS serves as a high-level mainframe document specifying the values of the achieved GLONASS performance characteristics plus the significant guaranteed margin. These, coupled with the signal reception environment and a priori estimation of user equipment performance characteristics, can further be translated into the performance that an end user can expect to achieve in his specific PVT solution.

This GLONASS OS PS is a basis for certification of GLONASS services and development of lower level standards for user receiver and GLONASS-based service, as well as for development of international standards like those of the International Civil Aviation Organization (ICAO), the International Maritime Organization (IMO) and others.

Use of the unified set of performance parameters and calculation methods for all GNSS — GLONASS, GPS, Galileo and BDS — is a conventional practice. The similar standards for GPS, Galileo and BDS have been published and are regularly updated.
In fact, this GLONASS OS PS is the second one after the ICD baseline interface between GLONASS and user receiver manufacturers and the GLONASS-based services developers. The OS PS establishes the minimum performance that can be achieved by users with a high level of trust based on the system’s long-term statistical history.

Signal-in-Space. This OS PS specifies standards for the GLONASS OS Signal-in-Space (SIS) performance neglecting receiver biases, signal propagation and reception biases (in terms of performance metrics used to specify system performance, that is, taking into account the GLONASS space segment and the GLONASS ground segment contributions to the performance). It can serve as a basis for certification of the GLONASS-based services and receivers incorporating GLONASS, including those used in aviation and other user domains.

The OS PS provides an overview of the GLONASS system and an overview of the GLONASS Open Service SIS. It specifies the standards for the performance characteristics of the channel of standard accuracy used to provide the Open Service, and lists the legal reference documents.

L3 CDMA. One of the most significant tasks is the harmonization of GLONASS user interfaces with respect to new L3 CDMA signals. The requirements related to the interface between the space segment of GLONASS and the navigation user segment for radio frequency links is established by the GLONASS ICDs.

The new version of ICD for CDMA L1, L2 and L3 signals to be broadcast by new-generation Glonass-K2 satellites was issued in 2016. However, the Glonass-M satellites (## 755-758) and the Glonass-K satellites currently in orbit transmit the L3 signal as per the L3 Open Access CDMA Radionavigation Signal Interface Control Document (Edition 1) of 2011.

In order to mitigate the above-mentioned discrepancies, five reference documents (Interface Control Documents for open-access signals) have been updated and prepared for publication. In addition, flight tests to verify new ionospheric and tropospheric delay models have been scheduled.

Incorporating More Data

The new ICDs for open access and authorized signals incorporate changes related to the introduction of additional data into the spare bits of the navigation message. This additional data is to be used by user receivers for better PVT solution purposes.

The updated versions of ICDs will incorporate:

• The mathematical ionospheric delay model and inclusion of the model parameter into the navigation message.
• The mathematical tropospheric delay model, which does not require that any specific parameters be included into the navigation message. It only employs data on the latitude of a user receiver location and the season (i.e., winter, spring, summer, and autumn).
• The attribute (or flag) to inform a user that a satellite is in the turn mode and its antenna phase center behavior is different from that when a satellite is in the sun orientation mode.
• Information about the types of signals broadcast on the L1, L2, and L3 frequencies; 5-bit field, in which the first three bits denote L1, L2, and L3 CDMA signals, respectively, while the 4th and the 5th bits denote L1 and L2 FDMA signals, respectively.
• A 5-bit field to be used to broadcast age of data (AOD) for time offsets in addition to the similar field used to broadcast AOD for ephemerides.

Backward Compatibility. The updated CDMA and FDMA ICDs will support the backward compatibility for the uninterrupted operation of the existing envelope of user equipment and the introduction of the ionospheric and tropospheric model parameters into the message spare capacity.

Constellation Refresh

The GLONASS constellation has been replenished steadily. Since 2013, we have been launching one to two satellites a year, and this year is not an exception. The launch on May 27 and the December launch will help sustain the nominal constellation. The Glonass-M satellites demonstrate good dynamics for the average operational life. Two satellites are well beyond their 10-year design life — their operational lifetime has exceeded 12 years. As some of the Glonass-M satellites grow older, their cesium frequency standards performance in terms of daily stability improves (see Figure 1).

Glonass-K. In 2020, the launch campaign for the Glonass-M satellites will come to its end. The Glonass-K satellites will come on stage with the first launch of Glonass-K-15 scheduled for the beginning of the next year. We are fully confident that this satellite will not disappoint our users.