1. Basic concepts of the GLONASS system
The Russian Global Navigation Satellite System (GLONASS) is based on a constellation of active satellites which continuously transmit coded signals in two frequency bands, which can be received by users anywhere on the Earth's surface to identify their position and velocity in real time based on ranging measurements.
The system is a counterpart to the United States Global Positioning System (GPS) and both systems share the same principles in the data transmission and positioning methods.
In 1982 the first GLONASS satellites were set into orbit, and the experimental work with GLONASS began. Over this time span, the system was tested, and different aspects were improved, including the satellites themselves. Although the initial plans pointed to 1991 for a complete operational system, the deployment of the full constellation of satellites has been completed in late 1995, early 1996.
The space segment of GLONASS, is formed by 24 satellites located on three orbital planes. Each satellite is identified by its slot number, which defines the orbital plane (1-8, 9-16,17-24) and the location within the plane. The three orbital planes are separated 120 degrees, and the satellites within the same orbit plane by 45 degrees. During 1996 two satellites have been decommissioned, and the current stand has 22 active satellites.
The GLONASS orbits are roughly circular orbits with an inclination of about 64.8 degrees, a semiaxis of 25440 Km and a period of 11h 15m 44s.
The ground control segment of GLONASS is entirely located in former Soviet Union territory. The Ground Control Center and Time Standards in Moscow and the telemetry and tracking stations in St. Petersburg, Ternopol, Eniseisk, Komsomolsk-na-Amure.
The coordinate system of the GLONASS satellite orbits is defined according to the PZ-90 system, formerly the Soviet Geodetic System 1985/1990. The time scale is defined as Russian UTC. As a difference from GPS, the GLONASS time system includes also leap seconds.
All satellites transmit simultaneously in two frequency bands to allow the user to correct for ionospheric delays on the transmitted signals. However, each satellite is allocated a particular frequency within the band, determined by the frequency channel number of the satellite. These different frequencies allow the user's receivers to identify the satellite. In the current set up, two satellites in the same orbit, occupying antipodal locations, transmit in exactly the same frequency, with a few exceptions.
The actual frequency of transmission can be derived from the channel number k by applying the following expressions:
Frequency band L1: f1(k) = 1602 MHz + k * 9/16 MHz
Frequency band L2: f2(k) = 1246 MHz + k * 7/16 MHz
Superimposed to the carrier frequency, the GLONASS satellites modulate their navigation message. Two modulations can be used for ranging purposes, the Coarse Acquisition code, with a chip length of 586.7 meters and the Precision code, of 58.67 meters. The satellites also transmit information about their ephemerides, almanac of the entire constellation and correction parameters to the time scale.
The ephemerides values are predicted from the Ground Control Center for a 24-h period, and the satellite transmits a new set of ephemerides every 30 minutes. The almanac is updated approximately once per day.
2. GLONASS research in the DLR-DFD Neustrelitz
In the DLR-DFD Neustrelitz we follow several research lines regarding GLONASS, most of which are integrated within the project "Development and Operation of compatible GLONASS monitoring stations in Neustrelitz and Moscow (GLOMO)".
The main highlights of our activities in this field are the following:
So far, five different GLONASS receivers have been operated and tested: ASN-16 (Institute of Radionavigation and Time, Russia), SHKIPER and ISDE-16 (Institute of Space Device Engineering, Russia), R100/R101 (3S Navigation, USA) and GG-24 (Ashtech Inc., USA).
A prototype 16-channel GLONASS/GPS receiver (ISDE-16) developed by the Russian Institute of Space Device Engineering for system monitoring is installed and operated at Neustrelitz. A twin receiver is in the process of installation in Russia, with a real-time data transfer between both receivers.
The GLONASS observables obtained from our monitoring receivers 3S_R101 (3S_Navigation, USA) and ISDE-16 (Institute of Space Device Engineering, Russia) are used for a real time evaluation of the observables quality and precision. The information, updated every hour with the latest observations, is available to external users in this WWW server.
Data from GLONASS and GPS are combined to obtain a precise evaluation of the delay introduced by the ionospheric Total Electron Content on the navigation observables. This information will be used to provide real-time corrections to differential GLONASS or GPS users.
The information service for GLONASS users provided by the DLR Neustrelitz Remote Sensing Ground Station.