GLONASS Receivers

GLONASS
Title	GLONASS Receivers
Edited by	GMV
Level	Basic
Year of Publication	2011

A GLONASS Receiver is a L-band radio processor capable of solving the navigation equations in order to determine the user position, velocity and precise time (PVT), by processing the signal broadcasted by GLONASS satellites.

GLONASS Receivers

A GLONASS Receiver is a device capable of determining the user position, velocity and precise time (PVT) by processing the signal broadcasted by satellites. Any navigation solution provided by a GNSS Receiver is based on the computation of its distance to a set of satellites, by means of extracting the propagation time of the incoming signals traveling through space at the speed of light, according to the satellite and receiver local clocks.

Notice that satellites are always in motion, so previous to obtaining the navigation message, the satellite’s signal is detected and tracked. The receiver’s functional blocks that perform these tasks are the antenna, the front-end and the baseband signal processing (in charge of acquiring and tracking the signal).

Once the signal is acquired and tracked, the receiver application decodes the navigation message and estimates the user position. The Navigation Message includes:

• Ephemeris parameters, needed to compute the satellite’s coordinates
• Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions
• Service Parameters with satellite health information
• Almanacs, needed for the acquisition of the signal by the receiver. It allows computing the position of all satellites but with a lower accuracy than the ephemeris

The ephemeris and clocks parameters are usually updated every half-an-hour, whereas the almanac is updated at least every six days.

For more information, please refer to article GLONASS General Introduction and GLONASS Interface Control Document which specifies parameters of interface between GLONASS space segment and user equipment in L1 and L2 Bands.

Particularities

Each GNSS system uses a specific Reference Frame; although a multi-constellation receiver is able to convert all information to the same common frame, a GLONASS-only receiver uses the GLONASS PZ-90 reference frame. In an analogous way, each system has its own time reference defined by the respective control segments; the time reference for GLONASS is called “GLONASS Time” (GLONASST).

Each GNSS System transmits its own navigation message, defined in the respective Signal In Space Interface Control Documents, SIS ICD. As an example, the satellites transmit information that allows the receiver compute their positions. For the case of GLONASS (unlike GPS and Galileo), the satellites transmit their position and velocity at a given time epoch. The GLONASS receiver then computes the satellite position based on these ephemeris parameters.

Another distinction regarding the transmitted navigation message with impact on the receiver is the ionospheric parameters transmitted to support the single frequency receiver in computing the ionospheric error; although GLONASS does not transmit this information, the receiver could re-use the parameters transmitted by GPS or Galileo systems and use them as applicable.

GNSS signals modulation, structure, navigation message contents and formats are often different among signals from the same system and from different systems. Most of these characteristics are easily implemented at the receiver (e.g. requiring only “software modifications”, such as the use of different PRN codes or the ability to cope with different message structures). The main difference among GNSS receivers falls into the specific characteristics that have impact at RF level, such as the Multiple Access Techniques employed. GLONASS employs FDMA techniques, making the receiver a little more complex at RF level ( than Galileo and GPS) in order to be able to cope with the different carrier frequencies – one for each satellite.

It should be noted that the current trend consists on facilitating the access of each system to the receivers, i.e. fomenting multi-constellation receivers. Hence, most discussions and agreements among the systems’ responsibles are conducted in the sense of taking this effort out of the user segment, focusing on compatibility and interoperability aspects in the system design.

Civil/Commercial use

Comparing to GPS system, GLONASS use in civil/commercial applications is rare. One of the main differences between GPS and GLONASS is that the former uses CDMA technique to separate the satellites while the later uses FDMA technique. The main impact at receiver level is that GLONASS receivers are in general more expensive since they require higher IF bandwidths and hence they need more complex hardware. The migration of GLONASS system towards CDMA techniques may reduce the cost at receiver level.

To improve this situation, the Russian government has been actively promoting GLONASS for civilian use. In 2011, the government announced that all passenger cars, large transport vehicles and vehicles transporting dangerous materials would be required to use GLONASS-equipped navigators. The tracking of this road traffic will be tied to road tax collection as well as to a roadside assistance in the event of an accident. In addition, the government has been pushing for all car manufacturers in Russia to make cars with GLONASS since 2011. This affects all car makers, including foreign brands like Ford and Toyota, which have car assembling facilities in Russia.

Commercial response to GLONASS improved accuracy is gaining momentum, and many GNSS Receivers manufacturers already have in their product offer GPS+GLONASS receivers:

• Qualcomm has announced the first GLONASS capable phone (MTS 945 from ZTE): "ZTE is first to market with a smartphone that supports both the GPS and GLONASS satellite systems, taking full advantage of the functionality which has been integrated into our Snapdragon MSM7x30 chipset and software”.
• In February 2011, ST-Ericsson launched “the world’s smallest receiver” to connect to both GPS and GLONASS satellites.
• Broadcom Corporation, a global leader in semiconductors for wired and wireless communications, announced two new GPS system-on-a-chip solutions that include support for the GLONASS Russian Navigation Satellite System.
• In April 2011, Sweden’s Swepos became the first foreign company to use Russia’s GLONASS positioning technology, due to Swepos’ conviction that it is better than GPS at northern latitudes.

Related articles

• GNSS Receivers General Introduction
• Generic Receiver Description
• System Design Details
• Receiver Characteristics
• Interfaces and Protocols