An Update on GLONASS: Those of us who use satellite receivers for surveying can purchase a pair of receivers that process signals from GPS satellites only or from both GPS and GLONASS constellations.

Galileo satellite receivers are at least three to five years away, but they will be compatible with GPS. For people who manufacture GPS receivers for hikers, emergency response personnel, etc., acquiring a multiple-constellation receiver is not as easy.

I received an E-mail magazine a few months ago that contained an article on “GLONASS for Civilians.” The article stated that Russia’s global satellite navigation system will become available to civilians in that country starting in January 2007, and will be offered to commercial customers nationwide (in Russia) in 2009. Until now, GLONASS had restrictions that prevented civilians from using the system.

What is the issue with GLONASS that has prevented the civilian community (consumers and commercial users not involved in surveying and similar applications) from having access to the system? Signal structure. The following are brief descriptions of the signal structure for both GPS and GLONASS:

  • GPS transmits different codes on the same frequency, referred to as the code division multiple access (CDMA) signals. Each of the signals transmitted by the different satellites in the L1 band has a unique pseudorandom noise (PRN) code.
  • GLONASS transmits a common code on different frequencies, referred to as frequency division multiple access (FDMA) technology.

In the surveying market, Topcon Positioning Systems (Livermore, Calif.) has manufactured GPS/GLONASS receivers for many years, and other manu-facturers of carrier-phase receivers now offer this option. The difference between FDMA and CDMA signals increases the technical difficulties in manufacturing, but with 14 GLONASS satellites combined with 31 GPS satellites, it is feasible to produce such a receiver.

Russia is now considering adding CDMA signals on the third frequency of GLONASS K satellites and at L1 on GLONASS M satellites. The first GLONASS K satellite is scheduled to be launched in 2009. The words used in the press release are that the system will “probably” be able to implement CDMA signals. Although Russia will lift restrictions, 18 satellites are necessary to provide navigation services over all of Russia. By 2009, it will have 24 satellites, which are needed for worldwide service. But there are problems; frozen fuel lines have delayed activation of two satellites that were launched in December 2005.



    Image courtesy of Lockheed Martin.

    Future GPS Launches Block IIR-M Satellites

    There are now three Block IIR-M GPS satellites on orbit. These are satellites with the L2C signal, the second civil signal on the L2 carrier. They are the following:

    • PRN 17, in slot C4, launched September 26, 2005.
    • PRN 31, in slot A2, launched September 25, 2006.
    • PRN 12, in slot B5, launched November 17, 2006.

    There were 10 Block IIR satellites launched before the decision was made to add the L2C signal (11 Block IIR satellites launched, but the first launch failed to reach orbit). Seven IIR-M satellites are to be launched over the next three years. GPS receiver manufacturers have developed receivers capable of positioning with L2C pseudoranges, but with only three satellites on orbit it will be a year or so before three or more satellites are visible at the same time. (In my June column, I will discuss the three L2C satellites in more detail and include graphics, using mission planning software, to show their location.)



    Block IIF Satellites

    It has been announced that the launch date for the first Block IIF GPS satellite will be between May 2008 and January 2009; Boeing is the prime contractor. Block IIF satellites will have a third civil frequency, L5, added to what is available on Block IIR-M satellites. These satellites will be positioned in slots in the six orbital planes of Block II.


    GPS III

    GPS III satellites are the latest satellites on the drawing board; the first planned launch is 2014. This will be a new constellation in three orbital planes. At this time, the major change reported will be a new civil signal, L1C, to replace the C/A code on Block II satellites.

    With all the changes coming in the satellite geodesy arena, I should be able to get enough information to make this column thought-provoking for the next few years.