Russian scientists and engineers are at work on a new code-division multiple-access signal format to be broadcast on a new GLONASS L3 signal. Taking an approach similar to that implemented on the newly designed GPS L5 signal, this will, once implemented across the constellation by new satellite launches, facilitate interoperability with and even eventually interchangeability among other GNSS signals, including of course GPS.
An article in the November issue of GPS World, authored by Alexander Povalyaev, the deputy head of division in JSC Russian Space Systems and a professor at the Moscow Aviation Institute, will give an outline and provide some details on a new flexible navigation message format proposed for use in the GLONASS CDMA signal under development. The format allows for relatively easy upgrades in the navigation message, if required.
Navigation messages developed and broadcast so far, by both GPS and GLONASS, are fixed, regular structures including pages (frames), subframes (rows), and words. Despite their simplicity, “such structures are very conservative indeed,” says Professor Povalyaev. The only possibility to update such navigation messages is restricted to the use of previously allocated backup frames. Increasing numbers of such frames make for ineffective use of navigation message transmission capacity. Conversely, the relatively small number of backup frames restricts the potential for future navigation message upgrades.
Prof. Povalyaev states that a comparison of data transmission via GLONASS and GPS, respectively, reveals that the data transmission rate in GLONASS is 5 times greater than in GPS. This explained by the higher redundancy of the GPS navigation message. In addition to approximately 11 percent of its subframes in backup, the GPS signal reserves fields for transmission of 32 satellite almanacs. As a result, Povalyaev believes that the GPS navigation-message transmission channel used inefficiently.
For GLONASS, the situation is different, with fewer backup bits in the navigation message, and fields reserved for transmission of only 24 satellite almanacs. This increases transmission channel efficiency but creates problems when it comes to updating the system, particularly in maintaining backward compatibility for previously manufactured user equipment. From this point of view, he says, a large number if backup frames in preferable.
He proposes a GLONASS navigation message with flexible row structure, as was used for the first time in the design of the GPS L5 signal. In this structure, the navigation message is formed as a variable row flow of different types. Each row type has a unique structure and contains specified information type, for example, ephemeris, almanacs of specific satellites, parameters of Earth pole movement models, parameters of ionospheric delay models, and so on. He goes on to describe how signal-processing disruptions in legacy user equipment can be avoided.
A flexible row structure of the navigation provides more effective use of transmission channel capacity. The main advantage of the flexible row structure is the possibility of its evolutional upgrade, meeting the requirements of backward compatibility.
Currently GLONASS uses signals with frequency separation in L1 (1592.9 – 1610 MHz) and L2 (1237.8 – 1256.8 MHz). The foreseen upgrade, already underway with one recently launched GLONASS satellite transmitting an L3 signal, will permit, in the long term, signals with code separation in L1, L2, and L3 (1190.35 – 1212.23 MHz).
Look for further details in the November issue of GPS World magazine.