The world’s GNSS systems are entering a phase of transformation — modernization of existing systems (the U.S. Global Positioning System and Russia’s GLONASS) and development of new systems (China’s BeiDou and Europe’s Galileo) that benefit from the lessons learned from the original GNSSs.
Notable among the modernization initiatives is an interest in implementing new satellite signal designs. These include the GPS L5, L2C, and L1C signals as well as those signals designed for Galileo and BeiDou. GLONASS designers are also working on modernized signals.
In many cases, these new signals adopt or build on features designed and being introduced as part of the modernization of GPS, the first and still most widely used GNSS. Among others, these include such innovations as higher transmit power to improve reception under challenging conditions, longer codes for a better cross-correlation between satellites signals, data-less pilot channels that facilitate long integrations and improve the sensitivity threshold, and secondary codes — short pseudorandom noise (PRN) codes to simplify the data synchronization.
A unique cooperative agreement signed in 2004 between the United States and the European Union calls for common use of a binary offset carrier (BOC) modulation at 1575.42 MHz. Under this agreement, Galileo and GPS system operators (the European Space Agency and the U.S. Air Force, respectively) are implementing two different versions of a multiplexed BOC(6,1,1/11) signal.
Although currently operating as a regional system, the Phase III plan for the BeiDou B1 civil signal also calls for shifting to the L1 frequency centered at 1575.42 MHz and transmitting a multiplex binary offset carrier (MBOC 6,1,1/11) modulation similar to the modernized GPS civil signal (L1C) and the Galileo L1 Open Service signal.
GLONASS says it will introduce CDMA signals at 1575.42 MHz, which has emerged as the common frequency for current and future civil signals, in place of the frequency division multiple access signals currently transmitted at higher frequencies. But the new signals’ specifications, including such parameters as data rate and signal structure, are still under development.
We asked A. J. Van Dierendonck, one of the pioneers in GPS system development with 40 years in the satellite navigation field, to comment on some of the innovations seen in these new signals. Dr. Van Dierendonck is a codeveloper of the L5 signal structure that will be carried by the GPS spacecraft beginning with the Block IIFs now being launched. He also participates in the US/ EU bilateral discussions that take place under the auspices of the 2004 agreement.