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New GNSS Frequencies, Advantages of M-Code, and the Benefits of a Solitary Galileo Satellite

“GNSS Solutions” is a regular column featuring questions and answers about technical aspects of GNSS. Readers are invited to send their questions to the columnists, Professor Gérard Lachapelle (lachapel@geomatics.ucalgary.ca) and Dr. Mark Petovello (mpetovello@geomatics.ucalgary.ca), Department of Geomatics Engineering, University of Calgary, who will find experts to answer them.

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Q: What are the major differences between Galileo and GPS current and forthcoming frequencies?

A: Galileo has been designed to be both independent and interoperable with other GNSSes, and particularly GPS. The search for interoperability makes Galileo look like GPS, while the desire of independence of both systems has the opposite effect.

As Prof. Günter Hein summarized in a previous “Working Papers” column in Inside GNSS, the degree of interoperability between the two systems will be a function of their compatibility with each other (and other GNSSes), the simplicity of the user segment, economic aspects, their independence, national security, and the vulnerability of a combined PVT (position, velocity, and time) solution.

The current frequency plan for Galileo and modernized GPS (GPS IIRM, IIF, and III) completely reflects this dual aspect of the two systems. At first sight, the choice of multiple carriers, of the frequency bands, of some central frequencies, and of the modulations — bi-phase shift key (BPSK) and binary offset carrier (BOC) — indicates a similar system structure. However, a closer look reveals some major differences in the frequency occupation (carrier frequencies, bandwidth, spectrum shape, interplexed signals) of both systems.

(For the rest of Olivier Julien and Christophe Macabiau’s answer to this question, please download the complete article using the PDF link above.)

Q: What are the major characteristics (improvements) of M-code relative to (over) the existing P-code?

A: The GPS M-code signal design began in 1997 and was concluded in 2001. The M-code signal was first broadcast from the GPS Block IIR-14(M) satellite that was launched on September 25, 2005.

All future GPS satellites will transmit M-code as well as the P(Y)-code signal, which is being retained for use by currently fielded military receivers. M-code is an integral part of GPS modernization and the key enabler for Defense Department’s Navigation Warfare program.

Although the proven capability of GPS’s P(Y)-code signal is impressive, the M-code signal offers essential improvements for warfighters of the future. The single most important characteristic of the M-code signal is its spectral separation from civil signals in the GPS L1 and L2 bands. This separation is achieved through the use of M-code’s binary offset carrier BOC(10,5) spreading modulation.

(For the rest of Col. Richard L. Reaser’s answer to this question, please download the complete article using the PDF link above.)

Q: A Galileo test satellite was recently launched. What information can be gathered from a single satellite?

A: The GIOVE-A (Galileo In-Orbit Validation Element – A) satellite was launched on December 28, 2005. Its main mission objectives are securing its frequency filing with the International Telecommunications Union, critical testing of payload elements, and detailed assessment of the receiver performance and environmental effects (multipath and robustness to interference).

GIOVE-A is transmitting test signals in all the three Galileo frequency bands: L1, E6, E5. However, only the signals in two frequency bands at a time are transmitted by GIOVE-A. The second satellite, GIOVE-B (to be launched later in 2006), shall be able to transmit in all three frequency bands simultaneously. All the Galileo modulations foreseen for major Galileo services (Open Service, Public Regulated Service, Commercial Service, Safety-Of-Life Service) shall be transmitted by GIOVE-A for the purpose of testing and validation of the signals.

(For the rest of Dr. Andrew Simsky’s answer to this question, please download the complete article using the PDF link above.)

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