All GNSS systems have to operate within specific parts of the radio spectrum allocated to them. The International Telecommunication Union (ITU) allocates the total share of radio navigation to the Radio Navigation Satellite Service. When the signals are used by the aviation community, the frequency band should also be shared on a co-primary basis with the Aeronautical Radio Navigation Service (ARNS). Allocation of GNSS frequency bands is complex since multiple services and users can coexist in the same range.
The International Telecommunications Union (ITU) is a United Nations agency coordinating the shared global use of the radio spectrum. It involves a wide variety of spectrum including those for television, radio, cell-phone, radar satellite broadcasting, etc., and even microwave ovens. ITU has been working in the allocation of the radio-frequency bands used by the radio navigation satellite services. Allocation agreements were output from the World Radio Communication Conferences in 2000 and 2003.
GNSS frequency bands
GNSS frequency bands in the range of 1 to 2 GHz (known as L-band) are suitable for mobile satellite systems as they suffer from little rain loss and offer lower path losses. Thus, all GNSS use signals in the lower parts of L-band. It should be noted that only the L1/E1 and L5/E5 signals use frequencies allocated to the ARNS.
The following figure shows the different GNSS frequency bands used by the different systems:
All GNSS satellites transmit navigation signals in two or more frequency bands. These signals contain ranging codes that allow receivers to compute their travelling time from satellite to receiver, along with navigation data, in order to know the satellite’s position at any time. The main signal characteristics are:
- Carrier frequency: Radio frequency sinusoidal signal at a given frequency.
- Ranging (or spreading) code: A pseudo random sequence of 0s and 1s that allows the receiver to measure the travel time of the signal from satellite to receiver. Often referred to as Pseudo-Random Noise (PRN) codes.
- Navigation data: A binary-coded message providing information on the satellite ephemeris, clock parameters, almanac, health status and other complementary information. Some signals, known as “pilot signals”, lack this component, thus offering better acquisition and tracking performances.
- A multiple access scheme is used to allow the harmonious sharing of a frequency band by several satellites. This technique is FDMA or Frequency Division Multiple Access for GLONASS, where satellites use a single spreading code but a different carrier frequency. For all other GNSS, the Code Division Multiple Access (CDMA) technique is used, with all satellites using the same carrier frequency but different spreading codes. Now, GLONASS is also working on CDMA signals for improved interoperability.
Users get access to GNSS services with one or multiple signals, which determine their position with a certain level of accuracy. Some services like the GPS “Standard Positioning Service” (SPS) or Galileo’s “Open Service” (OS) are availab for free to all users. All they need a suitable receiver to access those signals. Some others have a controlled access like the GPS “Precise Positioning Service” (PPS) or of GGNSS frequency bandalileo “Public Regulated Service” (PRS) which are restricted for governmental/military use. Galileo’s “Commercial Service” (CS), intended to provide enhanced performance to subscribed users, is also restricted and available on payment.