Electronic devices communicate by sending/receiving electromagnetic waves, called signals. Electronic signals transmitted by an electronic device travel through space arriving at all receivers available within their range. Then, how is it possible that so many electronic devices transmitting so many kinds of signals at the same time and receivers are able to hear only the ones that want to hear? Signal frequency is the answer to this question. In simple terms frequency is the number of waves that pass a fixed place in a given amount of time. The common way to measure frequency is to count the number of waves that occur in a second. The associated unit is hertz (abbreviated Hz) named after German physicist Heinrich Rudolf Hertz. Antennas are used to transmit and receive electronic signals at a certain frequency. Current technology allows a signal antenna to operate (transmit or receive) at multiple frequencies at different fractions of time.

Global Navigation Satellite Systems (GNSS) are no exception. Like all other electronic devices, GNSS systems also communicate through electronic signals, i.e., they transmit signals via satellites at certain frequencies and receivers with antennas tuned to those frequencies can collect those signals to process and determine position accurately. Following the successful deployment and operation of the pioneering Global Positioning System (GPS) by the US, other GNSS satellite constellations have been placed by other nations. Each GNSS system is designed to transmit signals at multiple frequencies to ensure reception. Available GNSS systems are listed below with corresponding signal names and broadcast frequencies.

Figure 1: Determining position using radio signals (c: speed of light).