An increasing number of applications and systems for communicating information nowadays makes use of the spread spectrum technique. The spread spectrum technique is a digital modulation technique in which a digital signal is spread over a wide frequency band so that it has a noise-like spectrum. This is done by breaking up each data bit of the digital signal into multiple sub-bits (commonly called chips or Pseudo Noise (PN) code bits) that are then modulated and up-converted to a carrier frequency. By using orthogonal codes for different communication links, the same frequency band can be used for different simultaneous communication links. Using the same PN code as the transmitter, a receiver can correlate the received, spread signal and reconstruct the data signal, while other receivers that use other codes or other transmission techniques cannot. One of the advantages of using the spread spectrum communication technique is the robustness to narrow band interference signals. Because spread-spectrum receivers are rapidly introduced in applications and systems meant for the consumer markets, the cost of the receiver system is a major determining factor in order to remain competitive.
A specific class of spread spectrum systems are devices and receivers for position determination. Such devices are gaining importance for both the consumer market and for high precision applications. Most of the existing systems nowadays are based on the American Global Positioning System (GPS) system. Because this is a military system, precise position determination can be at any moment made impossible by the satellite operator via deliberately introducing errors. Moreover, in many areas the number of visible satellites can be too limited to determine an accurate position. These two problems can be reduced by also using a second positioning system such as the Russian Global Orbiting Navigation Satellite System (GLONASS) system.
Combined GPS and GLONASS receivers have been reported. S. Riley, N. Howard, E. Aardoom, R. Daly, and P. Silvestrin, in "A combined GPS/GLONASS high precision receiver for space applications", ION-GPS 95, Palm Springs, USA, September 1995, disclose a prototype multichannel combined GPS/GLONASS receiver capable of simultaneously tracking the Course Acquisition and Precision codes. This receiver separates the GPS and GLONASS signals on two different chains of hardware components, one chain for the GPS signals, one chain for the GLONASS signals. Japanese patent application, JP7128423-950519, "Receiver Common to GPS and GLONASS" discloses a device that receives both GPS and GLONASS signals. These signals are converted into first IF signals by an image reject mixer. The oscillation frequency of a local oscillator is set to be at the middle between the GPS carrier frequency and the GLONASS carrier frequency. This receiver separates the GPS and GLONASS signals on two different chains of hardware components, one chain for the GPS signals, one chain for the GLONASS signals. These prior-art implementations of combined GPS/GLONASS receivers are complicated and hence costly.
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