Building a GPS System with Kaben’s Products and Technology
The Global Positioning System provides world wide geolocation for a user segment, through accurately synchronized transmissions from a network of satellites having accurately know orbits. Simplistically, since a user’s receiver will not be accurately synchronized with the satellites, a minimum of four satellite transmissions are required to determine a user’s latitude, longitude, altitude, and time offset.
At present, two GPS channels are used for geolocation; L1 at 1575.42 MHz, and L2 at 1.227.60 MHz. L1 is available to civilian users, through a C/A (Coarse / Acquisition) spread spectrum code, while both L1 and L2 are available to select military users, through a P (Y) (Precise Encrypted) faster spread spectrum code. The increased geolocation accuracy available to military users is achieved through the faster spread spectrum code, and through the ability to accurately compensate for ionospheric delay by receiving two channels, spaced in frequency, simultaneously.
In order to improve geolocation accuracy for the civilian user, auxiliary techniques have been introduced, such as the Wide Area Augmentation System and the Differential GPS system. In essence, these systems continuously measure the GPS location of precisely known ground-based receivers, and broadcast corrections to be used by other receivers, based on the measured location error.
Starting in 2008, a major improvement in geolocation accuracy will become available to civilian users, through the introduction of both an L1C and an L2C signal. For receivers receiving both L1C and L2C simultaneously, ionospheric delay estimation will no longer be needed, and exact correction for this delay will be available, yielding the auxiliary, improved accuracy techniques unnecessary. Further, by 2011, a new GPS channel, L5 at 1176.45 MHz will become operational for use as a civilian Safety-of-Life (SoL) system.
As a result, modern GPS receivers will not only need to receive multiple signals from 4 or more satellites (typically 12 to 20), but will also have to tune to at least three channel frequencies (L1, L2, and L5) simultaneously. The Kaben GPS receiver, based on its tunable, on-chip, SAW replacement filter capability, easily provides multiple satellite, multiple channel frequency front end reception for GPS receivers.
The Kaben GPS receiver is shown in the diagram where the three L1, L2, and L3 channels are delivered in a band spanning 1176 MHz (L5) to 1575 MHz (L1). These three channels are applied to the first SIF, where, in addition to providing front end filtering, the inherent frequency conversion (due to sampling decimation) is also used to equalize, and narrow the frequency span between the individual bands. At the output of the first SIF, L2 appears at 72 MHz, L5 at 123 MHz, and L1 at 276 MHz. The three signals are then applied to the second SIF which applies highly selective, band-pass filtering about each, as well as frequency conversion to a low output frequency. The resulting three sampled-analog signals appear equally spaced in frequency: L1 at 51 MHz, L5 at 102 MHz, and L2 at 153 MHz. The three GPS signals are then passed collectively to the ADC.
