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Issue #212 March 2008
Problem 2—What is the relationship, if any, between the GPS master clock and the GPS microwave carrier frequencies L1 and L2? Why are two different frequencies used?
Answer 2—The L1 carrier is 1,575.42 MHz, which is exactly 154 times the GPS master clock rate of 10.23 MHz.
The L2 carrier is 1,227.60 MHz, or 120 times the master clock.
Two frequencies are used so that receivers can make estimates of the bending effects of the ionosphere, which allows them to make corrections to their time-of-flight measurements. Both carriers are modulated with the coarse acquisition (C/A) signal.
Although the L1 and L2 satisfy all of the original goals of the GPS design, additional carrier frequencies are being investigated for newer functionality: L3 is at 1,381.05 MHz (135 × 10.23 MHz), L4 is at 1,841.40 MHz (180 × 10.23 MHz), and L5 is at 1,176.45 MHz (115 × 10.23 MHz).
Also, high-resolution GPS receivers can lock directly onto the carrier frequencies in order to establish their position more accurately. The carrier wavelength is just 19 or 24 cm, while the C/A “chip” wavelength (at 1.023 MHz) is 293 m. Such receivers can establish absolute position to within a few cm and make relative position measurements to a fraction of 1 cm.
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