1. Field of the Invention
The invention relates generally to GPS receivers and more particularly to a GPS timing receiver for providing a GPS-derived time signal having a fine incremental time resolution.
2. Description of the Prior Art
The global positioning system (GPS) is commonly used by GPS timing receivers for deriving a GPS time signal. Typically, the GPS-derived time signal has the form of pulse train having one pulse per second, each pulse coincident or having a known offset with the start of each second of GPS time. One application for the GPS-derived time signal is calibrating an atomic clock. Before GPS was available it was sometimes required to transport the atomic clock to the location of a time standard such as the National Bureau of Standards in Colorado in order to perform the calibration. Another application of the GPS-derived time signal is providing a precise time signal for timing TDMA and CDMA receivers in wireless and satellite systems. An important figure of merit for a GPS timing receiver is the fineness of the time steps (incremental time resolution) of the GPS-derived time signal. A fine time step or small incremental time resolution is necessary in order to minimize the time difference between the time provided by the GPS-derived time signal and the actual GPS time.
Existing GPS timing receivers operate by downconverting the GPS satellite signal to a GPS intermediate frequency (IF) signal having a certain pre-detection information bandwidth and then sampling the GPS IF signal with a clock signal. The sampled GPS IF signal is then correlated with an internally generated GPS replica signal to obtain correlation data. The time, repetition rate, and frequency of the GPS replica signal are derived from the clock signal and then adjusted by the correlation data to drive the GPS replica signal to correlate to the time, repetition rate, and frequency of the GPS IF signal. It is desirable for the GPS IF signal to have a narrow pre-detection information bandwidth in order to minimize jamming from spurious signals that are close in frequency to the GPS satellite signal and to achieve the best signal-to-noise ratio. The clock signal must have a frequency at least as high as the Nyquist sampling rule rate of two times the information bandwidth or information will be lost in the sampling process. In order to minimize cost and power consumption, the frequency of the clock signal is typically as low as practical above the Nyquist rate. The GPS-derived time signal is then generated from the clock signal and information in the correlation data using one of several known techniques.
In one technique, the GPS derived time signal is generated by frequency dividing the clock signal to the desired frequency and then delaying the divided signal in an analog delay line having several output taps. A digital signal representing the correlation data for the time difference between the GPS IF signal and the GPS replica signal then selects the output tap that minimizes the time difference. A fine incremental time resolution is obtained by having closely spaced output taps. This technique was used in a GPS receiver model "5000A Timer" that was commercially available from Trimble Navigation of Sunnyvale, Calif. beginning in 1983. However, a disadvantage of this technique is that a tapped analog delay line is required in order to obtain fine time resolution.
Another technique used in existing GPS receivers uses a voltage controlled oscillator (VCO) having a small tuning range for providing the clock signal. A digital signal representing the correlation data for the frequency difference between the GPS IF signal and the GPS replica signal is converted to an analog signal in a digital-to-analog converter (DAC) which is then used to drive the VCO to adjust the time and frequency of the clock signal so that the time, repetition rate, and frequency of the GPS replica signal matches the GPS IF signal. The clock signal is then frequency divided to get the desired frequency for the GPS-derived time signal. An example of such a GPS receiver is a model "4000S Surveyor" that was commercially available from Trimble Navigation beginning in 1987. A disadvantage of this technique is that absolute time can be lost in frequency dividing the clock signal. A further disadvantage is that the frequency resolution and accuracy of the GPS-derived time signal is limited by the resolution and accuracy of the DAC. Typically, the DAC is required to be monotonic with a resolution of more than sixteen bits. Such DACs are expensive.
In another technique, the GPS-derived time signal is obtained by frequency dividing the clock signal by a divide number that is dynamically adjusted according to the correlation data for the time difference between the GPS IF signal and the GPS replica signal. A disadvantage of this technique is that dynamically adjusting the divide number creates phase noise or jitter in the GPS-derived time signal. In yet another technique the GPS-derived time signal is obtained by frequency dividing the clock signal by a divide number that is fixed. The frequency divided signal is then digitally delayed in a circuit such as a shift register that is clocked by the clock signal. The incremental time resolution is determined by the period of the clock signal. A fine time resolution is obtained by having a high frequency clock signal. However, GPS receivers using this technique typically suffer from relatively poor jamming immunity and poor signal-to-noise ratio due to having a wide pre-detection information bandwidth corresponding to the high frequency clock signal.
All of the existing or known techniques for generating the GPS-derived time signal having a fine incremental time resolution suffer from one or several disadvantages. What is needed is a GPS timing receiver having a narrow pre-detection information bandwidth and providing a fine incremental time resolution without resorting to an analog delay line.