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A GPS Frequency and Time Standard

Global Posistioning System (GPS) frequency and time standards that represent a significant advantage in time keeping and frequency reference technology

March 1, 1998
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A GPS Frequency and Time Standard

Quartzlock UK Ltd.
Totnes, Devon, UK

The Global Positioning System (GPS) is used widely to determine location. If at least four of the system's satellites are in view (typically six or seven are in view at one time), position can be determined in four dimensions, including longitude, latitude, altitude and time. Because the GPS operates on the principle of precise time measurement, the satellites are equipped with an extremely accurate time source provided by onboard atomic clocks that are kept in time by ground monitoring and control stations.

This highly accurate time signal provides the time reference for the model 8 series GPS frequency and time standards. These high stability oscillators provide a reasonably priced, highly accurate reference for frequency calibration purposes in the laboratory and are suitable for use as a standard for time and frequency traceable to the UK National Physical Laboratory's (NPL) Universal Time Coordinated (UTC) time standard.

Error Sources

A number of technical challenges are associated with using the GPS satellite signals to derive an accurate frequency source. The most significant short-term error source is selective availability (SA). The SA method is used by the US military to deliberately downgrade the accuracy of the transmitted signal to limit the usability of the signal by an enemy or terrorist group. In addition, the reference signal transmitted by the satellites for civilian use (the coarse acquisition (C/A) code) provides only the standard position service, which produces UTC time data accurate to within 340 ns 95 percent of the time. In practical terms, SA presents a problem when frequency measurement accuracy is required to be greater than one part in 10E9. However, it has been determined from experimental data that the signal downgrading due to SA tends to zero over time. The mean difference between any two random readings separated by n seconds increases with n up to four minutes only, after which time remains constant indefinitely. Hence, it is relatively easy for this instrument to provide excellent long-term results over periods of 100,000 seconds or more.

For practical laboratory work as a frequency calibration reference, short- and medium-term accuracy is more useful but more difficult to achieve. The NPL found an extremely wide variation (over a factor of 100) in performance of GPS-based frequency standards in the medium term (between 100 and 10,000 seconds). However, accuracy as good as a few parts in 10E13 is achievable.

Short- and Medium-term Accuracy

Short- and medium-term accuracy depends on the design of the receiver and the accuracy of the LO. Model 8 uses a basic crystal oscillator, models 8A and 8A+ incorporate an oven-controlled crystal oscillator (OCXO) and model 8A-Rb incorporates a rubidium oscillator. All models exhibit particularly good medium-term performance and provide a 1 pps output as a time reference and a 10 MHz output for frequency calibration purposes.

To attain good performance, the standards employ several unique techniques. Instead of relying strictly on the C/A code, the instruments track both the frequency and phase of the carrier signal transmitted by the satellites, yielding good short-term accuracy. At 1.57542 GHz, the carrier provides a time and frequency resolution 10,000-times better than the C/A code alone.

A local reference crystal oscillator operates at 10.23 MHz, a frequency selected because it is a submultiple of the carrier frequency and a multiple of the pseudorandom number (PRN) code chipping frequency. The receiver locks this LO to the GPS time by first using its processor to control a 10 MHz signal derived from the local crystal or rubidium source and phase locking the 10.23 MHz oscillator to that signal.

The downconverter contains two intermediate stages operating at 102.3 MHz and 10.23 MHz. The satellite transmissions are more difficult to jam because the transmitted spectrum is spread over a broad bandwidth by modulating with a PRN code, making the signal's bandwidth approximately 2 MHz. A regenerated PRN code derived by a state machine that is driven by the onboard 68000 processor is cross correlated with this signal to derive the desired narrowband signal.

This resulting signal still suffers from Doppler shift due to the motion of the satellites. The Doppler shift is removed in a single-sideband mixer using a correction generated by the onboard processor. This corrected 10.23 MHz signal contains the C/A data only and is converted directly to a baseband signal composed of two quadrature components. When the LO is synchronized to the GPS carrier frequency, one of these components is zero and the other is ±1, according to the polarity of the C/A data stream. An analog/digital converter feeds the two components to the processor, which adjusts the LO until it is synchronized.

This process ensures an accurate short-term reference. However, this reference is still affected by SA. Medium-term stability is achieved and SA is overcome as a result of the performance of the LO, particularly in the case of the rubidium model and the choice of an appropriate algorithm for the control loop.

Configuration and Performance

The model 8 series frequency and time standards yield a frequency resolution that is better than the C/A-code-only detection by a factor of 10,000 using high resolution carrier phase measurements of the satellite signals. This capability enables the instruments to detect any LO frequency excursions almost instantly and allows for fast correction such that, even with a low cost crystal, the short-term stability is well controlled. These standard oscillators yield performance rivaling expensive multichannel receivers at an affordable price.

The instruments are able to track all the satellites that are in view, minimizing errors due to any single satellite. All of the satellites' user range accuracies are taken into account during the course of averaging, a process that reduces the SA effects considerably.

Figure 1 shows the standard receiver's simplified block diagram. The units utilize standard low cost, readily available components, ensuring ease of field repair.

The model 8 series standards are available with stability specifications of less than 1 x 10-12 to 3.5 x 10-13 from 100 to 300 seconds and 1.8 x 10-10 to 1.9 x 10-12 for 10 seconds. These specifications improve with the models 8A (OCXO) and 8A-Rb (rubidium oscillator). Standard outputs are 0.1, 1, 5 and 10 MHz (2.5 V into 50 W ) with 2048 kHz and 13 MHz (+13 dBm into 50 W ) as options. A 1 pps UTC sync output also is standard. Low distortion sine-wave and TTL-compatible outputs are offered.

The units are available in bench-top, rack-mounted and Euro-cassette configurations. A serial interface (RS232) is provided on the model 8A at 4800 or 19,200 baud via a nine-way D-type connector for connection to an IBM-compatible PC.

Applications for the frequency and time standards include use as laboratory standards for calibration, telecommunications synchronization, and cellular and personal communications network base station commissioning. In addition, the units can be used for timescale correction to UTC, time transfer applications and radio transmitter frequency referencing.

Conclusion

The model 8 series GPS frequency and time standards represent a significant advancement in time keeping and frequency reference technology. The instruments offer performance that is comparable to more expensive units. Additional information is available at the company's Web site at http://www.quartzlock.com.

Quartzlock UK Ltd.,
Totnes, Devon, UK
+44 (0)1803 862062.

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