GPS-based time and frequency solutions

The Global Positioning System (GPS) constellation consists of 24 or more satellites located in six orbital planes at an altitude of about 12,550 miles. The satellites are synchronized by onboard atomic clocks and continuously transmit ranging signals, their current time, and orbital dynamics at two frequencies in the L band, called L1 (1575.42MHz) and L2 (1227.6MHz). Civilian GPS usually uses only the L1 band, while surveying and military purposes require the use of both the L1 and L2 bands.

The GPS constellation is designed to ensure that at any given time, at least four satellites can be seen from any location on Earth. Using the range measurements of the four GPS satellites and the transmitted satellite orbit information, a GPS receiver can accurately determine the position and time. This is done mathematically by finding the intersection of four or more spheres centered around four or more satellites.

The GPS position solution can be used to determine the current latitude, longitude, and altitude to within 5 meters (using the L1 band only), and the timing is accurate to within 10 ns. If the GPS receiver is in a fixed position, the position solution can be fixed and distance measurements can be used to further improve the accuracy of the time solution. Similarly, a fixed-position receiver can obtain a time solution as long as it can see a satellite, which may occur when the receiver's view of the sky is partially obscured.

The GPS system is synchronized to its own time scale, GPS Time, which is provided by the United States Naval Observatory (USNO). USNO operates two master clock facilities that provide accurate time for the GPS system, one in Washington, D.C. (USNO) and the other in Colorado Springs County (AMC). GPS Time is also synchronized to Coordinated Universal Time (UTC), the international time standard. However, unlike UTC, which is corrected using leap seconds to compensate for changes in the Earth's rotation, GPS Time is a continuous time scale that does not require such corrections.

There are no time jumps in the GPS time scale, which simplifies continuous time and position solutions in GPS receivers. In 1980, UTC and GPS time were aligned. Since then, however, UTC has been periodically corrected by adding leap seconds, so GPS time is now 15 seconds ahead of UTC. The data broadcast by GPS satellites contains the UTC correction parameters and predictions of future leap seconds. This allows GPS receivers to calculate UTC time from GPS time.

GPS receivers output satellite information and time/position solutions in a standard format. The most common formats are defined in the NMEA0183 or NMEA2000 specifications. These specifications define sentences including visible satellites, satellite signal strength, GPS or UTC time solution, and position solution. Manufacturers may also define sentences to output data in a custom format or to provide access to special features of their GPS receivers.

GPS-based timing and synchronization solutions can be used as a time source or time reference, or as a simple time comparison method. In the past, high-end rack-mount systems were typically based on rubidium clock technology, which was a standard offering in this field, capable of maintaining extremely tight long-term stability characteristics with the accuracy of a master reference clock for weeks. Of course, high-end technology also comes with a high price. As you can imagine, a rack-mount unit of this type can cost thousands to tens of thousands of dollars.

However, now, thanks to advances in component design, manufacturing technology, and changing specifications, GPS-based time and synchronization solutions have become very popular in many different applications in a variety of different technology fields. From simple GPS timing receivers with low-volume prices of less than $75 to more complex GPS-based devices with excellent phase noise and holdover characteristics, priced at less than $500 each, GPS-based timing and synchronization are now available for any application.

The process begins by optimizing the timing requirements of the GPS receiver. The GPS timing output signal, or 1 pps (pulses per second), is typically accurate to nanoseconds or milliseconds, depending on the implementation, and is a direct reflection of the GPS time provided by the satellites. Some GPS timing receivers can also generate an accurate frequency output from the PPS.

Phase

Accurate time is an integral component of a GPS solution. A GPS timing receiver can determine position to within 1m and time to within nanoseconds by solving trigonometric equations for position and time. By continuously solving the GPS equations, the GPS time scale can be reconstructed locally (at the receiver). Process noise events - deviations from the GPS time scale, manifested as phase (time) errors. Because the receiver's local clock is synchronized to the GPS time scale, the receiver is able to produce an accurate 1 pulse per second 1pps output, which is only a few nanoseconds away from the GPS time scale.

Keep

During periods of time when no GPS signal is received, the receiver enters holdover mode, where it generates timing outputs based on the last known GPS time and clock drift solution and the stability of the local clock. By storing the last known good time and clock drift solution, the stability of the 1pps and frequency outputs depends only on the stability of the local oscillator.

For stringent holdover requirements, the receiver's temperature controlled oscillator (TCXO) can be replaced with an oven controlled oscillator (OCXO) with the required stability. Upon resuming normal operation from holdover mode, the 1pps and frequency outputs revert to the GPS calculated solution.

Advanced GPSDO Solutions

As the need for more specification-sensitive applications becomes more important in the market, more accurate and lower-cost GPS-based timing and frequency devices are becoming more and more common. Based on the standard GPS timing receiver, the more complex GPSDO (GPS locked crystal oscillator) adds phase-locked loop circuits and higher stability oscillators to improve the phase and holdover performance of the device.

These devices come in many different sizes and form factors, from units with through-hole pins for PCB mounting to units that are packaged and have standard connectors for easy integration. GPSDOs typically range in price from $200 to $1,000, depending on their form factor and features.

Now, we have access to highly accurate time and frequency information at a fraction of the cost of what it once was. Users no longer have to purchase expensive and complex rack-mount systems because a well-designed GPS receiver with PPS and frequency outputs can better meet their needs. Here are some simple guidelines to consider when choosing a GPS timing/frequency solution:

1. Confirm that your GPSDO solution vendor fully supports the GPS receiver portion of the device. If the vendor obtains the GPS receiver portion of its solution from another company, it most likely does not have the necessary GPS experience to properly support its customers.

2. Confirm that your GPSDO solution supplier designs and develops the oscillators used in its equipment. Likewise, if it does not have specific oscillator experience, it may not be able to provide direct support for customers' technical needs.

3. Verify that your GPSDO vendor has a long history of providing these types of GPS-based timing and frequency solutions. Integrating the components of these devices into a single design is not a simple task. Complex filtering and GPS firmware design are integral to the product. There are always some suppliers who just throw their solutions and any available parts without a comprehensive understanding of their products or the applications they will be integrated into.

4. Make sure your GPSDO supplier is familiar with the characteristics of your application. Before purchasing any GPSDO equipment, discuss your application with the supplier. Through a simple technical conversation, you will be able to know whether the supplier can provide the right support for your needs.

5. Make sure you understand exactly what the timing and/or frequency control technical requirements are for your application. The clearer you are about your performance requirements, the better chance you have of getting a GPS solution that is perfect for your application. Companies often overstate their requirements and end up purchasing expensive systems that exceed their actual needs.