What Are GPS Disciplined Oscillators (GPSDO)?

GPS disciplined oscillators (GPSDO) are some of today's most trusted and accurate sources of timing. These powerful devices (sometimes called GPS clocks) consist of a high-quality stable oscillator and a GPS receiver. The GPSDO works by disciplining (or steering) the oscillator output to a GPS device or GNSS satellite signal via a tracking loop.

GPSDO timing signals are accurate up to nanoseconds and are capable of generating frequency accuracies and stabilities up to parts per trillion. This makes GPSDOs very useful because satellite time signals must be very precise to provide positional accuracy for successful GPS and assured PNT navigation. 

How Does a GPSDO Work?

The disciplining mechanism in a GPSDO works in a similar way to a phase-locked loop (PLL). Some GPSDOs use PLLs; however, many are replaced with a microcontroller to compensate for frequency, temperature, and other environmental parameters.

The real timing magic of a GPSDO happens because of the combined efforts of the GPS signal and the quartz crystal oscillator.

GPS receivers have excellent long-term stability (characterized by Allan deviation). But their short-term stability is often degraded by many factors, including:

• The internal resolution of the one pulse per second (1PPS) reference timing circuits
• Signal propagation effects (such as multipath interference)
• Atmospheric conditions
• Other impairments

This is where the high-quality crystal oscillator (often an oven controlled crystal oscillator or OCXO) comes in. OCXOs offer better short-term stability, but are often exposed to thermal, aging, and other longer-term effects.

The combined efforts of the GPS receiver and the OCXO make a GPSDO an excellence source for overall frequency stability.

Related: GPSDOs for CubeSats & Other SmallSats: GPS Disciplined Oscillator Design & Architecture

What's the Difference Between GPSDOs and NCOs?

GPSDOs typically phase-align the internal flywheel oscillator to the GPS signal by using dividers to generate a 1PPS signal from the reference oscillator. 

Then, this 1PPS signal is compared to the GPS-generated 1PPS signal and uses the phase differences to control the local oscillator frequency in small adjustments.

This is the main difference between GPSDOs and numerically controlled oscillators (NCOs). 

Rather than disciplining an oscillator by frequency adjustments, NCOs typically use a free-running, low-cost crystal oscillator. They also adjust the output phase by digitally lengthening or shortening it many times per second in large phase steps. This ensures that the number of phase transitions per second (on average) is aligned to the GPS receiver reference source.

With that said, GPSDOs guarantee frequency accuracy... even in the presence of high phase noise and jitter. 

Related: Assured Positioning, Navigation, and Timing (PNT)

What Happens When a GPS Signal Becomes Unavailable?

When a reliable GPS signal becomes unavailable, the GPSDO goes into holdover mode. Holdover mode is where the GPSDO tries to maintain accurate timing using only the internal crystal oscillator. (This is why it's important to choose a high-quality oscillator from a trusted supplier). 

Sophisticated algorithms are used to compensate for the aging and temperature stability of the oscillator while the GPSDO is in holdover.

Related: Assured PNT: Going Beyond GPS Timing

Common GPSDO Applications

GPS disciplined oscillators can be used in a variety of applications. For starters, GPSDOs are used as the basis for Coordinated Universal Time (UTC) around the world. UTC is the official accepted standard for time and frequency.

GPSDOs are also used to provide synchronization to wireless base stations and serve well in standards laboratories as an alternative to cesium-based references.

Other common applications for GPSDOs include:

• Assured positioning, navigation, and timing (PNT) applications like passive radar
• Critical infrastructure
• Cube satellites and LEO satellite communication

Related: GPSDOs vs. Atomic Clocks: Which Work Best for LEO, MEO, & GEO Satellites? 

The Latest in GPSDO Technology

Bliley Technologies has applied our 85+ years of frequency control experience to make Atlas 1PPS one of the best performing GPSDOs on the market today. We also offer a variety of other great GPS disciplining solutions.