GPSDO
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A GPS clock, or GPS disciplined oscillator (GPSDO), is a combination of a
GPS The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide geol ...
receiver and a high-quality, stable oscillator such as a
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
or rubidium oscillator whose output is controlled to agree with the signals broadcast by GPS or other
GNSS A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are op ...
satellites. GPSDOs work well as a source of timing because the satellite time signals must be accurate in order to provide positional accuracy for GPS in navigation. These signals are accurate to nanoseconds and provide a good reference for timing applications.


Applications

GPSDOs serve as an indispensable source of timing in a range of applications, and some technology applications would not be practical without them. GPSDOs are used as the basis for
Coordinated Universal Time Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communicat ...
(UTC) around the world. UTC is the official accepted standard for time and frequency. UTC is controlled by the International Bureau of Weights and Measures ( BIPM). Timing centers around the world use GPS to align their own time scales to UTC. GPS based standards are used to provide synchronization to wireless base stations and serve well in standards laboratories as an alternative to cesium-based references. GPSDOs can be used to provide synchronization of multiple RF receivers, allowing for RF phase coherent operation among the receivers and applications, such as
passive radar Passive radar (also referred to as parasitic radar, passive coherent location, passive surveillance, and passive covert radar) is a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illu ...
and
ionosonde An ionosonde, or chirpsounder, is a special radar for the examination of the ionosphere. The basic ionosonde technology was invented in 1925 by Gregory Breit and Merle A. Tuve and further developed in the late 1920s by a number of prominent phys ...
s.


Operation

A GPSDO works by disciplining, or steering a high quality
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
or rubidium oscillator by locking the output to a GPS signal via a tracking loop. The disciplining mechanism works in a similar way to a
phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
(PLL), but in most GPSDOs the loop filter is replaced with a
microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...
that uses software to compensate for not only the phase and frequency changes of the local oscillator, but also for the "learned" effects of aging, temperature, and other environmental parameters. One of the keys to the usefulness of a GPSDO as a timing reference is the way it is able to combine the stability characteristics of the GPS signal and the oscillator controlled by the tracking loop. GPS receivers have excellent long-term stability (as characterized by their
Allan deviation The Allan variance (AVAR), also known as two-sample variance, is a measure of frequency stability in clocks, oscillators and amplifiers. It is named after David W. Allan and expressed mathematically as \sigma_y^2(\tau). The Allan deviation (ADE ...
) at averaging times greater than several hours. However, their short-term stability is degraded by limitations of the internal resolution of the one pulse per second (1PPS) reference timing circuits, signal propagation effects such as
multipath interference In radio communication, multipath is the radio propagation, propagation phenomenon that results in radio signals reaching the receiving antenna (electronics), antenna by two or more paths. Causes of multipath include atmospheric ducting, ionosph ...
, atmospheric conditions, and other impairments. On the other hand, a quality oven-controlled oscillator has better short-term stability but is susceptible to thermal, aging, and other long-term effects. A GPSDO aims to utilize the best of both sources, combining the short-term stability performance of the oscillator with the long-term stability of the GPS signals to give a reference source with excellent overall stability characteristics. 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 phase comparing this 1PPS signal to the GPS-generated 1PPS signal and using the phase differences to control the local oscillator frequency in small adjustments via the tracking loop. This differentiates GPSDOs from their cousins NCOs (
numerically controlled oscillator A numerically controlled oscillator (NCO) is a digital signal generator which creates a synchronous (i.e., clocked), discrete-time, discrete-valued representation of a waveform, usually sinusoidal. NCOs are often used in conjunction with a digital ...
). Rather than disciplining an oscillator via frequency adjustments, NCOs typically use a free-running, low-cost crystal oscillator and adjust the output phase by digitally lengthening or shortening the output phase many times per second in large phase steps assuring that on average the number of phase transitions per second is aligned to the GPS receiver reference source. This guarantees frequency accuracy at the expense of high phase noise and jitter, a degradation that true GPSDOs do not suffer. When the GPS signal becomes unavailable, the GPSDO goes into a state of holdover, where it tries to maintain accurate timing using only the internal oscillator. Sophisticated algorithms are used to compensate for the aging and temperature stability of the oscillator while the GPSDO is in holdover. The use of
Selective Availability The error analysis for the Global Positioning System is important for understanding how GPS works, and for knowing what magnitude of error should be expected. The GPS makes corrections for receiver clock errors and other effects but there are sti ...
(SA) prior to May 2000 restricted the accuracy of GPS signals available for civilian use and in turn presented challenges to the accuracy of GPSDO derived timing. The turning off of SA resulted in a significant increase in the accuracy that GPSDOs can offer. GPSDOs are capable of generating frequency accuracies and stabilities on the order of parts per billion for even entry-level, low-cost units, to parts per trillion for more advanced units within minutes after power-on, and are thus one of the highest-accuracy physically-derived reference standards available.


Form factor

GPSDOs could be: * Fully encapsulated, portable and standalone. * Board mounted. * Modular, connecting via external interface such as
PCIe PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe, is a high-speed standard used to connect hardware components inside computers. It is designed to replace older expansion bus standards such as Peripher ...
. The main difference is in the size and the power source. When standalone GPSDO may require an external power supply, board and modular GPSDOs can draw power from the
Motherboard A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...
.


References

{{Reflist Synchronization