Technology What is a GPS/Multi-GNSS Disciplined Oscillator (GPSDO/GNSSDO)?

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What is a GPS/Multi-GNSS Disciplined Oscillator (GPSDO/GNSSDO)?

The GNSS* satellites including GPS satellites have ultrahigh-performance atomic oscillators. The accuracy of the satellite’s atomic oscillators is controlled and maintained from the ground stations. The GPS/Multi-GNSS Disciplined Oscillator receives GPS/GNSS satellite signals and outputs highly accurate and precise time pulse (1PPS) and reference frequency by controlling its own oscillator with the precise time provided by the GPS/GNSS receiver. They are also called GPSDO(GPS Disciplined Oscillator)and GNSSDO(GNSS Disciplined Oscillator).

Periodical calibration work is not necessary since GPSDO/GNSSDO compensates time pulse and reference frequency continuously by using GPS/GNSS.

The GPSDO/GNSSDO controls the frequency output of its own oscillators (TCXO or OCXO) and achieves at a much lower cost, superior performance than an atomic oscillator (Rubidium or Cesium) replacement.

Now days GPSDO/GNSSDO are used extensively in large infrastructures requiring precise time, reference frequency and time synchronization, such as digital terrestrial broadcast base stations, cell phone base stations and wireless communication network systems.

*
GNSS (Global Navigation Satellite System)
GNSS (Global Navigation Satellite System) is a satellite system that is used for positioning by receiving its satellite signals. The GPS of United States, GLONASS of Russia and Galileo of Europe are now in operation or are about to start operation. Japan launched “Michibiki” of QZSS (Quasi-Zenith Satellite System) in 2010.

Example of frequency control

Interruption of satellite signals (Holdover function)

As explained above, a GPSDO/GNSSDO controls the frequency of its oscillator by using the received GPS/GNSS signals. However, exogenous noise such as jamming signal or failure of GPS/GNSS antenna by lightning strike may cause the GPS/GNSS receiver to loose lock with the GPS/GNSS satellite signals. A GPSDO/GNSSDO is used in infrastructures that must continue to operate through such interruptions. In these circumstances the GPSDO/GNSSDO shall continue to output highly accurate and precise timing (time pulse and reference frequency) for a certain period time (Holdover status). To achieve this requirement, the Holdover Function (Holdover Control) is available.

When the GPS/GNSS receiver looses lock, the GPSDO/GNSSDO outputs timing by using the intrinsic performance of its oscillator. Higher is the performance requirement in Holdover mode, the more expensive and larger size oscillator (OCXO vs TCXO) should be used in the GPSDO/GNSSDO.

Yet when the GPS/GNSS receiver looses lock, the GPSDO/GNSSDO oscillator is controlled to maintain timing performance by learning the frequency characteristic (Aging, temperature characteristic and etc) during GPS/GNSS lock condition thanks to Furuno’s Holdover Function. The Holdover performance is significantly defined by how accurately the GPSDO/GNSSDO learns its oscillator’s frequency characteristic. Furuno steps up the effort to improve oscillator control technology by providing Furuno GPSDO/GNSSDO to digital terrestrial broadcast base stations, cell phone base stations and wireless communication network systems which support foundations of countries.

The new Multi-GNSSDO GF-8705, which was announced on January 2015 achieves accuracy within 1.5μs (specification value), the world best-in-class holdover performance even if the GNSS receiver looses lock for 24 hours. The GNSSDO receives multiple GNSS satellites, such as GPS, GLONASS, QZSS and SBAS concurrently. As the GNSSDO receives GPS and GLONASS signals concurrently, from two different frequency bands, if the receiver looses the lock condition on either GPS or GLONASS, the GNSSDO can continue to control its oscillator by using the other satellite signals. This decreases the possibility to enter the holdover status.

Multi-GNSS disciplined oscillator GF-8705 1PPS accuracy [click to enlarge]

Steady status (GNSS receiver locks and 1PPS synchronizes with GNSS time)

GNSS receiver looses the lock condition for 24 hours

Furuno Advanced Multi-GNSS Disciplined Oscillator

Furuno Advanced Multi-GNSS Disciplined Oscillator supports concurrent reception of GPS, GLONASS, QZSS and SBAS. A variety of product line up is available depending on holdover performance, low phase noise performance of reference frequency, size and cost. Furuno Advanced GNSSDO supports Active Anti-Jamming functions and Multipath Mitigation functions and provides highly accurate and stable 1PPS synchronized with UTC time as well as highly stable frequency coherent to 1PPS.

Highly accurate and low phase noise model

* The GF-8701/GF-8702/GF-8703/GF-8704/GF-8705 use the same communication protocol.
The GF-8701, GF-8702 and GF-8703 have the same footprint and are pin-to-pin compatible. The GF-8704 and GF-8705 have the same footprint and are pin-to-pin compatible.
This enables user-replaceable depending on customer requirements.

Glossary of technical terms

The following is an explanation of the general technical terms which are commonly used in specifications for GPSDO/GNSSDO and for oscillators (TCXO and OCXO).

Nominal Frequency

It is the output frequency of a TCXO or OCXO which is specified by the oscillator manufacturer (Example: 10MHz).

Accuracy and Precision

Accuracy is the scale how far apart from the true value. Precision is the scale of variation in accuracy over a period of time. In general, these scales are expressed in ppm(10-6)or ppb(10-9) unit as relative to the value of the nominal frequency (true value). The following formula is used to calculate frequency accuracy.

Frequency accuracy= (F-Fs)/Fs
  Fs: Nominal frequency, F: Actual frequency

For example, the nominal frequency is 10,000,000Hz (10MHz) and the actual frequency is 10,000,001Hz, i.e. it is out of alignment by 1Hz.

Frequency accuracy =( 10000001 - 10000000 )/ 10000000 = 1x10-7 = 0.1ppm = 100ppb

Coherent

At 10MHz, if the number of cycle of the reference frequency wave between a 1PPS pulse and the next 1PPS pulse is constantly 10,000,000(10M), it means that the time pulse (1PPS) and the reference frequency are coherent. A lot of wireless communication systems such as cell phone base station require such feature.

Variety of Oscillators

Please refer to the chart for oscillator's frequency accuracy versus cost.
Furuno Multi-GNSSDO provides highly accurate and stable frequency comparable to an atomic oscillator thanks to Multi-GNSS signals reception and FURUNO’s Advanced Oscillator Control Function.

XO (= Crystal Oscillator)
A XO is an inexpensive oscillator, often used for in wristwatches.
TCXO (= Temperature Compensated Crystal Oscillator)
A TCXO is an oscillator which automatically compensates the alignment of its oscillator frequency for variation of ambient temperature.
OCXO (=Oven Controlled Crystal Oscillator)
An OCXO is an oscillator operated inside a temperature controlled oven which keeps the crystal and the oscillator at a constant temperature. This lowers the oscillator frequency variance.
Atomic Oscillator
An Atomic Oscillator is an oscillator which outputs high precision frequency based on the radiation frequency of atoms or molecules.

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