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GPS Time and Frequency Transfer Techniques a brief overview.

GPS time and frequency transfer is a method of enabling multiple sites share a precise reference time. GPS time and frequency transfer solves problems such as astronomical observatories correlating observed flashes or other phenomena with each other.

Multiple techniques have been developed, often transferring reference clock synchronization from one point to another, often over long distances. Accuracy approaching one nanosecond worldwide is economically practical for many applications. Radio-based navigation systems are frequently used as time and frequency transfer systems.

In some cases, multiple measurements are made over a period of time, and exact time synchronization is determined retrospectively.

1.       One Way

2.       Common View

3.       All In View

4.       Carrier Phase

5.       Time Transfer Using the Quartzlock E8000-TT
One Way

The one way GPS technique uses the signals obtained from a GPS receiver as the reference for a calibration. The GPS signals are used in real time to synchronize the local clock and GPS or UTC time. The purpose of the measurement is usually either to synchronize an on-time pulse, or to calibrate a frequency source. Before a receiver is used for measurements, it must complete its signal acquisition process. Part of the acquisition process is the antenna position survey. Unlike GPS navigation receivers (SAT NAVs), which compute position fixes while moving (often at a rate faster than one position fix per second), GPS time and frequency receivers normally do not move and therefore do not need to compute position fixes once the survey is completed.

Therefore, time and frequency receivers generally store a single position fix, and use that same position from then on. Many receivers automatically start a survey when they are turned on.

Once the signal acquisition is completed, an output signal from the receiver is connected to a measurement system. For time synchronization measurements, a 1 PPS signal from the receiver is generally used as an input to a time interval counter. For frequency measurements, a 10MHz frequency output from a GPSDO is used as an input to a phase comparator, or used as the external time base for test equipment such as frequency counters and signal generators.

Since the GPS satellites transmit signals that are steered to UTC, the long-term accuracy of a GPS receiver has always been excellent.

The time pulse accuracy (1PPS) is affected by delay in the antenna cable. This is about 4ns/m and can be allowed for by offsetting the 1PPS output. There is also a constellation dependant error between UTC and GPS time of up to ą15ns. A time receiver will typically have an accuracy of ą15ns (1sigma) to GPS time. A final accuracy of about ą 30ns to UTC can be achieved.
Common View

The common view method is a simple but elegant way to compare two clocks or oscillators located in different places. Unlike one-way measurements that compare a clock or oscillator to GPS, a common-view measurement compares two clocks or oscillators to each other.

The GPS satellite (S) serves as a single reference transmitter. The two clocks or oscillators being compared and are measured against two GPS receivers. The satellite is in common view of both receivers, and both simultaneously receive its signals. Each receiver compares the received signal to its local clock and records the data. The two receivers then exchange the data.

Common view directly compares two time and frequency standards. Errors from the two paths, that are common to the reference, cancel out, including the performance of the satellite clock.

The advantage of this technique is that it minimizes certain errors that might be present. The satellite clock errors are completely eliminated since they are common in both receivers. Ephemerides errors in the transmitted data and affecting to the computation of the paths are minimized. However, the main disadvantaged with respect to the one-way mode is that data between the receivers must be exchanged.

Common view requires a GPS receiver that can read a tracking schedule. This schedule tells the receiver when to start making measurements and which satellite to track. A receiver at another location makes measurements from the same satellite at the same time. The data collected at both sites are then exchanged and compared.
All in View

The All in view mode, can be used to synchronize clocks over widely separated distances. Unlike the Common view mode, the all in view mode does not require simultaneous observations by both stations; it only requires that each station observe as many satellites as possible during the day that its receiver can track.

The individual GPS time versus the local standard's time comparisons are put together over a period of time. The linear fit solution of these points is considered the offset of the GPS time from the local standard's time. Subtracting one local standard's offset time from the other yields the time difference between the two locations.

This method is more robust than the Common View Mode, because it observes significantly more satellites during the day. Therefore, it is more suitable for unattended synchronization systems because the offset values are more stable and the system is more robust to occasional data gaps since the offset is computed from several measurements.

The disadvantage is that post processing is required, and the time difference is not available in real time.
Carrier Phase

This technique uses both the L1 and L2 carrier frequencies instead of the codes transmitted by the satellites. It is important to note that carrier phase measurements can be one way measurements made in real time or post processed common view measurements.

The phase difference between the satellite oscillator and the receiver’s local oscillator is calculated. However, the phase observable is an ambiguous observable, the phase is measured modulo 2đ and only the fractional phase can be measured, whereas the pseudo range is an absolute observable. The absolute offset between the remote clocks is then only determined by the code information, while the carrier phases give a precise signal evolution. The use of the pseudo range information together with the carrier phase information increases the accuracy up to a factor 1000.

Since the carrier phase GPS technique requires geodetic GPS receivers as well as making corrections of the collected data using orbital, ionosphere and troposphere models and extensive post-processing, it is not practical to use for everyday measurements. However, the technique is used for experimental purposes and for international comparisons between primary frequency standards when the goal is to reduce the measurement uncertainty as much as possible.
Time Transfer Using the Quartzlock E8000-TT

Quartzlock has just enhanced its range of GPS Time and Frequency References with a new economical GPS All in View One Way Satellite Time and Frequency Transfer instrument the E8000-TT a GPS time and frequency reference with a smoothed 1PPS output synchronized to UTC.

Quartzlock are using the one way all in view mode, where each site is compared to an average of GPS time, derived by letting the receiver generate an average of all the satellites tracked. If the constellation is nearly the same at each site, then the result of the average is likely to be very close at each site.





Two or more E8000-TT units will provide remarkably accurate time transfer over medium baselines, up to several hundred kilometres.
E8000 design

The E8000 uses a commercial GPS timing receiver. This performs a self survey when moved to a new location, and stores the averaged position. The stored position is used when the unit is reset, and remains valid provided the unit is not moved.

With a valid stored position, the GPS receiver switches into over determined clock mode, and uses all satellites in view to provide the best possible estimate of GPS time, output as the rising edge of a pulse every second (1PPS).

The 1PPS output from the GPS receiver is phase modulated with a saw tooth with a peak amplitude of about 12ns. This is due to the finite clock resolution used in the GPS receiver.

The E8000 uses a Kalman filter to a) correct the local clock, which is an OCXO, and b) to smooth the 1PPS and remove the saw tooth modulation. The eventual 1PPS output from the E8000-TT has short term phase jitter of less than 1ns RMS.

GPS time can differ from UTC by up to ą15ns (1 hour averages), with occasional peaks at ą20ns (see NIST archived data). However when using short or medium baseline time transfer, both receivers will largely share the same constellation, and will therefore see the same offset from UTC.

Measurements on two E8000-TT receivers with co-sited antennas (zero baseline) have shown typical time differences (1 hour averages) of ą3ns, with occasional peaks at ą5ns. There is usually a fixed time difference of up to 30ns which can be removed after a calibration run. The 1PPS output from the E8000-TT receiver can be offset by up to ą500ms in 1ns steps.
Specification (typical results)

Using 2 Quartzlock E8000-TT receivers with identical co-sited antennas and cable lengths.

Fixed time difference before calibration:                            ą50ns maximum

Fixed time difference after calibration:                               ą5ns

Time difference variation (10 minute average):                ą10ns

Time difference variation (1 hour average)                        ą6ns

Analysis of 1PPS difference between 2 Quartzlock E8000A-TT

Smoothed version of data 1 hour moving average







Biuletyn Informacyjny  Quartzlock

Quartzlock A7-MX SSA Microwave Product Digest article -
Link PDF

Subskrypcja -  http://www.quartzlock.com/newsletter-signup.asp

QUARTZLOCK NEWSLETTER ~ September 2013

In this issue

1.      Reducing System Phase Noise

2.      Newsletter sign up




1.       Reducing System Phase Noise

What is phase noise and its effect.

Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities ("jitter"). Generally speaking, radio frequency engineers speak of the phase noise of an oscillator, whereas digital system engineers work with the jitter of a clock.

Phase noise is one of the causes of poor quality radio transmissions; it limits the operating range of radar and causes bit errors in Phase Shift Keyed digital modulation

Noise can have numerous adverse effects on system performance. Some of these effects are:

1.       It limits the ability to determine the current state and the predictability of precision oscillators

2.       It limits synchronization and synchronization accuracies;

3.       it can limit a receiver’s useful dynamic range, channel spacing, and selectivity;

4.       it can cause bit errors in digital communications systems;

5.       It can cause loss of lock, and limit acquisition and reacquisition capability in phase locked loop systems;

6.       It can limit radar performance, especially Doppler radar.

For example in surveillance, Doppler radars especially require low-noise oscillators. The velocity of the target and the radar frequency are primary determinants of the phase noise requirements. Slow-moving targets produce small Doppler shifts; therefore, low phase noise close to the carrier is required. To detect fast-moving targets, low noise far from the carrier is required. For example, when using an X-band radar to detect a 4kmjhour target (e.g., a slow moving vehicle), the noise 70 Hz from the carrier is the important parameter, whereas to detect supersonic aircraft, the noise beyond 10 kHz is important.

The combination of low flicker and low noise floor improves the bit error rate of a digital communication system for a given modulation scheme since the BER increases with the area under the phase noise curve. This small integrated noise or phase jitter similarly improves the resolution and probability of detection of radars and enhances the accuracy of distance measuring devices. Modern spectrum analyzers using low noise synthesized local oscillators have improved sufficiently to allow for the direct observation of sideband noise of fairly good sources. A higher performance reference would lower than local oscillator noise even further, making smaller measurement bandwidths feasible for direct measurement of all but the best sources.

The most commonly used reference frequency is 10MHz, however, work on 5MHz core oscillator with doubler are increasingly being considered. -123dBc/Hz @ 1Hz phase noise at 5MHz is a low cost option if the reference input allows, or doubled to 10MHz, even when doubler noise and loss are considered. -175dBc/Hz noise floor seems de-rigor for 2012.

The OCXO, however stable needs an external phenomenon to lock, for at least a 100x improvement in accuracy / drift. GPS and Rubidium are the obvious choices. Here the tune / lock line needs very careful design consideration. GPS effectively removes drift / year, 2E-12/day. Rubidium will achieve 4E-10 / year and improving Rubidium is antenna free, an advantage for many users as the GPS service cannot be guaranteed. Antennas come down in storms; wires get accidentally cut and connections become loose. Recent developments show that GPS signals can be disrupted due to solar events (solar flares) and Earth based signals (intentional and unintentional) operating in or near the GPS band.

The OCXO, however stable needs an external phenomenon to lock, for at least a 100X improvement in accuracy/drift. GPS and Rubidium are the obvious choices. Here the tune/lock line needs very careful design consideration. GPS effectively removes drift/year. Rubidium will achieve 4E-10/year and improving Rubidium is antenna free, an advantage for many users.

Add to the ideal reference a compatible low noise distribution amplifier, built into the reference with say 8 outputs then the complete RF and microwave system will see significant benefits:

1        All system instruments will now be on the same frequency

2        Most instruments will see a very significant improvement in phase noise (many instruments have noise floor problems of their own….outside reference control)

3        Considerable improvement in Allan Variance short term stability.

Typical instruments/system elements to benefit include:

Spectrum Analyzers, Digital Storage Oscilloscopes, Microwave Analyzers, Frequency Counters, Surveillance Receivers, Panoramic Receivers, Signal Stability Analyzers, Signal Sources, GPS Air Interface Simulators and GPS Simulation Systems. These are referencing applications. Calibration and standards laboratory uses are common.

Quartzlock has a number of low and ultra low noise references that will enable customer systems to take advantage of these benefits in both their applications and in house testing facilities.




E10-Y8

E10-Y8 Desk Top 8 Output Low Noise Rubidium Reference

E10-P Portable Rubidium Reference



E10-LN Low Noise Rubidium Reference



E5-6 Distribution Amplifier



E8-Y GPS Frequency & Time Reference



E10-MRX Sub Miniature Rubidium Oscillator



Desk Top Modules

In this issue QUARTZLOCK NEWSLETTER ~ November 2012

Reducing Instrument and Test System Noise

What is phase noise and its effect

Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities ("jitter"). Generally speaking, radio frequency engineers speak of the phase noise of an oscillator, whereas digital system engineers work with the jitter of a clock.

Phase noise is one of the causes of poor quality radio transmissions; it limits the operating range of radar and causes bit errors in Phase Shift Keyed digital modulation

Noise can have numerous adverse effects on system performance. Some of these effects are:

1.      It limits the ability to determine the current state and the predictability of precision oscillators
2.      It limits synchronization and synchronization accuracies;
3.      it can limit a receiver’s useful dynamic range, channel spacing, and selectivity;
4.      it can cause bit errors in digital communications systems;
5.      It can cause loss of lock, and limit acquisition and reacquisition capability in phase locked loop systems;
6.      It can limit radar performance, especially Doppler radar.

For example in surveillance, Doppler radars especially require low-noise oscillators. The velocity of the target and the radar frequency are primary determinants of the phase noise requirements. Slow-moving targets produce small Doppler shifts; therefore, low phase noise close to the carrier is required. To detect fast-moving targets, low noise far from the carrier is required. For example, when using an X-band radar to detect a 4kmjhour target (e.g., a slow moving vehicle), the noise 70 Hz from the carrier is the important parameter, whereas to detect supersonic aircraft, the noise beyond 10 kHz is important.

The combination of low flicker and low noise floor improves the bit error rate of a digital communication system for a given modulation scheme since the BER increases with the area under the phase noise curve. This small integrated noise or phase jitter similarly improves the resolution and probability of detection of radars and enhances the accuracy of distance measuring devices. Modern spectrum analyzers using low noise synthesized local oscillators have improved sufficiently to allow for the direct observation of sideband noise of fairly good sources. A higher performance reference would lower than local oscillator noise even further, making smaller measurement bandwidths feasible for direct measurement of all but the best sources.

The most commonly used reference frequency is 10MHz, however, work on 5MHz core oscillator with doubler are increasingly being considered. -123dBc/Hz @ 1Hz phase noise at 5MHz is a low cost option if the reference input allows, or doubled to 10MHz, even when doubler noise and loss are considered. -175dBc/Hz noise floor seems de-rigor for 2012.

The OCXO, however stable needs an external phenomenon to lock, for at least a 100x improvement in accuracy / drift. GPS and Rubidium are the obvious choices. Here the tune / lock line needs very careful design consideration. GPS effectively removes drift / year, 2E-12/day. Rubidium will achieve 4E-10 / year and improving Rubidium is antenna free, an advantage for many users as the GPS service cannot be guaranteed. Antennas come down in storms; wires get accidentally cut and connections become loose. Recent developments show that GPS signals can be disrupted due to solar events (solar flares) and Earth based signals (intentional and unintentional) operating in or near the GPS band.

The OCXO, however stable needs an external phenomenon to lock, for at least a 100X improvement in accuracy/drift. GPS and Rubidium are the obvious choices. Here the tune/lock line needs very careful design consideration. GPS effectively removes drift/year. Rubidium will achieve 4E-10/year and improving Rubidium is antenna free, an advantage for many users.

Add to the ideal reference a compatible low noise distribution amplifier, built into the reference with say 8 outputs then the complete RF and microwave system will see significant benefits:

1       All system instruments will now be on the same frequency
2       Most instruments will see a very significant improvement in phase noise (many instruments have noise floor problems of their own….outside reference control)
3       Considerable improvement in Allan Variance short term stability.

Typical instruments/system elements to benefit include:

Spectrum Analyzers, Digital Storage Oscilloscopes, Microwave Analyzers, Frequency Counters, Surveillance Receivers, Panoramic Receivers, Signal Stability Analyzers, Signal Sources, GPS Air Interface Simulators and GPS Simulation Systems. These are referencing applications. Calibration and standards laboratory uses are common.

Quartzlock has a number of low and ultra low noise references that will enable customer systems to take advantage of these benefits in both their applications and in house testing facilities.

In this issue (Summer 2012)

1.        Get More from Your Master Reference. 1
2.        Rubidium Frequency Standard. 1
3.        Newsletter sign up. 1

1.   Get More from Your Master Reference
The need to have synchronised frequency references at every workstation has moved on from being something that is an expensive luxury to a must have for all, however this does not mean that expensive reference oscillators have to be purchased and installed for each workstation. Quartzlock’s A5000 distribution amplifier enables twelve separate workstations to be connected to the same master reference ensuring that each workstation is synchronised to the master reference and each other. Therefore measurements and measurement errors are consistent throughout your facility.

2.   Rubidium Frequency Standard
The rubidium frequency standard, like its more expensive cousin, the hydrogen maser, may be operated either as a passive or as an active device. The passive rubidium frequency standard has proved the most useful, as it may be reduced to the smallest size whilst retaining excellent frequency stability. The applications for such a device abound in the communication, space and navigation fields.

The rubidium frequency standard may be thought of as consisting of a cell containing the rubidium in its vapour state, placed into a microwave cavity resonant at the hyperfine frequency of the ground state. Optical pumping ensures state selection. The cell contains a buffer gas primarily to inhibit wall relaxation and Doppler broadening. The Rubidium frequency standard essentially consists of a voltage controlled crystal oscillator, which is locked to a highly stable atomic transition in the ground state of the Rb87 atom.

There are several reasons why Rubidium has an important role to play as a frequency standard. Perhaps most significantly is its accuracy and stability. Moreover the stability of a Rubidium frequency standard over short time-scales “100s of seconds” betters that of Caesium (Caesium’s are more stable over longer time periods, in the regions of hours to years). After 100s the frequency stability of the best performing Quartzlock Rubidium is 5x10-12 and phase noise of the Quartzlock Rubidium is -110 dBc/Hz @ 1Hz from the carrier.

Due to its small size, low weight and environmental tolerance the Rubidium frequency standard is ideal for mobile applications. Indeed, Rubidium atomic clocks are being used in the GPS satellites. This is in part due to the extended life of the Rubidium physics package. The Rubidium is also extremely quick to reach operational performance, within 5mins reaching 5x10-10.

3.   Newsletter sign up
We’d like to send you occasional email newsletters with items of interest to the time and frequency community, press releases and new product information. To request the newsletter please sign up here. We will not send more than 1 newsletter a month on average.


 
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