Radio Astronomy SA

While several of our Observing Projects do not require precision frequency sources to operate effectively, it is near impossible for us to operate without precision time sources.  Since the introduction of global navigation satellite systems (GNSS) such as GPS; USA, GLONASS; Russia,  BeiDou; China & Galileo; EU,  anyone/anywhere can, using the correct receiver(s) determine precise time.  Additionally, reasonably accurate time can be achieved using computer protocols such as NTP and similar internet tools.  The attraction of GNSS as a precision time source can be enhanced by the addition of a disciplined reference oscillator to generate a high stability frequency source for synchronising multiple signal sources at any location.

Trimble’s Thunderbolt GPS Clock outputs a 10MHz reference signal and a 1 pulse per second signal with an over-determined solution synchronized to GPS or UTC time.  The 10MHz reference accommodates applications requiring sub-microsecond timing.  A single microprocessor performs both the GPS navigation and oscillator disciplining functions.  The GPS receiver is driven directly by the 10MHz output signal of the oscillator.  This is calibrated against the incoming GPS signal, with the resulting clock and frequency measurements fed into the oscillator frequency control algorithm.

In the upper image of the GPS Clock System, the Thunderbolt and the power supply it requires has been fitted into a 1U rack case.  All of our Trimble Thunderbolts use Lady Heather's Disciplined Oscillator Control software (V6) on various computer platforms to monitor/change the mode and characteristics of the Thunderbolt GPS Clock, as required for optimum performance.  The lower image illustrates some of the features available with Lady Heather, minus the whip and leather. 

One frequency critical observing project is the 23cm Hydrogen Line Project.  This system requires stable and precise frequency sources to measure doppler shifts between galactic objects in order to measure relativistic speeds between these objects.  The rackmounted GPS Clock System in the upper image comprises one of our Trimble Thunderbolts at the top, a custom built IRIG B Clock in the centre and the Symmetricon 2100TS.  The TymServe acts as a primary time server that broadcasts or responds to the specific time requests from client computers.  In a client/server mode, the NTP client sends a time request packet to the server, the server affixes its current time and returns the packet, and the client software processes the time data to adjust its local clock.  The TymServe’s accuracy -meaning its ability to synchronize time over the network - is typically one to 100 milliseconds, depending on the network configuration.  The time is obtained and tracked from one of four sources:  The Global Positioning System (GPS) satellite network;  Inter Range Instrumentation Group (IRIG) IRIG-B code;  National Institute of Standards and Technology (NIST) in the United States.  The time is adjusted, if necessary, by NIST to the correct international standard time, called Universal Coordinated Time (UTC).

The Rubidium Frequency StandardXSRM 238.4011.02 supplies a 5MHz output voltage whose frequency is very accurate, stable and of high spectral purity.  A very high short-term stability and extremely low long-term drift are ensured by the regulation of a highly stable crystal oscillator using the rubidium resonance frequency for reference.  Effects of temperature variation and external magnetic fields are suppressed since the elements that are responsive to such influences are protected by ovens or mumetal shields.
The XSRM can be used as a frequency standard wherever control with extreme stability and spectral purity of the frequency is required.  Fields of application are, for example, space research, extraterrestrial radio communications geodesy and radio-navigation, microwave spectroscopy, radar, control of TV transmitters with precision offset and of standard-frequency and standard-time systems.
Apart from the spectral lamp, the XSRM does not contain components that are subject to wear.  The spectral lamp has an average life expectancy of more than five years.  It can be replaced within a few minutes without requiring the set to be switched off.

The XSRM uses the atomic resonance frequency, 6.834 682 641GHz, of rubidium 87, which is extremely precise and scarcely influenced by ambient conditions, to regulate a highly stable 5MHz oscillator.  A cylindrical resonant cell which is surrounded by a cavity resonator is filled with a mixture of rubidium vapor and inert gas.  A spectral lamp containing the same mixture is excited by an RF generator and its light falls on a photo-diode after having passed through the resonant cell,

Duncan likes to make his own GPS/NTP Timeservers.
Blair used this link to David Taylor's website for his GPS/NTP Timeservers.
Peter used this link to Whit Reeve's website for his GPS/NTP Timeserver.

The solar powered Callisto project that is used to monitor radio outbursts from the Sun at our remote radio site was suffering from insufficient battery storage capacity to run everything overnight during winter.  The autonomous GPS Switch is located in the Spacecase mounted on the tower, controlling power to sections of the Phase 1 and Phase 2.5 spectrometers.

Blair built this neat circuit to cycle power to the receivers and low noise amplifiers, ON during sun up and OFF after the sun goes down.  Currently the ON and OFF times are hard coded, that may change at some stage.

He used an Arduino (lower LHS), uBolx NEO M6 GPS receiver (upper LHS), GPS antenna (the silvery thing in the middle), relay (upper RHS) and  a 12 volt to 5 volt power supply (lower RHS) to achieve an accurately timed power ON and OFF switch.

The Arduino code is available here.

Parts list with Altronics Cat. #
GPS module, Z6333x (uBlox Neo 6m)
Arduino Nano, Z6372
Relay module, Z6325
Voltage Buck module, Z6334 (switch mode 12 volt to 5 volt regulator)
Vero board, H0714
Header socket strip, P5384 (to allow the Arduino to be removed without soldering)