Feature: Clock and timing electronics
Figures 3: Illustration of a hydrogen maser on top of Mount Fuji [AI-generated image]
specific atoms into a cavity, and takes advantage of the natural behaviour of those atoms in the cavity to generate an incredibly stable frequency and time reference. Te longer stability is measured, the
more it improves through noise averaging. Eventually, somewhere around the million-second interval (weeks), the maser finally begins to show degradation with a predictable daily fractional error rate of less than one quadrillionth per day. Tat fractional error is equivalent in scale to comparing the diameter of a hydrogen atom to the length of the Panama Canal.
Measuring the speed of Voyager Going back to Voyager, the maser is critical for more than just measuring the range distance. Te velocity at which Voyager is speeding away from the Sun is measured by determining the Doppler frequency offset between the transmitted and received signals. Because the Earth is rotating about the Sun and its own axis, the relative velocity, and hence frequency,
is constantly changing. A compromise must be made between the desired precision of the measurement, which would typically improve with longer averaging times and the observation time, as the Earth’s simultaneous movement blurs the measurable frequency changes. NASA uses 60s observations as its
compromise window and achieves an astounding accuracy of 0.1mm/s. This means that at the transmitted frequency offset of 8.4GHz, a roughly 6mHz change must be measurable between the transmit and receive pulse. There are several other factors in the frequency offset besides the speed of the two objects, but the measurement can only be as precise as the clock itself. For this measurement, assuming we want the clock to contribute to a tenth of the measurement error, we can derive an order of magnitude engineering specification that states “the Allan deviation of the clock shall be no greater than 1E-13 at 60s averaging intervals”. Masers meet this requirement. For illustration, 1E-13 represents
28 November 2026
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fourteen orders of magnitude, roughly the difference in scale between the diameter of a hydrogen atom and the tallest building in the world, the Burj Khalifa. Imagine what surveyors (Mount Fuji), civil engineers (the Panama Canal) and structural engineers (the Burj Khalifa) could do with measurement tools of this precision.
Beyond deep-space navigation On a closing note, the maser isn’t just useful for measuring the location and speed of deep-space probes. Masers located at multiple observatories are used in celestial cartography. Once the position of the astronomical bodies is fixed on the star map, the same masers can then be used in a reverse process to measure millimetre changes in the distance between the observatories. If we place an observatory with a maser on top of Mount Fuji, a surveyor may not be able to measure the height to within a few hydrogen atoms, but hydrogen atoms would be used to make an incredibly accurate measurement possible.
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