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Comment by btoptical | Hacker News Reader

While impressive it's not clear why such a low phase noise oscillator is needed. This oscillator looks like it takes up lots of space and power. So some justification for the device should be presented. Furthermore its possible to create a robust FEC that would be tolerant of burst errors due to phase noise. So the paper is incomplete without some mention of phase noise and the FEC used.

Other than the high end parts used in this device, it is fairly low-tech compared to most modern communications systems. That is a good thing for something like this I guess.

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Keep in mind this was designed in 2002 using technology that was considered rock-solid & mature at that time. Spacecraft technology always seems outdated because of the lead times required: on something like New Horizons, it's between 15 and 20 years from component design to being used for its primary mission.

In addition, as another commenter noted, the reference oscillator is used for REX (the radiometry experiment on board New Horizons) and quality of gathered measurements for this experiment was one of the primary needs here: being able to use the component for the communications subsystem is just good power/mass engineering at work.

I believe the main reason is for navigation. They need a stable frequency reference because they use its delta, due to Doppler shifting, as the measure of the spacecraft's radial speed relative to earth.

From the paper, even this amazing stability isn't good enough for their navigation needs, by itself:

    7. NAVIGATION SUPPORT

    The downlink frequency from the New Horizons transceiver is
    derived from the USO, as discussed previously, but even this
    excellent frequency reference is not sufficient to support
    precise and accurate Doppler velocity measurements at the
    target level of 0.1 mm/s throughout the mission. In order to
    remove the impact of frequency drift and bias in the
    reference oscillator, New Horizons has implemented the
    noncoherent Doppler velocity measurement approach [2] which
    was successfully demonstrated in 2002 during the demanding
    Earth orbit phase of the CONTOUR mission [3].

They did mention justification: the uplink radio science experiment. Here's a paper covering that experiment: http://www.boulder.swri.edu/pkb/ssr/ssr-rex.pdf

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