AB9IL.net: Deep Space Communications Complex Tidbinbilla Tour

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Here is the first of three YouTube videos about the Deep Space Network (DSN) communications complex at Tidbinbilla, Australia. Each one shows the antennas and equipment up close, with some of the best explanations of low noise / super long range communications are accomplished.

One of the things I never knew was that Voyager probes actually transmit strong signals back to Earth, considering the extreme distances involved. There are probes at Mars and in Earth orbit which are weaker than the Voyagers!

The heavy 70 meter cassegrain antenna (DSS-43) transmits an incredibly narrow microwave beam, with a width far less than one degree wide. Aiming requires such precision that engineers created a buttery smooth azimuth-elevation platform which rides on a thin layer of oil.

DSN Picture Book: Majestic Deep Space Radio Antennas
A visual treat for anyone who enjoys fanatically precise giant structures.

For the second video, Richard Stephenson shows the ops center, where coordination with other DSN sites are managed and where communication tasks are handled. During filming, Mars was coming above the horizon and the operators were working the Maven probe. Voyager 2 was above the horizon, so the people were doing low rate comms and measurements on the signals.

DSN is like an amateur moonbounce station on steroids. System noise temperature for the Voyager 2 comms is typically between 17 to 19 degrees Kelvin. Positioning accuracy of the 70 meter antenna must be kept within a matter of millimeters. They'll lose the signal if the antenna just a couplee of centimeters off, if one measures at the edges. Even deformation due to dish sagging must be taken into account. Aiming accuracy is measured in millidegrees! Can you imagine the encoders used to measure antenna movements so small?

In the third video, Richard Stephenson compares the 34 meter beam waveguide antennas to the 70 meter dishes. NASA / JPL really likes the high data rates possible with the larger dish, but the 34 meter dishes have great maintenance and instrumentation advantages. Those smaller dishes use microwave mirrors to route signals down into an air conditioned instrumentation room at the antenna base. Easy peasy, with incredible precision - good enough for interferometry and beam forming.

Phasing and aiming is so important. Even the Lunar Reconnaissance orbiter, far closer to Earth than Mars probes, must specifically aim its high gain antenna at the DSN complex in use, otherwise DSN will not get a full strength signal. For its part, DSN antennas, with their narrow beams, must aim precisely at the spacecraft.

And then... A team of DSN techs work hard to characterize space probe receiver performance and doppler shifts, with lots of measurements, sweeps, chirps, and measurements of time delays.

There is also a visit down into one of the instrumentation rooms to see a transmitter's klystron unit, which generates the high power signals. Power levels are quite high, measured in megawatts, for hundreds of kilowatts of generated RF.

We learn that the incoming signals are mixed down to a standard in-house intermediate frequency of 300 MHz. In the past, that was for distribution to the various users. Nowdays, in the age of digital radio, the IF is digitized, and the sampled waveform is what is distributed to clients.

If you watch closely, you'll get a glimpse of the famous dish used to receive Neil Armstrong's first steps on the moon. The dish was originally at Honeysuckle Creek, but was fairly recently dismantled, moved, and reassembled in Tidbinbilla.

Pop Quiz: What frequency is used for the in-house time and frequency reference? Answer: 100 KHz. They have hydrogen masers which generate a precise microwave reference, which is divided down to a hundred kilohertz and fed to the various buildings. Whereas a lot of hams may have a GNSS based one pulse per second reference in the radio shack, DSN uses a hundred thousand pulses per second. Certainly, GNSS time is available for tasks which need it; a maser based 100kpps stream, synced to a GNSS 1pps stream would be pretty slick indeed.

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