The Pluto Perspective

One Score and One Year with New Horizons

In the early 1990s, I was a member of the Stanford University radio science group, led by Prof. Len Tyler. We were developing the means to perform uplink experiments across the solar system that needed special hardware in a spacecraft's radio system. Our principal incentive was that by using uplink - transmitting a signal from Earth to a spacecraft – we'd get much more sensitivity than by the traditional method of downlink, that is, sending the signal from the spacecraft.

We had pioneered prototypes of uplink radio receivers suitable for measuring, for example, very weak atmospheres occulted by an object between a spacecraft and Earth. When Alan Stern invited us to join him in a proposal for a Pluto mission, we eagerly accepted, as Pluto's atmosphere was an ideal application for our uplink experiments.

I was a Ph.D. thesis advisor to several students in our group whose dissertations covered the technology, methods and theoretical analysis of uplink experiments. One student in particular, Kamakshi Sivaramakrishnan, was interested in digital signal processing and digital filters. Her big challenge was to design and implement an ultra-linear, ultra-narrowband filter that could be used in a radiation- hardened Field Programmable Gate Array. It was Kamakshi's stunning achievement that she discovered a suitable design that fit into the one FPGAs we were allocated in the spacecraft's X-band receiver and became the heart of the Radio Science Experiment (REX) instrument on New Horizons.

At that time, we did not realize Kamakshi's digital filter would be ideal for an uplink bistatic radar experiment at Pluto, and then again at the Kuiper Belt object Arrokoth in 2019. Bistatic radar is where the transmitter and receiver are not the same; our case involved a powerful X-band signal transmitted from the NASA Deep Space Network stations on Earth, traveling across the solar system to "illuminate" Pluto in the region of Sputnik Planitia, then be scattered toward and received on New Horizons by REX in the spacecraft's X-band radio.

Now, one score and a year later, and having performed very successful radio occultations of Pluto's atmosphere with REX, retrieving the atmosphere's temperature and pressure down to the surface, and having done a very successful bistatic radar measurement of Sputnik Planitia's nitrogen-ice glacier properties, we celebrate the creativity and inspiration of our radio science graduate students, particularly the innovation and commitment of Kamakshi Sivaramakrishnan.