The 1970s ISEE-3 spacecraft is the target of the first-ever citizen scientist project to reclaim an abandoned spacecraft, crowdfunded in part by you. Now they're using the Deep Space Network to determine precise locational data, the first time the two have spoken since I carried change for pay phones.
Nicknamed the zombie spacecraft that wouldn't die, or the first hacked space probe, ISEE-3 is a spacecraft launched in 1978, repurposed through a series of missions in the 1980s (when it was renamed from ISEE-3 to ICE), and eventually abandoned by NASA in the 1990s. When its latest orbit brought it near Earth, a team of engineers captured imaginations with the dream of putting the craft back to work. After a successful crowdfunding campaign, they managed to get back into two-way communication, and are now figuring out the details of orbital manoeuvres to put ISEE-3 somewhere useful. But before they get too carried away with fancy orbital dynamics, they need to know exactly where ISEE-3 is. That's where getting time with NASA's Deep Space Network comes in handy, using the network to determine the spacecraft's exact position.
For an hour earlier today, the craft was ranged by antenna 24 of the Deep Space Network from the Goldstone station in California. At the time, other antenna at the same station were in communication with the Spitzer Space Telescope, the Juno craft exploring Jupiter, and the Messenger mission to Mercury. Meanwhile, Canberra station had a birthday exchange with the Lunar Reconnoissance Orbiter, and Madrid station had a multi-antenna conversation with the Mars Reconnoissance Orbiter.
The actual signal exchange is nothing exciting — the video below is a snippet of what it looked like — but I was pretty giddy to realize that the last time ISEE-3 had time on the Deep Space Network, we couldn't watch along live at home. In fact, the last communication pre-dates a lot of things. I don't know the exact date of last contact, but considering the craft was dropped into low-duty cycle in 1995, operations officially terminated in 1997, and the appropriate communications antenna were ripped out in 1999, I have some pretty solid estimates. The last time ISEE-3 was on the Deep Space Network, the internet hadn't yet been dominated by photographs of cats, and people were jumping on the hot new fad of AOL Instant Messenger. Friends was gaining popularity, Far Side was ending, and Star Wars was a single trilogy. Ellen DeGeneres came out, Rev. Jerry Falwell accused Teletubby Tinky Winky of being gay, Stanely Kubric was alive, and Justin Beiber was learning how to toddle around his living room. We had just barely realized rogue waves were a real thing, and had the nastiest El Niño on record. People were freaking out about the Y2K bug, and the European Union was so new that they hadn't yet adopted Euros. In other words, it's been a while since the last time the ICE designation was in active use.
The purpose of using the Deep Space Network to ping the spacecraft is to perform Doppler and ranging activities. In non-technical terms, that comes down to bouncing a signal to the craft, watching how it changes and how long it takes to return, and using that to measure the craft's exact position. That data will help the reboot team better calculate how to send it on a new trajectory in the most efficient and interesting manner possible.
But looking at the process in technical terms is weirdly riveting:
As a spacecraft built in the 1970s, ISEE-3 has the capacity for dual-frequency S/X downlinks, where a reference signal is used to build a pair of coherent downlink bands. The S/X refers to the frequency bands — the S-band uplink is at 2110 to 2120 MHz and downlink at 2290 to 2300 MHz, while the X-band uplink is at 7145 to 7190 MHz and downlink is at 8400 to 8450 MHz. ISEE-3 lacks the X-band uplink capacity, which wasn't added until 1989. Most modern craft use X-band only, or the even-higher frequency Ka-band. Higher frequencies allow for better communications performance and more accurate radiometric measurements as the shorter wavelengths are less impacted by charged particles and plasma. (Correction: Apparently, ISEE-3 is a bit funky and has a pair of S transponders instead; here's the details.)
The process starts by the ground station generating a signal of sinusoidal tones, that is phase-modulated before being transmitted to the spacecraft. The receiving spacecraft locks on, using a phase-locked loop to generate a coherent reference signal. The reference signal demodulates the ranging signal, which then gets passed through a low-bandpass filter to lop off the high end of the signal. This remnant of the ranging signal is then modulated onto the downlink, creating a coherent signal that has been frequency-dropped with respect to the original tones, and transmitted back to Earth.
Once received, everything is run through a computer model to determine the difference between the anticipated travel time and the actual travel time for a round-trip transit of the signal. Once we have the round-trip travel time, it's a simple matter to use the speed of light to calculate out the distance the signal traveled: the positional data. The frequency of the Earth-generated reference signal is compared to the modified signal returned from the craft to generate Doppler data. The Doppler data is used to measure how the range changes within the measured timeframe, adding velocity data to the positional data.
For a much, much more detailed look at using the Deep Space Network for determining spacecraft position, read this JPL monograph on the topic.
The spacecraft was not being entirely cooperative, with one of the transponders failing to respond properly to sweeping signals from the network. This isn't terribly surprising — as the project team teased on Twitter, "This is a bit like taking your 1978 Mazda into the shop for a tune-up." Even if they don't get all the data they need from this first session, the team has more windows lined up on the network on June 22nd and in the first week of July. In the meantime, they've been testing out the equipment, finding the spacecraft a bit temperamental but fully operational. The next major manoeuvre will be on June 21st when they attempt to boost the spacecraft rotation by just over an extra half-spin per minute to reach its full operational spin rate of 19.75 rotations per minute.
I am fascinated by all of this — using new technology to impersonate old technology to rebuild communications with a spacecraft that would otherwise be space junk, finding private partnerships to pursue scientific curiosity, and getting to watch all of it in real-time. Thank you to everyone who helped make it happen, from the engineers working in their converted McDonald's to the people scrounging up abandoned technical documentation to everyone who chipped in funding to make this possible. This is a "We're living in the future." moment that fills me with glee every time I think about it.