Todd J. Barber, Cassini lead propulsion engineer
Please pardon my lack of a February column—the shortest month zipped by even quicker than usual, thanks to my part-time commitments for the Mars Science Laboratory (MSL) mission. I can’t believe March is behind us as well, but I managed to squeeze in a late month interview with Gail Thomas, Cassini Lead Telecom Engineer, before April began. I now have a new appreciation for the complexity of the Cassini radio frequency subsystem (RFS), along with Gail’s most colorful background, and it’s a privilege to share it with you.
Following a career path atypical of most engineers, Ms. Thomas obtained a BA degree in music from UCLA, specializing in flute performance. However, she told me she always knew how engineers “looked at things” since her grandfather and father were trained as electrical and mechanical engineers, respectively. Her grandpa, like Gail, exhibited talents in both music and in technical areas. He hosted a Portland, Oregon, radio show and sang on the air, while simultaneously putting his E.E. degree to work in keeping the radio station running! With the arrival of the Great Depression, he headed south to pursue acting in Hollywood, with limited success. Again combining his expertise in music and engineering, he became a pioneer of sound engineering at Warner Brothers Studios. His son, Gail’s father, followed in his dad’s footsteps, too, working in the sound department at Universal Studios. Given the difficulty in finding an ideal job following her BA degree, Gail contemplated a career in the movie studio world and visited her dad, watching him work with music mixers in the sound department. This exposed her to electronics engineering, so she enrolled at Glendale College and then Cal State LA, studying electronics and computer technology. She began working at JPL as a summer hire (enabled via a softball connection) and then as an Academic Part-Time (APT) employee. This led her to a permanent position and she’s been here ever since.
After a brief stint in the radiation effects group when she first started, a telecom position opened up for Gail after about one year. Since then, Ms. Thomas worked on Cassini support equipment (back when it was CRAF-Cassini; long-time followers of Cassini might remember when it was part of a combined spacecraft series with the proposed but never-built Comet Rendezvous Asteroid Flyby spacecraft), before doing a similar job on the Mars Exploration Rovers (MER). This led to work in telecom mission operations for MER, MSL support equipment tasks, and eventually the lead telecom mission operations position on Cassini. Like me, Gail enjoys mission operations very much, especially the close proximity of other team players. She likened it to another real-time experience with which she is most familiar—playing in an orchestra. She also quipped it was important in telecom to be “ready for a solo when the time comes.” I also enjoyed her observation that obtaining coherent telemetry data, where the spacecraft locks up on a different uplink radio frequency than the downlinked frequency (in order to avoid interference between the two) is akin to harmonics in music. The parallels seem endless!
There are many parts to the Cassini RFS, and each subassembly could easily occupy a column, so I will only mention the highlights. One way to do this is to think about the flow of data, both uplinked from the ground and downlinked from the spacecraft. For uplink, an X-band radio signal is blasted at the speed of light to the Cassini high gain antenna, a 4-meter parabolic reflector provided by the Italian Space Agency (ASI). The signal passes through a diplexer (essentially a frequency filter) to a deep space transponder (DST). Many of the RFS components, including the DST, have built-in redundancy, cross-strapping capability, etc. From the DST, the signal moves into the command detector unit (CDU), which checks the sequence of bits (1’s and 0’s) to make sure the command stream is valid before being sent to Cassini’s command and data subsystem (CDS—Cassini’s main computers). The pathway from CDS to downlinked data is no less complex, starting with CDS talking to the telemetry control unit (TCU). The TCU is like the “brains” of the RFS and it configures the hardware for proper data downlink. From the TCU, data passes through the DST to the traveling wave tube amplifier (TWTA), which strengthens the signal before data is radiated from the HGA (i.e., sent “out the horn”). An Ultra-Stable Oscillator (USO) within this loop offers a temperature-compensated frequency standard that is stable to plus or minus six parts per million per year! This enables telemetry data to travel hundreds of millions of miles back to Earth to be picked up by the sensitive antennas of the Deep Space Network (DSN). There are also two low gain antennas (LGA-1 and LGA-2), incidentally, but we only used them early in the mission when we couldn’t point the HGA to Earth. However, we are planning to use one of them again for some future radio science activities.
One of Ms. Thomas’s responsibilities includes running the TFP (telecom forecast/prediction) tool, which predicts the available downlink data rate for a wide range of conditions. Uplink power is usually not affected by external variables, given the high power of the DSN transmitters, but downlink data may be affected by weather (the infamous “rain in Spain” among other things) and DSN antenna elevation angle, just to name a few. Generally, higher data rates are available at higher elevation angles, since there is less attenuation by Earth’s atmosphere. Cassini takes advantage of this phenomenon by bumping up the data rate, sequentially, for higher elevation angles. This is the infamous Cassini “wedding cake”—when one plots available data rate vs. time for a given downlink pass, it looks like the side view of a multi-tiered wedding cake. This technique is yet another clever way to help satisfy the voracious scientific appetites of the Cassini science teams, squeezing every last bit out of the deluge of Saturn science. Another of Gail’s duties is to monitor the link performance—sometimes RFS engineers at JPL spot telecommunication (downlink) issues before they are even spotted at the DSN sites. We that work the other subsystems on Cassini are happy that Gail “has our back” in this arena. Our knowledge of our subsystem’s health and safety is only as good as the latest available telemetry from RFS.
Many telecom engineers got their start through electronics kits in childhood, or particularly via ham radio. How delightful it was to learn of Gail’s less traditional journey to Cassini! Gail, I wish you all the best with the Cassini RFS and your wind ensemble, the Symphonic Winds of the Pacific. And if I haven’t said it lately (or at all)—thanks for providing years of reliable propulsion telemetry so I can effectively do my day job (between my own music gigs).