A TDRS Success Story: Reviving AIM with Morse Code

 

AIM spacecraft illustration

AIM was a goner. After more than a year of exhaustive efforts to save the spacecraft, the Aeronomy of Ice in the Mesosphere (AIM) mission was quickly going deaf. Several mitigation techniques had proven unsuccessful when NASA’s communications satellite delivered a unique solution: communicating with the spacecraft by mimicking Morse code.

AIM receiver image
AIM’s receiver problem: AIM’s receiver must fix onto all three locks from TDRS in the precise order shown above to receive command data successfully. AIM’s subcarrier lock is problematic, preventing the receiver from fixing onto the bit lock and, ultimately, blocking data from flowing in.
Credit: NASA Goddard/Seema Vithlani

Launched in 2007, AIM has one receiver and one transmitter to accept commands from the ground and send back science and spacecraft health data through 12 to 16 daily contacts with NASA’s Tracking and Data Relay Satellite (TDRS). But just days after launch, the receiver started to drop “out of lock” (see Image 1 to the left), causing AIM to intermittently lose its ability to accept commands from Earth.

No one knew what was wrong; sometimes, communications up to the spacecraft worked perfectly. Other times, the team on the ground would go days without reaching AIM, unable to send up critical navigation instructions and self-care commands.

Former AIM spacecraft communications engineer Neil Mallik said the team that built the spacecraft bus tried everything. At first, team members thought the problem was caused by temperature variations. They strategically planned commands to raise the temperature by turning on instruments and heaters, and every time AIM locked to TDRS, the AIM team issued those commands. They soon discovered raising the temperature wasn’t the solution.

Next, the team built autonomy into AIM. The mission had an existing backdoor contingency code that could guide the spacecraft in the event of various failure scenarios. The team planned to expand this backdoor code to incorporate every anticipated command – thousands in total – for the entirety of the 26-month mission’s remaining lifespan.

After nearly a year of writing code and anxiously waiting for subcarrier lock to be achieved, the team finished sending up new commands to AIM’s flight computer in increments, albeit acknowledging that much of the code may later need to be patched to account for unanticipated real-time events.

But while the ground could not always speak to AIM, AIM was sending back valuable science data. NASA extended the spacecraft’s lifetime from the planned 26-month study to a three-year mission. The code patches, given the uncertainty of spacecraft needs and due to AIM’s limited memory storage, were unfeasible for another two years.

AIM data image
AIM investigates mysterious “night-shining” clouds in the mesosphere.
Credit: NASA

Meanwhile, the communications problem was intensifying, and full receiver locks were becoming more and more sporadic. The AIM team went from going three to four days without contact, to six to 12 days. When an entirely unique idea finally sparked, AIM had not successfully received a single command to AIM for nearly 40 days.

Then, the team realized it could communicate with the spacecraft using a Morse code strategy, leveraging commands the AIM team had spent the last year programming.

TDRS offers two power modes (normal power mode and high power mode), which AIM receives as two distinct signal strengths. So if AIM could distinguish between the signal levels, the two signals could function as the binary elements that comprise Morse code (see Image 3 below).

AIM signal strength
The signal strengths act as the numbers “0” and “1” in binary code. So, as an example, imagine one element of a command code pre-programmed onto AIM’s flight computer was “0001”. TDRS could produce three “0s” at normal power mode in 20-second durations (or wait, wait, wait in Morse code), and then signify “1” using high power mode.
Credit: NASA

So how did they achieve this? Take another look at the graphic of AIM’s receiver in Image 1. Notice that while all three locks are necessary for the flight computer to accept valid data, only the carrier lock (which never experienced any problems) is necessary for delivering signal strength information.

Preceding application of this strategy, the team programmed the flight computer to accept commands via signal strength modulations.

The plan worked. The team could use Morse code to give commands for up to 24-day intervals, if needed. For the rest of the time, AIM was self-sufficient.

NASA extended the mission lifetime again, and to this day, TDRS keeps AIM going using the Morse code technique. The last time subcarrier lock was achieved in April 2017, the AIM team had gone 1,685 days – nearly five years – without being able to contact the spacecraft the traditional way.

By Seema Vithlani

 

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