This New Horizons blog is a team effort between Cathy Olkin, the co-principal investigator of the New Horizons Ralph instrument, and Ralph instrument engineer Eddie Weigle.
Just as it takes teamwork to fly a spacecraft to Pluto – even tasks like checking the commands that are sent to the spacecraft are done by a team – we decided to team up on this blog to take you behind-the-scenes of interplanetary spaceflight. Specifically, we’ll tell you how we check the commands for New Horizons’ Ralph instrument to make sure they will accomplish the desired science objectives.
Cathy: A command load is a set of commands that are transmitted to the spacecraft’s computers from Earth – in our case, sent through NASA’s largest Deep Space Network antennas – which control the spacecraft’s activities. Sometimes the command load covers a short period of time – maybe four days, or a week – but in other instances a single command load can span months, such as when New Horizons was in hibernation mode for much of the journey to Pluto.
For the time around closest approach to Pluto, we had one command load that executed commands over a nine-day time span ranging from one week before closest approach to two days after. These command loads are built by the science and mission operations teams and are then checked by the spacecraft engineers and instrument teams. That’s where we come in. We check the command loads for the Ralph instrument, a color camera and near-infrared imaging spectrometer.
We had more than 20 versions of this nine-day command load. The command load had 30,124 lines that needed to be checked! We checked that the instruments would carry out the desired science observations and that nothing would harm the spacecraft or instruments.
The first step in checking a command load is to compare where the instrument is pointing with the desired location. The Ralph instrument builds up images by scanning the field of view of the instrument across the target and I check that the pointing of the instrument is right by looking at a visualization of the commands using a tool call the Satellite Tool Kit.
Now, I will turn it over to Eddie to tell you more about how we check the command loads.
Eddie: Now we get to the fun stuff: making sure the Ralph instrument is doing what the scientists want. Prior to creating the command load, the Ralph science team confers and debates over the best possible ways to use the instrument. There are several facets to consider when deciding on the science. These discussions for creating Science Activity Plans (SAPs) must take into account the Ralph operating mode, where and when to point the instrument, the observation target, memory requirements, the type of data compression, and downlink time. Each SAP the science team approves is broken down into one or more “observations.” Each observation has a single purpose, and consists of a particular target, operating mode, and time.
With all this information, the command load is built. The load contains commands not just for Ralph, but for all the other instruments and subsystems as well, including the spacecraft itself. These command loads may contain thousands of commands, so to check each version of each load manually would be extremely time-consuming. So to aid our team in verifying the loads, we developed a Python script to analyze the full file. The script verifies that all the necessary commands are there to properly execute each observation.
After all the subsystem leads check and approve the command script, we still need to make sure the commands work as they should. So we run each command load on the New Horizons Operations Simulator – or “NHOPS”—a fancy name for a set of electronics that functions just like the spacecraft itself. Understanding the full complexity of the entire spacecraft typically goes beyond any individual instrument team, so we conduct a dry run to ensure all resources are properly used. To analyze the results of each NHOPS run, the Ralph team developed a web-based tool called Ralph Activity Manager (RAM).
RAM provides the team with end-to-end coverage for the commanding of the instrument. It not only includes the command checker I described earlier, but also correlates the spacecraft telemetry, the telemetry from the simulator tests, the command loads, and the science objectives. This allows us to easily track and manage all of the science goals, from the time we decide what to observe, until the data from those observations are on the ground.
And that, in a nutshell, is how we confirm the commanding on New Horizons is accurate – and how the mission team was not only able to deliver the goods on one of the great planetary encounters of our time, but also how we’ll continue to explore the farthest reaches of the solar system!