Flight Termination On a Bench

Most unmanned systems that fly have a system to make sure it doesn’t go where it shouldn’t (and some can very abruptly end the mission) - in other words, a Flight Termination System.  Like many things, it needs to be well tested - so a good test bench is a must.  We’ve been working on a test system that shares some similarities with the one in this blog, and it has some great capabilities.

Test case examples

Here’s what some test cases might look like.  The intent is to exercise the system and all the LRUs in concert, as if a real flight were happening.  The test steps look like this:

Test Case 1

• Begin flight - send signals indicating takeoff
  • Confirm proper operation by watching return signals over telemetry
• Wait for automatic stage separation
  • Watch for indication of this on telemetry stream
• Wait for automatic stage separation
  • Watch for indication of this on telemetry stream
• Flight termination - send signals over RF to flight termination system
  • Watch for it to process and trigger the end of the flight.

Once we can see that works correctly, we might build a similar case where we terminate the flight earlier, such as:

Test Case 2

• Begin flight - send signals indicating takeoff
  • Confirm proper operation by watching return signals over telemetry
• Wait for automatic stage separation
  • Watch for indication of this on telemetry stream
• Flight termination - send signals over RF to flight termination system
  • Watch for it to process and trigger the end of the flight.

From there, there’s variations we would want to make sure were addressed, such as:

Test Case 3

• Begin flight - send signals indicating takeoff
  • Confirm proper operation by watching return signals over telemetry
• Indicate early stage separation over simulated local communications
  • Watch for indication of this on telemetry stream
• Flight termination - send signals over RF to flight termination system
  • Watch for it to process and trigger the end of the flight.

And of course there are many other variations to watch out for.  Perhaps especially ones such as:

Test Case 4

• Flight termination - send signals over RF to flight termination system
  • Watch telemetry - it should ignore it since it is before launch
• Begin flight - send signals indicating takeoff
  • Confirm proper operation by watching return signals over telemetry
  • Perhaps there is a lockout or reset procedure - in which case that would need to be completed before a successful takeoff

Instrumentation

What sorts of interfaces do we control and measure in this test?

Tester outputs, system inputs

First, we need to simulate the environment to the system - so it depends on the tester simulating all of the interfaces external to the system.  Testers such as this actuate or control serial and analog interfaces, such as vibration sensors, voltage inputs, along with aerospace buses for interfaces to other subsystems.  RF signals are also generated, such as over-the-air commands from ground stations.

System outputs, tester inputs

Second, we need to measure the stimuli the system produces.  Things to measure often include RF measurements, but are broken down into two major categories - physical measurements and signaling.

RF physical layer measurements are geared towards measuring the characteristics of the signal itself.  This includes such measurements as spectral content, power levels, distortion, and intermodulation.  It requires expensive instrumentation class RF equipment.

RF signaling is using the RF signal, not measuring it.  It is not concerned with the physical measurements about the signal itself like the above, but the information communicated by it.  So signaling is primarily concerned with the demodulation and interpretation of the signal.  The RF quality of the instruments need not be as high, but often the software investment will be higher as it needs to implement the demodulation and protocol used by the system under test.  All of this is done with the end goal that the data sent by the flight system can be evaluated in the tests. 

Depending on the system, the instrumentation can generate as well as measure (physical layer) and interpret (signaling)  multiple data streams on multiple bands.  This system implements generation on one band, and both physical measurements and signaling on other bands.

Conclusion

Working on this system has been a blast for the Endigit team (pun intended but hopefully taken lightly). It is awesome to build and integrate and test such a wide range of technologies.