Propellant volume is gauged using acoustic resonances counting in a propellant tank based on spectral theory (Weyl's Law). This provides shape-invariant spectral characteristics enabling arbitrary shapes of the ullage/propellant, systematic control of accuracy, applicable to both conducting (MMH) and non-conducting propellants (Lox, LH2) and is non-intrusive. The technologies that enable the concept include sensors, actuators, electronics and data-processing and inference software. The concept of operations:

• Tap outside tank wall (mechanical actuator)
• This excites acoustic resonances in the liquid;
• Resonances are detected (via accelerometers)
• Information processed by electronics and peaks counting software,
• Infer volume of liquid based on Weyl's law.

• Technology is based on shape-invariant characteristics of the spectra, which enables mass-gauging in zero-gravity conditions (where alternative approaches do not apply)
• Minimal or no equipment is installed inside the tank
• Applicable to both conducting (MMH) and non-conducting propellants (LOx, LH2)