Metrology is critical in space exploration and rocket science as the performance of an instrument or vehicle has to be calibrated on earth to adequately carry out its task in space.
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The science of measurement is known as metrology. Every scientific device that is developed has to be calibrated against known quantities before it is deployed in the field. Calibration is a crucial component of measurement controls and is necessary to maintain the sufficiency of measuring and test equipment.
Metrology and Calibration
The controls and specifications needed to guarantee appropriate measurement quality and offer measurement assurance underpin the need for metrology. It includes everything related to measurement, including planning, carrying out, recording, assessing, and interpreting the results, as well as estimating measurement uncertainties. Scientific metrology in particular is concerned with the organization, creation, and upkeep of measurement standards.
International System of Units
The only system of measurement units that is accepted throughout the world is the International System of Units (or "the SI"). To ensure that our everyday measures stay similar and uniform globally, the SI defines units for many different types of measurement. The seven SI base units have been classified as "constants of nature" since 2020, allowing for future advancements in measurement precision and stability.
Known basic units are necessary when constructing any new instrument. As a simple example, one meter is a basic unit of length that is referenced when constructing physical devices. The definitions of the seven SI base units are:
- Kilogram - The kilogram is the weight unit that corresponds to the weight of the international kilogram prototype.
- Second - The second is the number of 9,192,631,770 radiation cycles that it took for the cesium 133 atom to change from its ground state's two hyperfine levels.
- Meter - The length of the meter is the distance that light would have traveled in a vacuum over a period of 1/299,792,458 seconds.
- Kelvin - The proportion of the triple point of water's thermodynamic temperature equal to 1/273.16 is known as the Kelvin unit of thermodynamic temperature.
- Mole - A system that has as many elementary units as there are atoms in .012 kg of the carbon 12 isotope is said to contain a mole of material (about 6.022x1023 atoms). When using the mole, it is necessary to specify the elementary entities, which can be atoms, molecules, ions, electrons, or other particles, or specific collections of such particles.
- Ampere - The ampere is the continuous current that produces a force of 2x10-7 Newton per meter of length between two parallel, straight conductors of infinite length and insignificant circular cross-sections put one meter apart in a vacuum.
- Candela - The candela is the amount of light emitted in a certain direction by a source with a radiant intensity of 1/683 watt per steradian and a monochromatic emission frequency of 540x1012 Hz.
Calibration is typically defined as a set of actions carried out in accordance with a clear, documented procedure that compares measurements made by an instrument to measurements made by a more precise instrument or standard, with the goal of identifying and reporting errors in the instrument tested or eliminating them by adjustment.
A simple example of calibration is initializing a telescope for stargazing. Typically in the Northern hemisphere, the telescope can be aimed at the North star and the coordinates calibrated to its location before exploring other objects in the sky. A more cutting-edge example is using the “second” base unit above to synchronize satellites orbiting the earth. Developed at the National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado, the cesium fountain atomic clock is the main time and frequency reference for the United States of America. All the satellites and other space exploration vehicles propelled from the US reference the NIST optical clock for reliable operation.
Metrology for Space Exploration
Currently, deep space navigation is conducted by sending signals from Earth to spacecraft, which subsequently send those signals back to Earth. On Earth, atomic clocks track how long it takes a signal to travel in both directions. Using this information human navigators on Earth can direct the spaceship to its destination.
Future deep space travel will require quicker methods for the astronauts inside a spaceship to know where they are. Ideally without having to send signals back to Earth. Especially when considering longer flights to explore the solar system. A spacecraft equipped with a deep space atomic clock would be able to instantly locate itself using its onboard navigation system after receiving a signal from Earth.
Similar to NIST hosting the standard base of time with the atomic clock, other laboratories are designated to carry out calibration and metrology for various other instrumentation. For example, the Metrology and Calibration Laboratory (MCL) at NASA's Marshall Space Flight Center in Huntsville, Alabama, employs scientists who perform precise measurements of space exploration apparatus, including the Space Launch System (SLS).
The SLS rocket, which produces more thrust than even the Saturn V rocket that carried the Apollo astronauts to the moon, is the most potent one ever created. It is over 100 meters tall making it taller than the Statue of Liberty in New York.
Such enormous rockets require accurate calibrations as they are built to function at high pressures and temperatures with space stations the size of six-bedroom homes that must sustain people spending years living and working in space.
Critical components of the SLS and space station are being tested with equipment that measures quantitatively. Hardware for SLS, including the intertank, liquid oxygen and hydrogen tanks, thermal protection systems, and other components, is calibrated at the MCL before testing. The Environmental Control and Life Support System, which supports the crew with essentials, is one of the life support systems that are manufactured and tested on the space station using the same calibration procedure.
Marshall's Laboratory is in charge of making sure that the measurement and testing tools used by its clients are precisely calibrated and have measurements that can be traced to a national metrological institute. The sort of measurement done at Marshall will be the same type of measurement made at another NASA location thanks to a consensus standard, also known as an intrinsic standard.
In order to successfully achieve NASA objectives, the MCL continuously collaborates with Marshall and other entities to develop the most technologically sophisticated measuring concepts and procedures. All measurement and test equipment, including specialized equipment for research and development and common equipment used in everyday operations and manufacture, are calibrated by its staff.
Success in NASA's exploration of the furthest reaches of space hinges on accurate and dependable measurements. These precise, exact measurements form the basis of NASA's exhaustive missions, which help guide decisions that lead to groundbreaking scientific advancements.
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References and Further Reading
Samuelson, A. (2019) Five Things to Know about NASA's Deep Space Atomic Clock.
[Online] NASA.GOV. Available at: https://www.nasa.gov/feature/jpl/five-things-to-know-about-nasas-deep-space-atomic-clock
Newton, K. (2017) Precise Measurements On Earth Ensure NASA’s Big Spacecraft Work In Space. [Online] NASA.GOV. Available at: https://www.nasa.gov/centers/marshall/news/news/releases/2016/precise-measurements-on-earth-ensure-nasa-s-big-spacecraft-work-in-space.html
Electronics-Notes. Seven Base SI Units: System International.
[Online] electronics-notes.com. Available at: https://www.electronics-notes.com/articles/basic_concepts/si-system-international/si-base-units.php
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