**A**

**acceleration**

> the rate at which an object's velocity changes with time

**active vibration isolation**

> see **vibration isolation - active**

**aliasing**

> a computational artifact inherent in all digital signal processing
> which has been preceded by an analog-to-digital conversion; it occurs
> when sampling an analog signal at points further apart than 1/(2B),
> where B is the bandwidth (in Hertz) of the signal being sampled,
> resulting in high frequency data content manifesting itself as errors
> at low frequencies; the only practical way to avoid aliasing errors in
> digital signal processing is by removing (e.g. by filtering) data with
> spectral content above the highest frequency of interest prior to the
> analog-to-digital conversion

**analog signal**

> a signal defined over all time with infinite precision; a
> continuous-time, continuous-amplitude signal

**anti-alias filtering**

> lowpass filtering of an analog signal to reduce the effects of
> aliasing

**ARIS**

Active Rack Isolation System for the International Space Station

**ASTRE**

> Accelerometre Spatiale Triaxiale Electrostatique sensor of the
> Microgravity Measurement Assembly

**atmospheric drag**

> deceleration of a vehicle caused by friction (drag) with the
> atmosphere

**attenuation**

> a measure of motion transmitted between two systems defined as the
> ratio of the isolated system motion to the support structure motion

**attitude maintenance**

> a condition of an orbital vehicle in which attitude control is
> maintained

**B**

**body coordinate system**

> a right-handed, orthogonal coordinate system with the +Xb axis
> extending forward (through the Orbiter nose) and the +Zb axis
> extending downward through the Orbiter belly; the +Yb axis extends to
> starboard

# C

**center of gravity**

> the balance point of an object in a steady gravitational (or
> acceleration) field

**center of mass**

> the centroid of the distributed mass of an article (e.g., a
> spacecraft)

**Control Unit (CU)**

> the central component of the SAMS-II instrument on the ISS

**cumulative RMS acceleration vs. frequency**

> a plot of the cumulative root-mean-square acceleration versus
> frequency quantifies the contributions of spectral components at and
> below a given frequency to the overall root-mean-square acceleration
> level for the time frame of interest

**cutoff frequency (f**c**)**

> frequency above which filtering is performed; highest frequency of
> interest

## D

**DC**

> electrical acronym for Direct Current that has been generalized to
> mean average or steady value

**deadband**

> allowable angular displacement from the desired flight mode, usually
> between 0.5 and 3 degrees; thruster firings are required for attitude
> control when the carrier exceeds the deadband

**digital signal**

> a signal defined only at sampling times with finite precision; a
> discrete-time, discrete-amplitude signal

**discrete Fourier transform (DFT)**

> discrete-time implementation of Fourier transform (see **Fourier
> transform**)

**dither**

> vibration employed in some mechanical systems to avoid stiction and to
> ensure smooth motion

**drop facility**

> research facility that creates a microgravity environment by
> permitting experiments to freefall through an enclosed vertical tube
> or shaft

## E

**encoding**

> assigning unique (binary) codes to quantized samples

**ergometer**

> a bicycle-type device used by crew members to maintain their physical
> fitness during a mission by means of pedaling

## F

**fast Fourier transform (FFT)**

> discrete-time implementation of Fourier transform that speeds
> computation when an integer power of two number of points is used (see
> **Fourier transform**)

**flight mode**

> attitude of the carrier with respect to the earth; expressed in terms
> of the Orbiter body axes, local vertical and velocity vector, e.g.,
> -ZLV,+YVV

**Fourier transform**

> a mathematical transform which resolves a time series into the sum of
> a DC (average) component and a series of sinusoids of different
> amplitude and frequency

**free drift**

> a condition of an orbital vehicle in which attitude control is
> disabled

**free-flyer**

> an unmanned orbiting spacecraft

**frequency resolution (**∆**f)**

> spacing in frequency between consecutive data points; limits ability
> to resolve adjacent spectral peaks (note dF=∆f)

**fundamental frequency**

> the lowest (base) frequency of a signal

## G

**g**

> an acceleration equal to Earth's gravitational acceleration at sea
> level (nominally 9.8 m/sec2)

**g-jitter**

see **microgravity environment**

**g-LIMIT**

> GLovebox Integrated Microgravity Isolation Technology microgravity
> vibration isolation system for the Microgravity Science Glovebox;
> developed by McDAC and NASA MSFC

**g vs. time plot**

> a two-dimensional graph, which shows acceleration magnitude versus
> time

**gravity gradient effects**

> the residual acceleration imposed by an off-center of mass location on
> an orbiting vehicle

## H

**harmonic**

> a frequency which is an integer multiple of the fundamental frequency

**HiRAP**

> High Resolution Accelerometer Package, an accelerometer managed by
> NASA JSC for use on the International Space Station as a component of
> MAMS

## I

**Interim Control Unit (ICU)**

> an initial central component of the SAMS-II instrument on the ISS

**International Space Station (ISS) Program**

> a cooperative venture between international partners to build a large
> orbiting laboratory in space

**interval average**

> a calculation method which averages a time interval of data points and
> produces one average value for the time interval

## J

**Japanese Experiment Module (JEM)**

> a module to be provided by NASDA for the ISS

## 

## K

**Ku band antenna**

a communication antenna attached to the forward wall of the Shuttle
cargo bay; this antenna typically operates with a 17 Hz dither vibration

### L

**life support systems**

> collection of pumps, valves, fans, etc. to provide circulation of
> vital materials (e.g. air, water) to the crew and other living
> organisms in an orbital spacecraft

**LV / LH / VV**

> local vertical, local horizontal, velocity vector; these may be used
> in specifying the attitude of an orbital vehicle

## M

**MAMS**

> Microgravity Acceleration Measurement System, an accelerometer managed
> by NASA JSC for use on the International Space Station

**mg (milli-g)**

> an acceleration equal to one-thousandth of Earth's gravitational
> acceleration at sea level

**µg (micro-g)**

> an acceleration equal to one-millionth of Earth's gravitational
> acceleration at sea level

**microgravity environment**

> an environment in which the effects of gravity are small compared to
> those experienced on Earth; a complex combination of accelerations
> typically present on an orbiting spacecraft or ground-based carrier
> (such as drop facility, parabolic flight aircraft, or sounding
> rocket); comprised of quasi-steady, oscillatory, and transient
> accelerations; historically referred to as g-jitter and residual
> acceleration; also not zero-gravity

**middeck**

> center deck of the Shuttle crew compartment which provides crew
> accommodations and contains three avionics equipment bays; small
> payloads may be accommodated in the middeck

**MIM**

> Microgravity Isolation Mount vibration isolation system developed by
> the Canadian Space Agency

**Mir Space Station**

> Russia's space station launched February 20, 1986; its name means
> peace or world in Russian; it is a manned, modular, permanent, and
> multi-mission station

**mission elapsed time (MET)**

> time basis for Shuttle missions; MET begins at zero days, zero hours,
> zero minutes, zero seconds at the time of Shuttle lift-off at launch

**MMA**

> Microgravity Measurement Assembly, an accelerometer managed by
> ESTEC/ESA for use on the Shuttle

**MMD**

> Microgravity Measuring Device, an accelerometer managed by NASA JSC
> for use on the Shuttle

**MPESS**

> Mission Peculiar Equipment Support Structure; experiment carrier
> mounted in the Shuttle cargo bay

## N

**NASDA**

> National Space Development Agency of Japan; Japan's space agency

**Nyquist criteria**

> sampling rate must be at least twice that of the highest frequency
> contained in the signal of interest

**Nyquist frequency (f**N**)**

> highest resolvable frequency; half the sampling rate (fN = fs/2)

## O

**OARE**

> Orbital Acceleration Research Experiment, a low-frequency
> accelerometer managed by NASA Glenn for use on Shuttle

**one third octave band RMS acceleration vs. frequency**

> a plot of one third octave band root-mean-square acceleration versus
> frequency that quantifies the spectral content in proportional
> bandwidth frequency bands for a given time interval of interest (100
> seconds for ISS vibratory requirements)

**Orbital Acceleration Research Experiment (OARE)**

> an accelerometer managed by NASA Glenn for use on Shuttle; measures
> acceleration in the quasi-steady regime

**Orbital Maneuvering System (OMS)**

> provides the thrust for orbit insertion, orbit circularization, orbit
> transfer, rendezvous, de-orbit, launch abort to orbit and launch abort
> once around

**Orbiter**

> a component of the National Space Transportation System (NSTS);
> individual vehicles are known by their Orbiter Vehicle (OV) numbers
> (e.g. OV-105); commonly referred to as the Shuttle (see **Space
> Shuttle**)

**oscillatory**

> a signal that varies above and below a mean value; acceleration
> components which are periodic in nature; for microgravity
> applications, accelerations with a frequency greater than 0.01 Hz;
> sources of oscillatory accelerations include: structural resonance,
> machinery operation, and crew activity

## P

**parabolic flight aircraft**

> research aircraft which creates microgravity conditions by flying a
> parabolic trajectory; about 40 periods of 15 to 20 seconds of
> microgravity can be flown during a single flight

**Parseval's theorem**

> equivalence between the root-mean-square value computed from the time
> domain signal to that computed from the frequency domain by taking the
> square root of the integral of the power spectral density

**passive vibration isolation**

> see **vibration isolation - passive**

**performance**

> a measure of how well a system achieves specified goals

**power spectral density (PSD)**

> a function that quantifies the distribution of power in a signal with
> respect to frequency

**Primary Reaction Control System (PRCS)**

> the RCS normally used for Shuttle maneuvers; PRCS jets provide 870
> pounds of vacuum thrust each

**Principal Component Spectral Analysis (PCSA)**

> a variation of spectral averaging and spectrogram analyses, which
> employs a peak detection algorithm to accumulate magnitude and
> frequency values of dominant or persistent spectral contributors and
> display them via a 2-D histogram

**Principal Investigator Microgravity Services (PIMS)**

> a project at NASA Glenn which analyzes microgravity environment data
> to support Principal Investigators and to characterize the
> microgravity environment of microgravity carriers

## Q

**QSAM**

> Quasi-Steady Acceleration Measurement, an accelerometer managed by DLR
> for use on the Shuttle and FOTON free flyer

**quantization**

> digitization; converting discrete-time, continuous-amplitude signal to
> discrete-time, discrete-amplitude signal

**quasi-steady**

> acceleration components which are unchanging or change very slowly;
> for microgravity applications, typical variations occurs on the order
> of an orbital period or more (approximately 90 minutes); sources of
> quasi-steady accelerations include aerodynamic drag, gravity gradient
> effects, and rotational effects

**Quasi-steady Three-dimensional Histogram (QTH)**

> a histogram technique which shows the variation of three dimensional
> accelerations in three two-dimensional plots

## R

**rack level isolation**

> the application of isolation to an entire rack structure; this is the
> standard design approach for the International Space Station

**Reaction Control System (RCS)**

> provides the thrust for Shuttle attitude (rotational) maneuvers
> (pitch, yaw and roll) and for small velocity changes along the orbiter
> axis (translation maneuvers)

**reference frame**

> a location against which an object\'s changes in position are
> observed; a set of lines or surfaces used for defining coordinates
> that define positions, directions, velocities, etc.

**residual acceleration**

> see **microgravity environment**

**RTS-EE**

> remote triaxial sensor head - electronics enclosure (SAMS-II)

**RTS-SE**

> remote triaxial sensor head - sensor enclosure (SAMS-II)

## S

**sample rate (f**s**)**

> rate at which analog signal is sampled (samples per second)

**sampling**

> discretization; converting an analog signal to a discrete-time,
> continuous-amplitude signal

**SAMS**

> Space Acceleration Measurement System, an accelerometer managed by
> NASA Glenn for use on Shuttle, Mir, and KC-135

**SAMS-FF**

> SAMS for Free Flyers, an accelerometer managed by NASA Glenn for use
> on sounding rockets, Shuttle, and KC-135

**SAMS-II**

> a second generation SAMS, an accelerometer managed by NASA Glenn for
> use on the ISS

**SIMO water dump**

> an action taken by the Shuttle when waste water and supply water are
> simultaneously vented out the port side of the fuselage

**smearing**

> an obscuring of the exact nature of frequency domain data, that
> results from the spectral averaging of non-stationary data

**sounding rocket**

> rocket which follows suborbital, parabolic path; used by microgravity
> researchers to achieve several minutes of reduced gravity conditions

**Space Shuttle**

> also known as the National Space Transportation System (NSTS) and
> Space Transportation System (STS); this system is composed of the
> Orbiter vehicles, the external tank, and the solid rocket boosters

**SPACEHAB module**

> a commercial pressurized laboratory module carried in the Shuttle
> cargo bay with shirt-sleeve working conditions for the crew

**Spacelab module**

> pressurized laboratory module carried in the Shuttle cargo bay with
> shirt-sleeve working conditions for the crew

**spectral averaging**

> averaging of power spectral densities (Welch\'s method) as a means of
> noise suppression

**spectral leakage**

> a by-product of convolution that limits ability to resolve
> closely-spaced spectral peaks

**spectrogram plot**

> a two-dimensional graph, which shows power spectral density magnitude
> (as color) versus frequency versus time; In other words, it displays
> how the power spectrum varies with time

**stability margin**

> a measure of how much uncertainty or variation can be tolerated with
> system stability maintained

**STABLE**

> Suppression of Transient Accelerations By LEvitation microgravity
> vibration isolation system developed by McDAC

**station keeping**

> see **attitude maintenance**

**structural coordinate system**

> a right-handed, orthogonal coordinate system with the +X0,axis
> extending aft (through the Orbiter tail) and the +Z0 axis extending
> upward out of the Orbiter payload bay; the +Y0 axis extends to
> starboard

**structural (natural) frequency**

> the lowest resonant frequency of a mechanical system

**sub-rack level isolation**

> the application of isolation to a specific payload within a rack or
> some other configuration

**supply water dump**

> an action taken by the Shuttle when supply water is vented out the
> port side of the fuselage

**T**

**Telescience Support Center (TSC)**

> a user's operation center remote from but connected to a primary
> operations center; full voice communication and mission video and data
> are available in a TSC

**temporal resolution (dT)**

> spacing in time between consecutive PSDs on a spectrogram; limits
> ability to resolve temporal changes (note dT does not equal ∆t)

**tolerance criterion**

> least stringent environment specification which will allow a
> successful experiment to take place; in this context, it is given in
> terms of the level of acceleration, including its maximum and/or
> minimum magnitude, orientation, frequency, duration, etc.

**transient**

> acceleration disturbances which are impulsive (short duration, high
> amplitude) in nature; sources of transient accelerations include:
> thruster firings, crew activity, and docking

**transmissibility function**

> the transfer function that relates the magnitude of the acceleration
> (or position) response of a system to the acceleration (or position)
> of the support structure

**treadmill**

> an exercise device used by crew members to maintain their physical
> fitness during a mission by means of walking and running

**trim mean filter (a.k.a. trimmean filter, trimmed mean filter)**

> a filtering technique typically applied to low frequency data which is
> intended to mitigate the effects of higher amplitude, higher frequency
> accelerations

## U

**Utilization Flight (UF)**

> a Shuttle mission to the ISS which carries re-supply material and/or
> experiment apparatus

## V

**Vernier Reaction Control System (VRCS)**

> the RCS are normally used for finite maneuvers and station keeping
> (long-time attitude hold); VRCS jets provide 24 pounds of vacuum
> thrust each

**vibration isolation**

> the reduction of vibration transmitted to/from an object from/to the
> surrounding structure

**vibration isolation - active**

> the use of active mechanical devices such as motors, actuators, and
> sensors, to reduce the transmission of vibrations by applying forces
> to cancel the measured motion

**vibration isolation - passive**

> the use of passive mechanical devices such as springs and dampers to
> reduce the transmission of vibrations

## W

**waste water dump**

> an action taken by the Shuttle when waste water is vented out the port
> side of the fuselage

## X

**X**b**, Y**b**, Z**b

> components of the Orbiter body coordinate system (see **body
> coordinate system**)

**X**0**, Y**0**, Z**0

> components of the Orbiter structural coordinate system (see
> **structural coordinate system**)

## Y

###  Z

**zero-gravity or zero-g**

> term historically used to describe the effect of the severely reduced
> gravity environment in a microgravity carrier, such as an orbiting
> spacecraft or a drop tower (see **microgravity environment**)
>
> ***NUMERIC***

**3-DMA**

> Three Dimensional Microgravity Accelerometer, an accelerometer managed
> by the University of Alabama in Huntsville for use on the Shuttle and
> sounding rockets