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Space Satellite Technology Background

Glossary

 

A quick guide to our most popular terminology.

Allan Variance

A measure of frequency stability using M samples, time T between measures and observation time. Also known as sigma-tau.

 

Angular Random Walk (ARW)

A measure of gyroscopic noise, and has a unit of deg/hour1/2 or deg/sec1/2.

 

Attitude and Heading Reference System (AHRS)

Consists of sensors on three axes that provide attitude information for aircraft, including roll, pitch, and yaw. Designed to replace traditional mechanical gyroscopic flight instruments.

 

Bandwidth

The gyroscope operates from DC (zero frequency) to some upper frequency (typically defined by the frequency in which the frequency response function magnitude is down by 3 dB).

 

Bias Instability

The random variation in bias as computed over specified finite sample time and averaging time intervals (e.g. over 100 s or 1 day).

 

Bias Thermal Sensitivity

The bounds within which the Bias may vary as the temperature varies across the operating temperature range, included in the “Over Operating Environments” specification Range:
The specified maximum input rate (°/sec) over which full performance will be provided.

 

Bias

The average over a specified time of accelerometer output measured at specified operating conditions that has no correlation with input acceleration or rotation. Bias is a signed quantity usually expressed in units of acceleration.

 

Clean Rooms

An environment free from dust and other contaminants, used chiefly for the manufacture of electronic components.

 

Composite Error

In general, any error that includes two or more sources; specific meaning is generated by naming the sources. More in detail, it is the maximum deviation of the output data from a specified output function. Composite error is due to the composite effects of hysteresis, resolution, nonlinearity, non-repeatability, and other uncertainties in the output data. Errors Sources are typically RSS’d to obtain a composite error.

 

Coriolis Vibratory Gyroscope (CVG)

A gyroscope that uses a vibrating structure to determine the rate of rotation with the Coriolis Effect.

 

Dynamically Tuned Gyroscopes (DTG)

A rotor suspended by a universal joint with flexure pivots, and the flexure spring stiffness is independent of spin rates.

 

Fibre Optic Gyroscope (FOG)

Uses the interference of light to detect mechanical rotations, making use of the Sagnac Effect.

 

Force-to-Rebalance (FTR)

Mode of operation for rate measuring gyroscopes in detail.

 

Full Range Output

The nominal voltage output for the specified full-range input. The actual voltage output will be a complex compilation of several parameters including scale factor, bias, linearity and temperature characteristics.

 

Hysteresis

The maximum difference, at a fixed temperature, between bias readings during a full range excursion of input rates – first measured at zero input rate after having just returned from the negative full range; and second, that bias reading measured at zero input rate after having just returned from the positive full range.

 

IEEE

The IEEE is a non-profit, technical professional association of more than 360,000 individual members in approximately 175 countries. Through its members, the IEEE is a leading authority in technical areas ranging from computer engineering, biomedical technology and telecommunications, to electric power, aerospace and consumer electronics, among others.

 

Inertial Measurement Unit (IMU)

An electronic device that measures and reports a body’s specific force, angular rate, and sometimes magnetic field surrounding the body, using a combination of gyroscopes and accelerometers.

 

International Standards Organization (ISO)

An organization devoted to the establishment of international standards for the design and fabrication of critical-environment applications. They developed 6 classes in their standards classification system, with class 1 as the cleanest environment.

 

International Traffic in Arms Regulation (ITAR)

A United States regulatory regime to restrict and control the export of Search, Detection, Navigation, Guidance, Aeronautical and Nautical System-related technologies to safeguard U.S. national security.

 

Mean Time Between Failures (MTBF)

The predicted elapsed time between inherent failures of a system during operation.

 

MEMS Gyroscopes

A class of gyroscopes that are printed onto circuit boards using photolithography, using oscillating components to detect the acceleration of vibrating objects.

 

Micro-Electro-Mechanical Systems (MEMS)

Technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements that are made using the techniques of microfabrication

 

Noise

Undesired perturbations in the accelerometer output signal which are generally uncorrelated with desired or anticipated input accelerations. Noise in the accelerometer output is of two types: intrinsic and seismic. Intrinsic noise is generated within the accelerometer and represents the limiting factor in making measurements. Intrinsic noise is random in nature and is characterized by a noise power spectral density (PSD) curve. Seismic noise is a true input acceleration (usually unanticipated by the user). It results from noise sources in the local environment (running motors, seismic shocks, etc.) and their transmission to the accelerometer through the mounting structure which supports the accelerometer.

 

Output Noise

Self-generated electrical noise is defined by its power-spectral-density (PSD) and given in volts-squared per Hz over a given range of frequency. This defines the distribution of noise power in the output of the instrument, on a per-Hz basis.

 

Repeatability

The closeness of agreement among measurements of the same variable, repeated under the same conditions, especially when changes in conditions occur or when the operation is interrupted between the measurements.

 

Resonator

An apparatus that increases the resonance of a sound, especially a hollow part of a musical instrument.

 

Ring Laser Gyroscope (RLG)

Consists of a ring laser having two independent counter-propagating resonant modes over the same path; the difference in the frequencies is used to detect rotation.

 

Scale Factor

The ratio of the change in output (in volts or amperes) to a unit change of the input (in units of acceleration); thus given in mA/g or V/g.

 

Sensing Element

Any device that receives a signal or stimulus (as heat or pressure or light or motion etc.) and responds to it in a distinctive manner.

 

Sensitivity

The ratio of a change in response to a change in an undesirable or secondary input (as the scale factor variation to a unit of power supply voltage change).

 

Shock

The limit of shock which the device will withstand without damage.

 

Start-up Time

The time required for the instrument to produce a usable rate output after power application.

 

Temperature Modeling

The process by which performance characteristics are measured over the operating temperature range and mathematically reduced to obtain constant and coefficients for a multi-order model. The model is normalized to one temperature (typically 20°C). The application of the model to raw data is referred to as ‘correction’.

 

Temperature Sensitivity

The sensitivity of a given performance characteristic (typically scale factor, bias, or axis misalignment) to operating temperature. Specified as worst-case value over the full operating temperature range. Expressed as the change of the characteristic per degree of temperature change; a signed quantity, typically in ppm/°C for scale factor, mg/°C for bias, and mrad/°C for axis misalignment.VibrationSurvival: The specified limit of random vibration (usually expressed in g-RMS over a defined frequency bandwidth) which the device will survive for a limited period of time without damage.

 

Vibration Rectification Coefficient (VRC)

A calculated number obtained by dividing the bias shift that occurs during vibration (rms)2 value of the applied input vibration. Usually expressed in mg/g2 units.

 

Vibration Rectification Error (VRE)

The apparent shift in accelerometer bias when subjected to sinusoidal or random vibration due to the accelerometers even order non-linearity. Usually expressed in mg or g units with a specified vibration applied.

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