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Term	Definition
Access area	The ability to collect information within a defined location by a specific sensor.
Bus	Also known as spacecraft bus. The portion of the spacecraft that supplies the support functions necessary for the payload to meet allocated system objectives. The support provided normally includes, power, C&DH, attitude knowledge and control, and other functions as necessary to meet objectives..
Collection Area	Same as Coverage
Constellation Satellites	Those satellites in the overall GPM Constellation of Satellites that provide passive rainfall measurements.
Core Satellite	The satellite developed by the GPM Project in association with NASDA that has Dual-frequency Precipitation Radar (DPR) and a microwave radiometer. Together these instruments provide rainfall measurement information.
Coverage	The portion of the Earth’s atmosphere and surface that can be accessed. Coverage by individual sensors will vary, and depends upon sensor design and spacecraft altitude. Normally referenced as a function of latitude or latitude bands.
Data Latency	The elapsed time between data collected by a sensor and its availability to a user. Any delays in users’ communication structure are not included within this requirement.
Data Streams	Rainfall measurements which are received from instruments capable of measuring rainfall. The information is typically unprocessed, aside from appending measurement-specific information such as spacecraft and ephemeris data. Data streams may come from Constellation Satellites or the Core Satellite, and from other sources of rainfall measurements, such as ground calibration sites.
Drop Size Distribution (DSD)	The number of aerosol, cloud, or rain droplets per specified size interval per unit volume over a specified range of sizes.
Dynamic Range	The operational range of an instrument in terms of its measurement sensitivity. The instrument ADC and the inherent noise in the background or instrument bound the lower range, whichever is dominant; the upper range is typically the point at which the input signal saturates the detector/receiver .
Field of Regard (FOR)	The area of Earth that a sensor can access over its normal span of motion. This does not include platform movement for a three-axis-stable platform normally pointed at the Earth. This may be discussed in either _ angle degrees or km2 of surface area.
Field of View (FOV)	The area of Earth that a sensor can collect from at any moment, but without moving the sensor. When the mission objective is centric about earth-based collection, then this may be discussed in either _ angle degrees or km2 of surface area.
Flexible Architecture	A GPM system level approach that supports evolution of the project to accommodate changes in available resources as the project matures.
Goal	A requirement which is significantly more difficult to meet than the target requirement but which, if met, would greatly enhance the utility of the data to the users. A "goal" value for a specified parameter is the minimum (maximum) of the range of preferred values for the parameter, where lower (higher) values of the parameter provide better performance or are otherwise more desirable. A design value falling between the target and goal values is desired by the GPM Project, and a value closer to the goal than the target is generally preferred, depending upon the impacts associated with approaching the goal. (See "target".)
GPM Constellation of Satellites	All spacecraft that provide rainfall measurements (data streams) used by the GPM Project. These spacecraft include both dedicated, GPM-launched spacecraft, and spacecraft launched by other programs, including operational weather satellites and environmental research satellites. This includes both the Constellation Satellites and the Core Satellite.
GPM-Dedicated Constellation Satellite	A constellation satellite developed and launched by the GPM Project, including its foreign partners. The on-board rainfall measurement instrument will most likely consist of a single microwave radiometer.
Ground Calibration Site	A dedicated, ground-based, rain measuring station with calibrated instrumentation able to provide accurate measurements of rain-related parameters. Also known as a Validation Site.
Half Power Beam Width (HPBW)	The HPBW is the angular width between the two directions of the microwave radiometer antenna at which the main beam gain function is one-half its maximum value within a plane containing the maximum gain of the main beam lobe. The HPBW may also be the physical size of the projection of that angle at the location where the measurement is taken.
Housekeeping	Functions such as orbit and attitude maintenance, navigation, power, command, telemetry and data handling, alignment, heater powers, temperature measurements, etc.
Launch Vehicle	The combination of one or more staged propulsion units, guidance control, fairing, and associated interface equipment necessary to launch a GPM spacecraft (launch vehicle payload) into orbit.
LCC Basis Value	The minimum design time that the NASA GPM space segment contribution will operate after a successful launch and initialization.
Life Cycle Cost (LCC)	Total cost of a system from pre-concept formulation through the LCC Basis value period. See also LCC Basis Value
Mean Mission Duration (MMD)	A probabilistic estimate identifying when a satellite will have a 50% chance of failing. This estimate is based on reliability calculations using individual subsystems and/or elements rolled up to the satellite level.
Measurement Accuracy (Bias)	The route-mean-square (rms) difference between the measured value of a parameter and the estimate of its true value. This is also the systematic bias associated with a measurement.
Measurement Data Latency	See Data Latency
Measurement Precision (Random Error)	The standard deviation of a measurement about its mean, measured value.
Mid-latitudes	The set of all locations on the earth's surface between 20 and 50 degrees north latitude and between 20 and 50 degrees south latitude.
Minimal Latitudinal Extent of GPM-Dedicated Constellation Satellite	Coverage as applied to the Constellation Satellites’ ability to collect information. It is driven by the inclination of the orbit. See also Coverage.
Minimum Latitudinal Extent of Core	Coverage as applied to the Cores Satellite’s ability to collect information. It is driven by the inclination of the orbit. See also Coverage.
Mission Sensors	Any sensor supporting the GPM mission objective. These sensors may be developed by the GPM Project and installed on GPM-dedicated constellation satellites, may be the source of rainfall measurement data streams from other constellation satellites, or may be sensors installed at GPM ground calibration sites.
On-Orbit Design Life	The period during which the NASA provided satellite or satellite components must meet all mission objectives. The definition uses the MMD to determine the on-orbit life.
Oversampling	Overlapping measurements within the same or subsequent scans of a sensor.
Payload (launch vehicle)	The portion of the total launch vehicle that will be deployed in space. In this context it is synonymous with spacecraft portion of the space segment.
Payload (spacecraft)	The combination of instrument and direct instrument support (e.g., processing, covers, antennas, thermal control) that works in conjunction with the Bus to meet space segment allocated requirements.
Percent Equatorial Data Coverage	The portion of the troposphere immediately above the equator that can be accessed over a specified period of time.
Precipitable Water	The total amount of water, water vapor and ice contained in a vertical column of the atmosphere.
Rainfall Rate	The amount of liquid water accumulated in mm/hour. For space systems, this value is extrapolated from instantaneous measurements.
Revisit Time	The time interval between successive measurements of the same area by a sensor of the GPM constellation. Because of the structure of the constellation, successive revisit times may not be equal, and the distributions of revisit times during a given period of time at different locations will be different. See also Sampling Period.
Sample Frequency	Frequency at which successive samples are taken from a single sensor during a pass or scan.
Sampling Period	See Revisit Time
Satellite	A spacecraft, including the spacecraft bus and its sensors, in orbit around the earth. Generally this refers to spacecraft in their intended operational orbit.
Snow Accumulation	The depth of the snow on the ground.
Snow Detection	The ability to measure the presence of snow on the ground and in the atmosphere. No indication of detection accuracy.
Snowfall Rate	The amount of snowfall accumulated in mm/hour. For space systems, this value is extrapolated from instantaneous measurements. Applies only to snow and not snow/rain mixtures.
Spacecraft	A manmade object that is designed to support one or more objectives. Normally includes a spacecraft bus and payload or instrument suite that is designed to operate in space.
Spatial Resolution	The minimum physical distance between two objects on the ground, or two atmospheric conditions at which a sensor is able to distinguish the objects. The horizontal spatial resolution for a microwave radiometer may be different from the HPBW of the antenna, and is a function of the scan geometry of the instrument and the signal processing applied to the measurement.
Spatial Variability	The uncertainty in spatial measurement reference of a collection sample. ()
Super Site	A validation site provided by GPM-NASA to support calibration and validation of satellite based rainfall measurements.
Target	Failure to meet a threshold requirement for a level 1 requirement is serious and may place the entire project at risk. A "target" value for a specified parameter is the maximum (minimum) of the range of preferred values for the parameter, where lower (higher) values of the parameter provide better performance or are otherwise more desirable. A design value falling between the target and goal values is desired by the GPM Project, and a value closer to the goal than the target is generally preferred, depending upon the impacts associated with approaching the goal. (See "goal".)
Telemetry	Health and status data of the satellite, including command authentication.
Users	The people, such as hydrological scientists and weather forecasters, who employ the obtained environmental data.
Validation Site	See ground calibration site

- Many of the above definitions are contingent on understanding what the scientist(s) think they want.
- The definitions will mature, expand, and have additions.

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