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absorption. The process by which electromagnetic radiation (EMR)
is assimilated and converted into other forms of energy, primarily heat.
Absorption takes place only on the EMR that enters a medium. A substance
that absorbs EMR may also be a medium of refraction, diffraction, or scattering;
however, these processes involve no energy retention or transformation
and are distinct from absorption.
absorption band. A range of wavelengths (or frequencies) of electromagnetic
radiation that is assimilated by the atmosphere or other substance.
acquisition. (1) Image captured by satellite sensor. (2) The process
of searching for and locking onto a received signal.
albedo. (1) The ratio of the amount of electromagnetic energy
reflected by a surface to the amount of energy incident upon it, often
expressed as a percentage. (2) The reflectivity of a body as compared
to that of a perfectly diffusing surface at the same distance from the
Sun, and normal to the incident radiation. Albedo may refer to the entire
solar spectrum or merely to the visible portion.
alignment data. Angular measurement of the physical position of
the optical axis with respect to the primary space vehicle reference axes.
altitude. Height above a datum, the datum usually being mean sea
level. Refers to point above the Earth's surface rather than those on
analog-to-digital conversion. The process of sampling continuous
analog signals in order to convert them into a stream of digital values.
ETM+ data undergo such a conversion prior to downlinking. Abbreviated
as A/D conversion.
angular velocity. Also called rotational velocity, it is the amount
of rotation that a spacecraft undergoes per unit time. For Landsat 7 it
is equal to 1.059 mrad/sec ((233 paths/cycle * 2*pi*1000 mrad/path) /
(16 days/cycle * 86400 sec/day)).
angle of drift. The angle between the heading of the axis of a
craft and its ground track.
anomaly. A deviation from the norm.
aperture. An opening that admits electromagnetic radiation to
a detector or film. An example would be the lens diaphragm opening in
apogee. The point in the orbit of a heavenly body, especially
of a manmade satellite, at which it is farthest from the Earth.
ascending node. The point at which the orbit of an earth satellite
intersects the plane of the equator going from south to north.
at-aperture-radiance. The radiance at the aperture of the sensor.
attenuation. The reduction in the intensity of radiation with
distance from its source due to atmospheric absorption and/or scattering.
It does not include the inverse-square decrease of intensity of radiation
with distance from the source.
attitude. The angular orientation of a spacecraft as determined
by the relationship between its axes and some reference line or plane
or some fixed system of axes. Usually, Y is used for the axis that defines
the direction of flight, x for the crosstrack axis, perpendicular to the
direction of flight, and z for the vertical axis. Roll is the deviation
from the vertical (the angle between the z-axis of the vehicle and the
vertical axis, or angular rotation around the y-axis). Pitch is the angular
rotation around the x-axis. Yaw is rotation around the z-axis.
azimuth. The arc of the horizon measured clockwise from the north
point to the point referenced. Expressed in degrees. Azimuth indicates
direction, not location.
background. Any effect in a sensor or other apparatus or system
above which the phenomenon of interest must manifest itself before it
can be observed.
band sequential. A format that arranges the data by band such
that all of the data from band 1 followed by all of the data from band
band, spectral. An interval in the electromagnetic spectrum defined
by two wavelengths, frequencies, or wave numbers. With Landsat, bands
designate the specific wavelength intervals at which images are acquired.
BCH. An error detection and correction scheme named after its
inventors Bose, Chanduri, and Hochergan.
black body. An ideal body which, if it existed, would be a perfect
absorber and a perfect radiator, absorbing all incident radiation, reflecting
none, and emitting radiation at wavelengths. In remote sensing, the exitance
curves of black bodies at various temperatures can be used to model naturally
occurring phenomena like solar radiation and terrestrial emmitance.
brightness value. In Landsat parlance, a number in the range of
0-255 that is related to the amount of planetary radiance striking a sensor's
calibration data. In remote sensing, measurements pertaining to
the spectral or geometric characteristics of a sensor or radiation source.
Calibration data are obtained through the use of a fixed energy source
such as a calibration lamp, a temperature plate, or a geometric test pattern.
The application of calibration data to restore measurements to their true
values is called rectification.
coherent noise. The noise associated with periodic signals arising
from power supplies, transmitters and clock signal typically.
color. That property of an object which is dependent on the wavelength
of the light it reflects or, in the case of a luminescent body, the wavelength
of the light it emits. If, in either case, this light is of a single wavelength,
the color seen is a pure spectral color, but, if the light of two or more
wavelengths is emitted, the color will be mixed. White light is a balanced
mixture of all the visible spectral colors.
color composite. A color image produced by the combination of
three individual monochrome images in which each is assigned a given color.
For ETM+ data, if blue is assigned to band 1, green assigned to band 2,
and red assigned to band 3, a true color image will result.
cubic convolution. A high-order resampling technique is which
the brightness value of a pixel in a corrected image is interpolated from
the brightness values of the 16 nearest pixels around the location of
the corrected pixel.
data capture. The receipt and storage of return link mission data
at the CADU level.
data continuity. A NASA requirement to ensure that Landsat 7 data
are compatible to those obtained by earlier Landsat satellites.
data granule. The increment of image data stored in the archive,
i.e. an interval, swath, or WRS scene.
data loads. Data and command transfers from the MOC to the onboard
dark shutter image data. The image data obtained from ETM+ detectors
when the calibration shutter obscures the detectors from incident electromagnetic
descending node. The point at which the orbit of an earth satellite
intersects the plane of the equator going from north to south.
detector. The composite circuitry supporting the development of
a single output data sample.
detector sample. The process of determining the transfer characteristics
(detector mean output as a function of incident exposure) for each detector
digital terrain elevation data (DTED). Digital information produced
by DMA which provides a uniform matrix of terrain elevation values. DTED
is commonly used to terrain correct Landsat data.
distortion. A change in scale from one part of an image to another.
dwell time. Refers to the momentary time interval during which
a detector is able to, or allowed to, sense incoming electromagnetic radiation
within its intended instantaneous field of view.
dynamic range. The ratio of the maximum signal to the smallest
EDC. Earth Resources Observation System Data Center is a national
archive, production, distribution and research facility for remotely sensed
data and other geographic information. (see EROS)
electromagnetic radiation. Energy emitted as result of changes
in atomic and molecular energy states and propagated through space at
the speed of light.
electromagnetic spectrum. The system that classifies, according
to wavelength, all energy (from short cosmic to long radio) that moves,
harmonically, at the constant velocity of light.
elevation. Vertical distance from the datum, usually mean sea
level, to a point or object on the Earth's surface.
emission. With respect to electromagnetic radiation, the process
by which a body emits electromagnetic radiation as a consequence of its
kinetic temperature only.
emissivity. Ratio of radiation emitted by a surface to the radiation
emitted by a black body at the same temperature under similar conditions.
May be expressed as total emissivity (for all wavelengths), spectral emissivity
(as a function of wavelength), or goniometric emissivity (as a function
Enhanced Thematic Mapper Plus (ETM+). The ETM+ is a fixed-position
nadir viewing whisk-broom instrument. The viewing swath is produced by
means of an oscillating mirror system that sweeps across track as the
sensor field of view moves forward along-track due to satellite motion.
ETM+ scene. A set of ETM+ observations that covers 170 km in width
by 185 km in length and is centered about a WRS vertex.
engineering data. All data available on-board about health, safety,
environment or status of the platform and instruments.
ephemeris. A set of data that provides the assigned places of
a celestial body (including a manmade satellite) for regular intervals.
Ephemeris data help to characterize the conditions under which remote
sensing data are collected and may be used to correct the sensor data
prior to analysis.
EROS. The Earth Resources Observation System was established in
the early 1970?s under the Department of Interior? U.S. Geological Survey,
to receive, process and distribute data from the United States? Landsat
satellite sensors and from airborne mapping cameras.
field-of-view. The solid angle through which an instrument is
sensitive to radiation. See effective resolution element, instantaneous
field of view, resolution.
focal length. In a camera, the distance measured along the optical
axis from the optical center of the lens to the plane at which the image
of a very distant object is brought into focus.
focal plane. In a sensor, the plane occupied by the detectors,
and on which the radiances sensed are incident.
frame. For Landsat 7, a frame is one Virtual Channel Data Unit
with a frame synchronizer pattern (frame marker) attached. This is the
same as a Channel Access Data Unit (CADU).
geocentric. Any coordinate frame whose origin is relative to the
Earth's center of mass.
geometric correction. The transformation of image data, such as
Landsat data, to match spatial relationships as they are on the Earth.
Includes correction for band-to-band offsets, line length, Earth rotation,
and detector-to-detector sampling delay. For ETM+ data, a distinction
is made between data that have been geometrically corrected using systematic,
or predicted, values and data that have been geometrically corrected using
precise ground control point data and elevations models.
geodetic coordinates. Quantities which define the position of
a point on the spheroid of reference (for example, the Earth) with respect
to the planes of the geodetic equator and of a reference meridian. Commonly
expressed in terms of latitude and longitude.
geodetic accuracy. A measure of how closely a point on the Earth
can be located relative to its true absolute location.
geosynchronous. An Earth satellite orbit in which the satellite
remains in a fixed position over a geographic location on Earth.
Global Position System (GPS). A constellation of satellites that
can be used to determine accurately the orbit data of satellites.
ground control point (GCP). A geographic feature of known location
that is recognizable on images and can be used to determine geometric
correction functions for those images.
ground track. The vertical projection of the actual flight path
of a plane or space vehicle onto the surface of the Earth.
ground truth. Data which are acquired from field checks, high-resolution
remote sensing data, or other sources of known data. Ground truth is used
as the basis for making decisions on training areas and evaluating classification
housekeeping data. All data available onboard about health, safety,
environment, or status of the platform and instruments.
hue. The attribute of a color that differentiates it from gray
of the same brilliance and that allows it to be classed as blue, green,
red, or intermediate shades of these colors.
image. The recorded representation of an object produced by optical,
electro-optical, optical-mechanical, or electronic means. It is the term
generally used when the electromagnetic radiation emitted or reflected
from a scene is not directly recorded on photographic film.
image enhancement. Any one of a group of operations which improves
the interpretability of an image or the detectability of targets or categories
in the image. These operations include contrast enhancement, edge enhancement,
spatial filtering, image smoothing, and image sharpening.
image restoration. A process by which a degraded image is restored
to its original condition. Image restoration is possible only to the extent
that the degradation transform is mathematically invertable.
image-to-image registration. The registration between images taken
at different times.
image transformation. A function or operator which takes an image
as input and produces an image as its output. Depending on the transform
chosen, the input and output images may appear entirely different and
have different intrepretations. Fourier, Hadamard, and Karhunen-Love transforms
as well as various spatial filters, are examples of frequently used image
infrared. Pertaining to energy in the 0.7 - 100 μm wavelength
region of the electromagnetic spectrum. For remote sensing, the infrared
wavelengths are often subdivided into near infrared (0.7 - 1.3 μm),
middle infrared (1.3-3.0 μm), and far infrared (7.0 - 15 μm).
Far infrared is sometimes referred to as thermal or emissive infrared.
instantaneous field of view (IFOV). The solid angle through which
a detector is sensitive to radiation. In a scanning system this refers
to the solid angle subtended by the detector when the scanning motion
is stopped. Instantaneous field of view is commonly expressed in milliradians.
IFOV also refers to the ground area covered by this solid angle.
international ground station (IGS). Any Landsat ground station
not belonging to the United States.
interval. Is a scheduled ETM+ image period along a WRS path, and
may be from 1 to 90 full scenes in length.
irradiance. The measure, in units of power, of radiant flux incident
on a surface.
jitter. Small rapid variations in a variable (such as a waveform)
due to deliberate or accidental electrical or mechanical disturbances
or to changes in the supply of voltages, in the characteristics of components.
Jitter effects arising from the oscillating mirrors and other movable
parts aboard the Landsat spacecraft are often a cause of certain anomalies
in the image data received and must be compensated for by the ground processing
K-band. A radio frequency band extending from approximately 12.5
to 36 gigahertz.
kernel. In the spatial domain, a kernel is a MxM operator which
can be used in the convolution or multiplication with a NxN image to accentuate
certain features or properties of an image. A kernel can also be represented
in the frequency domain as a Fourier transform.
L-band. A radio frequency band extending from approximately 1.0
to 2.0 gigahertz.
Landsat 7. Consists of the spacecraft and the ETM+ payload.
level 0. Space vehicle or instrument data at full space-time resolution
with space-to-ground communication artifacts removed.
light, transmitted. Light that has traveled through a medium without
being absorbed or scattered.
long term acquisition plan. The tasking of the sensor using cloud
predictions to optimize the acquisition of cloud free scenes.
lookup table. An array of values from which functions corresponding
to a given argument can be obtained.
major frame. For ETM+, a major frame period is one complete scan
of the ETM+ scan mirror (either direction), which includes not only the
period during a scan but also the turnaround interval when the scan mirror
changes direction for the next scan.
map projection. Any systematic arrangement of meridians and parallels
portraying the curved surface of a sphere or spheroid upon a plane.
metadata. An archived set of descriptive information about a scene
and the parent sub-interval that provides a user with geographic coverage,
date of acquisition, sun angles, could cover, gain states, and other quality
minor frame. For ETM+ major frames are partitioned into minor
frames which is the most fundamental element of the data stream structure
in which specific data measurands (e.g. imagery, PCD, time codes) are
mirror scan correction data. This data includes scan start time,
first half scan time error, second half scan time error, scan direction,
and any other data which is required to perform mirror scan correction.
modulate. To vary, or control, the frequency, phase, or amplitude
of an electromagnetic wave or other variable.
modulation transfer function (MTF). The modulation transfer function
of an imaging system measures the spatial frequency modulation response
of the system. As an imaging system processes or records an image, the
contrast modulation of the processed or recorded image is different from
the input image. The MTF can be thought of as a curve, indicating for
each spatial frequency the ratio of the contrast modulation of the output
image to the contrast modulation of the input image. It is formally defined
as the magnitude of the Fourier transform of the line spread function
of the imaging system.
mosaic. An image made by piecing together individual images covering
multiplexer. An electronic device which permits the transmission
of multiple messages simultaneously on one communication channel.
multispectral. Generally denotes remote sensing in two or more
spectral bands, such as visible and infrared.
nadir. That point on the celestial sphere vertically below the
observer, or 180° from the zenith.
narrowband data. The data includes the command or forward ranging
in the narrowband forward link, and the telemetry or return ranging in
the narrowband return link.
near infrared. The preferred term for the shorter wavelengths
in the infrared region extending from about 0.7 μm (visible red)
to about 3 μm. The longer wavelength end grades into the middle infrared.
Sometimes called solar infrared, as it is only available for use during
the daylight hours. Also known as the shortwave infrared (SWIR).
node. Either of the two points at which the orbit of a heavenly
body intersects a given plane, especially the plane of ecliptic. With
respect to Landsat, the orbital nodes occur at the equator, one on the
descending, or daylight, track of the orbit and the other on the ascending,
or nighttime, track.
noise. Any unwanted disturbance affecting a measurement (as of
a frequency band), especially that which degrades the information-bearing
quality of the data of interest.
Nyquist interval. The maximum time interval between equally spaced
samples of a signal that will enable the signal waveform to be completely
determined. The Nyquist interval is equal to the reciprocal of twice the
highest frequency component of the sampled signal.
Nyquist's theorem: A theorem, developed by H. Nyquist, which states
than an analog signal waveform may be uniquely reconstructed, without
error, from samples taken at equal time intervals. The sampling rate must
be equal to, or greater than, twice the highest frequency component in
the analog signal.
optical transfer function (OTF). A mathematical statement that
describes the relationship between the input and the output of an imaging
system. When the transfer function operates on the input, the output is
obtained. Given any two of these three entities, the third can be obtained.
orbit adjust. The adding to or taking away of orbital velocity.
This is normally done to maintain altitude or orbit phasing relationships.
orbital period. The interval in time between successive passages
(orbits) of a satellite through a reference plane.
orthorectified. Describing an image in which terrain relief distortions
have been removed.
panchromatic. A single band covering a broad range of wavelengths;
usually used in context of collecting information from the whole visible
parallax. The apparent change in the position of one object, or
point, with respect to another, when viewed from different angles.
path. The longitudinal center line of a Landsat scene of a Landsat
scene, corresponding to the center of an orbital track. Sequential numbers
from east to west are assigned to 233 nominal satellite tracks for Landsat
7. Path numbers are used with row numbers to designate nominal scene center
payload. That part of a spacecraft (e.g. ETM+) that is separate
from the equipment or operations necessary to maintain the spacecraft
payload correction data. Image support data imbedded in the wideband
data stream. Includes satellite attitude, ephemeris, time, angular displacement
sensor (ADS) data and payload state.
perigee. The point in the orbit of a heavenly body (e.g. satellite)
at which it is nearest the Earth.
pixel. Picture element provided by a single detector scene sample
pitch. The rotation of a spacecraft about the horizontal axis
normal to its longitudinal axis (in the along-track direction) so as to
cause a nose-up or nose-down attitude.
polar stereographic. An azimuthal stereographic projection commonly
used with Landsat data acquired about 65° latitude. In this projection,
the meridians are straight lines converging at the pole (central point),
and lines of latitude are concentric circles about this point. Like the
UTM projection, the polar stereographic is a conformal projection, meaning
that angular relationships are preserved.
pole wander. The apparent motion in the poles of the Earth relative
to inertial coordinate system. Changes in moments of inertia are due to
changes in moments of density due primarily to tides and liquid mass.
The National Imager and Mapping Agency (NIMA) generates pole wander data
which are used by the Landsat 7 system in the conversion of downlinked
ephemeris from inertial to fixed reference, during Level 0R processing.
precision correction. Post-processed geometric correction of satellite
data using ground control points to correlate the spacecraft's predicted
position with its actual geodetic position.
prime meridian. Meridian of longitude 0 degrees, used as the origin
for measurements of longitude. The meridian of Greenwich, England, is
the internationally accepted prime meridian on most charts.
quantization level. The number of numerical values used to represent
a continuous quantity.
quaternion. A vector of four components; the position is contained
in the first three components and an associated scalar rater is located
in the last component of this four element vector.
radian. The angle subtended by an arc of a circle equal in length
to the radius of the circle: 57.3°
radiance. Measure of the energy radiated by an object. In general,
radiance is a function of viewing angle and spectral wavelength and is
expressed as energy per solid angle.
Rayleigh scattering. Selective scattering of light in the atmosphere
by particles that are small compared with the wavelength of light.
reflectance. The ratio of the radiant energy reflected by a body
to that incident upon it. In general, reflectance is a function of the
incident angle of the energy, viewing angle of the sensor, spectral wavelength
and bandwidth, and the nature of the object.
registration. The process of geometrically aligning two or more
sets of image data such that resolution cells for a common ground area
can be digitally or visually superimposed.
roll. The rotation of a spacecraft about its longitudinal axis
(in the along-track direction) so as to cause a side-up or side-down attitude.
The roll axis is referred to as the y axis.
row. The latitudinal (nominal) center line of a Landsat scene.
Row 1 is at latitude 80° 47'N, row 60 is at the equator, and row 122
is at latitude 81° 51'S. In total there are 248 rows.
sampling rate. The number of samples taken per unit time, i.e.,
the rate at which signals are sampled for subsequent use, such as for
modulation, coding, and quantization.
saturation. The condition where energy flux exceeds the sensitivity
range of a detector.
S band. A radio frequency band extending from approximately 2.0
to 4.0 gigahertz.
sidelap. The extent of lateral overlap between images acquired
over adjacent ground tracks.
signal-to-noise ratio. The ratio of the level of the information-bearing
signal power to the level of the noise power. More precisely, the signal-to-noise
ratio of the mean DN to the standard deviation in DN. This is a temporal
noise definition in that the mean DN is the time averaged value and the
standard deviation in DN is the standard deviation in the time series.
space oblique mercator. A variation on the basic mercator map
projection based on the dynamics of satellite motion. The movements of
the satellite, sensor, and the Earth, expressed as functions of time,
are used to calculate which latitudes and longitudes on the Earth correspond
to locations in the projection plane.
spectral band. An interval in the electromagnetic spectrum defined
by two wavelengths, frequencies, or wave numbers.
spectral response. The response of a material as a function of
wavelength to incident electromagnetic energy, particularly in terms of
the measurable energy reflected from and emitted by the material.
spectral signature. The quantitative measurement of the properties
of an object at one or several wavelength intervals. Spectral signature
analysis techniques use the variation in the spectral reflectance or emittance
of objects as a method of identifying the objects.
steradian. A unit of measure of solid angles. Formally, it is
the angle subtended at the center of the sphere by a portion of the surface
whose area is equal to the square of the radius of the sphere. There are
4 pi steradians in a sphere.
subinterval. Is a contiguous segment of raw wideband data received
during a Landsat 7 contact period. Subintervals are caused by breaks in
the wideband datastream due to communication dropouts and/or the inability
of the spacecraft to transmit a complete observation (interval) within
a single Landsat 7 contact period. The largest possible subinterval is
35 full scenes long with a partial scene preamble and postamble. The smallest
possible subinterval is a single ETM+ scene.
sun elevation angle. The angle of the Sun above the horizon.
solar zenith angle. Reciprocal of the sun elevation angle.
sun synchronous. An Earth satellite orbit in which the orbital
plane remains at a fixed angle with respect to the Sun, precessing through
360° during the period of a year.
swath. Refers to the 185 kilometer wide ETM+ imaging ground track.
telemetry. The science of measuring a quantity, transmitting the
measured value to a distant station, and there, interpreting or recording
the quantity measured.
temporal. Pertaining to, concerned with, or limited by time.
temporal resolution. The expected repeat time between measurements
over the same location.
thermal band. A general term for intermediate and long wavelength
infrared-emitted radiation, as contrasted to short wavelength reflected
infrared radiation. In practice, generally refers to infrared radiation
emitted in the 3-5 μm and 9-14 μm atmospheric windows.
thermal infrared. The preferred term for the middle wavelength
ranges of the infrared region extending roughly from 3 μm at the
end of the near infrared, to about 15 or 20 m where the far infrared commences.
In practice the limits represent the the envelope of energy emitted by
the Earth behaving as a graybody with a surface temperature around 290
K. Seen from space, the radiance envelope has several brighter bands corresponding
to windows in the atmospheric absorption bands. The thermal band most
used in remote sensing extends from 8 to 15 μm.
time, Greenwich mean. Mean solar time of the meridian of Greenwich,
England (longitude 0), used by most navigators and adopted as the prime
basis of standard time throughout the world. Abbreviated GMT.
time, mean Sun. The mean Sun time at a given location on the Earth
is determined by the distance in longitude from the Greenwich meridian.
The mean Sun time at any location is determined by dividing the difference
in longitude from Greenwich (in degrees, moving east) by 15 and adding
the result to the current GMT. This will be mean Sun time relative to
Greenwich, expressed in hours.
transmittance. The ratio of the energy per unit time per unit
area (radiant power density) transmitted through an object to the energy
per unit time per unit area incident on the object. In general, transmittance
is a function of the incident angle of the energy, viewing angle of the
sensor, spectral wavelength and bandwidth, and the nature of the object.
ultraviolet radiation. Electromagnetic radiation of shorter wavelength
than visible radiation but longer than X-rays; roughly, radiation in the
wavelength interval between 10 and 4,000 angstroms.
umbra. The complete or perfect shadow of an opaque body, as a
planet, where the light from the source of illumination is completely
universal transverse mercator. A widely used map projection employing
a series of identical projections around the world in the intermediate
latitudes, each covering 6 degrees of longitude and oriented to a meridian.
The UTM projection is characterized by its property of conformality, meaning
that it preserves scale and angular relationships well, and by the ease
with which it allows a useful rectangular grid to be superimposed on it.
The UTM projection is most commonly used with landsat data.
UT1-UTC time correction data. Universal Time (UT) 1 is determined
from observations of stellar transits to determine local mean sidereal
time corrected to remove the effects of polar motion. Universal Time Coordinated
(UTC) is defined to be equal to that of the International System used
for atomic time, but it is kept with .9 seconds of UT1 by periodic leap-second
virtual channel data unit (VCDU). The CCSDS protocol data unit
consisting of a fixed length data structure. It is used for bidirectionally
space/ground communications on a CCSDS virtual channel.
visible radiation. Electromagnetic radiation of the wavelength
interval to which the human eye is sensitive; the spectral interval from
approximately 0.4 to 0.7 μm.
wavelength. Wavelength = 1/frequency. In general, the mean distance
between maximums (or minimums) of roughly periodic pattern. Specifically,
the shortest distance between particles moving in the same phase of oscillation
in a wave disturbance.
world geodetic system (WGS). The reference Earth model used by
the Landsat 7 system.
worldwide reference system. A global indexing system for Landsat
data which is based on nominal scene centers defined by path and row coordinates.
X-band. A radio frequency band extending from approximately 8.0
to 12.5 gigahertz.
yaw. The rotation of a spacecraft about its vertical axis so as
to cause the spacecraft's longitudinal axis to deviate left or right from
the direction of flight. The yaw axis is referred to as the z axis.
zenith. The point in the celestial sphere that is exactly overhead.