Automatic Picture Transmission, a way of transmitting analogue images,
as used by NOAA and Meteosat for example. Higher quality digital images
are transmitted on a different frequency, see HRPT and PDUS respectively.
AVHRR
Advanced Very High Resolution Radiometer, carried aboard the NOAA range of
polar-orbiting satellites. The AVHRR is a very old instrument design, so the
term "very high resolution" must be taken in that context! See our
AVHRR document for more details.
Band
A single range of the electromagnetic spectrum as detected by a sensor.
See also Sensor and Channel.
Channel
A single range of the electromagnetic spectrum as detected by a sensor.
The name Channel refers to the transmission to earth of the data sampled from
that band. See also Band.
Coverage
The area on the ground seen by a satellite at a particular position.
Polar-orbiting satellites can in fact only see a single
line going from left to right as the scanner rotates. The coverage is determined
by the angles between which the scanner rotates (the scanner angle).
The other determinant of the coverage is of course the movement of the
satellite over the earth and the ability of the receiving station to receive
its transmissions during that movement. Reception starts when the satellite
comes over the station's horizon and ends on the opposite horizon. The
coverage is the area seen by the satellite at each of those horizons plus
the area in between. See also pass and equator
crossing angle. See also projection. See
also our coverage document.
CZCS
Coastal Zone Colour Scanner, a scanner carried aboard the Nimbus-7 satellite.
See our CZCS document for more details.
DMSP
The USA's Defense Meteorological Satellite Programme, a series of satellites
notable for their ability to image city lights at night-time.
Equator crossing angle
The longitude (angle measured in degrees west from the Greenwich meridian)
of the point directly below the satellite when it crosses the equator.
An important reference point because, given that the orbital characteristics
of the satellite are known, its position and thus the extent of its coverage
can be calculated for any point along its path. Because the earth is rotating
underneath the satellite, and the altitude has been chosen to given an
orbit of nearly 24 hours, the angle at which the satellite crosses
the equator will be different for each subsequent orbit. See also Coverage.
GAC
Global Area Coverage is the term for one-quarter resolution (4km per pixel)
AVHRR imagery recorded on tape recorders aboard the NOAA
satellites. The limited resolution means that imagery from one full orbit can
be recorded. It is later transmitted to one of the NOAA ground stations, in
contrast to HRPT which is transmitted in real-time.
See also LAC.
Geostationary
A satellite which appears to remain at a fixed location above the earth,
cf. Polar orbiting satellites. A satellite in a geostationary orbit is in
fact orbiting the earth at the same rate at which the earth is rotating.
Meteosat is a satellite in a geostationary orbit, the NOAA satellites
received here at Dundee are polar orbiting.
GMS
Japanese geostationary satellite positioned over Japan / Australia;
see also Meteosat and GOES.
GMT
Greenwich Mean Time. Now replaced by the term UTC.
GOES
American geostationary satellites, GOES-E is positioned over the
USA / South America and GOES-W is positioned over the Pacific Ocean.
See also Meteosat and GMS.
HRI
High Resolution Imagery. A method of encoding high resolution image data
from the Meteosat satellite along with all the
satellite calibration and state-of-health data.
HRPT
High Resolution Picture Transmission. A method of encoding high resolution
image data from the AVHRR scanner along with all the
satellite calibration and state-of-health data.
IR
Infra-Red light, also known as thermal because it can give an indication
of the temperature of the observed object. Wavelengths range from 10.5 to
12.5 µm. See also Visible and Water Vapour.
IODC
Indian Ocean Data Coverage is the name given to an old satellite Meteosat-5
which has been moved from 0 degrees East to 63 degrees East in order to provide
images centred over the Indian Ocean.
LAC
Local Area Coverage is the term for full-resolution
AVHRR imagery recorded on tape recorders aboard the NOAA
satellites. It is later transmitted to one of the NOAA ground stations, in
contrast to HRPT which is transmitted in real-time.
See also GAC.
Meteosat
European Meteorological Satellite, in a geostationary orbit, administered
by EUMETSAT. Meteosat is positioned over Africa / Europe; see also GMS and GOES.
METOP
A series of satellites for both US and European Meteorological Agencies,
carrying scanners from both regions, including the AVHRR from the US. Replaces
the NOAA series of satellites.
MODIS
Moderate Resolution Imaging Spectroradiometer, a scanner carried aboard
the EOS range of NASA satellites. See MODIS
for more details.
MVISR
Multichannel Visible and Infrared Scan Radiometer, a scanner carried
aboard the Chinese FengYun series of satellites.
NASA
National Aeronautics and Space Administration (USA). See the
NASA web site for more details.
NDVI
Normalised Difference Vegetation Index. Image data from channels 2
and 1 combined using an algorithm that is designed to indicate areas of
vegetation.
NOAA
National Oceanic and Atmospheric Administration (USA). See the
NOAA web site for more details.
NPOESS
The National Polar-orbiting Operational Environmental Satellite System
in the USA. NPOESS is the name given to a series of satellites controlled
by three US government agencies; NASA, and the departments of Commerce and
Defense. Replaces the DMSP series of defense satellites, the NOAA satellites,
and the NASA satellites.
See the NPOESS page for more details.
NPP
The NPOESS Preparatory Project, a satellite to bridge the gap
between the NOAA series of AVHRR satellites, the NASA series of EOS satellites,
and the NPOESS joint mission series.
See the NPP page for more details.
Pass
The reception of satellite data from the time it comes over the horizon
of the receiving station to the time it disappears over the opposite horizon.
The pass can refer to that particular part of the satellite's orbit
or to the image received during that time.
PDUS
Primary Data User Station; the name for the equipment used to receive
digital data from Meteosat. Analogue images are also transmitted by Meteosat
(SDUS - Secondary DUS) but these are of lower quality, resolution and
usefulness for scientific applications, and are not received at Dundee.
Polar-orbiting satellite
A satellite which orbits the earth, travelling roughly pole-to-pole.
The orbit is in fact inclined so that it may go one side of the North Pole
and the opposite side of the South Pole. Cf. Geostationary
satellites. See also projection.
Projection
A method used to flatten a 3-D object (the earth in this case) so that
it can be viewed in two dimensions. See also our description of projections
used by the satellite station.
QuickLook
A reduced-resolution image of a pass. The image
may have a reduced pixel resolution or a reduced colour resolution. Dundee
quicklooks are reduced by 6 in the former and by 1.25 in the latter for AVHRR.
They are also linearised to remove the effects of the curvature of the earth.
Quicklooks can be used to check for cloud or surface features before ordering
high resolution (HRPT) data. Cf. thumbnail.
Scanner
The instrument carried aboard the satellite which views the earth.
Examples are AVHRR, CZCS,
SeaWiFS and MODIS.
Typically the scanner is simultaneously sensitive to a number of parts (bands)
of the visible and infra-red spectrum. It will rotate, scanning the earth
directly underneath the satellite (the sub-satellite point) from left to
right. The data will then be transmitted to earth as one scan-line, typically
1000 to 2000 points across, each point having values for the parts of the
spectrum seen. By the time the scanner comes back to the start the satellite
will have progressed so a different part of the earth is seen.
See also Sensor.
SeaWiFS
Sea-viewing Wide Field-of-view Sensor. See SeaWiFS
for more details.
Sensor
The part of the satellite's scanner which is sensitive to light (visible
or otherwise). Each scanner may carry multiple sensors, which may be used to
sense different bands or to increase the spatial resolution.
See also Scanner. See also sensor comparison.
Thumbnail
A reduced-resolution image of a quicklook,
small enough that many can be viewed on a page at once to allow comparisions
to be made.
TIP
TIROS Information Processor, handles instrument data signals and status
telemetry from NOAA satellites.
TIROS
Television Infra-Red Orbiting Satellite a fore-runner to the NOAA series.
UTC
Coordinated Universal Time. All satellite times are reported in UTC
rather than in local time or daylight-saving (summer) time.
See also GMT.
VIIRS
The Visible/Infrared Imager/Radiometer Suite, a scanner carried aboard
the NPP / NPOESS series of satellites.
Visible
Visible wavelengths range from 0.4 to 0.7 µm.
If only one sensor receives from these wavelengths you will see
grayscales, not colour. Colour requires scanners with sensors at each
of the wavelengths for the colours to be combined.
See also IR and WV.
Water Vapour
Water Vapour absorption can be detected by scanning at wavelength
from 5.7 µm to 7.1 µm. See also IR and Visible.
