Landsat, 40 años mirando un planeta cambiante

Representación de uno de los satélites LANDSATEste pasado lunes la NASA, en colaboración con el servicio geológico estadounidense USGS, lanzaba el octavo satélite de la serie Landsat, una de las míticas misiones de la historia espacial. Desde las alturas completará su objetivo que, al igual que sus anteriores hermanos, es la observación detallada de nuestro planeta.

Mares, océanos, ríos, bosques, nubes, volcanes, islas… cualquier punto de la geografía de la Tierra o de su atmósfera es atentamente observado por estos satélites que giran en órbita heliosincrónica a 705 kilómetros sobre nuestras cabezas realizando un vuelta completa cada 99 minutos.

Desde 1972, año en el que se lanzó el primer Landsat, el proyecto nos ha proporcionado una inmensa cantidad de datos e imágenes que además están a disposición de cualquier persona o científico que quiera acceder a ellos puesto que se ofrecen libremente en abierto desde numerosas webs oficiales. Como muestra podéis disfrutar de la galería especial que USGS ha elaborado para este 40 aniversario.

Pero este gran programa posee un impresionante valor añadido fruto del tiempo que lleva funcionando. Haber sido testigo de excepción de todos los rincones de la Tierra durante 40 años nos ofrece un archivo sin igual en el que poder comprobar los cambios que se han realizado en nuestro planeta durante todo este tiempo.

Si os parece vamos a realizar un pequeño recorrido por estas cuatro últimas décadas de la mano del programa Landsat para descubrir la evolución viva de nuestro entorno.

Comparación del Mar de Aral desde 1973 hasta 2009

La imagen que podéis observar es la comparación de la desecación del Mar Aral en los últimos 34 años. A la izquierda la imagen correspondiente a 1973 y a la derecha, con apenas un 12% de lo que en su día fue, la fotografía de 2009.

En palabras de Michael Freilich, director de la División de Ciencias de la Tierra en la NASA, estos 40 años del programa LandSat han proporcionado el registro más consistente y fiable del panorama cambiante de la Tierra. Y si existe una zona en nuestro planeta donde, desgraciadamente, podemos observar la evolución del tiempo y de la mano del hombre esa es la Selva amazónica.

El nuevo satélite de la serie LandSat continuará con la espectacular labor desarrollada por sus antecesores y en este caso seguro que la mejorará puesto que es capaz de tomar imágenes con una resolución espacial de 30 metros enviando más de 400 fotografías cada día… un importantísimo colaborador espacial para conocer a fondo los cambios que se producen en nuestro planeta a vista de pájaro.

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Hemisferio Sur, mucho hielo.


El invierno del hemisferio sur se manifiesta en la congelación de las aguas que rodean la Antártida. Este año (línea roja) la extensión del hielo marino está batiendo el récord desde que se tienen mediciones satelitarias. Como es normal, los medios de comunicación no dan cuenta de ello.

Earth’s gravity pictured in ‘HD’

By Jonathan Amos
Science correspondent, BBC News, Bergen

It is one of the most exquisite views we have ever had of the Earth.


This colourful new map traces the subtle but all pervasive influence the pull of gravity has across the globe.

Known as a geoid, it essentially defines where the level surface is on our planet; it tells us which way is “up” and which way is “down”. It is drawn from delicate measurements made by Europe’s Goce satellite, which flies so low it comes perilously close to falling out of the sky.

Scientists say the data gathered by the spacecraft will have numerous applications. One key beneficiary will be climate studies because the geoid can help researchers understand better how the great mass of ocean water is moving heat around the world.

The new map was presented here in Norway’s second city at a special Earth observation (EO) symposium dedicated to the data being acquired by Goce and other European Space Agency (Esa) missions.

Imaginary ball

Launched in 2009, the sleek satellite flies pole to pole at an altitude of just 254.9km – the lowest orbit of any research satellite in operation today.

The spacecraft carries three pairs of precision-built platinum blocks inside its gradiometer instrument that sense accelerations which are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth.

This has allowed it to map the almost imperceptible differences in the pull exerted by the mass of the planet from one place to the next – from the great mountain ranges to the deepest ocean trenches.

Two months of observations have now been fashioned into what scientists call the geoid.

…Put a ball on this hypothetical surface and it will not roll – even though it appears to have “slopes”. These slopes can be seen in the colours which mark how the global level diverges from the generalised (an ellipsoid) shape of the Earth.

In the North Atlantic, around Iceland, the level sits about 80m above the surface of the ellipsoid; in the Indian Ocean it sits about 100m below.

MAPPING THE DIFFERENT EFFECTS OF GRAVITY

  • 1. Earth is a slightly flattened sphere – it is ellipsoidal in shape
  • 2. Goce senses tiny variations in the pull of gravity over Earth
  • 3. The data is used to construct an idealised surface, or geoid
  • 4. It traces gravity of equal ‘potential’; balls won’t roll on its ‘slopes’
  • 5. It is the shape the oceans would take without winds and currents
  • 6. So, comparing sea level and geoid data reveals ocean behaviour
  • 7. Gravity changes can betray magma movements under volcanoes
  • 8. A precise geoid underpins a universal height system for the world
  • 9. Gravity data can also reveal how much mass is lost by ice sheets
  • The geoid is of paramount interest to oceanographers because it is the shape the world’s seas would adopt if there were no tides, no winds and no currents.

    If researchers then subtract the geoid from the actual observed behaviour of the oceans, the scale of these other influences becomes apparent.

    This is information critical to climate modellers who try to represent the way the oceans manage the transfer of energy around the planet.

    How the UAH Global Temperatures Are Produced

    I am still receiving questions about the method by which the satellite microwave measurements are calibrated to get atmospheric temperatures. The confusion seems to have arisen because Christopher Monckton has claimed that our satellite data must be tied to the surface thermometer data, and after Climategate (as well all know) those traditional measurements have become suspect. So, time for a little tutorial.

    NASA’S AQUA SATELLITE
    The UAH global temperatures currently being produced come from the Advanced Microwave Sounding Unit (AMSU) flying on NASA’s Aqua satellite. AMSU is located on the bottom of the spacecraft (seen below); the AMSR-E instrument that I serve as the U.S. Science Team Leader for is the one on top of the satellite with the big dish.
    aqua_night_pacific

    Aqua has been operational since mid-2002, and is in a sun-synchronous orbit that crosses the equator at about 1:30 am and pm local solar time. The following image illustrates how AMSU, a cross-track scanner, continuously paints out an image below the spacecraft (actually, this image comes from the MODIS visible and infrared imager on Aqua, but the scanning geometry is basically the same):
    Aqua-MODIS-swaths

    HOW MICROWAVE RADIOMETERS WORK
    Microwave temperature sounders like AMSU measure the very low levels of thermal microwave radiation emitted by molecular oxygen in the 50 to 60 GHz oxygen absorption complex. This is somewhat analogous to infrared temperature sounders (for instance, the Atmospheric InfraRed Sounder, AIRS, also on Aqua) which measure thermal emission by carbon dioxide in the atmosphere.

    As the instrument scans across the subtrack of the satellite, the radiometer’s antenna views thirty separate ‘footprints’, nominally 50 km in diameter, each over over a 50 millisecond ‘integration time’. At these microwave frequencies, the intensity of thermally-emitted radiation measured by the instrument is directly proportional to the temperature of the oxygen molecules. The instrument actually measures a voltage, which is digitized by the radiometer and recorded as a certain number of digital counts. It is those digital counts which are recorded on board the spacecraft and then downlinked to satellite tracking stations in the Arctic.

    HOW THE DATA ARE CALIBRATED TO TEMPERATURES
    Now for the important part: How are these instrument digitized voltages calibrated in terms of temperature?

    Once every Earth scan, the radiometer antenna looks at a “warm calibration target” inside the instrument whose temperature is continuously monitored with several platinum resistance thermometers (PRTs). PRTs work somewhat like a thermistor, but are more accurate and more stable. Each PRT has its own calibration curve based upon laboratory tests.

    The temperature of the warm calibration target is allowed to float with the rest of the instrument, and it typically changes by several degrees during a single orbit, as the satellite travels in and out of sunlight. While this warm calibration point provides a radiometer digitized voltage measurement and the temperature that goes along with it, how do we use that information to determine what temperatures corresponds to the radiometer measurements when looking at the Earth?

    A second calibration point is needed, at the cold end of the temperature scale. For that, the radiometer antenna is pointed at the cosmic background, which is assumed to radiate at 2.7 Kelvin degrees. These two calibration points are then used to interpolate to the Earth-viewing measurements, which then provides the calibrated “brightness temperatures”. This is illustrated in the following graph:
    radiometer-calibration-graph

    The response of the AMSU is slightly non-linear, so the calibration curve in the above graph actually has slight curvature to it. Back when all we had were Microwave Sounding Units (MSU), we had to assume the instruments were linear due to a lack of sufficient pre-launch test data to determine their nonlinearity. Because of various radiometer-related and antenna-related factors, the absolute accuracy of the calibrated Earth-viewing temperatures are probably not much better than 1 deg. C. While this sounds like it would be unusable for climate monitoring, the important thing is that the instruments be very stable over time; an absolute accuracy error of this size is irrelevant for climate monitoring, as long as sufficient data are available from successive satellites so that the newer satellites can be calibrated to the older satellites’ measurements.

    WHAT LAYERS OF THE ATMOSPHERE ARE MEASURED?

    For AMSU channel 5 that we use for tropospheric temperature monitoring, that brightness temperature is very close to the vertically-averaged temperature through a fairly deep layer of the atmosphere. The vertical profiles of each channel’s relative sensitivity to temperature (’weighting functions’) are shown in the following plot:
    AMSU-weighting-functions

    These weighting functions are for the nadir (straight-down) views of the instrument, and all increase in altitude as the instrument scans farther away from nadir. AMSU channel 5 is used for our middle tropospheric temperature (MT) estimate; we use a weighted difference between the various view angles of channel 5 to probe lower in the atmosphere, which a fairly sharp weighting function which is for our lower-tropospheric (LT) temperature estimate. We use AMSU channel 9 for monitoring of lower stratospheric (LS) temperatures.

    For those channels whose weighting functions intersect the surface, a portion of the total measured microwave thermal emission signal comes from the surface. AMSU channels 1, 2, and 15 are considered “window” channels because the atmosphere is essentially clear, so virtually all of the measured microwave radiation comes from the surface. While this sounds like a good way to measure surface temperature, it turns out that the microwave ‘emissivity’ of the surface (it’s ability to emit microwave energy) is so variable that it is difficult to accurately measure surface temperatures using such measurements. The variable emissivity problem is the smallest for well-vegetated surfaces, and largest for snow-covered surfaces. While the microwave emissivity of the ocean surfaces around 50 GHz is more stable, it just happens to have a temperature dependence which almost exactly cancels out any sensitivity to surface temperature.

    POST-PROCESSING OF DATA AT UAH
    The millions of calibrated brightness temperature measurements are averaged in space and time, for instance monthly averages in 2.5 degree latitude bands. I have FORTRAN programs I have written to do this. I then pass the averages to John Christy, who inter-calibrates the different satellites’ AMSUs during periods when two or more satellites are operating (which is always the case).

    The biggest problems we have had creating a data record with long-term stability is orbit decay of the satellites carrying the MSU and AMSU instruments. Before the Aqua satellite was launched in 2002, all other satellites carrying MSUs or AMSUs had orbits which decayed over time. The decay results from the fact that there is a small amount of atmospheric drag on the satellites, so they very slowly fall in altitude over time. This leads to 3 problems for obtaining a stable long-term record of temperature.

    (1) Orbit Altitude Effect on LT The first is a spurious cooling signal in our lower tropospheric (LT) temperature product, which depends upon differencing measurements at different view angles. As the satellite falls, the angle at which the instrument views the surface changes slightly. The correction for this is fairly straightforward, and is applied to both our dataset and to the similar datasets produced by Frank Wentz and Carl Mears at Remote Sensing Systems (RSS). This adjustment is not needed for the Aqua satellite since it carries extra fuel which is used to maintain the orbit.

    (2) Diurnal Drift Effect The second problem caused by orbit decay is that the nominal local observation time begins to drift. As a result, the measurements can increasingly be from a warmer or cooler time of day after a few years on-orbit. Luckily, this almost always happened when another satellite operating at the same time had a relatively stable observation time, allowing us to quantify the effect. Nevertheless, the correction isn’t perfect, and so leads to some uncertainty. [Instead of this empirical correction we make to the UAH products, RSS uses the day-night cycle of temperatures created by a climate model to do the adjustment for time-of-day.] This adjustment is not necessary for the Aqua AMSU.

    (3) Instrument Body Temperature Effect. As the satellite orbit decays, the solar illumination of the spacecraft changes, which then can alter the physical temperature of the instrument itself. For some unknown reason, it turns out that most of the microwave radiometers’ calibrated Earth-viewing temperatures are slightly influenced by the temperature of the instrument itself…which should not be the case. One possibility is that the exact microwave frequency band which the instrument observes at changes slightly as the instrument warms or cools, which then leads to weighting functions that move up and down in the atmosphere with instrument temperature. Since tropospheric temperature falls off by about 7 deg. C for every 1 km in altitude, it is important for the ‘local oscillators’ governing the frequency band sensed to be very stable, so that the altitude of the layer sensed does not change over time. This effect is, once again, empirically removed based upon comparisons to another satellite whose instrument shows little or no instrument temperature effect. The biggest concern is the long-term changes in instrument temperature, not the changes within an orbit. Since the Aqua satellite does not drift, the solar illumination does not change and and so there is no long-term change in the instrument’s temperature to correct for.

    One can imagine all kinds of lesser issues that might affect the long-term stability of the satellite record. For instance, since there have been ten successive satellites, most of which had to be calibrated to the one before it with some non-zero error, there is the possibility of a small ‘random walk’ component to the 30+ year data record. Fortunately, John Christy has spent a lot of time comparing our datasets to radiosonde (weather balloon) datasets, and finds very good long-term agreement.

    Satellite measurements show our quiet sun is cooling the upper thermosphere

    The TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) missionData from the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) mission are being used to understand the climate of the upper atmosphere. Credit: NASA

    From NASA News. New measurements from a NASA satellite show a dramatic cooling in the upper atmosphere that correlates with the declining phase of the current solar cycle. For the first time, researchers can show a timely link between the Sun and the climate of Earth’s thermosphere, the region above 100 km, an essential step in making accurate predictions of climate change in the high atmosphere.

    Scientists from NASA’s Langley Research Center and Hampton University in Hampton, Va., and the National Center for Atmospheric Research in Boulder, Colo., presented these results at the fall meeting of the American Geophysical Union in San Francisco from Dec. 14 to 18.

    Earth’s thermosphere and mesosphere have been the least explored regions of the atmosphere. The NASA Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) mission was developed to explore the Earth’s atmosphere above 60 km altitude and was launched in December 2001. One of four instruments on the TIMED mission, the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, was specifically designed to measure the energy budget of the mesosphere and lower thermosphere. The SABER dataset now covers eight years of data and has already provided some basic insight into the heat budget of the thermosphere on a variety of timescales.