A new wind constellation: the Cyclone Global Navigation Satellite System (CYGNSS)

CYGNSS_AutoF

This is a new concept for scatterometry, using the reflection of GPS signals from the ocean surface to study severe storms and cyclones. CYGNSS is a constellation of eight micro-satellites that will make accurate measurements of ocean surface winds in order to better understand the life-cycle of tropical cyclones, typhoons and hurricanes. Why eight? Because the satellites do not have to carry a transmitter, but instead rely on the GPS satellites to provide the transmitted energy. This means that the individual satellites weigh about 170 kg, and that eight of them can be accommodated in one launch. This is a great idea, scheduled for launch in 2016, and I’ll write more about it in the coming weeks.

Here is a reference for CYGNSS: http://earthobservatory.nasa.gov/blogs/fromthefield/category/cygnss/?src=eoa-features

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K-band Radar Interferometric Topographic Measurements with GLISTIN: dial-in seminar on Thursday, May 21

This seminar might be of interest to readers working with space borne radars:
The NASA Cryo Remote Sensing from Space (CRSS) webinars resume this week after a longer-than-planned hiatus. Please join us on Thursday, May 21, at 3:00 PM EDT. Scott Hensley from JPL will talk about K-band Radar Interferometric Topographic Measurements with the GLISTIN Radar (http://airbornescience.jpl.nasa.gov/instruments/glistin/).
Webex information is below. Please dial in to the telecon line provided for audio.

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What would happen if you put a smart phone into orbit?

HTC_Nexus

Consider a smartphone: it has a camera, GPS, radio transmitter/receiver and a fair bit of computer memory. What would happen if you put it into orbit? Could it serve as a satellite? In 2010, a group of engineers at the NASA Ames Research Center tried just that. Here is a portion of an article from Science, describing the launch of several smartphones

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New satellite AltiKa radar could find 100,000 underwater mountains

“This article in Science, by Eric Hand,  describes the use of the French AltiKa altimeter on the Indian SIRAL satellites to map the location of seamounts. A detailed account is given at the following link.

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A question from a reader about the Rayleigh criterion

Question from a reader:

I am working my way through “An Introduction to Ocean Remote Sensing” and I have a question on the Rayleigh Criterion, page 73, equation (3.31) This gives the surface resolution of the human eye as 0.2 mm at a range of 1 m.

I must have missed something, but my question is how can I discern a human hair, width 10 micro-metres or a dust mote floating in the sunlight?

My answer:

Good question! So first let’s look at the definition of resolution. Consider two candles a small distance apart that we view through a slit.

two_candles

Each candle generates a diffraction pattern, so that if the candles have an angular resolution that is less than that determined from the Rayleigh criterion, then we will be not be able to see that there are two candles.

small_candle

Now let’s blow out one of the candles and move it far away. Can we still see it? The answer depends on the brightness of the candle relative to its background. If the candle is sufficiently bright, even though the subtended angle is much smaller than that given by the Rayleigh criterion, it will excite a retinal sensor. This is the solution to the dust mote problem (note that I am following http://en.wikipedia.org/wiki/Visual_acuity).

The human hair presents a similar problem and solution. From Wikipedia, “The smallest detectable visual angle produced by a single fine dark line against a uniformly illuminated background is also much less than foveal cone size or regular visual acuity. In this case, under optimal conditions, the limit is about 0.5 arc seconds or only about 2% of the diameter of a foveal cone.” The eye has the ability to detect small changes in brightness, even though the angular width of the object is much less than that subtended by the retinal sensor, and less than the angular width given by Rayleigh criterion. Two hairs side-by-side however, would suffer diffractional blurring.

Finally, try it! Draw a couple of lines on a sheet of white paper, then set the paper at distance greater than that determined from the Rayleigh criterion. Can you distinguish the lines?

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The SSALTO/DUACS Multimission Altimeter Processing System

duacs_global_nrt_madt_merged_h_latest_glo_madt_n0_t0

The French SSALTO/DUACS multimission altimeter processing system combines data from all altimetric satellites to produce a near-global product of absolute and relative sea surface height. SSALTO stands for Segment Sol multi-missions dALTimetrie, d’orbitographie et de localisation precise; DUACS stands for Data Unification and Altimeter Combination System, or Developing Use of Altimetry for Climate Studies. The data set provides a near-global data set of geostrophic currents (idea for this post is courtesy Lei Wang).

SSALTO/DUACS processes data from all altimeter missions, including the Chinese HY-2A, Saral/AltiKa, Cryosat-2, OSTM/Jason-2, Jason-1, Topex/Poseidon, Envisat, GFO, ERS- 1&2 and even Geosat. From each satellite, SSALTO/DUACS uses the Interim Geophysical Data Record (IGDR), where these are 1-second averages available within 3 days of data collection, from all altimeters. For greater accuracy, the reader should use the GDR, which is a validated record available with a 6-week time lag. Examination of the image shows that the combined product provides data north into Arctic and south to the Antarctic coast. The website for the system and directions on obtaining data are at http://www.aviso.altimetry.fr/en/data/product-information/information-about-mono-and-multi-mission-processing/ssaltoduacs-multimission-altimeter-products.html; the handbook for the data set is at http://www.aviso.oceanobs.com/fileadmin/documents/data/tools/hdbk_duacs.pdf

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Air Force is building a new radar on Kwajalein Atoll to track space debris

cobra-dane2sm

(generic image of phased-array radar)

(From Science, 9 January 2015, article by Ilima Loomis)

“An estimated 500,000 pieces of space junk—old satellites, rocket parts, debris from collisions—swarm in orbit around Earth. Much of it is potentially deadly: NASA officials say anything larger than 1 centimeter in diameter poses a threat to the International Space Station. But current tracking systems can generally only watch objects 10 cm or larger, and the U.S. government currently follows less than 5% of space hazards—just 23,000 objects. That should change with the addition of a powerful new Air Force radar system, scheduled to break ground this month on Kwajalein Atoll in the Marshall Islands. When it comes online in 2019, the flood of information passed along to nonmilitary spacecraft operators will bring reassurance—but also some wrenching choices about which hazards to ignore.”

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