Wednesday, April 28, 2010

USGS对inSAR的介绍

What is InSAR?

InSAR stands for Interferometric Synthetic Aperture Radar. This is thus a remote sensing technique that uses radar satellite images. Those radar satellite (ERS1, ERS2, JERS, IRS or Radarsat) shoot constantly beams of radar waves towards the earth and record them after they bounced back off the Earth's surface.

Two information compose the images. One contains how much of the wave bounced back off to the satellite (signal intensity). That depends on how much of the wave has been absorbed on the way and how much has been reflected in the direction of the satellite.

The second information is the 'phase' of the wave. When a wave travels through space, we can think of it as a hand on a clock. It starts on 12 when the wave leaves the satellite. The 'hand' (phase) keeps running round and round the clock until the wave reaches the ground. When the wave hits the ground, the hand stops and indicating a certain 'time' or 'phase'. When the wave comes back to the satellite, it tells the satellite on what value the hand/phase got stopped.

Every point in a satellite image (pixel) is carrying those two information: the intensity and and the phase. The intensity can be used to characterize the material in which the surface the wave bounced off is made of and what orientation it has. Oil leaks on the sea, for instance, can be spotted in that way. They look much smoother than the water surrounding.

The phase is used in another way. When the radar satellite revisists the exact same portion of the Earth, the phase image should be identical. If it is not the case, then something has been going on. And by combining those two images, we can measure how much and where the ground has moved.

Our research focuses on tectonic and volcanic ground deformations. And sometimes, very unexpectedly, we find that human impact is much more important than we would have thought before.

源文档


How satellite radar interferometry works?

Return signal from satellite holds the key

The technical details of how and why radar interferometry works are rooted in physics and radar engineering, but for our purposes a much simpler explanation will suffice. A pulse of radar energy is successively emitted from a satellite (left), scattered by the Earth's surface, and recorded back at the satellite (right). The radar energy received by the satellite contains two important types of information.

Figures from T. Freeman, Jet Propulsion Laboratory
The first type information is encoded in the strength or amplitude(振幅强度) of the return signal, which is influenced by various physical properties of the surface including ground slope, particle size (i.e., sand versus boulders), and soil moisture. The ERS satellites record return signal strength from a continuous swath of the Earth's surface about 100 km wide (60 mi) wide, and scientists on the ground assemble this information in the form of a radar image. The image is a portrayal of the surface that resembles a conventional photograph in some ways, but not entirely. Think of the difference between a conventional photo and an infrared image, which shows warm areas as bright irrespective of their brightness in visible light. Radar images differ from conventional photos in a similar way.

The second type of information contained in the return radar signal has to do with the round trip distance(往返距离) from the satellite to the ground and back again. We can think of a radar pulse as an invisible tape measure calibrated in units of the radar wavelength. We call the fractional part of the round trip distance the phase of the return signal. For the ERS satellites, the radar wavelength is 5.66 cm (2.2 inches).

If we were able to acquire two radar images at different times from exactly the same vantage point in space and compare them, any movement of the ground surface toward or away from the satellite would show up as a phase difference(相位差) between the images. For example, if a point on the ground moved toward the satellite (mostly upward) by one-half wavelength, the phase of the return signal from that point would increase by one full wavelength relative to the first image. It isn't possible to steer a satellite accurately enough to return it to exactly the same point in space on different orbits, but it's relatively easy to get within a few hundred feet and then do the necessary geometric corrections.

Combining or "interfering" images from different satellite passes

It turns out that the most accurate way to measure small phase changes(相变) is to combine two images together after all of the necessary corrections have been made. This process is sometimes called "interfering"(干涉) the images, because combining two waves causes them to either reinforce or cancel one another, depending on the relative phases. For example, you may have observed interference between two sources of water waves on a pond. Now imagine that we can keep track of all the places where two radar images reinforce one another(相互加强), and all the places they cancel one another(相互抵消). We'll represent the first case as a red pixel in a new image that we'll create, and the second case as a blue pixel. Intermediate cases(中间情形) will be represented as intermediate colors of the spectrum from red to blue. The resulting image is called an interferogram.

The properties of waves are such that we can't tell the difference between waves that reinforce one another because they are exactly in phase with(同相) one another, or out of phase(异相) by any number of whole wavelengths (1, 2, 3...). As a result, an interferogram for an area that domed upward during the time interval between two radar images would show a concentric pattern(同心纹) of color bands(色带), called fringes(干涉条纹), not unlike the contours on a topographic map (above). In this case, though, each fringe would represent just one-half wavelength of surface movement toward the satellite--nearly 3 cm for ERS (just over an inch). To determine the total amount of movement, we only have to count the number of fringes. Our geodetic camera is ready to track volcano deformation from space!

源文档

Sunday, April 18, 2010

同门

同 门
2010年4月18日 17:40
今天下午专门抽时间(本来正在自习室上自习)去二里庄公交站充公交卡,里面只剩三块钱了。路上特堵,三站路用了半个多小时,结果又碰上公交公司一个特2的工作人员,翻看了一下我递过去的学生证,以非常强势的口吻说:“你的学生证没注册,不能充。”Fuck!学生证注册与否管你屁事,只要我的公交卡在有效期内就有充值的权利。所以说,有些人哪怕只有屁大点的权力,她就能把它发挥到极致,撑能发威装大娘。没办法,不给充就回去吧,改天去牡丹园那边看看能充吗,不行就找学校给盖个“注册”章了。于是去等车,但越想越不爽,专门抽时间过来竟然不给充,又折回去到窗口A试试,结果这位阿姨看了一下俺的学生证,啥都没说就给充了,让我再次坚信了窗口B那人就是一个ZB的SB。
  充完后心情好爽,不用再折腾着去牡丹园充或者去学校盖章了。50¥应该能用到年底吧,一周刷五天,一天刷两次,一次刷两毛……北京就这点好处,公交地铁都比其他城市要便宜,地铁才2块,在上海这是坐公交的价格。在回去的时候因为用手机发推坐过了一站,到五道口下的,那就晃悠着再回去吧。五道口这地儿老外太多,但乞丐也多,政府也不救助一下,让他们整日在老外的眼皮底下行乞,多给天朝丢脸呀。
  快到学校的时候,突然发现同届的ZC和上届的S师姐往这边走,前几天跟师姐聊天她还在昆明呢,怎么回来了?忙上前喊了一下,仨人在路边聊起来了。ZC上来就说俺胡子该刮了,木想到还有人替俺形象着想,真是感激涕零。原来师姐今天刚从昆明飞回来,要准备论文答辩了,毕业后就去昆明那家单位工作。她是投奔她男友去的,而且工作好像是他男友的导师给介绍的,让俺听了很泪奔,这导师咋就这么好呢?还负责给学生的对象介绍工作,再想想俺们,惨!
  现在感觉上研究生最大的收获之一是结识了一帮亲如兄弟姐妹的同门,尤其我们本届的6个人,关系特别好,相互之间非常照顾。记得刚开学时,我没带床垫,只带的褥子、床单和被子,交际甚广的同门Bush很快就给搞了一个来,当时自己不在宿舍,他送货上门,让我感觉非常温馨。后来又是Bush给我和隔壁的同门XY搞了两个收音机,考英语听力的时候要用。总之,同门Bush是一个非常热心、乐于助人的大好人。可惜老板对他很失望,因为他热心班级事务,还爱打个篮球,有时就把老板的事给忘一边了。记得也没什么大事,就是有次整理机房,我们几个都去了,而他去打球去了,而且老板又从师兄那知道他去打球了,于是把他一阵臭骂和数落,每次批他的时候几乎都把这事翻出来。
  住在我隔壁的同门XY和我一样,是从某所211高校考过来的,是个特别可靠的哥们,没事的时候我就去他们宿舍串门。XY最大的特点是说话特别逗,印象最深的有两次。第一次还是大家都在工作室给老板干活的时候。某天中午一起去吃饭,出了工作室上卫生间,解决完毕我正要洗手,他来了句:“洗什么手啊?还不信任自己吗?”对啊,必须信任自己,那就不洗了。还有一次是下届复试时他本科的一个师妹报的我们老板,但老板想调剂给其他导师,他给引荐了一下,老板见人家小姑娘长得不错,可能想给高我们两届的大师兄创造机会,问XY:“这丫头有男朋友了吗?”他灰常痛快的来了句:“有了,但是远水解不了近渴嘛,估计马上就分了吧……”我们听到后直接笑喷了。“那这不行……”,老板说。最后把这小姑娘调剂了。还有一次XY的GF和他闹别扭,可能嫌他不浪漫,这家伙在QQ上向我这个老光棍儿求救:XX,什么是浪漫?我女朋友在跟我闹腾呢。我无语,随便来句:浪漫有鸟用?可以当饭吃啊?好好工作,安安稳稳过日子就行了。他来了句:“哦,原来浪漫就是好好过日子啊!我这就告诉她。”想想就挺搞笑,这种事问我那纯粹听我扯淡。
  再说说同门ZC,一个有点假小子气质的天津女孩,一直在工作室跟老板做项目,很同情她,因为真的非常累。她也是本校的,对老板的脾气秉性有所了解,所以许多事情她都提醒我们一下,不要让老板在某些事上抓住小辫子。记得XY宿舍的Job还对她有想法(后来知道这个特老实的娃几乎对我们班女生都有过想法,牛逼吧),又要她的电话,又是QQ的。当时不确信她有没有BF,但像我们这种狼多肉少,连学校里的蚂蚁都是公的的工科学校,不是残废估计都剩不下吧。其实人家早有了,已经在天津什么工商局工作了,我还见过。有次他来北京,在我们宿舍住了一两天,当时还以为只是普通的高中同学,因为介绍的时候ZC说这是她的朋友,后来知道好像天津人说的朋友就是指男女朋友。
  我们这届还有一个女生,大家都称呼她川儿或者阿川,我的一个老乡评价她是一个特甜的女生。据说她老爸是中南大学毕业的,老妈是高中教师,所以为人处世比较得体,咱这种两腿沾满泥巴的土老冒和人家不在一个档次。后来大家跟老板一起吃饭的时候,老板老是夸奖她,虽然最后一名但比谁都强。川儿比较有想法,在实习单位帮一个领导做事,感觉学不到东西就跟老板商量提前一年毕业并考博。前段时间考的P大,结果英语没过线,今天本校的博士考试刚结束,应该问题不大。在跟老板相处这件事上我应该向她和其他同门学习,当初老板在我们这届重点培养我,结果由于个人能力、对未来的考虑、不现实的理想和对自由的渴望及对束缚的厌恶,我逃离了,老板对此很失望。还好下届的师弟能力挺强的,老板不用担心接不上茬儿。我没有为自己的决定后悔过,也许不明智,但当时也是无奈的选择,没有其他选项可选了。
  最后是SH了,干活踏实但有时因为责任心太强而有点push。有次老板让他负责一块小项目,然后他分给大家做。任务分好后一般限定多长时间完成就可以了,但他老是催,结果把HJ师姐催火了,俩人在工作室吵起来了,弄得有点不高兴。俺这个师姐有点泼辣,心直口快,但对于那个活,和我一样,做得比较慢。我和SH待的时间比较长,因为老板把我们俩和ZC留在身边做项目,平时都是一起去吃饭。那次跟师姐吵了后很打击他,说以后再也不负责这臭烂事了。其实SH特别能干,虽然也是和我一样爱抱怨。总之,比我要踏实,我心比天高,命比纸薄,坐得住但做不进去,他既坐得住,又能干活。现在去昆明帮老板的一个朋友做项目,据说挺辛苦的,但机会应该比跟着老板要多,因为在昆明那边是负责人,项目开会的时候都能去参加。
  我为拥有这样的同门而感到幸运,也许这才是我硕士期间最大的财富。或许他们知道我压力比较大,所以在老板为难我的时候经常安慰我,尤其是俩女同门,让我想得开,其实我哪有那么经不起打击。
  再就是感谢今天遇到的S师姐,大家一起做过项目,比较了解。其实我们这届最喜欢的就是这个师姐(上届一共四个,还有一个更漂亮的,特会打扮……),活泼开朗,待人亲切,爱在工作室闹腾,像个小孩似的,大师兄对她是垂涎三尺,可惜人家早就名花有主了。S师姐让我感到亲切除了她开朗随和的性格还因为她曾经在某人向老板打我小报告时替我说过公道话,也许她不知道我已经知道此事了。她一直笑话我傻,不知有其中缘由否,今天把公交卡充值这事说给她们听,她来了句“你这不不傻啊?”。许多事自以为做得天衣无缝,实则隔墙有耳。那个几十平米的工作室曾让我很崩溃,知人知面不知心,我太惑于表象而被蒙蔽了双眼,我若待下去只能被废掉。
  马上就要毕业了,大家朝着自己的目标努力奋斗吧!祝福你们,我亲爱的同门们!
  That's all.

Wednesday, April 14, 2010

如果可以

如果可以

来生就做一只小鸟

自由自在地飞翔

累了就在树枝上小憩

饿了就啄个草籽

渴了就饮口溪水

无牵亦无挂

孤独也潇洒

Tuesday, April 13, 2010

转: 谈一场为了结婚的恋爱

从别处转来的文章……很温暖的文字。最喜欢最后一句:

也许将来的某一天我也会坚定如此的对一个人说:你敢天长,我就敢地久。

毛泽东语录里教育我们说,不以结婚为目的的谈恋爱,都是耍流氓。

那么,在那最后一次之前,我们都在不断地耍流氓。

因为寂寞,因为不安,甚至纯粹地为了恋爱而恋爱,那些写在青春岁月里斑驳的痕迹,是伤过了也爱过了的记忆。然后终究将消散了去再也不复返。时间像在做一道减法题,每个人固执坚持不忘的回忆原来那么不堪一击,全部消散在未来的幸福里。

所以,在某一天,我们会遇见这样一个人。

你们可以相遇得很平淡亦或很离奇,你们可以一见如故亦或没有言语。但重要的是你们遇见了,遇见彼此的那一刻起你们开始相信奇迹。这个人,也许任性坏脾气也不算美丽,也许不帅气也没有人民币,但是真的不要紧。生命中有一个人,只要遇见了,就好了。

因为她你开始改变审美,不再一味只爱大曲线的MM。她的任何表情在你的眼里都显得明媚。

因为他你开始变换品味,不再追求绅士和地位,习惯他犯傻的行为和满身的汗水味。

因为彼此你们都坚信自己摆脱了庸俗,不谈房子不谈车子只谈恋爱就那么地想守住一辈子。

因为这个人的出现,我们宁愿赌上一世的姻缘。把未来日子里所有的桃花运都交付给你,换你跟我永远在一起,好不好?

因为那一个人,你开始那么仔细地,计划未来。

你们可以每天守在身边也可以异地恋,你们可以经常见面亦或靠通讯相连。但是选择在一起的那一瞬间便都不再害怕艰难。

你们开始不在乎口袋里有没有太多的钱,你们可以一起去吃大餐或者拉面,但最美味的,却永远是桌子对面那张或许吃相不太好看的脸。

因为那个人的出现,我们不再喊着单身万岁,我们不再认为甜蜜是很肤浅的东西。我们开始发现原来自己有太多的幻想,原来自己也仍然坚信地久天长。

不是因为年龄的增长,不是因为岁月的流浪,突然想在还有无尽青春的年华里说那一句,我们结婚吧,好不好?

我们结婚吧,好不好。那样我们就可以不再对着电话诉说想念,就可以每天清晨起来看见你的睡脸,然后一起吃一顿不太丰盛却很温暖的早餐。

我们结婚吧,好不好。那样我们就可以光明正大地在楼下拥抱,而不用担心被老爸老妈和邻居看到。然后我们十指紧扣,到处逍遥。

我们结婚吧,好不好。那样我们就可以拿着民政局发的红色小本子四处炫耀,我们可以在房间里挂满结婚照,看着看着就会不自觉地微笑。

一生就这么一次,谈一场以结婚为目的的恋爱吧。

不再因为任性而不肯低头,不再因为固执而轻言分手。最后地坚信一次,一直走,就可以到白头。

亲爱的,谈一场不耍流氓的恋爱,好不好?然后就那样相守,在来往的流年里,岁月安好。

惟愿这一生,执子之手,与子偕老。

你敢天长,我就敢地久。