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The TanDEM-X Mission - An Overview

The TanDEM-X Mission: An Overview

Hauke Fiedler #1, Gerhard Krieger *1, Manfred Zink #1, Marwan Younis *1, Markus Bachmann #1, Sigurd Huber *1,

Hajnsek #1, Alberto Moreira #1

Institute of Radio Frequency and Radar Systems DLR-Oberpfaffenhofen, 82234 Wessling, Germany

Hauke.Fiedler@dlr.de

Abstract — TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is an innovative spaceborne radar interferometer mission that was approved for full implementation by the German Space Agency in spring 2006. This paper gives an overview of the TanDEM-X mission concept,

the data acquisition strategy with optimized performance, and

provides some examples for new imaging techniques and applications.

I. I NTRODUCTION

The primary objective of the TanDEM-X mission is the generation of a world-wide, consistent, timely, and high precision digital elevation model aligned with the HRTI-3 specification as the basis for a wide range of scientific research, as well as for operational, commercial DEM production [1]. This goal will be achieved by means of a second, TerraSAR-X like satellite flying in close orbit configuration with TerraSAR-X. Both satellites will then act as a large single-pass SAR interferometer with the opportunity for flexible baseline selection. This enables the acquisition of highly accurate cross-track and along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal de-correlation and atmospheric disturbances. Besides the primary goal of the mission, several secondary mission objectives based on along-track interferometry as well as new techniques with bistatic SAR have been defined which represent an important and innovative asset of the mission. TanDEM-X will be implemented in the framework of a public-private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH, as for TerraSAR-X. The launch of TanDEM-X is planned for September 2009.

II. M ISSION C ONCEPT

The TanDEM-X mission is an extension of the TerraSAR-X mission, co-flying a second satellite of nearly identical capability in a close formation. The TerraSAR-X satellite (TSX), as basis for TanDEM-X, is not only a high performance SAR system with respect to SAR image and operational features, but it has already built in all necessary features required for the implementation of the TanDEM-X mission. Examples are additional X-band horn antennas for inter-satellite phase synchronization, the availability of a dual-frequency GPS receiver for precise orbit determination, excellent RF phase stability of the SAR instrument, and PRF synchronization based on GPS as a common time reference. The second satellite (TDX) will be as much as possible a rebuild of TSX with only minor modifications like an

additional cold gas propulsion system for formation fine tuning and an additional S-band receiver to enable a reception of status and GPS position information broadcast by TSX. This guarantees a low development risk and it offers the possibility for a flexible share of operational functions among

the two satellites. The TDX satellite will be designed for a

nominal lifetime of 5? years and has a nominal overlap with TSX of 3 years. Note in this context that TSX holds consumables and resources for up to seven years of operation, allowing for a potential prolongation of the overlap and the TanDEM-X mission duration.

The instruments on both satellites are advanced high resolution X-band synthetic aperture radars based on active phased array technology, which can be operated in Spotlight, Stripmap, and ScanSAR mode with full polarization capability [2]. The center frequency of the instruments is 9.65 GHz with a selectable SAR chirp bandwidth of up to 300 MHz. The active phased array antenna, which has an overall aperture size of 4.8 m x 0.7 m, is fixed mounted to the spacecraft body and incorporates 12 panels with 32 dual-pol waveguide sub-arrays each. This enables agile beam pointing and flexible beam shaping.

A. Interferometric Data Acquisition

Interferometric data acquisition with the TanDEM-X satellite formation can be achieved in different operational modes: Examples are Bistatic, Monostatic, and Alternating Bistatic operation which are illustrated in Figure 1. The three interferometric configurations may further be combined with different TSX and TDX SAR imaging modes like Stripmap, ScanSAR, Spotlight, and Sliding Spotlight. Operational DEM generation is planned to be performed using the bistatic InSAR stripmap mode shown in Figure 1 in the middle. This mode uses either TSX or TDX as a transmitter to illuminate a common radar footprint on the Earth's surface. The scattered signal is then recorded by both satellites simultaneously. This simultaneous data acquisition makes dual use of the available transmit power and is mandatory to avoid possible errors from temporal decorrelation and atmospheric disturbances.

B. Relative Phase Referencing

A peculiarity of the bistatic data acquisition is the use of independent oscillators for modulation and demodulation of the radar pulses. Any deviation between the two oscillators will hence cause a residual modulation of the recorded azimuth signal. The impact of oscillator phase noise in bistatic SAR has been analyzed in [3] where it is shown that oscillator

Irena

Fig. 1 Data Acquisition modes for TanDEM-X: Pursuit monostatic mode (left), bistatic mode (middle), and alternating bistatic mode (right).

noise may cause significant errors in both the interferometric phase and SAR focusing. The stringent requirements for interferometric phase stability in the bistatic mode will hence require an appropriate relative phase referencing between the two SAR instruments or an operation in the alternating bistatic mode. For TanDEM-X, a dedicated inter-satellite X-band synchronization link will be established by a mutual exchange of radar pulses between the two satellites. For this, the nominal bistatic SAR data acquisition is shortly interrupted, and a radar pulse is redirected from the main SAR antenna to one of six dedicated synchronization horn antennas mounted on each spacecraft. The pulse is then recorded by the other satellite which in turn transmits a short synchronization pulse. By this, a bidirectional link between the two radar instruments will be established which allows for mutual phase referencing without exact knowledge of the actual distance between the satellites. On ground, a correction signal can then be derived from the recorded synchronization pulses which compensates the oscillator induced phase errors in the bistatic SAR signal. The performance of such a synchronization link has been investigated in [4].

C. Orbit Configuration and Formation Flying The TanDEM-X operational scenario requires a coordinated operation of two satellites flying in close formation. The adjustment parameters for the formation are the inclination vectors, realized by the right ascension of the

ascending node and inclination, the eccentricity vectors, and

the relative perigee also called libration phase angle. With

these parameters, several options have been investigated during the phase A study, and the HELIX satellite formation shown in Figure 2 has finally been selected for operational DEM generation. This formation combines an out-of-plane (horizontal) orbital displacement by different ascending nodes with a radial (vertical) separation by different eccentricity vectors resulting in a helix like relative movement of the satellites along the orbit. Since there exists no crossing of the satellite orbits, one may now allow for arbitrary shifts of the satellites along their orbits. This enables a safe spacecraft operation without the necessity for autonomous control. It is

furthermore possible to optimize the along-track displacement

at predefined latitudes for different applications: cross-track

interferometry will aim at along-track baselines which are as short as possible to ensure an optimum overlap of the Doppler spectra and to avoid temporal de-correlation in vegetated areas, while other applications like along-track interferometry or super resolution require selectable along-track baselines in the range from hundred meters up to several kilometers. A fine

Cross-track baselines.

The HELIX formation enables a complete coverage of the Earth with an optimised height of ambiguity. This may be achieved by e.g. a drifting HELIX concept, in which the horizontal displacement will be enlarged e.g. linearly during a fixed time span. This may be obtained by e.g. a small inclination offset of one satellite. The effective baseline will increase, hence resulting in a stable height of ambiguity depending on time, geographical latitude and/or incident angles. Southern and northern latitudes can be mapped with the same formation setting by using ascending orbits for one and descending orbits for the other hemisphere, as illustrated in Figure 2 on the right. A fine tuning of the cross-track baselines can be achieved by taking advantage of the natural rotation of the eccentricity vectors due to secular disturbances, also called motion of libration. The phases of this libration can be kept in a fixed relative position with small manoeuvres

using the cold gas thrusters on a daily basis, while major formation changes as well as a duplication of the orbit keeping manoeuvres required by TSX will be performed by the hot gas thrusters. III. P ERFORMANCE A NALYSIS

This section investigates the interferometric performance of TanDEM-X. For this, an interferometric data acquisition in bistatic stripmap mode will be assumed. Major factors which affect the relative height accuracy are the radiometric sensitivity of each SAR instrument, range and azimuth ambiguities, quantization noise, processing and co-registration errors as well as surface and volume decorrelation, scaled by the baseline length [5]. The key quantity in estimating the interferometric performance is the coherence which has been computed by the product Temp Vol Az Geo Coreg Amb Quant SNR tot γγγγγγγγγ???????=where the right hand side describes the different error

contributions due to the limited SNR ( γSNR ), quantization

(γQuant ), ambiguities (γAmb ), limited coregistration accuracy (γCoreg ), baseline decorrelation (γGeo ), relative shift of Doppler spectra (γAz ), volume decorrelation (γVol ), and temporal decorrelation (γTemp ). Each of these terms has been evaluated and Figure 3 shows the result of the interferometric performance analysis for two different ambiguous heights

NH (desc )

SH (asc

corresponding to different baseline lengths in the order of

(dashed). The solid curve shows the error resulting from the combination of

multiple swaths. All errors are point-to-point errors for a 90% confidence interval.

IV. D OWN L INK C ONCEPT For mapping the whole Earth within ~1 year, an average orbit acquisition time of ~170 s is required. One year corresponds to ~33 repeat cycles. Due to the limitation of mass memory onboard both satellites, the recorded data should be dumped as fast as possible. To dump these data, 110.000 s of down link per repeat cycle is required, which can be achieved by ~ three ground stations. These ground stations should be placed such that after recording data a ground station is visible and the data may be dumped in the same orbit. Due to the land mass distribution and the data acquisition concept (which assumes a recording of data in ascending orbits on the northern hemisphere, see above), two ground stations should be placed at high northern latitudes and one station at high southern latitude. Furthermore, the northern ground stations should be separated such that they will not share a common overlap of possible dumps. The southern station should be placed near South America or South Africa because in these descending orbits most of the data on the southern hemisphere will be collected. An example for a ground station network fulfilling the above constraints would be e.g. Kiruna, Inuvik, and O’Higgins.

V. N EW I MAGING T ECHNIQUES The TanDEM-X mission will provide the remote sensing scientific community with a unique data set to exploit the capability of new bistatic radar techniques and to apply these innovative techniques for enhanced geo- and biophysical parameter retrieval.

A. Along-Track Interferometry TanDEM-X is predestinated for along-track interferometry (ATI), which compares the phase of two complex SAR images acquired in identical geometries but separated by a

short time interval. This technique is well suited for

monitoring dynamic processes on the Earth's surface. As

outlined in Section 2, it is possible to adjust the along-track displacement between the two satellites from almost zero to several kilometers. By this, it becomes possible to adapt the

ATI sensitivity of TanDEM-X to a wide range of radial velocities. The HELIX formation enables even a minimization of the across-track component for a given latitude and incident angle. Along-track interferometry can furthermore be performed by the so-called dual-receive antenna mode in each of the two tandem satellites, which provides additional along-track baselines of 2.4 m [10]. The combination of short and long baseline ATI data acquisitions can be used to improve both the detection and localisation of moving objects and to resolve phase ambiguities from high-velocity scatterers. TanDEM-X provides hence a capable along-track SAR interferometer with four phase centers. Potential applications are Ground Moving Target Indication (GMTI), the measurement of ocean currents, and the monitoring of sea ice drift. B. Very Large Baseline Cross-Track Interferometry

Very large baseline interferometry takes advantage of the

high RF bandwidth of the TSX and TDX satellites which allows for coherent data acquisitions with baselines of up to

km and more. Note that less than 5% of the maximum possible (critical) baseline length is used during nominal DEM data acquisition. Large baseline interferograms can hence significantly improve the height accuracy, but the associated low height of ambiguity requires a combination of multiple interferograms with different baseline lengths to resolve phase ambiguities. By this, it becomes possible to derive DEMs with HRTI-4 like accuracy on a local or even regional scale. Further opportunities arise from a comparison of multiple large baseline TanDEM-X interferograms acquired during different passes of the satellite formation. This provides a very sensitive measure for vertical scene and structure changes. Potential applications are a detection of the grounding line which separates the shelf from the inland ice in polar regions, monitoring of vegetation growth, mapping of atmospheric water vapour with high spatial resolution, measurement of snow accumulation or the detection of anthropogenic changes of the environment, e.g. due to deforestation. Note that most

of these combinations rely on a comparison of two or more single-pass (large baseline) cross-track interferograms and do hence not necessarily require coherence between the different

passes. Further information can be gained from an evaluation

of coherence changes between different passes, potentially augmented by polarimetric information. This could for instance reveal even slight changes in the soil and vegetation structure reflecting vegetation growth and loss, freezing and

thawing, fire destruction, human activities, and so on. TanDEM-X enables hence the entry into a new age of

interferometric (and tomographic) processing techniques as it was ERS-1/2 for the development of classical repeat-pass SAR interferometry. C. Polarimetric SAR Interferometry

Polarimetric SAR interferometry combines interferometric with polarimetric measurements to gain additional information

from semi-transparent volume scatterers [6]. This allows e.g.

for the extraction of important biophysical parameters like vegetation density and vegetation height. Fully polarimetric operation uses the split antenna and is susceptible to ambiguities. This can be avoided by reducing the processed azimuth bandwidth and/or by limiting the swath width. Figure 4 illustrates the achievable performance of a simulated scenario for TanDEM-X. This analysis is based on the Random Volume over Ground (RVoG) [6] model assuming a vegetation layer with a height of 1.2 m and an extinction coefficient of 4 dB/m. The dashed line indicates the height variation of the interferometric phase centre with different polarisations (corresponding to a variation of μ on the abscissa). The green tube shows the height errors due to volume decorrelation for an effective baseline of 4 km and an independent post-spacing of 30 m x 30 m. The blue tube shows additional errors due to the limited system accuracy and the red tube indicates potential errors in case of temporal decorrelation caused by a possible along-track separation between the two satellites (here: γTemp = 0.7). The performance analysis predicts a sufficient phase centre separation to enable a successful retrieval of important vegetation parameters like volume height, extinction, etc. TanDEM-X will be the first mission to demonstrate this technique in a single-pass data acquisition mode.

Fig. 4 Vertical separation of interferometric phase centers in TanDEM-X as a function of the ground-to-volume scattering ratio μ (cf. [7]). The vegetation height is 1.2m, the ground-to-volume ratio μ ranegs from -7 dB to 3 dB and the extinction is 4 dB/m.

D. Bistatic SAR Imaging

Bistatic SAR imaging provides additional observables for the extraction of important scene and target parameters [8]. TanDEM-X allows for the simultaneous acquisition of bistatic and monostatic images in a single data take to obtain a highly informative set of multi-angle observations. A quantitative evaluation of the bistatic radar cross-section (RCS) and a comparison with its monostatic equivalents facilitates the detection and recognition of targets. The segmentation and classification in radar images is expected to be substantially improved by comparing the spatial statistics of mono- and

bistatic scattering coefficients. This is also supported by the joint airborne bistatic radar experiments performed by DLR and ONERA, which revealed significant changes of the scattering behavior for both artificial and natural targets even in case of rather small bistatic angles [9]. A joint evaluation of mono- and bistatic SAR images could furthermore be used to isolate different scattering mechanisms, like e.g. a distinction between highly directive dihedral returns from more isotropic volume scattering. Bistatic SAR imaging has moreover potential for the retrieval of sea state parameters, the estimation of surface roughness and terrain slope, as well as stereogrammetric, meteorological and atmospheric applications. Innovative processing algorithms will be required to exploit all these capabilities. The bistatic data acquired with TanDEM-X will hence provide a unique data source to improve our understanding of bistatic imaging and its exploitation for future remote sensing applications. Data takes with large bistatic angles are planned at the beginning and at the end of the TanDEM-X mission where the satellites are separated from each other by several tenths of kilometers. E. Digital Beamforming and Super Resolution

Digital Beamforming combines the RF signals from a set of small non-directional antennas to simulate a large directional antenna. Due to the split antennas and dual receiver channels of TSX and TDX, four phase centers can be obtained in a tandem mode. An appropriate combination of the multiple Rx signals enables then an efficient suppression of azimuth ambiguities [11]. By this, it is possible to demonstrate the capabilities of high resolution wide swath SAR imaging. This technique could also be of interest for advanced interferometric SAR modes like the alternating bistatic mode where it allows for a reduction of the PRF, thereby resolving potential timing and ambiguity conflicts. TanDEM-X will be the first configuration which demonstrates this highly innovative technique from space.

Another promising technique is super resolution. This technique exploits the fact that the signals received by the two satellites have different aspect angles for each scattering point on the ground. In consequence, the two ground range and/or azimuth spectra are shifted relative to each other. A coherent combination of the signals yields then a wider spectrum which corresponds to an improved spatial resolution. This technique requires a cross-track and/or along-track separation in the order of 5 to 10 km. Such baselines will be available in later mission phases after the standard DEM data acquisition has been completed.

VI. C ONCLUSION

TanDEM-X is a highly innovative bistatic single-pass SAR interferometer which allows for the acquisition of unique remote sensing data products. The achievable height accuracy for global DEM generation with TanDEM-X is mainly limited by the height of ambiguity that can finally be processed during phase unwrapping. The standard HRTI-3 DEM is a reasonable

compromise between performance, processing, and data

acquisition effort. A mission scenario has been developed which enables the acquisition of a global HRTI-3 DEM within less than three years [5]. This concept includes several data takes with different baselines, different incident angles, and data takes from ascending and descending orbits to deal with difficult terrain like mountains, valleys, tall vegetation, etc. The TanDEM-X mission concept allocates also sufficient acquisition time and satellite resources to secondary mission goals like along-track interferometry or the demonstration of new bistatic radar techniques.

In 2006, the TanDEM-X mission has been approved for realization by means of a public private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH. The launch of the TanDEM-X satellite is planned for spring 2009 which ensures at least three years of joint operation with the TerraSAR-X satellite. Current work includes the preparation of a detailed data acquisition plan which has also to take into account potential conflicts between the TerraSAR-X and the TanDEM-X mission, the development and analysis of a calibration concept [12], the design of a multibaseline InSAR processor, the compilation of a science plan as well as performance investigations for the other innovative TanDEM-X imaging modes.

A CKNOWLEDGMENT

The authors would like to thank the TanDEM-X team. The TanDEM-X project is partly funded by the German Federal Ministry for Economics and Technology (F?rderkennzeichen 50 EE 0601).

R EFERENCES

[1] A. Moreira, G. Krieger, I. Hajnsek, M. Werner, D. Hounam, S. Riegger,

E. Settelmeyer, "TanDEM-X: A TerraSAR-X Add-On Satellite for

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[2]M. Stangl, R. Werninghaus, B. Schweizer, C. Fischer, M. Brandfass, J.

Mittermayer, H. Breit, "TerraSAR-X technologies and first results,"

IEE Proc. - Radar, Sonar and Navigation, vol. 153, pp. 86-95, 2006. [3]G. Krieger, M. Younis, "Impact of Oscillator Noise in Bistatic and

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Zink, B. Wessel, "Development of TanDEM-X DEM Calibration Concept," in Proc. EURAD, Munich, Germany, 2007.

a,an和the用法区别

a，an和the用法区别 a，an和the 用法你们都了解了吗?今天给大家带来a，an 和the 用法，希望能够给帮助到大家，下面就和大家分享，来欣赏一下吧。英语无捷径|| a，an和the 用法区别，记住就不要再犯错了! 1. 不定冠词 a 还是an 判断用a还是用an的依据是其后的单词的发音，而不是字母!以元音音素开头的单词前用an来修饰，以辅音音素开头的单词前用a来修饰。常用的搭配规律： 1) 多数以元音字母(a、e、i、o、u)开头的单词，首音节都是发元音：I atean an apple yesterday. It’s sweet.我昨天吃了一个很甜的苹果。An American usually speaks English.美国人通常讲英语。Driving carefully helps avoid an accident.谨慎驾驶有助于避免事故。Learn how to survive anearthquake.学习如何在地震中生存。He went to aninterview yesterday.他昨天去面试了。She is cutting anonion.她在切洋葱。

2) 元音字母U开头的单词，首音节可能发元音或者辅音[ju:] 【j】a university [junivsiti] studenta unique [junik] styleA university is where you do your degree.大学是你获得学位的地方。Have you ever seena UFO/an unidentified flying object?你见过不明飞行物吗? 3) 记住三个h开头却不发音的名词：hour，honst，honourHalf an hour has passed.半小时过去了。Its an honour to be invited to your dinner.很荣幸被邀请参加你的晚宴。 4)European 欧洲人，欧洲的，元音字母e不发音，首音节是[ju:]He is a European, and he has lived in Hangzhou for 10 years. 他是欧洲人，在杭州住了10年。 5)如果名词前有形容词做定语修饰，则我们看形容词是辅音还是元音开头。There stands an oak tree in front of the house.房子前面有一棵橡树。An important lessonIve learned from it is never to give up.我从中学到的重要一课是永不放弃。He drives an old car.他开一辆旧车。In those years he was just an unknown pianist.在时的他只是一个默默无闻的钢琴家。An honest man never lies.诚实的人从不说谎。 2. 定冠词the 定冠词the : 表示特指某事物或人

英语语法的五种基本句型

教学目标： 1.让学生掌握英语学习中的五种基本句型； 2.学会分辨句子属于哪种句型； 3.懂得使用不同的句型造句；教学重点与难点： 1.五种句型的分析与理解； 2.句型4（主+动+宾+补）与句型5（主+动+宾+宾）的掌握与比较；教学方法： 1.ppt演示；课堂练习：一、下列的句子属于哪种类型 1. The sun rises 2. She is walking along the lake. 3. I like this book very much. 4. That man seems kind 5. He bought his sister a piano. 6. She kept us waiting for over three hours. 7. Let me give you a hand. 8. We tried to make her happy. 二、请说出五个句子，并说出属于哪种句型家庭作业： 1.每个句型各举出3个句子 2.翻译句子

导入主题：我们现在能用英语写简单的作文，也可以用英语进行简单的交流与沟通，可是大家知道英语中有多少种基本句型吗？那这些句型分别是什么？（让学生自由思考、讨论，引出今天的课题，英语学习中的五种基本句型）1.Subject (主语) ＋Verb (谓语) 此句型的特点是:谓语动词是不及物动词,本身能表达完整的意思,后面不需跟宾语,但有时可跟副词、介词短语等作状语。如: He laughed. John has read widely. He lives in London. 2. Subject(主语) ＋Verb (谓语) ＋Object (宾语) 此句型的特点是:谓语动词是及物动词,不能表达完整的意思,必须跟有一个宾语。如: Our team beat all the others. 3. Subject (主语) ＋Link. V(系动词) ＋Predicate(表语) 此句型的特点是:谓语动词是连系动词,不能表达完整的意思,必须加上一个表明主语特征、身份、状态的表语。常见的系动词有:be(是),become(成为),get(变得),turn(变得),grow(变得),look(看起来),feel(感到),smell(闻起来),taste(尝起来),sound(听起来),seem(似乎),keep(保持),stay(保持)等。如: The rose smells sweet. 4. Subject(主语)＋Verb (动词)＋Object (宾语)＋Complement(补语) 此句型的特点是:谓语动词虽然跟有一个宾语,但意思还不完整,必须加上另外一个成分(宾语补足语)对宾语进行补充说明。可以用作宾语补足语的有:名词、形容词、不定式、动名词、分词、介词短语等。如: We must keep our school clean. They made him their monitor. 5. Subject(主语)＋Verb(谓语)＋Indirect object(间接宾语)＋Direct object (直接宾语) 此句型的特点是:谓语动词跟有两个宾语,这两个宾语都是动作的对象或承受者,其中指人的是间接宾语,指物的是直接宾语。当间接宾语放在直接宾语之后时,通常需要加介词for或to。可跟双宾语的动词有:answer,bring,buy,find,get,give,lend,make,pass,pay,send,show,sing,take,teach,tell, write等。如: Mr. Li told us an interesting story. Would you please give this dictionary to Li Hua？【注】S=Subject(主语). V=Verb(谓语动词). P=Predicative(表语). O=Object(宾语).

介词from的语法特点与用法习惯

介词?f rom的语法特点与用法习惯 1.不要根据汉语意思在及物动词后误加介词?from。如：他上个星期离开中国去日本了。误：?H e left from China for Japan last week. 正：?H e left Chine for Japan last week. 另外，也不要根据汉语意思错用介词?from。如：太阳从东方升起，从西方落下。误：?T he sun rises from the east and sets from the west. 正：?T he sun rises in the east and sets in the west. 2.f rom虽然本身是介词，但它有时也可接介词短语作宾语。如： Choose a book from among these. 从这些书中选一本吧。 A man stepped out from behind the wall. 一个人从墙后走出来。比较： I took it from the bed. 我从床那儿（或床上）拿的。 I took it from under the bed. 我从床下拿的。注意，下面一句用了?from where（引导非限制性定语从句），而未用?f rom which，其中的where=i n the tree，即?from where=f rom in the tree。如： He hid himself in a tree, from where he could see the enemy in the distance. 他躲在一棵树上，从那儿他可以看到远处的敌人。 3.有时其后可接?w hen, where引导的宾语从句，此时可视为其前省略了?t he time, the place。如： He didn’t speak to me from when we moved in. 从我们迁入之时起，他没和我说过话。

Seated的用法小结

Seated的用法小结 seated是一个比较特别的过去分词，说它特殊一是因为它的词性尚有不确定性——它有时是过去分词，有时又具有形容词的性质，像是一个形容词；二是因为这样一个很少引人注意的过去分词，在近几年的高考英语考题中经常“露脸”，一下子变成了一个热点词汇。下面我们先来看几道高考题： 1. Please remain __________ until the plane has come to a complete stop. (山东卷) A. to seat B. to be seated C. seating D. seated 2. Please remain __________; the winner of the prize will be announced soon. (辽宁卷) A. seating B. seated C. to seat D. to be seated 3. Can those _________ at the back of the classroom hear me? (福建卷) A. seat B. sit C. seated D. sat 对于seated的用法，首先要从动词seat说起。同学们可能只知道seat的名词用法，即只知道它表示“座位”。其实，seat还可用作动词，且是一个典型的及物动词，其意为“给某人座位”“让人坐”或“能容纳……”句式：sb be seated 或seat sb / oneself 。如： Seat the boy next to his brother. 让那个孩子坐在他哥哥旁边。 We can seat 300 in the auditorium. 我们这个礼堂可容纳300人。

aanthe的用法区别修订版

a a n t h e的用法区别集团标准化小组：[VVOPPT-JOPP28-JPPTL98-LOPPNN]

三人行教育a,a n,t h e的用法区别（一）a, an的用法 1.a：用于辅音音素（不是辅音字母）开头的词前，表示单数的数量“一”。 a book a desk 2.an：用于元音音素（不是元音字母）开头的词前，表示单数的数量“一”。 an apple a big apple an old man 3.元音字母：以元音字母Aa, Ee, Ii, Oo开头的单词基本都是以元音音素开头 Uu较特殊 4.辅音字母：26个字母中，除了5个元音字母，其他是辅音字母 5.特殊情况：an umbrella（u发元音） a useful english book （u发辅音） an hour（h是辅音字母，但它不发音）练习一：用a, an, the填空 1._____book 2._____apple 3._____orange 4._____uncle 5._____eraser 6._____ math book 7._____new book 8._____old book 9._____teacher 10.______English 11.____ hour 12._____good teacher 13.____idea 14.____good idea 15.____island 16.____ astronaut （二）the的用法 1.一般用于特指：The girl is my sister. 那个女孩是我的妹妹。

2.第二次出现：There is a book on the desk. The book is mine. 书桌上有一本书，那本书是我的。 3.世界上独一无二：the moon月亮 the earth地球the Great Wall长城the sun太阳 4.固定的词组：on the desk在书桌上 5.形容词最高级前：Jenny is the oldest girl in the class. 在班里珍妮是年纪最大的女孩。练习二：选择 ( )1.That’s _____ island. I like to go there. A.a B.an C.the ( )2.I have ___ new book. Jenny has ___ old book. A.a/ an B.an/ a C.an/ an ( )3.This is ___ orange. That’s ____big orange. A.a/ an B.an/ a C.an/ an ( )4.Yesterday I studied English for ____ hour. A.an B.the C.a ( )5.This is a bag. ____ bag is Tom’s. A.A B.An C.The ( )6.___ man is Lisa’s father. A.A B.The C.An ( )7.____ Great Wall is in China. A.A B.The C.An ( )8.My English book is in ___ desk. A.the B.an C./ ( )9.She has ___ idea. It’s a good idea. A.a B.an C.the 练习三：a, an, the填空

aan和the的用法

a a n和t h e的用法球类运动前面不用冠词在操场上是固定搭配ontheplayground （一）不定冠词：a∕an的用法： ⑴表示一个例：Shehasacleverson.她有个聪明的儿子。 ⑵表示每个例：wehave3Englishclassesaweek.我们每周上3次英语课。 ⑶表示某个例：Thebookis∕waswrittenbyastudent.这本书是一个学生写的。 ⑷表示某类之一例：Iamateacher,heisadoctor.我是一名老师，他是一名医生。 ⑸第一次提到的人或物用不定冠词表示，再次提到时用定冠词。例：Ihaveabike,thebikeisgreen.我有一辆自行车，这辆自行车是绿色的。 ⑹用于可数名词单数形式前，表示类别。例：Ateachermustlovehisstudent.老师应该爱学生。 ⑺用于表示价格，速度，比率，时间等意义的名词前例：3timesaday.一天三次 10yuanameter.10元一米

⑻用于抽象名词前，表示一种… 例：anewculture一种新文化 ⑼用于句型：“a∕an+Mr.∕Mrs.∕Miss.+姓氏”中例：aMr.Wang一位姓王的先生（不认识） Mr.Wang王先生（认识） ⑽用于某些短语中例：alotof许多，大量 haveagoodtime玩的开心，过的愉快（二）定冠词the的用法： ⑴表示特定的人或事物例：Thebookonthedeskismine.桌子上的书是我的。 ⑵表示听话人，说话人彼此都很熟悉的人或事物例：WhereisTom汤姆在哪儿？ Heisintheroom.他在屋里。 ⑶第一次提到的人或物用不定冠词表示，再次提到时用定冠词。例：Ihaveabike,thebikeisgreen.我有一辆自行车，这辆自行车是绿色的。 ⑷表示世界上独一无二的东西（专有名词除外）例：Thesun太阳

英语常用口语及语法句型

常用口语： 1. I’m not myself！我烦透了！ 2. Don’t bother me！别烦我！ 3. Give me five more minutes, please. 再给我五分钟时间，好吗？ 4. How did you sleep? 你睡的怎么样？ 5. Don’t hog the bathroom! 别占着卫生间了！ 6. Don’t hog the shower! 别占着浴室了！ 7. Don’t hog my girlfriend! 别缠着我的女朋友了！ 8. Get outta there! 快出来！ 9. I will treat you. 我请客。 10. What are you in the mood for? 你想吃什么？ 11. Who is gonna drive? / Who’s driving? 谁来开车？ 12. You know what I mean? 你明白我的意思吗？ 13. Could you run that by me again?

你能再说一遍吗？ 14. So what you are trying to say is... 那么，你想说的是…… 15. Whadja do last night? 昨晚你干嘛去了？ Whadja=What did you 16. Didja have a good time? 玩的开心吗？ didja=did you 17. Where wouldja like to go tonight? 今晚你想上哪儿？ Wouldja=Would you 18. I am running late. 我要迟到了。 19. I’ve gotta get outta here. 我得离开这儿了。 20. I’ve gotta catch the bus. 我要去赶公共汽车了。 21. gotta=got to wanna=want to gonna=going to 22. Yo—taxi! 嗨，出租车！ 23. Where to ? (你)要去哪儿？

动名词的语法特征及用法

动名词的语法特征及用法动名词由动词加－ing词尾构成，既有名词的特征，又有动词的特征。了解动名词的语法特征可帮助学习者深入理解动名词的意义，从而正确使用动名词。一、动名词的名词特征动名词的名词特征表现在它可在句子中当名词来用，作主语、宾语、表语、定语。例如： Beating a child will do more harm than good．打孩子弊大于利。（作主语） Do you mind answering my question？你不介意回答我的问题吧？（作宾语） To keep money that you have found is stealing．把拾到的钱留起来是偷盗行为。（作表语） No one is allowed to speak aloud in the reading room．阅览室里不许大声说话。（作定语）在动名词担任这些句子成分时，学习者需注意的是： 1、有些动词后只能用动名词作宾语，构成固定搭配，需特别记忆。常见的这类动词有：admit（承认），advise（建议），allow（允许）， appreciate（感激），avoid（避免），can't help（禁不住），consider（考虑），deny（否认），dislike（不喜欢），enjoy（喜欢），escape（逃脱），excuse（原谅），feel like（想要），finish（结束），give up（放弃），imagine（想象），involve（包含），keep（保持），mind（介意），miss（错过），permit（允许），practise（练习），quit（停止），recollect （记得），recommend（推荐），suggest（建议），stop（停止），resent（对……感到愤恨、怨恨），risk（冒……危险），cannot stand（受不了）等。例如： We do not permit smoking in the office．我们不允许在办公室吸烟。 In fighting the fire，he risked being burnt to death．在救火中，他冒着被烧死的危险。 She denied having stolen anything．她否认偷过任何东西。 I suggest doing it in a different way．我建议换一个方法做这件事。 2、动名词常用于一些固定句型中，常见的有：It is no use ／no good．．．；It is a waste oftime．．．；It is fun ／nice ／good．．．；There isno．．．（不可以／不可能……）等。例如： It is no use asking him．He doesn't know any more than you do．问他也没用，他并不比你知道得更多。 It's no fun being lost in rain．在雨中迷路可不是好玩的。 It's a waste of time your reasoning with him．你和他讲道理是在浪费时间。

冠词a,an,the的用法

想说“这个/那个”的时候用the, 想说“（任意）一个”的时候用a。专指：专有名词KFC专指肯德基地名国家名什么的独一无二的事物类指:凡是同一类：都是苹果有大有小，the big one ，the small one 泛指：很不具体的，无特别指定对象，是和“特指”相对的；例如a girl就是一女孩，普通吧。此时也可是泛指一类人。a beautiful girl is like a evil. 特指：有特别指定对象，和“泛指”相对。the girl可能是上文提到过或者对话人都心知肚明的女孩，是有一个具体对象的. 冠词是用在名词前面，帮助说明名词所指的人或事物，是泛指还是特指的词。冠词是一种虚词。冠词分不定冠词（The Indefinite Article）和定冠词（The Definite Article）a, an是不定冠词，the 是定冠词。 an, a是不定冠词，仅用在单数可数名词前面，表示“一”的意义，但不强调数目观念。a用在以辅音（指辅音音素）开头的词前，an用在以元音（指元素音素）开头的词前不定冠词的用法： 1. 表示人或事物的某一类 A steel worker makes steel. A plane is a machine that can fly. 2. 表示某一类人或事物中的任何一个。 This is an apple. His father is a teacher. 3. 泛指某人或某物，但不具体说明何人何物。 A comrade is waiting for you downstairs. I met an old man on my way to school. 4. 表示“一个”的意思

全国公共英语三级常见语法

第一节动词的时态一、一般现在时： 1、由when、as soon as、the minute、the moment、till、until等引起的时间状语从句，以及由if、unless、provided that等引起的条件状语从句常常用一般现在时态表示将来的动作，而主句则用一般将来时态。例：They will go home for winter vocation as soon as they finish their exams. 2、当表示普遍的真理或者众所周知的客观事实，常常用一般现在时态。例：The earth is round. 地球是圆的。二、一般过去时：区分三个短语的用法： 1、used to do sth：过去常常做某事。 2、be/get used to doing sth：习惯做某事。 3、be used to do sth：被用于做某事。三、一般将来时： 1、be to+动词原形：表示安排或计划好了的动作。例：The Third-Ring Road is to be open to traffic before National Day. 2、be about to+动词原形：表示即将发生的动作。例：The lecture is about to begin.讲座即将开始。 3、一些表示动作趋势，如开始、终结，以及一些表示动作方向，如往来的动词，常常用现在进行时态表示按照安排将于将来发生的事情，这类动词常见的有如：start，go，leave，come，arrive等。例：We are leaving for Beijing tomorrow. 我们明天动身去北京。四、进行时态：重点区分when和while引起的时间状语的用法。 When表示时间上的点，在考试中其引导的时间状语从句多翻译为“这时？”，主句多用进行时态；while引导的时间状语从句多翻译为“正当……时”，该从句用进行时态。例：One of the guards was sleeping when the general came in, which made him very angry. I fell and hurt myself while I was playing tennis. 五、现在完成时：

aan和the用法精讲和练习

a, an, the的用法冠词是虚词，本身不能单独使用，也没有词义，它用在名词的前面，帮助指明名词的含义。英语中的冠词有三种，一种是定冠词（the Definite Article），另一种是不定冠词（the Indefinite Article），还有一种是零冠词（Zero Article）。 an, a是不定冠词，仅用在单数可数名词前面，表示“一”的意义，但不强调数目观念。a用在以辅音（指辅音音素）开头的词前， an用在以元音（指元素音素）开头的词前。the是定冠词，修饰特指名词翻译成“这个”。如果泛指某物,用a,/an,具体指某物的话，用the. 注意：（1）当我们使用an时，条件有三：①这个名词的读音必须是以元音音素开头--即它的音标的第一个音素是元音，而不是说它是以元音字母开头。②它必须是个可数名词。③它还必须是个单数名词。我们常常见到这类用法： a university 一所大学 an hour 一个小时 an orange 一只桔子 an engineer 一位工程师 an ordinary man一个普通人 an honest person一位诚实的人

a boy, a city, a girl, a useful animal , an old man, an honest boy, a bad apple, a tall elephant, a university(虽然u 是元音字母，但不读元音), an hour 一个小时 (虽然h 不是元音，但单词读音是元音开头) 不定冠词有a和an两种：a用于辅音音素开头的词前，an用于元音音素开头的词前。例如： a boy, a city, a girl, a useful animal , an old man, an honest boy, a bad apple, a tall elephant, a university(虽然u 是元音字母，但不读元音), an hour 一个小时 (虽然h 不是元音，但单词读音是元音开头) 1．指某类人或事物中的任何一个。 An elephant is bigger than a horse. A car runs faster than a bike. （2）指人或事物,用来表示“—”的意思，但不强调数的观念，只说明名词为不特定者。指人或事物即不具体说明是何人何物。。例如： There are seven days in a week. We have three meals a day. A teacher is looking for you. We work five days a week.

英语最基础的语法句型有哪些

英语最基础的语法句型有哪些英语语法是针对英语语言进行研究后，英语语法系统地总结归纳出来的一系列语言规则。英语语法的精髓在于掌握语言的使用。 1 英语五种基本句型结构一、句型1：Subject (主语) ＋Verb (谓语) 这种句型中的动词大多是不及物动词，所谓不及物动词，就是这种动词后不可以直接接宾语。常见的动词如：work, sing, swim, fish, jump, arrive, come, die, disappear, cry, happen 等。如： 1) Li Ming works very hard.李明学习很努力。 2) The accident happened yesterday afternoon.事故是昨天下午发生的。 3）Spring is coming. 4) We have lived in the city for ten years. 二、句型2：Subject (主语) ＋Link. V(系动词) ＋Predicate(表语) 这种句型主要用来表示主语的特点、身份等。其系动词一般可分为下列两类： (1)表示状态。这样的词有：be, look, seem, smell, taste, sound, keep 等。如： 1) This kind of food tastes delicious.这种食物吃起来很可口。 2) He looked worried just now.刚才他看上去有些焦急。 (2)表示变化。这类系动词有：become, turn, get, grow, go 等。如： 1) Spring comes. It is getting warmer and warmer.春天到了，天气变得越来越暖和。 2) The tree has grown much taller than before.这棵树比以前长得高多了。

法语语法-名词的特点和用法

{1} 1. 名词（le nom, le substantif）的特点名词是实体词，用以表达人、物或某种概念，如：le chauffeur（司机），le camion（卡车），la beauté（美丽）等。法语的名词各有性别，有的属阳性，如：le soleil（太阳），le courage（勇敢），有的属阴性，如：la lune（月亮），la vie（生活）。名词还有单数和复数，形式不同，如：un ami（一个朋友），des amis（几个朋友）。法语名词前面一般要加限定词（le déterminant），限定词可以是数词、主有形容词，批示形容词或冠词。除数词外，均应和被限定性名词、数一致，如：la révolution（革命），un empire （一个帝国），cermarins（这些水手），mon frère（我的兄弟）。https://www.sodocs.net/doc/a67389196.html, 大部分名词具有多义性，在文中的意义要根据上下文才能确定，如： C’est une pluie torrentielle.（这是一场倾盆大雨。） Lorsque rentre la petite fille, c’est sur elle une pluie de baisers.（当小姑娘回家时，大家都拥上去亲吻她）。第一例, pluie是本义，第二例, pluie是上引申意义。 2. 普通名词和专有名词（le nom commun et le nom propre）普通名词表示人、物或概念的总类，如：un officier（军官），un pays（国家），une montagne （山），la vaillance（勇敢、正直）。专有名词指特指的人、物或概念，如：la France（法国）。专有名词也有单、复数；阴阳性。如：un Chinois（一个中国男人），une Chinoise（一个中国女人），des Chinois（一些中国人）。 3. 普通名词和专有名词的相互转化（le passage d’une catégorie àl’autre）普通名词可转化为专有名词，如：报刊名：l’Aube（黎明报），l’Humanité（人道报），l’Observateur（观察家报）等报刊名称是专有名词，但它们是从普通名词l’aube（黎明），I’humanité（人道），l’Observateur（观察家）借用来的。专有名词也可以转化为普通名词，意义有所延伸，其中许多还保持第一个字母大写的形式，如商品名：le champagne（香槟酒），une Renault（雷诺车），le Bourgogne（布尔戈涅洒）。以上三例分别来自专有名词la Champagne（香槟省），Renault（雷诺，姓），la Bourgogne（布尔戈涅地区）。 4. 具体名词和抽象名词（les noms concrèts et les noms abstraits）

试题专题练习不定冠词(含解析)

不定冠词 1、（2016?天津）Tianjin is beautiful city in north of China．（）A．a；a B．a；the C．the；不填D．不填；the 【考点】不定冠词（a，an）；定冠词（the）．【分析】天津是中国北方的一个美丽的城市．【解答】答案：B 不定冠词a，an 表示泛指；定冠词the 表示特指或者再次提到；根据Tianjin is beautiful city，可知是首次提到，且为泛指一个美丽的城市． beautiful 第一个音/b/是辅音，应该用a；根据in north of China．在中国的北部，是特指，应该加定冠词the，故选：B． 2、（2016?河北）I have _______ pet cat．It is so cute．（） A．a B．an C．the D．不填【考点】不定冠词（a，an）．【分析】我有一只宠物猫．它很可爱．【解答】答案：A a，an 表示泛指；the 表示特指或者再次提到；零冠词用于一些特殊的结构中．根据It is so cute．它很可爱．推测可知上文是应该是：我有一只宠物猫．是泛指．an用于第一个音节是元音的音素前，a用于第一个音节是辅音的音素前；pet 第一个音节/p/是辅音，应该用a，故选：A 3、（2016?重庆）Ciqikou is______famous place in Chongqing．（） A．a B．an C．the D．/ 【考点】不定冠词（a，an）．【分析】磁器口是重庆的一个著名的地方．【解答】答案：A．根据句意，这里表示一个有名的地方，要用不定冠词a/an，表示数量"一"，元音音素前用an，辅音音素前用a，famous/?fe?m?s/中字母f发/f/，辅音音素，即a famous place一个有名的地方．定冠词the，表示特指，故选：A． 4、（2016?济宁）_____ apple a day keeps the doctor away．（） A．A B．An C．The D．/ 【考点】不定冠词（a，an）．【分析】一日一苹果，医生远离我．【解答】答案为B．不定冠词（a，an）表示泛指，定冠词（the）表示特指．不定冠词a、an是"一个"的意思．a与an 的区别是a用于辅音音素前，an则用于元音音素前．句子中apple是泛指苹果，而apple的音标为['?p（?）l]，apple是以元音音素开头，故答案为B．5、（2016?重庆）Mary wants to be ________ good doctor when she grows up．（）A．a B．an C．the D．/ 【考点】不定冠词（a，an）．【分析】当玛丽长大之后，她想成为一名好医生．【解答】答案为A．不定冠词（a，an）表示泛指，定冠词（the）表示特指．句子的good doctor （好医生）是属于泛指任何一位好医生，故要用不定冠词．不定冠词a、an是"一个"的意思．a 与an 的区别是a用于辅音音素前，an则用于元音音素前．good的音标为[g?d]，good以辅音音素开头，故要用a，故答案为A 6、（2016?青岛）David is ______ eight-year-old boy with short black hair．（）A．/ B．a C．an D．the 【考点】不定冠词（a，an）．【分析】David是一个黑色短发的八岁男孩．

小学语法归纳a, an, the的用法

a, an, the的用法（一）a, an的用法 1.a：用于辅音音素（不是辅音字母）开头的词前，表示单数的数量：“一”。但小学阶段学生一般理解为：单词开头第一个不是元音用a a book 一本书 a desk一张书桌 2.an：用于元音音素（不是元音字母）开头的词前，表示单词的数量“一”。但小学阶段学生一般理解为：单词第一个字母是元音字母用an an apple 一个苹果 a big apple一个大苹果an old man一位老人 3.元音字母：Aa, Ee, Ii, Oo, Uu 4.特殊情况：an hour（h是辅音字母，但它不发音） 5.辅音字母：26个字母中，除了5个元音字母，其他是辅音字母练习一：用a, an, the填空 1._____book 2._____apple 3._____orange 4._____uncle 5._____eraser 6._____math book 7._____new book 8._____old book 9._____teacher 10.______English 11.____ hour 12._____good teacher 13.____idea 14.____good idea 15.____island 16.____ astronaut （二）the的用法 1.一般用于特指：The girl is my sister. 那个女孩是我的妹妹。 2.第二次出现：There is a book on the desk. The book is mine. 书桌上有一本书，那本书是我的。 3.世界上独一无二：the moon月亮the earth地球the Great Wall长城the sun太阳 4.固定的词组：on the desk在书桌上 5.形容词最高级前：Jenny is the oldest girl in the class. 在班里珍妮是年纪最大的女孩。练习二：选择 ( )1.That’s _____ island. I like to go there. A.a B.an C.the ( )2.I have ___ new book. Jenny has ___ old book. A.a/ an B.an/ a C.an/ an ( )3.This is ___ orange. That’s ____big orange. A.a/ an B.an/ a C.an/ an ( )4.Yesterday I studied English for ____ hour. A.an B.the C.a ( )5.This is a bag. ____ bag is Tom’s. A.A B.An C.The ( )6.___ man is Lisa’s father. A.A B.The C.An ( )7.____ Great Wall is in China. A.A B.The C.An ( )8.My English book is in ___ desk. A.the B.an C./ ( )9.She has ___ idea. It’s a good idea. A.a B.an C.the 练习三：a, an, the填空 1.Tom wants to be _____ scientist(科学家).He wants to go to ___ moon. 2.Jenny has ______ uncle and _____ cousin. 3.I have _____ eraser. _____ eraser is in my bag. 4._____ boy is Tom’s brother. 5.The bird was here ______ hour ago. 6.There is _____ English book on the desk. ______ English book is Tom’s. 7.I have _____ book. It is _____ English book. And it has _____ goos story. _____ book is in my bag. 8.There is _____ cat under the tree. _____ cat is playing.

常见系动词的分类及使用特点

常见系动词的分类及使用特点系动词词义不完整，在句中不能单独使用（除省略句外），后面必须接有表语，系动词和表语一起构成合成谓语。常见的系动词大致可分为三类。第一类：表示特征或状态的，有 be, look, feel, seem, appear, smell, taste, sound, turn out（结果是、证明是）等。 You'll be all right soon. You don't look very well. I feel rather cold. He seems to be ill. It appears that he is unhappy. The roses smell sweet. The mixture tasted horrible. How sweet the music sounds! The day turned out (to be）a fine one. 第二类：表示从一种状态到另一种状态的变化，有 become, get, grow, turn, fall, go, come, run 等。 He became a world-famous scientist. It is getting warmer and warmer. It grew dark. The food has turned bad. Yesterday he suddenly fell ill. Mary's face went red. His dream has come true. The boy's blood ran cold. 第三类：表示保持状态的，有keep, remain, continue 等。 Keep quiet, children! The weather continued fine for a long time. It remains to be proved. 系动词后的表语可以是名词、代词、数词、形容词、分词、动名词、不定式、副词、介词短语、词组、从句，系动词 be 可用于上述所有情况。如： The people are the real heroes. (名词) That's something we have always to keep in mind. (代词) She is often the first to come here. (数词) She is pretty and wise. (形容词). The news was surprising. (分词) His job is teaching English. (动名词) The only method is to give the child more help. (不定式） I must be off now. (副词) The bridge is under construction. (介词短语) That would be a great weight off my mind. (词组) This is why he was late. (从句) 系动词的使用特点： 1、所有的系动词都可接形容词作表语，此处略举数例。