衛星測高 (satellite altimetry)

衛星測高技術自1969 年以來發展至今,已有四十多年的歷史。 其觀測原理是利用測高衛星上的測高儀連續發射雷達波至地球表面,再讓衛星上的接收裝置接收經由地表反射回來的回波,經量測這段時間差及返回的波形即可計算出衛星至地表的距離。起初,由於海水表面較為平滑穩定且對於雷達訊號反射率較好,因此測高衛星被用於測量海平面高度,目前對於海水面變化的研究越趨成熟,觀測精度可達公分級。而陸地表面因為有植被、建物等覆蓋,且對於雷達訊號反射率較差,造成測高衛星在陸地上觀測精度也相對較差,所以近年來學者們紛紛研究發展出如波型重定等較有效的演算法,來提升陸地觀測精度。由於測高衛星能提供連續、高精度且大範圍的資料,有助於監測海水面變化及研究海洋大地水準面、重力場異常等,近年來更因多位學者陸續發展各種波形重定演算法,因此也有學者開始利用衛星測高技術來研究冰層變化及地表高程變化等等。

Satellite altimetry technology has more than 40 years of history so far since 1969. The observation principle is that satellite altimeter continuously launch radar wave to the Earth's surface, and then let the receiver on the satellite to receive echoes back through the ground, measuring the time difference, which can be calculated Satellite to the surface of the distance. At first, because the surface of the sea is relatively smooth and stable, the radar signal reflectivity is good. So the satellite is used to measure the sea level, the current development in the sea surface has been more mature, observation accuracy is up to the nominal level. In the mean time, because of vegetation, building and other coverage, the radar signal reflectivity of land surface is poor; resulting in high altitude satellite observations on the land has relatively bad accuracy. So in recent years, scholars have studied the development of wave type to enhance the accuracy of land observation. The altitude satellite can provide continuous, high-precision and large-scale information to help monitor the sea surface changes and the study of marine geoid, gravity field anomaly. In recent years more than a number of scholars have developed a variety of waveform algorithms. So there are scholars began to use satellite altimetry technology to study the ice changes and surface elevation changes and so on.

衛星定軌 (Satellite Orbit Determination)

利用獲得之衛星追蹤觀測數據,進行衛星軌道的確定。傳統天體力學中,衛星定軌模型是一符合牛頓力學且無擾動的簡單二體問題, 惟現實中之定軌模型為一具有擾動的運動問題,故衛星真實軌道與運動微分方程所得之積分軌道間存有偏差。 為了獲得最接近真實軌道的積分軌道,必須根據大量的衛星觀測資料,同時考慮各項衛星擾動力 (如地球引力位、多體擾動、大氣阻力、太陽輻射壓、地球輻射壓、地球固體潮、海潮、相對論效應等)可能擾動偏差之影響, 利用參數計算方式進行精密衛星軌道的確定。

The satellite orbit determination is carried out using the obtained satellite tracking observation data. In the traditional celestial mechanics, the satellite orbit model is a simple two-body problem with Newtonian mechanics without disturbance. However, the orbit model in the reality is a perturbed motion problem, so there is a deviation between the integral orbit of the real orbit of the satellite and the differential equation of motion. In order to obtain the real orbit as accurate as possible, it is necessary to consider the satellite disturbance force (such as earth gravitational force, multi-body disturbance, atmospheric resistance, solar radiation pressure, earth radiation pressure, earth tide, tide, relativistic effect, etc.). And also consider the effect of disturbance deviation, and the use of parameter calculation method to determine the precise satellite orbit.

重力測量

What is "Gravimetry"?

Gravimetry is a surveying technique to measure or observe strength of gravity field. The unit of gravity is the "acceleration", which is represented as gal (10^-2 metre per second squared), mgal (10^-5 metre per second squared) or μgal (10^-8 metre per second squared). The main method to obtain gravity signals includes land-base, shipborne, airborne, satellite, and gravimetry. The instruments for collecting gravity measurements are usually divided into relative, absolute, and superconducting gravimeters. The information of gravity field is very useful for geophysics, geodesy, oceanography, geology, and even national defense, etc. For geodesists, the importance of the gravity is that it can help with obtaining a high quality geoid model; For oceanographers, the gravity is useful for the investigation of currents, tides, and sea surface topography; For geophysicists, the gravity can be used to understand the characteristics of the earth's interior sources; For geologists, the gravity is expected to aid in monitoring crustal deformation. Therefore, gravity modeling with high accuracy is always a primary goal for geoscientists.


Terrestrial Time-varying gravimetry

(時變重力)

The contemporary development of time-varying gravimetry is an offspring from the static gravimetry thanks to introduction of new generation of gravimeters, which are far more capable in terms of precision, stability and temporal consistency. These features extend the data spectrum to study the Earth since migrations of mass in the deep or deformation on surface leave its trace on gravitational signal, which serves as valuable clues to answer more scientific secrecies. As a result, no longer being confined to the missions for geodetic datum, time-varying gravimetry can contribute much on exploring various dynamics of the Earth.

 

The Laboratory of Geodesy and Geodynamics houses and operates two kinds of state-of-the-art time-varying gravimeters. First kind is the super-conducting gravimeter. One of these observatory-based instruments, T48, is set up in the tunnel laboratory at Hsinchu, while the other one, T49, is at a monitoring station on Mt. Yang-Ming. Superconducting gravimeters have excellent performance (in terms of precision and drift) in the band of the Earth tide and long wave length seismic signals. Gravimetrical data collected within this band include the surficial signals such as tide, atmospheric and hydrological effects, and also the signal form the deep Earth-like core motion and normal mode excited by Earthquake. The data collected by super-conducting gravimeters for the past years are precious records to study respective subject. It has contributed remarkable result in a task to identify an active fault nearby T48 observatory.

 

The other kind of vital gravimeters in Laboratory of Geodesy and Geodynamics is the two absolute gravimeters. Implementing the principal of a free-fall experiment, an absolute gravimeter collect gravity observation of the bandwidth from Earth tide to the infinity. It has the tremendous stability across the temporal domain which makes decent reference in time-varying monitoring missions. Most important of all, it is mobile capable of shipping out to a site designated to monitor a scientific event. Two absolute gravimeters, labeled #224 and #231 respectively, are operating in the laboratory. These two instruments have been deployed around Taiwan for many scientific missions, including the monitoring for Orogenic activities, land subsidence, and volcanic activities. Recently, the employment of absolute gravimetry has shown an admirable potential in groundwater hydrology, providing a new non-intrusive approach in monitoring the storage of aquifers.

合成孔徑雷達干涉量測技術 (INSAR)

近年來航太科技快速發展,地表變形監測也進入了太空時代,其中又以能全天候、大範圍快速面狀監測的「合成孔徑雷達干涉量測技術」 (Synthetic Aperture Radar Interferometry, InSAR)最受注目,其原理說明如圖1。

In recent years, the rapid development of aerospace technology, surface deformation monitoring has entered the space age, the all-weather, large-scale rapid surface monitoring of "synthetic aperture radar interferometry technology" (Synthetic Aperture Radar Interferometry, InSAR) attracts the most attention. The principle is described in following figures.

圖1. InSAR原理圖(本圖片為網路擷取)

Figure 1. InSAR schematic (this picture is adopted from internet)

 

合成孔徑雷達(Synthetic Aperture Radar, SAR)屬微波成像雷達,透過合成孔徑原理提升影像的平面解析力以獲得高解析度影像。 InSAR技術的基本量測原理係藉由衛星運行軌道的重複性及固定性,將相同區域內,兩組或多組不同時間獲取的SAR影像資料進行精準的幾何校正, 再經由干涉演算、分析所發展出的大地測量技術,其量測精度可達公釐等級, 目前已廣泛應用於地表變形測量(圖2&圖4)及地形測繪(圖3)等地球環境監測相關領域之中。

Synthetic Aperture Radar (SAR) is a microwave imaging radar, through the synthetic aperture principle to enhance the plane of the image resolution to obtain high-resolution images. The basic measurement principle of InSAR technology is based on the repeatability and fixability of the orbit of the satellite. The geometric precision of SAR image data obtained from two or more groups at different times in the same area are calibrated. The development of geodetic technology, with accuracy up to the level of precision, has been widely used in surface deformation measurement (Figure 2 & Figure 4), terrain mapping (Figure 3), and other areas of the Earth's environmental monitoring.

圖2. 同震變形圖 

Figure 2. Coseismic deformation diagram

圖3. ERS-1/2 Tandem Mission Result: DEM of Europe processed at DLR

Figure 3. ERS-1/2 Tandem Mission Result: DEM of Europe processed at DLR

圖4. 濁水溪沖積扇時間序列變形圖與年平均下陷速率圖 

Figure 4. Time-series deformation diagram and annual average subsidence rate of turbid water alluvial fan