那么，怎样提高射电望远镜的分辨率呢？ 对单天线射电望远镜来说，天线的直径越大分辨率越高 。但是天线的直径难于作得很大，目前单天线的最大直径 小于300米，对于波长较长的射电波段分辨率仍然很低， 因此就提出了使用两架射电望远镜构成的射电干涉仪。对 射电干涉仪来说，两个天线的最大间距越大分辨率越高。 另外，在天线的直径或者两天线的间距一定时，接收的无 线电波长越短分辨率越高。
NIST Chip-Scale Atomic Clock
• On Aug. 30, 2004 • about the size of a grain of rice (1.5 millimeters on a side and 4 millimeters high), consume less than 75 thousandths of a watt (enabling the clock to be operated on batteries) and are stable to one part in 10 -10, equivalent to gaining or losing just one second every 300 years.
• the physics package will be integrated with an external oscillator and control circuitry into a finished clock about 1 cm3 in size.
Part 2. VLBI
-Very Long Baseline Interferometry
• Definition of Atomic Second :
地面状态的铯133原子对应于两个超精细能级跃迁 9 192 631 770个辐射周期的持续时间。
• 科学家当前正在研制更高精度的原子钟： 1 second in 10 billion years
Atomic Fountains(原子喷泉钟)
1968 -- NBS-4, the world’s most stable cesium clock, is completed. This clock was used into the 1990s as part of the NIST time system.
1972 -- NBS-5, an advanced cesium beam device, is completed and serves as the primary standard
The uncertainty of NIST-F1 is continually improving. In 2000 the uncertainty was about 1 x 10-15, but as of the summer of 2005, the uncertainty has been reduced to about 5 x 10-16, which means it would neither gain nor lose a second in more than 60 million years! It is now approximately ten times more accurate than NIST-7, a cesium beam atomic clock that served as the United State's primary time and frequency standard from 1993-1999.
分辨率
指区分两个彼此靠近射电源的能力，分辨率越高就 能将越近的两个射电源分开。利用射电望远镜进行 观测时其角分辨率可用下列公式来估算：

D
(2-1)
式中 为角分辨率， 为射电望远镜所接收的 无线电信号的波长，通常为13cm和3.6cm, D 为射 电望远镜接收天线的口径刘万科 博士
武汉大学测绘学院 卫星应用工程研究所 2008年09月
空间大地测量学
内 容 提 要
1. 原子钟（Atomic Clock） 2. 甚长基线干涉测量（VLBI） 3. 激光测卫(SLR） 4. 卫星测高(Satellite Altimetry)
5. 多普勒技术(Doppler Technique）
1993 -- NIST-7 comes on line; eventually, it achieves an uncertainty of 5 x 10-15, or 20 times more accurate than NBS-6.
1999 ---- NIST-F1 begins operation with an uncertainty of 1.7 x 1015, or accuracy to about one second in 20 million years, making it one of the most accurate clocks ever made (a distinction shared with similar standards in France and Germany).
NBS-1
1954 -- NBS-1 is moved to NIST’s new laboratories in Boulder, Colorado. 1955 --The National Physical Laboratory in England builds the first cesium-beam(铯原子束)clock used as a calibration source. 1958 -- Commercial cesium clocks become available, costing $20,000 each. 1959 -- NBS-1 goes into regular service as NIST's primary frequency standard. 1960 -- NBS-2 is inaugurated in Boulder; it can run for long periods unattended and is used to calibrate secondary standards.
/cesium/atomichistory.htm
1945 -- Isidor Rabi, a physics professor at Columbia University, suggests a clock could be made from a technique he developed in the 1930's called atomic beam magnetic resonance. 1949 -- Using Rabi’s technique, NIST (National Institute of Standards and Technology) announces the world’s first atomic clock using the ammonia molecule as the source of vibrations. 1952 -- NIST completes the first accurate measurement of the frequency of the cesium clock resonance. The apparatus for this measurement is named NBS-1.
6. 卫星跟踪卫星(SST)
Part 1. Atomic Clock
• The National Physics Laboratory in England developed the first accurate caesium atomic clock in 1955 • In 1967 the International Bureau of Weights and Measures (BIPM) adopted the atomic definition for an SI second
NBS-5
1975 -- NBS-6 begins operation; an outgrowth of NBS-5, it is one of the world’s most accurate atomic clocks, neither gaining nor losing one second in 300,000 years.
喷 泉 原 子 钟 内 部 构 造 图
Video Demonstration of How a Cesium Fountain Works
（喷泉钟的动画演示，请用鼠标点击上述画面）
NIST-F1 Cesium Fountain Atomic Clock
The Primary Time and Frequency Standard for the United States
Rubidium clock Hydrogen maser clock
Office of Naval Research ---'matchbox' atomic clock
• one second every 10,000 years
• Ultra-miniature Rubidium (Rb) Atomic Clock, 40 cm3
Galileo atomic clocks
Galileo satellites : rubidium atomic frequency standards and passive hydrogen masers. The stability of the rubidium clock is so good that it would lose only three seconds in one million years, while the passive hydrogen maser is even more stable and it would lose only one second in three million years.