holyphoenix
资深会员
注册日期: Aug 2002
来自: 湖南
发帖数: 742 |
 SETI 科学通讯 #2
通讯 #2
2000年5月4日
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A First Quick Look at Data
Author: Dan Werthimer
Translator: holyphoenix ([email protected])
1. 多重检测搜索
在这最近的月份中，基于被看见两次或者更多次的射电信号，我们已经一直在搜索科学数据。因为 SETI@home 的参与者们已经分析了1.5年的从 Arecibo 望远镜传来的数据，所以天空中很多区域已经为我们观测了两次或者更多次。
我们搜索在天空中某个同一地点并且同一频率重现的信号。我们也搜索那些根据很多个月观测所发现的显著频率漂移的信号。信号可能的频率漂移是因为多普勒效应——在朝向或者背离地球运动、有着自转和公转的行星上的发送器，由于其加速度所产生的在被观测到的频率上的漂移。也许另一种文明会校正他们的动作（motion。有更好的翻译么？），如果他们故意向我们发送信号；当然他们的发送器频率会一直不变的，假设我们说明了我们行星的运动。但是我们不知道是否地外文明会校正他们的动作，因此我们要搜索漂移的和非漂移的两种信号。
至今为止，我们已经鉴定了两对强烈的高斯信号，两者都被不同的观测所看见。我们也已经发现三个三重的更弱的高斯信号，每个信号都被三个不同的观测所看见（we've identified two pairs of strong gaussian shaped signals, both seen on two different observations. We've also found three trios of weaker gaussian signals, each signal seen on three different observations.）。我们正在进一步检验这些数据，并且将检验一些甚至更弱的信号。但是这些多重检测多确定为来自噪音，也就是说没有来自另一个文明的信号。我们正在通过一个有5千万高斯信号的数据库进行搜索，其中很少的信号将会随机的在同一地点同一频率发生。我们已经检验了数以千计的在我们25年的搜索中发现的这类多重检测，并且至今为止它们一直被证明为噪音或者来自地球的射电污染，没有是来自地外的信号。
2. 脉冲检测
我们已经开发了一个可以搜索脉冲射电信号的新版本的 SETI@home 屏幕保护程序。我们希望在下个月能够对这个新程序进行 beta 测试，并且在今年夏天的某个时候能够对所有的 SETI@home 参与者开放。
新的脉冲软件搜索有规律的彼此隔开的射电脉冲。很像 light house（light house 是个什么东西？）发送有规则的闪光，另一个文明可能发送规则的射电脉冲。如果我们将这些射电波转化成声波，你可能听见“滴答、滴答、滴答……”。
脉冲走索软件使用两种不同的算法——一种算法搜索在时间上均匀隔开的三脉冲。这个三重算法不需要太多计算机时间，但是它只能检测到强大的脉冲信号。第二种算法，由 Eric Korpela 开发，能够找到非常微弱的信号，如果它们有规则的被重复数十次、数百次、数千次。
新的脉冲搜索代码给每个 work unit 增加了若干个小时的计算量。无论如何，当我们结合了来自脉冲搜索所增加的额外的计算负担和来自优化的加速，我们也已经优化了2.04版的分析软件，新的v3.0不会花费比v2.04更长的时间，并且我们将提供对全新类信号的搜索。
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Newsletter #2
May 4, 2000
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A First Quick Look at Data
Dan Werthimer
1. Multiple Detection Search
During the last month we've been searching the science data base for radio signals seen two or more times. Since the SETI@home participants have already analyzed 1.5 years of data from the Arecibo telescope, there are already many places on the sky we've observed two or more times.
We search for signals that recur at almost exactly the same place on the sky and almost exactly the same frequency. We also search for signals whose frequency may have drifted significantly over the many months between observations. Signals might drift in frequency because of the "doppler effect" - a transmitter on a rotating and revolving planet will move toward and away from earth, and this acceleration produces a shift in the observed frequency. Another civilization might correct for their motion if they are deliberately transmitting a signal towards us; then their transmitter's frequency would remain steady, assuming we account for our own planet's motion. But we don't know if extraterrestrial civilizations will correct for their motion, so we look for both drifting and non-drifting signals.
So far, we've identified two pairs of strong gaussian shaped signals, both seen on two different observations. We've also found three trios of weaker gaussian signals, each signal seen on three different observations. We are examining these data further, and also examining some even weaker signals. But these multiple detections are almost certainly from noise, and not signals from another civilization. We are searching through a data base of 50 million gaussian signals, and a few of these 50 million signals will randomly occur at almost the same place and the almost the same frequency. We've examined thousands of these kinds of multiple detections in our 25 years of searching, and so far they've always turned out to be noise or radio pollution from terrestrials, not extraterrestrials.
2. Pulse Detection
We've also been developing a new version of the SETI@home screensaver program that searches for pulsing radio signals. We hope to beta test this new program next month and make it available for all SETI@home participants sometime this summer.
The new pulse software searches for regularly spaced radio pulses. Much like a light house sends out regular optical flashes, another civilization might send out regular radio pulses. If we converted these radio waves into sound waves, you might hear "click, click, click...".
The pulse hunting software uses two different algorithms - one algorithm searches for three pulses evenly spaced in time. This "triplet" algorithm doesn't take much computer time, but it can only detect powerful pulses. The second algorithm, developed by Eric Korpela, can find very weak pulses if they are regularly repeated tens, hundreds or thousands of times.
The new pulse searching code adds several hours of computation to each work unit. However, we've also been optimizing version 2.04 analysis software, and when we combine the extra computational burden from the pulse search and the speed up from the optimization, the new version 3.0 doesn't take that much longer than version 2.04, and will allow a search for a whole new class of signals.
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