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什么是抛物线形SAR?

SAR工作原理

What does SAR-A stand for?

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什么是抛物线形SAR?

我们从SAR抛物线指标英文全称Stop and Reverse知道它有两层含义。

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1.5微波遥感(三)

侧视雷达成像原理与工作过程

真实孔径雷达的距离向分辨率可以通过脉冲压缩技术提高,提高方位向分辨率可以通过加大天线孔径和使用波长较短的电磁波来提高。但是微波波长是存在一定范围的,同时也要考虑波长范围是否在大气窗口以内,因此改变孔径是另一个可行的办法。但是搭载真实的大尺度天线花费较高,可行性又不高,因此SAR就是用来解决RAR的孔径问题的。

SAR工作原理

合成孔径雷达的距离向分辨率同样可以通过脉冲压缩技术提高,而孔径越小,方位向分辨率的值越小,即方位向分辨率越高(记得我大一学到这里的时候整个人都惊呆了,与真实孔径雷达完全相反了)

几种地类表面的散射特性

首先看一下这张图的横纵轴,横轴的 \theta 表示雷达的入射角,纵轴为散射系数,越大表示散射特性越强。

植被的散射特性

不同作物的散射特性

Parabolic SAR

In stock and securities market technical analysis, parabolic SAR (parabolic stop and reverse) is a method devised by J. Welles Wilder, Jr., to find potential reversals in the market price direction of traded goods such as securities or currency exchanges such as forex. [1] It is a trend-following (lagging) indicator and may be used to set a trailing stop loss or determine entry or 什么是抛物线形SAR? exit points based on prices tending to stay within a parabolic curve during a strong trend. Similar to option theory's concept of time decay, the concept draws on the idea that "time is the enemy". Thus, unless a security can continue to generate more profits over time, it should be liquidated. The indicator generally works only in trending markets, and creates "whipsaws" during ranging or, sideways phases. Therefore, Wilder recommends first establishing 什么是抛物线形SAR? the direction or change in direction of the trend through the use of parabolic SAR, and then using a different indicator such as the Average Directional Index to determine the strength of the trend.

A parabola below the price is generally bullish, while a parabola above is generally bearish. A parabola below the price may be used as support, whereas a parabola above the price may represent resistance. [2]

Construction

The parabolic SAR is calculated almost independently for each trend in the price. When the price is in an uptrend, the SAR emerges below the price and converges upwards towards it. Similarly, on a downtrend, the SAR emerges above the price and converges downwards. At each step within a trend, the SAR is calculated one period in advance. That is, tomorrow's SAR value is built using data available today. The general formula used for this is:

[math]\displaystyle< _ = _n + \alpha ( EP - _n ) >[/math] ,

where SARn and SARn+1 represent the current period and the next period's SAR values, respectively.

EP (the extreme point) is a record kept during each trend that represents the highest value reached by the price during the current uptrend – or lowest value during a downtrend. During each period, if a new maximum (or minimum) is observed, the EP is updated with that value.

The α value represents the acceleration factor. Usually, this is set initially to a value of 0.02, but can be chosen by the trader. This factor is increased by 0.02 each time a new EP is recorded, which means that every time a new EP is observed, it will make the acceleration factor go up. The rate will then quicken to a point where the SAR converges towards the price. To prevent it from getting too large, a maximum 什么是抛物线形SAR? value for the acceleration factor is normally set to 0.20. The traders can set these numbers depending on their trading style and the instruments being traded. Generally, it is preferable in stocks trading to set the acceleration factor to 0.01, so that it is not too sensitive to local 什么是抛物线形SAR? decreases. For commodity or currency trading, the preferred value is 0.02.

The SAR is 什么是抛物线形SAR? calculated in this manner for each new period. However, two special cases will modify the SAR value:

  • If the next period's SAR value is inside (or beyond) the current period or the previous period's price range, the SAR must be set to the closest price bound. For example, if in an upward trend, the new SAR value is calculated and if it results to be more than today's or yesterday's lowest price, it must be set equal to that lower boundary.
  • If the next period's SAR value is inside (or beyond) the next period's price range, a new trend direction is then signaled. The SAR must then switch sides.

Upon a trend switch, the first SAR value for this new trend is set to the last EP recorded on the prior 什么是抛物线形SAR? trend, EP is then reset accordingly to this period's maximum, and the acceleration factor is reset to its initial value of 0.02.

Statistical Results

The parabolic SAR showed 什么是抛物线形SAR? results at a 95% confidence level in a study of 17 years of data. [3]