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我们从SAR抛物线指标英文全称Stop and Reverse知道它有两层含义。
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。
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 什么是抛物线形SAR？ 什么是抛物线形SAR？ forex.  It is a trend-following (lagging) indicator and may be used to set a trailing stop loss or determine entry or exit points based on prices tending to 什么是抛物线形SAR？ stay within a parabolic curve during a strong trend. Similar to option theory's concept 什么是抛物线形SAR？ 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 什么是抛物线形SAR？ liquidated. The indicator generally works only in trending markets, and creates "whipsaws" during ranging or, 什么是抛物线形SAR？ sideways phases. Therefore, Wilder recommends first establishing the direction or change in direction of the trend through the use of parabolic SAR, and then using a different indicator such 什么是抛物线形SAR？ 什么是抛物线形SAR？ as the Average Directional Index to determine the strength of the trend.
A parabola 什么是抛物线形SAR？ 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. 
The parabolic SAR is calculated almost independently for each trend in 什么是抛物线形SAR？ 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 什么是抛物线形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.什么是抛物线形SAR？
The α value represents the acceleration factor. Usually, 什么是抛物线形SAR？ this is set initially to a value 什么是抛物线形SAR？ 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 什么是抛物线形SAR？ it is not too sensitive to local decreases. For commodity or currency trading, the preferred value is 0.02.
The SAR is 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) 什么是抛物线形SAR？ 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. 什么是抛物线形SAR？
- If the next period's SAR value is 什么是抛物线形SAR？ inside (or beyond) the next period's 什么是抛物线形SAR？ 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 trend, EP is then reset accordingly to this period's maximum, and the acceleration factor is reset to its initial value of 0.02.
The parabolic SAR showed results at a 95% confidence level in a study of 17 years of data.