TOA estimation allows the measurement of distance, thus enabling localization. Here, multiple base nodes collaborate to localize a target node via triangulation [3] .

It is assumed that the positions of all base nodes are known. If these nodes are dynamic, a positioning technique such as GPS is used to allow base nodes to localize their positions (GPS – TOA positioning). In some circumstances, multiple base nodes may cooperate to find their own position before any attempt to localize a target node [4] . 

TOA估計可以被使用進行定位，但是我們的假設條件是所有基節點的位置是已知的。所謂的基節點的意思是對目標進行定位的測量站。

如果基節點是動態的，那麼我們就要藉助於全球定位系統（GPS）確定基節點的位置。這種情況下的定位稱為GPS—TOA定位系統。

還有一些環境下，多個基節點可以通過相互合作確定他們自己的位置。

這就是說，如果使用TOA進行定位，我們就必須提前知道測量站（基節點）自身的位置，之後才能對目標進行定位。

Assuming known positions of base nodes and a coplanar scenario, three base nodes and three measurements of distances (TOA) are required to localize a target node (see Fig. 1.2 a).

假設已知基節點位置且為共面場景，需要三個基本節點和三個距離測量（TOA）來定位目標節點（參見圖1.2a）。

In a non - coplanar case, four base nodes are required.

非共面場景，需要四個基節點。

Using the measurement of distance, the position of a target node is localized within a sphere of radius with the receiver i at the center of the sphere (where   is directly proportional to the TOA

使用距離測量，目標節點的位置定位在半徑範圍內，接收器i位於球體的中心（其中與TOA 成正比，如圖1.2a所示。目標節點的定位 可以由使用主站的基節點或由目標節點本身執行。（理解下這句話）

Figure 1.2 (a) Operation of TOA

Although TOA seems to be a robust technique, it has a few drawbacks [5] :

1. It requires all nodes (base nodes and target nodes) to precisely synchronize: A small timing error may lead to a large error in the calculation of the distance . 2. The transmitted signal must be labeled with a time stamp in order to allow the base node to determine the time at which the signal was initiated at the target node. This additional time stamp increases the complexity of the transmitted signal and may lead to an additional source of error. 3. The positions of the base nodes should be known; thus, either static nodes or GPS - equipped dynamic nodes should be used.

雖然TOA似乎是一種強大的技術，但它有一些缺點[5]：

1.它要求所有節點（基本節點和目標節點）精確同步：小的定時誤差可能導致距離計算中的大誤差。

2.必須用時間戳標記傳送的訊號，以便允許基節點確定在目標節點處發起訊號的時間。 這種額外的時間標記增加了傳送訊號的複雜性，並可能導致額外的錯誤源。

3.應知道基節點的位置; 因此，應該使用靜態節點或配備GPS的動態節點。