| Abstract: | 無線通訊與感測技術的快速發展使得無線感測網路成為一門新興的科技，無線感測網路的應用也因此被廣泛地討論。在無線感測網路中物件追蹤是一個重要的議題，它包含了許多的應用，例如軍事上入侵者的偵測、野生動物棲息地的監控等。物件追蹤包含了幾個重要的步驟，例如事件的偵測、目標物的辨識、位置的估算等，在無線感測網路中，當物件的位置被估算出來之後，為了能讓使用者去查詢物件的位置，或者為了能讓感測到物件的感測器做回報，一個位置管理機制是需要的。本計畫的主題即討論無線感測網路上的位置管理機制。我們所提出的位置管理機制利用了無線感測網路的網路內資料處理的能力，以分散式的方式去執行物件位置的更新與查詢。而由於感測資料的不精確是感測網路先天上特有的特性，因此我們在此計畫中會去思考在使用者可容忍一些誤差的網路環境下，位置管理機制該如何達成的問題。
位置管理機制通常包含了兩個基本的操作：位置更新與位置查詢。一般而言，位置更新是在當一個物件從一個感測區域移動到另一個感測區域時發生，而位置查詢則是當使用者有需要知道物件的位置而發生查詢。無線感測網路處理位置更新與查詢的方法有很多，一個處理更新與查詢最簡單的做法是將使用者的查詢送給網路上的每一個感測節點，而偵測到物件的感測器在收到查詢後就會回覆物件的位置給資料匯集端(Sink)，我們可以發現在這個方法中，感測器不需要主動地做位置的更新，然而，很顯然這個方法在網路規模很大或者是查詢頻率很高時會非常地沒有效率，因為使用者的查詢必須被廣播到整個網路上。第二種處理更新與查詢的方法則是要求感測器偵測到物件時就必須主動地將物件的新位置傳送給資料匯集端，如此資料匯集端就隨時有物件的位置資訊，因此資料匯集端本身就可以馬上回覆使用者的查詢，而不再需要將查詢送到感測網路上，然而這方法在物件移動相當頻繁時會產生許多的位置更新訊息，因此我們可以很明顯地看出上述兩個方法是各有利弊。在本計畫申請人之前發表過的論文中，已針對此問題提出了一個樹狀結構的位置管理機制，然而在那些論文中，我們是假設使用者必須得到精確的位置資訊。
然而在物件移動的環境中要獲得物件正確的位置資訊幾乎是不可能的，原因除了定位技術本身就不是很準確，物件的移動與資料傳送的延遲也都使得查詢者得到的位置資訊並不精確。然而幸運的是在物件追蹤的應用中，不精確的位置資訊通常是能夠被容忍的，例如生物科學家為了要追蹤某動物時，生物學家可能只需要知道大概的移動方向即可而不需要知道正確的位置，除此之外，當科學家只是為了要觀察動物的日常作息時，幾個小時前的位置資訊對科學家們仍是有用的資訊。因此我們將在此計畫中提出一個位置管理機制去支援能夠容忍不精確位置資訊的物件追蹤感測網路。
The rapid progress of wireless communication and embedded micro-sensing MEMS technologies has made wireless sensor networks (WSNs) possible. Applications of WSNs have been studied widely. Object tracking is one of the important issues of WSNs, which has applications in such as military intrusion detection and habitat monitoring. The key steps involved in object tracking include event detection, target classification, and location estimation. In a WSN, when the locations of objects are successfully determined, a location management mechanism for reporting objects' locations and disseminating users' queries is required. The main theme of this project is location management. The proposed location management mechanism explores the in-network data processing capability of WSNs by executing distributed location updates and queries inside the network. Since inaccuracy of sensing data is inherent for WSNs, we consider the scenarios where users can tolerate a certain degree of imprecision in their query results.
Object tracking typically involves two basic operations: update and query. In general, updates of an object's location are initiated when the object moves from one sensor to another. A query is invoked each time when there is a need to find the location of an interested object. Location updates and queries may be done in various ways. A naive way for delivering a query is to flood the whole network. The sensor whose sensing range contains the queried object will reply to the query. Clearly, this approach is inefficient because a considerable amount of energy will be consumed when the network scale is large or when the query rate is high. Alternatively, if all location information is stored at a specific sensor (e.g., the sink), no flooding is needed. But whenever a movement is detected, update messages have to be sent. One drawback is that when objects move frequently, abundant update messages will be generated. The cost is not justified when the query rate is low. Clearly, these are tradeoffs. The project applicant has published some papers on this problem. Specially, the project applicant developed a tree-based location management mechanism. However, in those papers, the PI assumes that the users always need the accurate location information.
In moving object environments, maintaining the exact locations of objects anytime is almost infeasible. Not only the positioning results are error-prone, but also the data transfer delay and object mobility make the locations of objects inaccurate. Fortunately, imprecision is tolerable in many object tracking applications. For example, when life scientists intend to track an animal, it may be sufficient to know its moving direction rather than its exact location. In addition, the location information recorded several hours ago, instead of at the current time, may be still available for the life scientists to understand the animal's daily life. Therefore, in this project, we intend to develop an in-network location management mechanism to support imprecision-tolerant queries for object tracking sensor networks. |
