GPS Positioning Modes Định vị với GPS có thể được thực hiện bởi một trong hai cách: định vị điểm hoặc vị trí tương đối. Điểm định vị GPS sử dụng một máy thu GPS rằng các biện pháp pseudoranges mã để xác định vị trí của userA ngay lập tức, miễn là bốn hoặc nhiều vệ tinh có thể nhìn thấy ở người nhận. Dự kiến sẽ ngang vị trí chính xác nhận C / A mã dân sự đã đi xuống từ khoảng 100m (2 drms) khi sẵn sàng chọn lọc trên, khoảng 22 triệu (2 drms) trong trường. | 5 GPS Positioning Modes Positioning with GPS can be performed by either of two ways point positioning or relative positioning. GPS point positioning employs one GPS receiver that measures the code pseudoranges to determine the user s position instantaneously as long as four or more satellites are visible at the receiver. The expected horizontal positioning accuracy from the civilian C A-code receivers has gone down from about 100m 2 drms when selective availability was on to about 22m 2 drms in the absence of selective availability 1 . GPS point positioning is used mainly when a relatively low accuracy is required. This includes recreation applications and low-accuracy navigation. GPS relative positioning however employs two GPS receivers simultaneously tracking the same satellites. If both receivers track at least four common satellites a positioning accuracy level of the order of a subcentimeter to a few meters can be obtained 2 . Carrier-phase or and pseudorange measurements can be used in GPS relative positioning depending on the accuracy requirements. The former provides the highest possible accuracy. GPS relative positioning can be made in either real-time or postmission modes. GPS relative positioning is used for high-accuracy applications such as surveying and mapping GIS and precise navigation. 69 70 Introduction to GPS GPS point positioning GPS point positioning also known as standalone or autonomous positioning involves only one GPS receiver. That is one GPS receiver simultaneously tracks four or more GPS satellites to determine its own coordinates with respect to the center of the Earth Figure . Almost all of the GPS receivers currently available on the market are capable of displaying their point-positioning coordinates. To determine the receiver s point position at any time the satellite coordinates as well as a minimum of four ranges to four satellites are required 2 . The receiver gets the satellite coordinates through the navigation message
