A key aspect of a well-functioning indoor positioning system (IPS) is getting the system set up correctly. It's crucial since the quality of the installation directly determines the positioning accuracy. This post will give you an overview on how to measure the anchor coordinates correctly. This is the basis of positioning your object with 5-30 cm accuracy using the KIO RTLS.

The performance of any UWB-based IPS using time-of-flight measurement depends on the accuracy of anchor placement measurements. Even a small mistake can cumulatively add up to a significant error in the tag’s position calculations. Therefore, we always recommend using the services of professional land surveyors to guarantee the maximum accuracy of anchor coordinates, especially in a full-scale setup.

However, hiring a land surveyor can be expensive and unreasonable for a small scale IPS setup. The more affordable and almost equally good option is to use a simple tape measure or a hand-held laser distance meter. Here’s a short step-by-step guide on how to get the anchor measurements right the first time.

Step 1: Install the anchors in your tracking area

Begin by identifying the tracking area. It may be a single room or a part of a larger area. If your area exceeds the maximum dimensions of a single KIO RTLS tracking cell (up to 1000 m2) you can always add more anchors to form a multi-cell setup.

Usually, it is best to install the anchors near the corners of the tracking area. The anchors can be placed by fixing them flat on the wall or by using wall mounts. Connect all the required cabling to power the anchors and enable data transmission.

Step 2: Measuring coordinates where walls act as reference surfaces

After mounting the anchors, first determine a point in your tracking area that will be your zero coordinate point in the X-, Y-axes (0;0). This reference point is how you identify the location of all the anchors. In 3D tracking, you’d also have to add the Z-coordinate for height but we’re not using it in this example.

If your tracking area has perpendicular straight walls that can act as X- and Y-axes, then your reference point is the corner of the intersection of these two.

In a rectangular room, it is quite straightforward to determine the anchor coordinates (see figure 1 below). If you have mounted the anchors flat on the wall near each corner of the room, then you just have to measure the distances against respective walls to acquire the X and Y coordinates of each of the anchors. Make sure you also adjust your measurements, if you’re using wall mounts for anchors.

Figure 1: Determining anchor coordinates in a room with reference surfaces.

Step 3. Measuring coordinates if there aren’t any reference surfaces

In a room without reference points and where anchors don’t form a rectangle, it is difficult to make sure you’re measuring the X or Y coordinate of a distant anchor correctly. For example, if you’re setting up a temporary tracking area with anchors mounted on tripods. To get it right, you need to use some well-known trigonometric formulas.

First, fix the anchors on the tripods and measure all the necessary distances between them. We recommend using one anchor as the zero coordinate and also defining the X-axis (or Y-axis) between two anchors. The distances you measured between three anchors now form a triangle. Knowing the length of each side of the triangle you can calculate the inner angles using the formulas presented in figure 2.

Figure 2: Formulas for calculating the inner angles of a triangle.

In figure 3 you can see an example of how it works in practice. We have chosen the anchor A as the zero coordinate and the line between anchors A and B as the X-axis, which already gives us the coordinates for two anchors – A(0;0) and B(c;0).  The distance from A to C and B to C become the other two sides of a triangle. Now we can calculate the inner angle β by using the formula from figure 2.

Figure 3. Necessary steps before calculating the anchor coordinates.

Once you know the β, it’s possible to calculate the two unknown distances that make up the coordinates for the anchor C. A walkthrough with calculations for the missing values is shown on figure 4. Follow the same logic to determine the coordinates for the rest of the anchors.

 

Figure 4. Formulas for calculating the coordinates for anchor C.

Although it might seem daunting at first, it’s actually quite straightforward to calculate the anchor positions, especially if you have reference points in the room. We have worked with different projects and can assist you in this process. Contact us if you are having difficulties measuring the anchor coordinates or need help with some of the steps described above.

 

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