110

## Results and Discussion

0.018

Std. dev. X Std. dev. Y

0.017

## Standard Deviation (pixels)

0.016

0.015

0.014

0.013

0.012

0.011

0.01

0.009 0.5

1

1.5

2

2.5

3

Distance (m)

Figure 8.9 The centroid variation increases approximately linearly with distance. The mean stan- dard deviation of all visible markers was used in this plot. Outliers were removed.

generated from the mean of only two markers and is considered to poorly represent the general performance of the system. The standard deviations are approximately linearly proportional to the distance between 0.6 m and 3 m as shown by the two trend lines. The

spike observed at 1 m the measurement (see

is considered anomalous, ppendix G).

resulting

from

an

external

disturbance

of

The trend between 0.6 m and 3 m for the standard deviation given in Figure 8.9b in X is

σ_{X}(D) = 1.40 × 10

^{3}D + 10.4 × 10

3

,

(8.6)

and the standard deviation in Y is

σ_{Y}(D) = 1.30 × 10

^{3}D + 10.3 × 10

3

.

(8.7)

8.2.5

# Long term stability

Factors that could affect stability include temperature and ambient lighting. To determine whether there is any drift in the centroid data over time a long term measurement was made using each of the three Black Spot modules. Data were collected for approximately 8 hours throughout a working day. Each module was stationary and faced the stationary test beacon. Data from each module were not captured concurrently so cannot be compared di- rectly but the conditions are believed to be sufficiently similar to make the statistics broadly