Figure 24
Comparison of GPS Results with TSS Results
As stated at the beginning of this report, NOS' Coast Survey installed a TSS 335B heave, pitch, and roll device in the engine room of the ship and correlated NGS' GPS results with their results obtained from the device. Figure 23 depicts the low frequency AC component of pitch obtained from the TSS device with the GPS pitch results overlayed for comparison. The low frequency AC component of pitch was isolated by subtracting the mean value of the GPS and TSS data sequence from each sample in the respective sequence. This was followed by passing a 61-second boxcar window over the then zero-mean fluctuating pitch signals. The ship speed was subjected to the 61-second boxcar filtering. Figure 24 depicts ship heading and the high frequency AC roll component from the TSS-335B device with the GPS roll results overlayed for comparison. The AC component of roll was isolated by subtracting the mean value of the GPS and TSS data sequence from each sample in the respective sequence. This was followed by passing a 1.5 second boxcar window over the then zero-mean fluctuating roll signals. The ship heading was subjected to the 1.5 second boxcar filtering. The figures indicate that GPS is capturing the pitch and roll of the ship fairly well. There are some differences between the two sets of results because GPS responds immediately to changes in the vessel's speed and direction, while the TSS-335B unit has errors caused by uncertainties in measurement of vertical acceleration, uncertainties in the position of the TSS relative to the roll/pitch center of the ship, phase distortion introduced by the high pass filter, and settling and transient errors of the high pass filter excited by dynamic draft. A notable difference between GPS and TSS occurred during the ship's rapid turn, captured in figure 24.