Using the BUTTONWOOD as a Tide Gauge

Another part of the test was to demonstrate that GPS can be used to measure changes in water levels. A GPS receiver on the ship and the receiver at the base station collected data continually for 5 hours in the evening of the first day. There are several Physical Oceanographic Real-Time Systems (PORTS) installed in the San Francisco Bay area. PORTS was developed by NOS scientists and engineers. The system measures currents, water levels, and other physical conditions. By providing information about actual conditions, PORTS allows pilots and masters to use the Bay's channels to their fullest extent, while simultaneously increasing safety and minimizing the need for additional dredging (Scherer and Beaumariage 1988). Mariners now can access PORTS information by cellular telephone before entering the Bay. Table 1 is a sample output of the San Francisco Bay PORTS.

Table 1

San Francisco Bay PORTS NOAA/National Ocean Service at 10:49 am PST November 6, 1996 ............................................................................... --- TIDES --- : - CURRENTS -(F)lood, (S)lack, (E)bb, Temp. Golden Gate 3.8 ft.,Falling : Golden Gate , 54o F Alameda : Oakland Richmond 4.3 ft.,Falling : Richmond Port Chicago 4.1 ft. : Benicia Bridge , 58o F ...................................:........................................... --- SALINITY --- Surface Bottom : --- WATER TEMP --- Surface Bottom Benicia Bridge 17.3 psu 17.3 psu : Benicia Bridge 57o F 57o F Grizzly Bay (10.9 psu) 10.9 psu : Grizzly Bay (56o F) 56o F Honker Bay : Honker Bay .......................................:....................................... ---METEOROLOGICAL --- Wind Speed/Dir Air Pressure Air Temp Golden Gate 3 knots from N , gusts to 4 1025 mb 54o F Alameda Richmond Calm 1025 mb 58o F Port Chicago 5 knots from W , gusts to 8 1025 mb 57o F ............................................................................... To receive a description of PORTS, please contact NOAA at 301-713-2801

Figures 17a and 17b depict the water level values (6-minute means) above mean lower low water at the Golden Gate and Alameda PORTS during the 5 hours that the GPS equipment on the ship was collecting data. Figure 17c depicts the water level in the ellipsoid height system relative to the antenna on the BUTTONWOOD. Figure 18a is a plot of all three water level values where the height of the water value of the BUTTONWOOD was arbitrarily offset so its values would fall between 0 and 1 meter. Figure 18b is a plot of all three water level values where the water value of the BUTTONWOOD was set equal to Alameda at GPS time 285,840 (23:24:00 local time) for comparison purposes. This time was selected because that is when the water level values obtained from the Golden Gate and Alameda PORTS were equal (see figure 18a), and since the BUTTONWOOD was between these two stations, it was assumed that its value, at that time, was equal to the other two.

Figure 18b indicates the obvious fact that the water level at the ship is not the same as the water level at either PORTS. There is more water under the keel of the ship at GPS time 276,000 (20:40:00 local time) than there is at the Golden Gate Bridge and there is less water under the keel than there is at Alameda. What does this mean to the shipping industry? This means that the ship becomes a real-time water level gauge and if the bottom of the channel is accurately known, the pilot of the ship knows how far the bottom of the keel is from the bottom of the channel. If the pilot knows that there is another 12 cm of clearance, the ship can safely be allowed to squat more (increase its speed) and can get in and out of the port quicker.

Figure 17a

Figure 17b

Figure 17c

Figure 18a

Figure 18b