Surface Circulation Study 

of Waters Near 

Ochlockonee Bay, Florida 

by 

Peter Lazarevich 

27 July 2007 

Florida State University 

Department of Oceanography 

Tallahassee, Florida 32306-4320 U.S.A 
1. Introduction 

This report presents the design and fabrication of a GPS/radio-tracking system for 

measuring surface circulation, and an analysis of the field data collected by our group during 

May and June, 2007 in the Ochlockonee Bay area of Northwest Florida. The purpose of this 

study was to observe circulation patterns that could explain the occurrence and timing of 

bacterial contamination at nearby beaches. These sites are regularly monitored by the Florida 

Department of Health as part of their “Florida Healthy Beaches Program”, which was initiated in 

1998. In particular, Mashes Sands Beach, Wakulla County, has been monitored since 2000 and 

over that time there have been several advisories and warnings issued for contamination. At the 

present time, July 2007, Mashes Sands Beach has been under an advisory since mid-June. Our 

field work began prior to this advisory, and continued for several weeks afterwards. 

2. Study Area 

Ochlockonee Bay (Figure 1) is located within the Big Bend region of Northwest Florida, 

about 80 km south of Tallahassee, Florida. Freshwater input comes primarily from two rivers on 

the western end, the Ochlockonee and Sopchoppy Rivers, and it is open to Apalachee Bay on the 

eastern end. Ochlockonee Bay also has several tidal creeks and shallow marshes. Its basin is 

about 10 km long and 3 km wide, with depths ranging from 3 m to 7 m in the channel and from 

1 m to 2 m outside. There are extensive shoals throughout, and, at low tide, some become 

exposed. Mashes Sands Beach is located just outside of Ochlockonee Bay. A shoal extends 

some 3 km away from the beach. North of Mashes Sands, waters are also quite shallow. Broad 

marshes separate several, relatively closed-off bays. Dickson Bay, located directly north of 

Mashes Sands Beach, is one example. 

Figure 1. The study area. 
3. Drifter 

The original design goal was to make a drifter capable of measuring currents over the 

course of several weeks, with GPS fixes recorded every hour or better and transmitted by radio 

to a base station for real-time monitoring and recovery. This design relies upon three main 

components: a GPS receiver, radio transceiver, and a microprocessor controller, each available 

commercially as OEM components. Selection of the components was made based upon a 

compromise between battery efficiency, small-profile, and light-weight. However, due to the 

short duration of this project, additional consideration was given to selecting components that 

could be integrated and developed with ease. In the end, the following components were 

selected: a Lassen iQ GPS receiver from Trimble, an AC4790-200 transceiver from Aerocomm, 

and a BS2pe Microprocessor from Parallax. Table 1 lists the major drifter components and their 

specifications. 

Component Manufacturer and model Specifications 

GPS receiver Trimble, Lassen iQ 27 mA current draw (@ 3.3 V) 

NMEA output (GGPA string) 

Reacquisition time < 5 seconds 

External antenna 

Radio transceiver Aerocomm, AC4790-200 50 mA current draw (@ 3.3 V) 

100 mW output power 

4 mile range (advertised) 

Peer-to-peer protocol 

Frequency Hopping Spread Spectrum 

902 – 928 MHz frequency band 

76.8 Kbs data rate (baud rate) 

External half-wave dipole antenna 

Microprocessor Parallax, BS2pe 15 mA current draw in run mode (@ 5 V) 

40 uA current draw in sleep mode 

Programmable in BASIC 

32 Kbyte memory 

I/O lines for component control 

Battery AA Portable Power, 18650 2000 mAh each 

Li-Ion, rechargeable 

Voltage Regulator Maxim, MAX883 5 V regulated output 

200 mA output limit 

Table 1. Parts list of the drifter – major components, their manufacturer and model, and specifications. 
Twelve drifters and one base station were built, as shown in Figure 2. The drifters were 

programmed to acquire a GPS fix every two minutes, store the fix into memory, and then wake 

up at selected times and broadcast a radio message that included the last GPS fix made. The 

base station was programmed to constantly listen for these transmissions and output the 

messages via RS-232 for display on a computer. 

Figure 2. The twelve drifters and base station, being prepped prior to a deployment. 

The drifters were designed to be compact and low-profile in order to avoid being seen on 

the water by boaters. Figure 3 shows what an (unpainted) drifter looked like in the water. It was 

barely visible at 100 m. During field tests, it was found that the drifter was quite stable in the 

water, and that both the GPS and radio antennas remained slightly above the water line. 

Figure 3. View of the drifter (center of the photo) in the water from a distance of 100 m, plus a close-up zoom. 
4. Observations 

A total of nine deployments were made: five in Ochlockonee Bay, two in Dickson Bay, 

one in Ochlockonee River, and one in Apalachee Bay. The first deployment was a short test of 

the drifter system. The second, third, and fourth deployments were made at the same time that 

water samples were collected by the University of South Florida (USF) group. The remaining 

deployments were longer in duration and exhibited interesting circulation patterns. The table 

below summarizes the deployments. 

Deployment Date Location Duration Number of Drifters 

Number (2007) (hours) Deployed Recovered 

1 May 8 Ochlockonee Bay 0.67 2 2 

2 May 9 Ochlockonee River 1.00 3 3 

3 June 6 Ochlockonee Bay 1.25 8 8 

4 June 6 Ochlockonee Bay 0.92 8 8 

5 June 7 Dickson Bay 2.25 8 8 

6 June 25 Dickson Bay 10.83 12 11 

7 June 27 Ochlockonee Bay 22.92 10 6 

8 June 29 Apalachee Bay 3.67 6 6 

9 June 30 Ochlockonee Bay 6.92 6 6 

Table 2. Listing of the drifter deployments. 

In the figures that follow, drifter tracks from the nine deployments are plotted on maps 

showing the deployment and recovery locations, and 5-minute interpolated positions. 

Bathymetry contours and a recent aerial photo of the area are added. The aerial helps to identify 

submerged shoals, which show up as a lighter color than the deeper water. In addition, tidal 

information is also presented, as the circulation is dominated by the tides. The tidal phase is 

represented by a color-coding of the interpolated positions. Red dots indicate an incoming tide 

and blue dots indicate an outgoing tide. 
4.1. Deployment #1, 08 May 2007 

The first deployment was a short test of the drifter system, just inside the mouth of 

Ochlockonee Bay. Two drifters were deployed simultaneously, about 20 m apart, at a channel 

marker which helped to serve as a visual reference. They were kept in sight at all times, and 

radio contact was maintained throughout the trial. An outgoing tide carried the drifters out 

towards the mouth of Ochlockonee Bay. During the 40 minutes in the water, there was no 

appreciable relative motion between the drifters. 

Figure 4. Drifter tracks for deployment #1, 08 May 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, and are all blue for an outgoing tide. The red dots with black border are the locations where 

the USF team collected water samples. 
4.2. Deployment #2, 09 May 2007 

The second deployment was at a USF water sampling site, in the Ochlockonee River. 

Three drifters were deployed simultaneously, about 20 m apart. They were in the water for 

60 minutes, and in that time one of the drifters washed up along the river’s southern shore. 

While there was an outgoing tide in Ochlockonee Bay, the drifters traveled upriver, most likely 

due to a substantial phase lag in the tidal forcing at this location. 

Figure 5. Drifter tracks for deployment #2, 09 May 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, and are all blue for an outgoing tide. The red dot with black border is the location where the 

USF team collected a water sample. 
4.3. Deployment #3, 06 June 2007 

The third deployment was near USF water sampling sites, offshore of Mashes Sands 

Beach. Eight drifters were deployed simultaneously, in two groups, about 50 m apart within 

each group. An outgoing tide carried the drifters out towards the mouth of Ochlockonee Bay. 

They were in the water for 75 minutes, and some relative motion was observed. 

Figure 6. Drifter tracks for deployment #3, 06 June 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, and are all blue for an outgoing tide. The red dots with black border are the locations where 

the USF team collected water samples. 
4.4. Deployment #4, 06 June 2007 

Later, on June 6, another deployment was made nearby Mashes Sands Beach. Eight 

drifters were deployed simultaneously, about 50 m apart. They were in the water for 55 minutes, 

carried out on an outgoing tide. At recovery, two drifters had grounded along the northern shore 

of Ochlockonee Bay, three were still tracking currents with Ochlockonee Bay, and the other 

three had traveled north around Ochlockonee Point. This deployment clearly demonstrates how 

varied currents can be around a coastline and over shoals. 

Figure 7. Drifter tracks for deployment #4, 06 June 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, and are all blue for an outgoing tide. The red dots with black border are the locations where 

the USF team collected water samples. 
4.5. Deployment #5, 07 June 2007 

The fifth deployment was within Dickson Bay, north of Mashes Sands Beach. Eight 

drifters were deployed simultaneously, approximately 100 m apart. Although the predicted tides 

indicated an outgoing tide, the drifters moved north, in apparent opposition to the tides. 

However, they were traveling towards deeper waters in the channel to the north. This could be 

how waters in Dickson Bay predominately flow during an outgoing tide, instead of to the 

southeast over the shoals. They were in the water for about 2 hours and 15 minutes 

Figure 8. Drifter tracks for deployment #5, 07 June 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, and are all blue for an outgoing tide. 
4.6. Deployment #6, 25 June 2007 

The sixth deployment was outside Dickson Bay. All twelve drifters were deployed 

simultaneously, about 200 m apart. They were deployed on the incoming tide and were carried a 

distance of about 4 km back into Dickson Bay. Remarkably, the tracks converged, indicating 

that the flow in Dickson Bay was primarily directed in towards the channel. The drifters 

remained relatively close together, almost for the duration of the incoming tide; some separation 

was observed as several became grounded or were trapped around shoals. Later, when the tides 

reversed, the drifter separation generally increased. At recovery, six drifters had grounded along 

the coast, two were nearly grounded on shoals, three were tracking currents, and one was not 

heard from and therefore lost. This deployment lasted 10 hours and 50 minutes. 

Figure 9. Drifter tracks for deployment #6, 25 June 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, blue for an outgoing tide and red for an incoming tide. 
4.7. Deployment #7, 27 June 2007 

The seventh deployment was made at the mouth of Ochlockonee Bay. Ten drifters were 

deployed in sequential pairs, approximately 2 m apart in distance and 10 minutes in time. They 

were deployed on an outgoing tide and were initially carried towards Ochlockonee Bay. They 

then turned to the southeast. By the end of the outgoing tide, the drifters were approximately 

1.5 km away from the deployment site. Around this time some drifters show a break in their 

tracks. The most likely explanation is the drifters encountered an exposed shoal and turned onto 

their sides, submerging the GPS antenna and interrupting all drifter activity. Later, when the 

tides reversed, the floats came off the shoal and were carried into Ochlockonee Bay. 

Unfortunately, four of the drifters were lost. Another four drifters failed toward the end of their 

missions, but were recovered by chance, close to the two drifters that continued working until 

they were located 10 km inside Ochlockonee Bay. In the figure, the cyan stars east of the bridge 

represent the location of last valid GPS for the four failed units. This was the longest 

deployment, lasting 22 hours and 55 minutes. 

Figure 10. Drifter tracks for deployment #7, 27 June 2007. Deployment sites are indicated with the green stars. 

Here, cyan stars indicate either the recovery site or location of last valid GPS fix. The white lines indicate the drifter 

trajectories. Dots represent the 5-minute interpolated positions, blue for an outgoing tide and red for an incoming 

tide. The red dots with black border are the locations where the USF team collected water samples. 
4.8. Deployment #8, 29 June 2007 

The eighth deployment was well outside Dickson Bay, in Apalachee Bay. Six drifters 

were deployed in sequential pairs, approximately 2 m apart in distance and 15 minutes in time. 

An outgoing tide carried the drifters out towards the southeast. They were deployed for 3 hours 

and 40 minutes, and no significant relative motion was observed. The trial was ended early due 

to increasingly rough waves. 

Figure 11. Drifter tracks for deployment #8, 29 June 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, and are all blue for an outgoing tide. The red dots with black border are the locations where 

the USF team collected water samples. 
4.9. Deployment #9, 30 June 2007 

The final deployment was made in the center of Ochlockonee Bay. Six drifters were 

deployed in sequential pairs, approximately 2 m apart in distance and 10 minutes in time. They 

were released on an outgoing tide and traveled towards the southeast, until they became 

temporarily grounded on an exposed shoal. Later, when the tides reversed, the floats came off 

the shoal. Two pairs then turned northward and were carried about 4 km back inside 

Ochlockonee Bay while the other pair continued to the southeast and eventually were carried into 

a tidal creek, Chaires Creek. The trial lasted for 6 hours and 55 minutes. Apart from the 

separation caused by the exposed shoal, the drifter pairs remained remarkably close together. 

Figure 12. Drifter tracks for deployment #9, 30 June 2007. Deployment and recovery sites are indicated with the 

green and cyan stars, respectively. The white lines indicate the drifter trajectories. Dots represent the 5-minute 

interpolated positions, blue for an outgoing tide and red for an incoming tide. The red dots with black border are the 

locations where the USF team collected water samples. 
5. Analysis 

Tidal information was obtained from the NOAA predicted tide tables, location on the 

internet at: http://tidesandcurrents.noaa.gov/. Although predicted tides for Ochlockonee Bay 

were not available, there were data for nearby locations- Turkey Point and Shell Point. These 

two sites are roughly equal distance from Ochlockonee Bay, and the average of the two was used 

to determine predicted tides for the study area. The following three figures show the predicted 

tides for Ochlockonee Bay during the time of the deployments. 

Figure 13. Ochlockonee Bay tides (red line) for deployments #1 and #2 (blue dots). 

Figure 14. Ochlockonee Bay tides (red line) for deployments #3 through #5 (blue dots). 
Figure 15. Ochlockonee tides Bay (red line) for deployments #6 through #9 (blue dots). 

Drifter velocities were calculated for all drifter tracks. Figure 16 shows the histogram of 

all velocities, computed at all 5-minuted interpolated positions. The sample had a total of 

2865 velocities, with a mean of 20.0 cm/s. 

Figure 16. Histogram of all drifter velocities. 
6. Conclusions 

Overall, the GPS/radio drifter system performed quite well. The GPS unit provided 

accurate positioning at a relatively low-power cost. The radio performed below expectations, 

however, as the effective range was only 0.5 km. This was primarily due to the transmitting 

baud rate (76.8 Kbs) and the Frequency Hopping Spread Spectrum operating mode. While these 

capabilities are desired for large data throughput and data security, they require a higher signalto-noise ratio for successful transmissions. A future version of the drifter would include a 

different transceiver in order to achieve the desired range. 

Other changes to the design would include a new microcontroller that operated on 3.3 V 

instead of the 5 V the current design implements. This would simplify the design and improve 

battery efficiency. Also, a short, wide housing would enable the drifter to operate while on a 

shoal, instead of having the GPS antenna submerge as it has been found to do. Table 3 

summarizes the performance of the current version of the drifter and suggests what is possible 

with a future design. 

Specification Current version Future version 

Battery lifetime 3 days 15 days 

Number of GPS fixes 1790 10,000 

Range 0.5 km 5 km 

Weight 0.61 kg 0.5 kg 

Height 26 cm (46 cm with antenna) 10 cm (20 cm with antenna) 

Width 7 cm 18 cm 

Table 3. Drifter performance. 

7. Acknowledgements 

I would like to thank my stepfather, Bill Cochran, for his technical guidance in the drifter 

design. The project greatly benefited from his practical approach towards hardware development 

and software programming. The Florida State University Coastal and Marine Laboratory 

provided the boats needed to carry out the field work. Stephanie Fahrny, Mike Lavender, and 

Joel Valdez (Florida State University) assisted in the recovery of the lost drifters in 

deployment #7. Funding for this project was provided by the Florida Department of Health, 

grant #091004-523-020947.