An Ocean Surface Current Following Drifting Buoy OPERATION MANUAL

Clearwater Instrumentation, Inc.
304 Pleasant Street
Watertown, MA 02181
924-2708 Tel (617) 924-2724 Fax

TABLE OF CONTENTS

1. INTRODUCTION
   1. Clearwater Instrumentation, Inc.
   2. ClearSat-15-B Meteorological Drifter
2. Requirements
   1. Life Expectancy
   2. Data Transmission
      1. DBCP 1993 SVP Barometer Argos Data Format
      2. DBCP 1996 SVP Barometer Argos Data Format
   3. Pressure Sensor Accuracy
   4. Temperature Accuracy
   5. Drogue Design
   6. Spherical surface float
   7. Holey Sock Drogue
   8. Drogue On-Off Sensor
   9. Delivery Requirements

FIGURES

Figure 1. WOCE/SVP BAROMETER PORT
Figure 2. AIR TEMPERATURE PROBE

1. INTRODUCTION

The ClearSat-15-B is a WOCE/SVP barometer drifter which satisfies requirements of oceanographers for a drogued satellite tracked buoy that closely follows surface currents, and of meteorologists for instrumentation capable of long term, accurate measurements of barometric pressure, air temperature and sea surface temperature on the open ocean. The ClearSat-15-B incorporates WOCE/TOGA physical design attributes to promote accurate water following by the drifter and longevity in the demanding the sea surface environment. The drifter is instrumented with Clearwater’s ARGOS certified PTT, the ClearSat-2.0 powered by an 8 MHz 68HC11 microprocessor. CMOS circuitry, power switching, and software that implements sophisticated scheduling to minimize power consumption and prolong battery life. The ClearSat-2.0 can perform highly accurate analog measurements with its 16-bit A/D converter, easily interfaces with analog and digital transducers, and stores complex data records in its 32K random access memory.

1.a Clearwater Instrumentation, Inc.

Since Clearwater began making drifters in 1989, our goal has been to run a successful company and to serve effectively the oceanographic community. We have done this by actively participating in the design and innovation of ARGOS tracked surface drifters, by continually seeking to improve drifter durability, and by producing these instruments at a reasonable price through aggressive cost reduction. Since 1989 Clearwater Instrumentation, Inc. has delivered almost 1000 ARGOS tracked drifters to oceanographers throughout the United States, Europe, and Asia. Our instruments have proven their durability by consistently surviving the longest periods in environments as diverse as tropical oceans and sub-polar seas. At the same time we have met our goals to improve our instruments, control costs, and develop new instrumentation. We believe our success depends on two factors. We work closely with oceanographers in addressing their instrumentation needs, which are determined by their research goals, environmental conditions of the area being investigated, and by budget constraints. Also, we have an experienced, well-trained staff of engineers and assemblers to design and build our instruments.

1.b ClearSat-15-B Meteorological Drifter

The Clearwater ClearSat-15-B provides researchers an oceanographic and meteorological drogued drifting buoy by combining proven sensor technology and drifting buoy design. The ClearSat-15-B is the result of extensive design and testing which coordinated by the WOCE Global Drifter Center at Scripps Institution of Oceanography (University of California, La Jolla, California). The physical configuration of the ClearSat-15-B is like that of the ClearSat-15, Clearwater’s surface drifter which meets WOCE/TOGA specifications for an oceanographic drifting buoy. WOCE/TOGA surface drifters have thousands of buoy years of experience in programs operated throughout the world's oceans. Instead of the 13 in surface sphere used by WOCE/TOGA surface drifters the ClearSat-15-B has a 16 in diameter surface sphere to accommodate the additional battery power needed for long term continuous transmission to the Tyros satellite. The tether and drogue specifications are the same as the WOCE/TOGA design: the drogue is a seven section holey sock centered at 15 m depth and provides a drag area ration greater than 40. In order to measure barometric pressure and air temperature, the surface sphere is fitted with instrumentation developed by the Global Drifter Center and the United Kingdom Meteorological Centre. Sensor design specifics for the ClearSat-15-B are discussed below.

2. Requirements

The instrument offered here meets or exceed the requirements for a drifting buoy that accurately follows surface currents, measures barometric pressure, air temperature and sea surface temperature, and can be deployed from aircraft. Where it is possible, information is presented to illustrate the design of the ClearSat-15-B.

2.a Life Expectancy

The physical design of the ClearSat-15-B is like the WOCE/SVP drifter which has seen extensive use in the world's oceans. These drifters have a life expectancy of over 400 days. However, because the ClearSat-15-B must operate continuously, compared to the one third duty cycle of WOCE/SVP drifters, it are supplied with a 112 AH 12VDC power pack.

2.b Data Transmission

ClearSat Barometer drifters have been manufactured with two DBCP SVP Barometer Argos data formats. The DBCP 1993 format stores 24 hourly atmospheric pressure reading in a 4 Argos messages (256 bits per message, or page). This earlier format is the DBCP 1993 SVP Barometer Argos Data Format. When DBCP realized that even with a one-third duty cycle (three hours on followed by six hours off) there was a very high rate of data redundancy, they formulated a format which stores the twelve previous atmospheric pressure readings in two Argos messages of 128 bits each (DBCP 1996 SVP Barometer Argos Data Format).

2.b.i DBCP 1993 SVP Barometer Argos Data Format

Until recently ClearSat Barometer drifters formatted their data according to the DBCP. The DBCP format is attached in Appendix I. The DBCP format provides 24 hours of hourly barometric pressure, sea surface temperature, drogue status, battery voltage, allocates space for up to three additional sensors, time stamps barometric pressure, and provides checksums. The DBCP data specification also recommends a de-spiking routine for processing barometric pressure measurement. 130 samples are taken each hour at a 1/sec rate. The de-spiking algorithm rejects anomolous high readings and takes the median of acceptable values to remove the slight variation caused by the buoy being lifted and dropped by waves. Locations are computed and reported by Service ARGOS with the data stream transmitted by the drifter. An example of Argos data is found in

2.b.ii DBCP 1996 SVP Barometer Argos Data Format

2.c Pressure Sensor Accuracy

The ClearSat barometer drifter utilizes the AIR Intellisensor II SB barometer (800-1060 Mb) which measures pressure to ± 0.01 mb; accuracy is ± 0.5 mb for 1 year. Intellisensor I specifications are presented in Appendix IV which also contains a comparison of the units with a precision barometer for the temperature range of -23 to 48 C and 775 to 1075 mb. The ClearSat 1.1 PTT communicates with the AIR Intellisensor over a digital interface (TTL).

Air pressure is conveyed to the input of the pressure sensor with a port arrangement designed by the WOCE Global Drifter Center at Scripps Institution of Oceanography (University of California, La Jolla, California). The port has been tested for the equivalent of several years in instruments released of the coast of California and in the Atlantic. Atmospheric pressure is admitted into the surface sphere through baffled port mounted on top of the surface float. The port promotes accurate barometric pressure sampling (minimizes pressure aberrations cause by passage of wind past the surface sphere and reduces pivot effects with proper port inlet design, yet ensures that water does not enter the tubing leading to the pressure sensor). Figure 1 depicts the barometer port.

2.d Temperature Accuracy

Air and sea surface temperature are measured with linear thermistor probes accurate to greater than ± 0.1 C. Sea surface temperature is measured with a probe described in WOCE/SVP specifications. Air temperature is measured by using a thin narrow thermistor probe (6.0 in x .0145 in dia.) placed on the top of the barometric pressure port (Figure 2). The fatigue resistant wire and stress relief of the probe where it attaches to the barometer port increase probe life. The thermistor is completely waterproof: it is potted in reflective white, thermally conductive epoxy and encased in a thin layer of Teflon at the tip of the probe. Recent work by the UK Meteorological Office indicates that such a probe can measure air temperature without the need for radiation shielding. Mounting the probe on top of the barometer port keeps it as dry as possible, and the thermistor wires can be brought into the surface sphere through the passage for air pressure. The mean distance between the probe top and sea surface is 22 in. Since a drifter with barometer port was observed infrequently submerged just over the top of the surface sphere, even in 12 foot seas, the additional height should keep the probe out of the water more than 90 % of the time and permit dry bulb temperature measurements for a corresponding fraction of operation.

Sea surface and air temperature probe calibrations and accuracy are verified for the operating range: 0 - 40 C.

The ClearSat 1.1 PTT measures temperature with a 16 bits A/D converter. For sea surface temperature WOCE/TOGA specifications are employed: one reading per minute, average 15 readings, and update SST buffer. Air temperature measurements conform to DBCP specifications. Note that DBCP temperature reporting format requires temperature to be reported to ± 0.1 C in 10 bits.

To demonstrate ClearSat thermistor probe accuracy, measurements for two probes selected at random from company inventory and reported by PTT through Service ARGOS are included in Appendix III, along with barometric pressure measurements. For this test the probes were deployed in air and no special precautions were used to ensure that their temperatures were the same. Temperature measurements differ by less than 0.05 C.

2.e Drogue Design

The ClearSat barometer drifter is a subset of the WOCE/TOGA surface drifter which was designed specifically to track ocean currents at 15 m depth with an accuracy of greater than 2 cm/sec. Clearwater’s WOCE/TOGA surface drifter is called the ClearSat-15. The ClearSat-15 drifter conforms to the WOCE/SVP surface drifters specification. This surface drifter is characterized by a spherical surface float, high drag-area ratio (greater than 40:1),, low tether tension at the surface float, and a holey-sock drogue centered at 15 meters below the surface . Field studies and numerical simulations have demonstrated that these drifter design features minimize slip, the velocity of the drifter relative to the layer that is being tracked,,. Subsurface floatation on the tether below the surface sphere minimizes static tension on the tether attachment to the surface sphere.

2.f Spherical surface float

The surface float is a 16 in ABS sphere. This size is necessary to accommodate the additional batteries needed to provide for continuous transmission as compared to the ClearSat-15 which operates for one third of the time (24 hours on, 48 hours off, as required for most WOCE programs).

2.g Holey Sock Drogue

The drogue is an eight section holey sock drogue centered at 15 meters depth. This drogue provides a calculated drag area ratio of greater than 40.

2.h Drogue On-Off Sensor

Drogue presence is indicated by a cumulative measurement of surface sphere submergence. Because the holey sock drogue keeps the surface sphere from rapidly following the sea surface as waves pass, the sphere is occasionally submerged slightly below the surface. If the drogue come off, the surface sphere closely follows the surface and seldom submerges. The ClearSat 1.1 PTT interrogates two contacts near the top of the surface sphere every second to determine if they are in seawater. This record of submergence is accumulated for a 30 minutes period and is reported as a submergence/10. The range is 0 (never submerged) to 179 (1800 seconds submerged). Typical values indicating drogue on are between 30 to 100.

2.i Delivery Requirements.

Because each ARGOS-tracked drifter requires programming determined by its ARGOS PTT ID number and mission requirements, an order with the ARGOS PTT ID numbers must be placed at least 60 days prior to delivery.

1993 DBCP SVP Barometer Argos DATA Format

1993 SVP Barometer Drifter Sampling Scheme and Transmission Format

Sensor Sampling

Pressure Sampling, (P1, P2, if installed as Sensor2). sampled for 160 seconds just before new transmission

• Interval between pressure samples = (Air => approx. 1 second, Time to 1st sample after initial command is 1.2 seconds, because AIR takes first sample and throws it away then averages the next four ), (Kavlico => 1 second exactly)
• Number of pressure samples = 160
• De-spiking algorithm
  Take the median of the lowest 10 points of 160. Then calculate the median of the points within the entire set of 160 points that are within 1 mb of the 10 point median.
• Number of sensors monitored = 2
  AIR-SB-1A => Asynchronous Serial Output
  Kavlico P512 => 0.5 - 4.5 V Output
• Store as 12 bit count, 0 => 800.0 mb and 4095 => 1209.5 mb, with a 0.1 mb resolution.
• Pressure data is sampled hourly without regard to the transmission schedule. "Age" is the time elapsed relative to most recent pressure sample (P1i). For example a P1i-8 pressure sample is the 8 hour old pressure sample relative to the current pressure, P1i.
• If 15 or more of the 160 samples include communication errors, the sampled set is deemed corrupt. The count transmitted for corrupt sets is zero.

Sea Surface Temperature Sampling (SST), sampled for 15 minutes just before new transmission

• Interval between temperature samples = 60 seconds
• Number of temperature samples averaged = 15
• Number of thermistors monitored = 1 (YSI 44018 or equivalent)
• Store as 10 bit count

Salt Water Switch (Drogue), sample for 30 minutes just before new transmission

• Length of Salt Water Switch data collection = 30 minutes
• Scaling Factor divided into number of seconds in the water to fit 8-bits = 10.

Duty Cycle

Drifters should have option to be on 1 hours followed by off j hours with,

0 hours < I < 72 hours
0 hours < j < 72hours
i & j are both integers

Repetition Rate

90 ± 6 seconds

DBCP 1996 WOCE SVP Barometer Argos Data Format