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Report generation and data display from the NMT memory tag: A software demonstration

Michael Barthelow, Philip Ekstrom and Stan Moberly
NMT International, Ltd.
P.O. Box 427, Ben Nevis Road
Shaw Island, Washington 98286 USA

ABSTRACT

The NMT Memory Tag promises to provide researchers with fisheries data never before obtainable. Implanted in a large pelagic fish such as a tuna or swordfish, the tag will record depth, internal temperature, external temperature and sunlight intensity every two minutes for as long as 10-12 years, enabling researchers to determine the position of the fish from day to day and giving them a unique window on the fish’s behaviour.

1. INTRODUCTION

1.1 Tag Description

The Northwest Marine Technology (NMT) Memory Tag is primarily intended to be attached to or implanted in a large pelagic fish such as a tuna or swordfish to determine and record its position and key environmental variables every day for a period of years. When recovered and interrogated it will yield that position information along with statistical summaries of all data measured and a detailed time series record of a portion of the mission.

The tag is housed in a stainless-steel cylinder 16 mm in diameter by 100 mm long and weighs 52 grams. A thin, flexible measurement stalk 2 mm in diameter is attached to one end (stalk length is custom, based on users' needs), and when the tag is implanted the stalk extends through the skin of the animal to trail in the water. The end of the stalk houses the light sensor and the sensor for external temperature. The pressure sensor and the sensor for internal temperature are in the body of the tag.

The tag uses times of sunrise and sunset to determine longitude and the sea temperatures at standard depths to determine latitude. This approach is presented in Smith and Goodman (1986). As first proposed by NMT in 1987, the tag does intelligent onboard data processing to reduce memory requirements. The tag extracts measured data from each day and records in a day log the comparatively small amount of information required to fix position, plus some additional diagnostic information such as noon light intensity and water clarity. The resulting day log is compact enough so that it can log every day of a multi-year mission and never lose data for lack of memory space.

Either NMT or the user can handle the tasks of initially setting parameters and of reading data from a recovered tag. Host software running on an IBM-compatible personal computer communicates with the tag through an adapter to do either task. The data file from a recovered tag is initially in compact binary form, maximum size 257K, which is easy to handle and store. A second program translates the compact data record into a set of printable reports. A third program provides graphical display of the data for easy browsing, and generates various kinds of graphical output.

2. MEMORY TAG DATA LOGS

The tag combines 3 types of logging to maximize the information gained from a long mission. The tag is dormant much of the time, but wakes up every 128 seconds, takes measurements and stores them into a Time Series Log. Every 24 hours the tag reduces the day’s data to a summary sufficient to help determine the tag’s position, and stores this information into a Day Log. All readings are also tallied and stored in a Histogram Log, which maintains a record of the number of times each possible value was measured in each of the data channels. While the Time Series Log may overflow on a long mission, resulting in lost data, the Day Log and Histogram Log cannot overflow, and will preserve information from every data measurement taken since the tag was initialized.

2.1 Day Log

The Day Log is the primary position record, and holds a twelve-byte entry for each day of a mission. Primary quantities recorded are the times of sunrise and sunset, and the temperatures at the surface, at 61 m (200 ft) depth, and at 122 m (400 ft). These standard depths can be changed as a user option. The record also contains some engineering information, and a cross-reference to the day’s data in the time series log. Seven years’ worth of storage space is allocated to the primary day log. Even after this initial log space fills up, no day log entries are lost. Instead the log is expanded into space otherwise used for time series records, converting time series log into day log. This overwrites 1.6 days (1100 records) of the oldest time series for every additional year of mission beyond the nominal seven years.

2.2 Histogram Log

The most compact data summary is a set of histograms which tally the number of occurrences of every possible data value in each measurement channel. These tallies are based on all measurements taken since the tag was initialized. For example, one can learn from the histograms the extreme measured values of depth and temperature and the fraction of time the animal spent, say for example, below 250 metres depth.

2.3 Time Series Log

The remaining memory, 215 kilobytes, is devoted to a Time Series Log containing four-byte records of raw data. The user may specify the rate at which data is recorded by setting the Time Series Log Interval (N). Data is always measured every 128 seconds, 675 times per day; the user can choose to record one out of every N of these measurements. For example, if the user chooses to record every seventh measurement (N=7), a new record will be added about every 15 minutes.

The Time Series Log has two parts. Section A fills up with the first data measured and does not change thereafter. Section B fills after Section A, but when it is full, new data begins to overwrite the oldest data in that section. Thus, the Time Series Log preserves the very first data measured, and the last data measured. The user may choose how much of the available space to devote to each section.

3. TAG COMMUNICATION KIT

Tags are delivered with the clock and all parameters set as requested by the user, and are ready to use. There is no way to turn them off, so it is not possible to release an inactive tag. The NMT Memory Tag Connection Kit contains the necessary hardware and software to communicate with the tag and read its data. The two software applications supplied with the kit, HOST.EXE and READABLE.EXE run under MS-DOS on a IBM-compatible personal computer. This communication kit has been successfully tested on a wide variety of computers.

Using HOST.EXE, the user may adjust the tag’s parameters to custom-fit its operation to the mission’s need and clear the logs just before the tag is released. When the tag is recovered at the end of a mission, HOST.EXE is used to download the stored data from the tag into a compact data file.

The data download yields a binary file, size 257 Kbytes or less, intended for easy storage on the floppy disk of the minimal portable computer which is likely to be along in the field. READABLE.EXE is used to translate the compact binary data into a set of formatted text files, which are easily printed or imported into any word processing program. The reports contain control codes which allow the user to easily write a custom program to read the reports from disk and perform further processing of the data.

4. GRAPHICAL DISPLAY

Still another program called DATAPLOT, presently under development and nearing completion, displays the log contents graphically. This program runs under MS Windows and allows the user to browse through the three logs, zooming in on areas of interest. Individual graphs can be printed directly or saved as Windows metafiles for import into other Windows applications.

5. SUMMARY

The NMT Memory Tag is more than a simple data logger. It has sensors that are carefully adapted to solve the real problems of fixing the position of pelagic fish. It does intelligent data processing onboard and keeps a day log compact enough so that it never loses position information to data overflow. Finally, it is part of a complete system of hardware and software which presents data to the user in a convenient form, ready to use.

One of the challenges of the bounty of data returned by the Memory Tag is finding an efficient way to scan all the data and isolate those parts which are of the most interest. This problem is solved by a third application called DATAPLOT, currently under development and nearing completion. DATAPLOT, running under Microsoft Windows, allows the user to browse through a graphical representation of the Memory Tag data, zooming in and out of the details of the logs. The graphs may be printed directly, or saved to disk as Windows metafiles and imported into other Windows applications.

6. REFERENCES CITED

Smith P., and D. Goodman. 1986. Determining fish movements from an "archival tag": Precision of geographical positions made from a time series of swimming temperature and depth. NOAA Tech. Memo. NMFS-SWFC La Jolla (60): 1-13.


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