Global Positioning System Navigation Option OPERATION MANUAL

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

TABLE OF CONTENTS ClearSat Argos FAD EZ-Marker Buoy Operation Starting and stopping buoy transmissions Buoy transmission cycles Altering buoy transmission cycles Sea surface temperature option Power pack ARGOS message data conversion Argos technical file Data conversion Byte 0, Duty Cycles Byte 1, Battery Level Bytes 2 and 3, Temperature Battery care and replacement

Tables

Figures

1. ClearSat Argos FAD EZ-Marker Buoy.

The ClearSat Argos FAD EZ-Marker Buoy is a satellite-tracked buoy which facilitates the location of drifting objects anywhere in the world's oceans. When the buoy is activated, an Argos transmitter (PTT) sends a brief radio signal (401.65 MHz), which allows the buoy to be tracked by low earth orbit satellites. The high frequency permits use of a compact, low-profile antenna which fits inside the 30-cm. diameter spherical hull. The entire buoy projects no more than 20 cm above the sea surface. In addition, the Argos message format needs only brief transmissions (320 milliseconds) every 90 seconds to acquire high quality locations and accurate data transmissions.

Transmitted PTT signals are received by a CLS/Argos uplink receiver on NOAA Tiros satellites when they pass over the buoy. CLS/Argos decodes the signals, computes a buoy location and makes the information available to the user in a variety of forms, including e-mail and phone-accessible database. The computed locations are evaluated statistically and classed according to their calculated accuracy. Location quality, or class, is affected by RF (PTT) signal strength and quality, number of messages received during the satellite pass, altitude of the pass, and sea state. Location classes vary from 1 to 3. Location 3, the most precise class, is accurate to 150 meters (two standard deviations); location 2 to 350 meters; and location 1 to 1 kilometer. Accuracy refers to the statistical distribution of measured locations about the actual location. NOAA Tiros satellites are in near-polar sun-synchronous orbits at 300 to 400 miles altitude. They complete an orbit around the earth once every 90 minutes. There are usually a minimum of two Tyros satellites receiving Argos PTT signals at any time. A PTT at the equator sees no less than six passes a day. The frequency of satellite passes increases poleward.

2. OPERATION.

Buoy operation and Argos PTT transmissions are controlled by a magnetic reed switch inside the buoy hull (see Figure 1 Magnet on sphere flat. and Figure 2 Magnet off sphere flat.), and metal contacts on top of the hull. A magnet provided with the buoy is attached to a small flat area on the hull, underneath which is the reed switch. The magnet used alone starts and stops PTT transmission. The magnet and contacts together are used to set PTT transmission cycles. The figures to the right show the location of the flat area on the base of the sphere with the magnet in place, and without it.
	Figure 1: Magnet on	Figure 2: Magnet off

2.a STARTING AND STOPPING BUOY TRANSMISSIONS.

The buoy is shipped with a magnet attached as shown above. This prevents the buoy from transmitting before deployment, which would drain the batteries and might incur Argos charges if the PTT signals were picked up by the satellite. After the magnet is removed from the buoy, the transmitter will start PTT transmissions in approximately 90 seconds. PTT transmissions consist of brief bursts spaced approximately 90 seconds apart. Replacing the magnet on the flat area will stop transmissions in approximately 30 seconds. The magnet must be removed for the buoy to transmit. The magnet must be in place to keep the buoy turned off.

2.b BUOY TRANSMISSION CYCLES.

Operating in its default mode, Mode 0, the marker buoy will transmit a brief message approximately every 90 seconds, 24 hours a day. CLS/Argos requires intervals between successive transmissions to be varied between 84 and 96 seconds in 1-second steps. Twenty-four hour transmission is called the "continuous" transmission duty cycle. This is the factory default setting.

In addition to continuous transmission, there are four duty cycles with sleep modes of various lengths. The duty cycle can be changed by the user to reduce the number of transmissions by suppressing transmission for intervals of 12, 16, 24, or 48 hours at a time. The period when the buoy is not transmitting is called "sleep mode." For instance, when the transmission mode is set to Mode 1, the buoy transmits approximately every 90 seconds for 8 hours, then ceases transmission, or "sleeps," for 16 hours. Important information about transmission modes is summarized in Table 2.4.

2.c ALTERING BUOY TRANSMISSION CYCLES.

When the transmission mode is reset, the new duty cycle of Argos transmission is immediately activated. Transmission mode is not reset when the buoy is turned off for an extended period by replacing the magnet on the hull.

Duty cycles can only be altered by following the steps detailed below. Changing the duty cycle requires a magnet and length of wire to short (electrically connect) the contacts on the top of the float.

1. Buoy must be turned on and operating (broadcasting or "sleeping"). To turn on the buoy, remove the magnet and wait for at least 60 seconds.
2. Place magnet on the flat area on the bottom of the sphere. After exactly 30 seconds, proceed immediately to the next step, leaving the magnet in place.
3. Short the contacts on the top of the float for the number of seconds corresponding to the desired duty cycle in Table 2.4, then remove the shorting wire. For example, to initiate Mode 1 (8 hours of transmission followed by 16 hours of "sleeping"), short the contacts for between 5 and 10 seconds.
4. Wait at least 30 seconds after removing the shorting wire, then remove magnet.
5. Verify that the desired duty cycle has been set by checking byte 0 in Argos message (see Section 5, DATA CONVERSION).

3. SEA SURFACE TEMPERATURE OPTION.

If the marker buoy has the sea surface temperature option, there will be a stainless steel thermistor probe next to the flat area that covers the reed switch. The PTT controller samples the thermistor once a minute and calculates an average sea surface temperature every 15 minutes. Before the end of the first temperature averaging interval, the PTT reports no temperature. Afterwards, temperature averages are updated every 15 minutes and are reported by the PTT in the second and third bytes of the transmitted data string. Conversion of bytes to temperature is discussed below in Section 5.2, DATA CONVERSION.

4. POWER PACK.

The PTT is powered by a 13.5 VDC 56 AH alkaline pack. There is sufficient power to operate the buoy in the continuous mode for about one year. Table 4.1 shows the available battery power associated with decreasing voltage.

5. ARGOS MESSAGE DATA CONVERSION.

The options installed on a Marker Buoy affect the Technical File submitted to Argos and how the Argos message is read. If you have and uplink receiver programmed to decode Argos messages, refer to your manufacturer's manual for how to access the data message from the buoy .

5.a Argos technical file.

The user should include information in the following Tables, ClearSat Argos Marker Buoy with Sea Surface Temperature and ClearSat Argos Marker Buoy without Sea Surface Temperature in the Argos application. Use the table appropriate to the selected options.

5.b DATA CONVERSION.

The ClearSat Argos Marker Buoy reports a four byte data string, For example:

010 135 147 064

5.c Byte 0, Duty Cycles.

The first byte is duty cycle (see section 2.c, ALTERING BUOY TRANSMISSION CYCLES.).

5.d Byte 1, Battery Level.

The second byte reports battery level. The decimal value is divided by 10 to obtain the battery voltage. See Table 2 Battery Life to relate voltage to remaining useful battery life.

5.e Bytes 2 and 3, Temperature.

The third and fourth bytes are sea surface temperature in degrees Celsius for the last 15 minute sampling interval. Ten bits are needed to obtain an accuracy of ±0.1 °C. Since ARGOS words are eight bits, all eight bits of the third word and the two most significant bits of the fourth word are used to obtain a ten bit number. You can request ARGOS to report the temperature as a decimal and they will perform the conversion, which is described below. Temperature is derived from those ten bits by converting them into a decimal number, or count, which is used in a linear regression equation. The regression coefficients are slope, A1, equals 0.05 and intercept, A0, equals -5.0. Here is an example of how to interpret the temperature reading for the data string 000 000 147 064. To obtain temperature "Counts" convert 147 and 064 to binary numbers:

147 (decimal) = 10010011; 64 (dec.) = 01000000

Concatenate the two most significant bits of the second temperature value to the right hand side of the first temperature value and evaluate as a decimal number:

10010011 concatenated with 01 = 1001001101 (bin) = 589 (dec).

Calculate the temperature from the regression equation:

A1 A0

24.45 (C) = (0.05000) x (589) - 5.000

You may quickly evaluate the temperature data (this method is just as precise as the explanation given above) as follows:

4 x (first data) + 0 (for second data = 000)
4 x (first data) + 1 (for second data = 064)
4 x (first data) + 2 (for second data = 128)
4 x (first data) + 3 (for second data = 192)

Using the previous example:

589 = 4 x 147 + 1

6. Battery Care and Replacement.