OPERATION MANUAL

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

TABLE OF CONTENTS

1. CLEARWATER ARGOS/GPS MARKER BUOY.

2. OPERATION.

   1. Starting and Stopping Buoy Transmissions.

   2. Altering Buoy Transmission Cycles

3. SEA SURFACE TEMPERATURE OPTION.

4. GPS OPTION.

5. POWER PACK.

6. ARGOS MESSAGE DATA CONVERSION.

   1. Argos technical file.

   2. Data Conversion for GPS Bytes 1 - 24.

      1. GPS Argos message.

      2. General information: Bytes 1 - 4.

      3. Date Bytes: 5 - 8

      4. Universal Time Bytes: 9 - 12.

      5. Latitude Bytes: 13 - 16.

      6. Longitude Bytes: 17 - 20.

      7. Checksums Bytes: 21-24.

   3. Data Conversion for Temperature (Optional) Bytes: 25-28.

   7. BATTERY CARE AND REPLACEMENT.

1. ARGOS/GPS MARKER BUOY.

The ClearSat Argos/GPS 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 from the buoy are received by CLS/Argos uplink receivers 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 email 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 Tiros 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.

Locations of the GPS Marker Buoy are provided by CLS/Argos and by a GPS (Global Positioning System) engine controlled by the marker buoy's microprocessor. Buoy locations are calculated and disseminated by CLS/Argos; Argos locations are accurate to between 300 and 1000 m, depending on the quality of the location. The GPS engine obtains locations which are transmitted by the buoy's radio in an Argos message. GPS locations are generally accurate to less than 100 m. By using transmitted information from CLS/Argos, the user can employ Argos or GPS positioning to follow the buoy. If the user comes within 10 km of the buoy, s/he may use an Argos uplink receiver to obtain transmissions directly from the buoy. Transmissions received in real time include the latest GPS position, which may be used to guide the user to the buoy.

2. OPERATION

Buoy operation and Argos PTT transmissions are controlled by a magnetic reed switch inside the buoy hull, 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 is used to control 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.

2.a STARTING AND STOPPING BUOY TRANSMISSIONS.

The marker 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 marker 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 2.2. 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 a length of wire to short (electrically connect) the contacts on the top of the float.

• 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.
• 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.
• Short the contacts on the top of the float for the number of seconds corresponding to the desired duty cycle in Table 2.2, 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.
• Wait at least 30 seconds after removing the shorting wire, then remove magnet.
• Verify that desired duty cycle has been obtained by checking byte 0 in Argos message (see Section 6, ARGOS MESSAGE 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 6, DATA CONVERSION.

4. GPS Option.

The GPS (Global Positioning System) option allows the buoy to acquire and transmit highly accurate positions using the GPS satellite system, which is operated by the U.S. government. GPS positions are obtained approximately every 30 minutes, after which the Argos message is updated to reflect the most recent position. The Argos message reports the year, month, date, UT (Universal Time, accurate to 1 second), and latitude and longitude to +/-0.0001 degree. This close location measurement affords full GPS accuracy (approximately 20 meters) when selective availability is in force. Conversion of the Argos message to date, time and position is discussed below in Section 6, ARGOS MESSAGE DATA CONVERSION.

5. Power Pack.

The Argos Marker Buoy PTT is powered by a 13.5 VDC 56 AH alkaline pack. Marker buoys with GPS are powered by a 12 VDC 112 AH alkaline pack. Each internal parallel unit is diode-protected so that if one of its cells fails, the other packs will not discharge through the failed pack. Table 4.1, Battery Life, shows the available battery power associated with decreasing voltage.

6. ARGOS MESSAGE DATA CONVERSION.

The options installed on a marker buoy affect the technical file submitted to Argos, as well as how the Argos message is read. If you have an uplink receiver programmed to decode Argos messages, refer to your manufacturer's manual for how to access the data message from the buoy.

6.a ARGOS Technical File.

The user should include information in the following Table GPS/Argos Technical File in the Argos application.

6.b Data Conversion for GPS Bytes 1 - 24.

Marker buoys equipped with the GPS option report in their Argos message the most recent information obtained from the onboard Argos GPS engine. The data report the year, month, day and UT time, and the latitude and longitude. The information is updated approximately every hour. A general evaluation of the format is presented in Table Argos/GPS Data Format.

6.b.i GPS Argos message.

A typical Argos data message from the GPS Marker Buoy, with each byte converted to decimal, might look like this:

008 115 001 000
096 001 010 017
018 057 040 025
000 042 036 078
001 071 019 094
019 036 035 027
000 000 147 064

The interpretation of the data is summarized in Table 6.1.

6.b.ii General information: Bytes 1 - 4.

008 115 001 000

Byte 1 = 008, Transmission duty cycle is 8 hour on 16 hours off.

Byte 2 = 115. Battery voltage is 11.5.

Byte 3 = 1. Figure of merit is 1. Figure of Merit is a measure of convergence of

Table GPS/Argos Technical File

Table Argos/GPS Data Format

navigation solution which is calculated by the GPS engine. Typical results are between 1 and 3:

1 > 7.5 meters
2 > 7.6 to 12.5 meters
3 > 12.6 to 25 meters
4 > 26 to 50 meters
5 > 51 to 250 meters
6 > 251 to 500 meters
7 > 500 meters or more

Byte 4 = 0. There are no error messages.

6.b.iii Date Bytes: 5 - 8

096 001 010 017

Year = 1996, Month = 01 (January), Day = 10 Checksum (2) = 17
or, 01-10-1996. To evaluate the checksum:

• Convert each bytes 5, 6 and 7 to hexadecimal: 60_H, 1_H, A_H
• Sum the digits: 6_H + 0_H + 1_H + A_H = 11_H
• Convert to decimal: 11_H = 17. Data are verified because checksum is 17.

6.b.iv Universal Time Bytes: 9 - 12.

018 036 046 025

Hours = 18, Minutes = 36, Seconds = 46, Checksum(3) = 25
or, 18:36:46. Evaluate the checksum as in Section 6.1.4.

• Convert each bytes 9, 10, and 11 to hexadecimal: 12_H, 24_H, 2E_H
• Sum the digits: 1_H + 2_H + 2_H+ 4_H + 2_H + e_H = 19_H
• Convert to decimal: 19_H = 25. Data are verified because checksum is 25.

6.b.v Latitude Bytes: 13 - 16.

000 042 036 78

North (0), 42.3678^o

6.b.vi Longitude Bytes: 17 - 20.

001 071 019 092

West(1) 71.1992^o

6.b.vii Checksums Bytes: 21-24.

019 032 033 021

Checksum(1) = 19, Checksum(4 = 32), Checksum(5) = 33, Total Checksum = 21

Checksum(1) is evaluated by:

• Convert bytes 1, 2, 3, and 4 to hexadecimal:
• 008 115 001 000 become 8_H, 73_H, 1_{H ,} 0_H
• Sum the digits: 8_H + 7_H + 3_H+ 1_H + 0_H = 19_H
• Convert to decimal: 19_H = 25. Data are verified because checksum is 25.

Checksum(4) is evaluated by:

• Convert bytes 13, 14, 15, and 16 to hexadecimal:
• 000 042 036 78 become 0_H, 2A_H, 24_{H ,} 4E_H
• Sum the digits: 2_H + A_H+ 2_H + 4_H + 4_H+ E_H = 24_H
• Convert to decimal: 24_H = 36. Data are verified because checksum is 36.

Checksum(5) is evaluated by:

• Convert bytes 17, 18, 19, and 20 to hexadecimal:
• 001 071 019 092 become 1_H, 47_H, 13_{H ,} 5C_H
• Sum the digits: 1_H + 4_H + 7_H+ 1_H + 3_H + 5_H+ C_H = 33_H
• Convert to decimal: 19_H = 25. Data are not verified because checksum is 33.

Total Checksum is evaluated by:

• Convert checksum(1), checksum(2), checksum(3), checksum(4), and checksum(5), or bytes 21, 8, 12, 22, and 23 to hexadecimal:
• 19 17 25 36 35 become 13_H, 11_H, 19_{H ,} 24_H, 23_H
• Sum the digits: 1_H + 3_H + 1_H+ 1_H + 1_H + 9_H + 2_H+ 4_H + 2_H+ 3_H = 1B _H
• Convert to decimal: 1B_H = 27. Data are verified because the total checksum, Byte 24, is 27.

6.c Data Conversion for Temperature (Optional) Bytes: 25-28.

Bytes 25 and 26 are reserved and will be zero. Bytes 27 and 28 report temperature, if this option is selected. Ten bits are used to obtain the accuracy (±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 words by converting them into a decimal number, or count, which is used in a linear regression equation. Regression coefficients for your drifters are A₁ = slope and A₁ = intercept.

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 where the coefficients are:

A₁ = 0.05 and A₀ = -5.00

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

You may quickly evaluate the temperature data (this way 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: