SOUNDING INSTRUCTIONS 1978
The primary purpose of these instructions is to establish uniformity in the execution and documentation of bathymetric surveys accomplished with the satellite navigation system aboard MV Cape Pillar. The instructions provide guidelines for experienced personnel engaged in sounding operations and can be used for training new personnel.
II General Description
The aim of the exercise is to relate water depths and ship’s position so that a Bathymetric map can be produced.
The SATNAV system is utilised in sounding operations to provide real time navigation information enabling the navigator to sail along a desired track. At prescribed fix times the echogram is automatically marked and latitude and longitude recorded so that post plotting of manuscripts can be carried out. The system dead-reckons using Doppler sonar velocity information and gyrocompass headings. The dead-reckoning position is updated periodically to correspond to satellite fix positions obtained by Doppler tracking of TRANSIT satellites.
In order to reduce soundings to a common datum - Mean Sea Level (MSL) - tidal information is required in the survey project areas at 1/2 to 1 hourly intervals depending on the tide range. Heights are obtained from BMTRs (Bottom Mounted Tidal Recorder) and applied to the raw echogram readings.
A 7 metre launch equipped with radar and echosounder is used for inshore areas, shoal areas and consort sounding.
A Mini-Ranger, Radio Positioning System is also used on the Cape Pillar and Launch in more complex areas such as around small islands, shoals, etc.
The basic position reference for the SATNAV system is obtained from the U.S. Navy Navigation Satellite System (NNSS) often called TRANSIT. Transit satellites have been in service constantly since January 1964. In 1967, the U.S. Government decided to release details of the system for commercial use, and equipment was available for sale in 1968. The Transit system provides the only means currently available to obtain position fixing in a world geodetic datum.
The Transit satellites, of which there are currently six, are in approximately circular polar orbits at altitudes of about 600 nautical miles. They form a “birdcage” within which the earth rotates, each satellite having an orbital period of approximately 1-3/4 hours. Whenever a satellite passes above the horizon of the user, an opportunity to obtain a position fix exists.
The satellites radiate very stable 400 and 150 megahertz signals which are phase modulated with information of several types. Part of the message is classified military data, while the part referred to as "message set A" contains unclassified navigation data which precisely describes the position of the satellite as a function of time.
As the satellites pass each of the four tracking stations, located in Maine, Minnesota, California, and Hawaii, the changing Doppler frequency shift on the signals transmitted by the satellites is recorded as a function of time. This Doppler data is transmitted to the computing center at Point Muga, California. There, by means of the best current model of the earth's gravity field, the Doppler data is used to determine each satellite's orbit and to predict future orbital positions. The predicted orbital positions are formatted into a navigation message and are provided to one of two injection stations located in Minnesota and California. These stations transmit the navigation message to the appropriate satellite for storage in its memory system. These injections occur approximately every 12 hours.
The satellites travel roughly four miles per second, and this motion with respect to an observer on the earth, causes a Doppler shift in the radiated 400 and 150 MHz signals. This Doppler shift is measured by the 702A receiver providing an indication of the change in slant range between the observer and the satellite. The slant range data and the satellite position data are combined in the navigation computer with estimates of the boat position during the satellite pass to determine the position of the receiver on the earth.
The purpose in transmitting two signals at different frequencies, both of which are Doppler shifted and both of which carry the navigation data, is to minimize the effects of ionospheric interference with the Doppler shift. Since the ionosphere affects the two frequencies differently, the computer program may compensate for ionospheric effect if both Doppler signals are available. There is a sharp increase in position fix accuracy when a dual channel receiver is used compared to a single channel receiver which would receive only one of the two transmitted frequencies.
Satellite position fixing under optimal conditions provides measurements precise enough to be used for geodetic survey purposes. Equipment of this type is capable of fixed site accuracy of 2 meters. This accuracy depends on the tracking of multiple satellite passes and subsequent fix computation based on the collected Doppler counts and tracked satellite orbits, rather than the real-time transmitted satellite orbits.
The Magnavox 702A dual channel receivers are used for real-time computation and are typically capable of RMS accuracy of 32 meters or less.
The navigation message contained in the satellite's memory system is transmitted to the ground as phase modulation on the transmitted carrier frequencies. A complete navigation message is transmitted every two minutes, beginning and ending at the instant of each even GMT minute. By receiving the transmitted signals, the user can obtain the following information needed to compute a position fix :
The Doppler shift on the received signals.
The satellite's navigation message.
Accurate even-minute time marks.
The, Doppler shift is a measure of the change in slant range between the satellite and the satellite receiver. This change is clearly a function of the receiver motion during the pass as well as the satellite motion, and this receiver motion is quite significant in producing an accurate fix. It is therefore necessary that estimates of boat position during a pass be made, and that these estimates of boat motion be used in the navigation solution.
The position fix process consists of comparing the estimated slant range changes between the approximate boat position and known satellite positions with the actual slant range changes defined by the Doppler shift, and iteratively adjusting the boat positions until the best match is obtained between estimated and actual slant range changes. Thus, although the position fix is expressed as a latitude and longitude at a specific time, the fix process in fact adjusts the entire boat path during the satellite pass and produces a "delta latitude" and "delta longitude which may be added to the old estimates of latitude and longitude at some point during the pass to produce a new "fix latitude" and "fix longitude".
The System hardware comprises the following :
Magnavox 702A Satellite Receiver
Magnavox 610 Doppler Sonar
BWD 539 B Oscilloscope
Velocimeter and Thermister
Sperry Mark 227 Gyrocompass
HP 2100 A Digital Computer
HP High-speed Photoreader
CRT Display Unit
Atlas Deso 10 Echosounder
HP 975 Calculator
DNT-2 Tide Recorder
Aanderra Tide Recorder
Power supply 8.5 KVA x 4 KVA Generators
Magnavox 702 A Satellite Receiver
The Magnavox 702 A is an automatic satellite tracking receiver used in conjunction with the US Navy Navigation Satellite System (TRANSIT) to locate a user's coordinate position.
The receiver performs three primary functions :
receiving and decoding messages transmitted by the TRANSIT satellites;
deriving Doppler data from the satellite transmission, and
formatting the satellite message and Doppler data for transmission to the HP 2100 A computer.
Magnavox 610 Doppler Sonar
The measurement of ship velocity is based on a well-known phenomenon of Doppler shift which is a change in signal frequency caused by relative motion between a transmitter and a receiver. With Doppler sonar, which has the transmitter and the receiver located on the same moving platform, the relative motion between ship's hull and the bottom insonified by sonar beam causes a measurable shift in the frequency of the echo.
The equipment transmits pulses of high frequency (150 KHz, duration 55 millisecond) sound in four narrow beams toward the ocean bottom. Signal reflected from the bottom, or from scattering particles in the water when depths are in excess of 300 metres, are recovered, amplified, and tracked in frequency. The Doppler frequency shift of the return signal is a direct measure of velocity. The two pairs of beams give velocity information relative to the fore-aft and port-starboard axes of the ship. Information from opposing beams is complemental and in combination tends to cancel certain error items. The fore-aft, port-starboard velocity vectors are used to compute along-track and drift angle. The course made good is determined by adding the drift angle to the gyrocompass reading.
BWD 539 B Oscilloscope
The oscilloscope is used to display signal returns from the Doppler sonar, enabling the operator to monitor the equipment's performance. A noisy return indicates deep water or ship’s speed too high. The length of return indicates water depth. A short return indicates shallow water or water track; a check with the echosounder verifies mode.
The velocimeter consists of a pump, a small chamber containing a calibrated sound path, and an electronic transmitting and timing unit. In operation, water is pumped in through a gate valve in the hull of the ship to maintain a continuous flow through the chamber. The electronic unit measures the time required for a signal to travel the known distance around the calibrated sound path in the chamber. The velocity of sound in water is thus determined directly. The velocity of sound in water is required to compute ship's velocity, from Doppler sonar data because the measured Doppler shift is a function of velocity of sound in water.
Doppler sonar velocity indication is influenced by tilt of the sonar transducers. Pitch and trim angles affect measurement of forward motion, while roll and list affect sidewise motion. The effect is always a reduction in apparent speed. The inclinometer determines pitch and roll angles for correction of the sonar velocity readings. The equipment consists of two pendulums attached to the transducer shaft so that one pendulum swings in the direction of the fore-aft axis and the other along the port-starboard axis.
Changes in the inclination of the transducer shaft cause the pendulums to move which in turn causes the voltage across two variable resistors to vary. Changes in voltage are transmitted to the computer where they are converted to pitch and roll angles.
Sperry Mark 227 Gyrocompass
The Mark 227 Gyrocompass is essentially a gyroscope to which has been added control devices that make it seek and continuously align itself with the meridian and point to true north. Gyrocompass operation depends on gyroscopic inertia and precession and the earth’s rotation and gravitation.
The gyrocompass is physically aligned with the fore-aft axis of the ship so that gyrocompass readings indicate the direction that the fore-aft axis is pointing. This information is transmitted to the computer where it is added to the drift angle to determine the course made good by the ship.
HP 2100 A Digital Computer
The satellite receiver, Doppler sonar, and gyrocompass are interfaced to a 12K HP 2100 A computer. The computer processes input data from these sensors as it is acquired to compute and output navigation information in real time. The input, computation and output operations are controlled by a comprehensive software package developed by Magnavox. The computer also maintains system timing with inputs from a precise 5 MHz oscillator in the receiver and very accurate time marks received from the TRANSIT satellites.
ASR 43 Teleprinter
The teleprinter is the primary means of communicating with the computer. During sounding operations, navigation information is automatically printed out on the teletype to provide hard copy for post analysis of results. The operator is able to exercise a number of options available in the computer program by typing in the appropriate command on the Teleprinter. It is also possible to load a program into the computer via the Teleprinter. It is also possible to load a program into the computer via the Teleprinter tape reader although this is normally done with the HP High-Speed Photo Electric Tape Reader.
HP High Speed Photo Electric Tape Reader
This equipment is used to enter a program in punch paper tape format into the computer.
CRT Display Unit
Real time navigation information from the computer is displayed on a small CRT unit. The information, which is updated every second, is used to navigate the ship along a desired track.
Atlas Deso 10 Echosounder
The echosounder consists of two units: a recorder and a control unit. Supersonic sound waves are used for measuring the depth. Short sound pulses are emitted by 33 KHz and 210 KHz transducers mounted in the ship’s hull. Part of the energy is reflected by the bottom and returns as an echo to the same transducer which operates as transmitter and receiver thus avoiding angular errors in shallow water. The time between emission of the sound pulse and return of its echo determines the depth. Depths up to 1400 metres can be measured.
This is basically a crystal oscillator and counter which provides accurate timing for triggering the echosounder at fix time. The Automark is syncronised with the SATNAV clock on an even hour and should not need further adjustment unless a timing drift of more than a few seconds occurs.
This instrument is mounted on the launch and used to provide ranges between the launch and reference position e.g. ship, ground mark etc.
The Miniranger consists of a Receiver-Transmitter Assembly with Omnidirectional Antenna, a Range Console and two Radar Transponders with Sector Antennas. It operates on the basic principle of pulse radar and uses a C-band transmitter unit (located on the mobile unit) to interrogate the two transponders. The elapsed time between the transmitted interrogation produced by the transmitter and the reply received by the unit from each transponder is used as a basis for determining the range in metres to each transponder. The system is line of sight, displays ranges directly to an accuracy of about 5 metres and can range to about 50 nautical miles.
HP 97S Calculator
The calculator is based around the HP-97 programmable printing calculator and uses BCD interfacing to gather data from a wide range of instruments. 224 program steps are available. The HP 97S is interfaced with the Miniranger to provide real-time positioning. It is also used to adjust dead-reckoning positions at Satellite update time.
DNT 2 Tide Recorder
The tide recorder is a completely self-contained instrument which records pressure variations in the water caused by tidal changes. The underwater case is made of ABS plastic, and houses the power supply, pressure transducer, electronic unit, crystal clock and chart recorder. The recorder is capable of operating for about 6 weeks in water depths of approximately 20 metres. As the instrument has to be level on the seabed, the water depth is governed by the depth that divers can operate in. A graduated paper chart roll containing tide trace and hourly time marks is detailed from this instrument. Hourly tide heights can be extracted and MSL calculated in the field.
TGR-3 Aanderra Instruments Depth Recorder
The Aanderra TGR-3 tide recorder is a self-contained depth recorder that can take one depth, temperature, and time recording each hour for a year in 270 metres of water.
The depth is detected by pressure changing the natural resonant frequency of a quartz crystal. Temperature is detected the same way, and all readings are recorded on a ¼ inch magnetic tape which is later removed and played back directly into a computer for decoding and application of the calibration constants of the, quartz sensors. Hand decoding is not considered feasible because the recordings are all coded in binary, and the number of recordings per hour can be only set at binary multiples of four hours down to about 3.75 minutes. Real-time data can be printed out on a specially adapted Cannon calculator, but this is only used to see that the instrument is still operating correctly when recovered.
The pressure case, is made of special corrosion-resistant alloys and it is covered with a special plastic. To achieve the small size, small current drain, and high density, low speed recording, nearly all the mechanical parts are specially made. The instrument does not have to be levelled on the seabed and can be lowered from the ship without divers.
Power Supply 8.5 KVA x 4 KVA Generator
The ship’s power supply is 220 DC and great care must be taken so that no NATMAP equipment is connected to the main supply. AC power for the operations room is supplied by an 8.5 KVA motor generator. A 4 KVA motor generator is available as a standby unit. Power is monitored and controlled from the operation room. Any power problems should be reported immediately to the Electronics Technician and the Chief Engineer.
Starting up order :
Receiver Self Test
Computer Switch Options
Velocimeter and Thermister
DNT 2 Recorder
STARTING UP PROCEDURES
This should be done while the ship is tied up. The equipment should be set up in the following order as the gyrocompass and the receiver oscillator take 4-6 hours to stabilise.
240 AC power supply on
Power to Magnavox console - receive on standby
Power to Gyrocompass
Positioning of sonar transducer in sea chest