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491 Echo Sounder Operations Manual

Welcome to the Northstar 491

Congratulations on your purchase of the Northstar 491 Echo
Sounder. The 491 connects with a Northstar navigator and a
third-party transducer to add echo sounding capability to the
Northstar 957, 958, 961, and 962 navigators. See the

Northstar

491 Installation Manual

(part number GM495) for

recommended transducers.

You may be adding the 491 to your existing Northstar
navigator for echo sounding, or you may have purchased your
navigator complete with echo sounding. In either case, the 491
should be installed and configured by a qualified marine
technician.

To get started right away, Section 2, ”Quick Start: Using the
Echo Sounder,” starting on page 2-11.

Using this manual

Organization

Chapter 1 introduces you to the 491 echo sounder and to this
operations manual.

Chapter 2 gives you a quick-start introduction to get you
started with echo sounding as soon as the 491 has been
installed and configured.

Chapter 3 tells how to adjust the echo sounder’s display—the
gain, clutter, colors, etc.

CAUTION!

Proper installation and configuration of your Northstar
491 is of utmost importance for accurate and effective
performance under a variety of conditions. To get the
best performance from your 491, Northstar strongly rec-
ommends installation and set-up by a qualified marine
technician. For installation procedures, see the Northstar
491 Echo Sounder Installation Manual (Part Number
GM495).

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Summary of Contents for 491

Page 1: ...7 958 961 and 962 navigators For the Northstar 6000i information in Section 6 of its manual can be used instead Revision A Part Number GM494 Northstar a unit of Brunswick New Technologies Marine Electronics 30 Sudbury Road Acton Massachusetts 01720 www NorthstarNav com Service 978 897 6600 Sales 800 628 4487 ...

Page 2: ... Adjusting the Echo Sounder Picture 27 The picture menu 28 Adjusting the gain strong echoes 28 Adjusting the clutter weak echoes 29 Adjusting the signal level weak echoes 30 Suppressing interference the noise limiter 31 Selecting the color bar hue and white level 32 4 Understanding Depth Settings 35 Viewing the seabed depth 36 Viewing the depth marker 36 Changing the depth settings 36 Setting the ...

Page 3: ...s using SoundTrac 52 Marking fish as waypoints 53 Navigating to a past echo position 53 7 Setting the Alarms 55 Understanding the alarms 56 Setting the fish alarm 56 Setting the bottom alarm 58 Setting the temperature alarm 60 8 Troubleshooting 491 Operations 63 Troubleshooting the echo sounder 64 Checking the transducer 65 ...

Page 4: ......

Page 5: ... 1 2 Technical support 1 4 How echo sounding works 1 5 Powering the 491 on and off 1 7 Setting up the 491 1 7 This chapter introduces the Northstar 491 Operations Manual explains how to contact Northstar for technical support and service and describes the 491 s basic set up and power on off procedures ...

Page 6: ...n To get started right away Section 2 Quick Start Using the Echo Sounder starting on page 2 11 Using this manual Organization Chapter 1 introduces you to the 491 echo sounder and to this operations manual Chapter 2 gives you a quick start introduction to get you started with echo sounding as soon as the 491 has been installed and configured Chapter 3 tells how to adjust the echo sounder s display ...

Page 7: ...pecific conventions are used to provide a consistent way to recognize certain information as follows is the universal symbol used for caution and warning information that pertains to your personal safety or pre vents damage to the 491 The CAUTION and WARNING boxes contain critical information please read them NOTE is the type style used for titles of reader notes These notes contain valuable infor...

Page 8: ...t to the Northstar factory for diagnosis and repair When describing an operating problem be as complete and accurate as possible Be sure to have the serial number available whenever contacting or corresponding with either your dealer or the Northstar factory The 491 s serial number is located at the top of the unit If you need help feel free to call the Northstar Service Department at 978 897 6600...

Page 9: ... your 491 How echo sounding works An echo sounder produces high voltage electrical pulses that the transducer converts into sound waves called pings The sound waves are sent downward through the water When a sound wave hits and reflects off an object such as fish debris seaweed or the seabed the reflected sound wave an echo returns back to the transducer along an imaginary vertical line called a w...

Page 10: ... 958 6000i Navigator 491 module GPS antenna Transducer Ship s Power 10 foot cable Northstar supplied 10 foot cable Northstar supplied 33 foot cable part of transducer 961 962 Processor Control Head 491 module GPS antenna Transducer Ship s Power 10 foot cable Northstar supplied 10 foot cable Northstar supplied 33 foot cable part of transducer ...

Page 11: ...des setting up the following options the transducer s depth the speed and temperature the calibration for the speed sensor the calibration for the temperature sensor the gain calibration for the transducer You may wish to change the following options from time to time to match your personal preferences depth units feet meters or fathoms on the screen temperature units Celsius or Fahrenheit on the ...

Page 12: ...t now Using the 491 with the 957 958 Before starting you should double check that the navigator s AUX port is set to the Sounder option 1 Press STAR until you reach the OPTIONS SERVICE INFO screen 2 Press the PORT SETUP OPTIONS key 3 Check that the AUX PORT option is set to Sounder If not set it now ...

Page 13: ...HO SCREEN Gain Clutter Noise Limiter Hue Signal level Return Shift Range Bottom box Return Alarms Advance Speed Snapshot A Scope Data Return Fish alarm Bottom alarm Temp alarm Return Bottom box Temp box Temp graph Speed Return Picture settings Depth settings More settings Alarm settings Data on screen Display Rename Archive Erase Return Snapshot ...

Page 14: ...ore ECHO SCREEN Gain Clutter Noise Limiter Hue Signal level Return Shift Range Bottom box Return Alarms Advance Speed Video A Scope Data Return Fish alarm Bottom alarm Temp alarm Return Temp box Temp graph Bottom box Speed Return Picture settings Depth settings More settings Alarm settings Data on screen Video on Return Video setup ...

Page 15: ...s picture 2 12 Using demo mode 2 14 Introducing the displayed information 2 15 Using automatic mode 2 16 Changing the frequency of echoes 2 17 Zooming in and out 2 19 Setup functions 2 22 This chapter shows you how to start using the echo sounder and explains several of the echo sounder s important features ...

Page 16: ... To activate SoundTrac press the CURSOR PAD left or right on the ECHO screen The position marker appears controlled by the CURSOR PAD and movement of echoes to the left stops The data boxes at the bottom of the screen turn white and refer to the position marker rather than to your current position Press the green LIVE DATA key to return to Depth Scale Depth box Water temperature box Water temperat...

Page 17: ...ou must press STAR to clear alarms that don t automatically clear The SAVE and MOB functions always work normally Moreover on the 961 962 whenever you press either of these keys the corre sponding SAVE or MOB waypoints will be displayed automatically on the echo sounder s picture as well as on the CHART screen after you press CHART Depth Scale Depth box Water temperature box Water temperature scal...

Page 18: ...ave demo mode repeat this procedure selecting OFF instead of ON Displaying echo sounder data on the CHART screen You can overlay a smaller version of the echo screen onto the CHART screen For the 961 962 To overlay the echo screen 1 Press CHART to display the CHART screen then press OVERLAY 2 At the CHART OVERLAY SELECTION window press SHOW 490 Both chart views A and B will display live echo sound...

Page 19: ...ale on the right side depth marker press the CURSOR PAD to scroll the marker down water temperature digital display if temperature sensor is connected can be turned off water temperature graph if temperature sensor is con nected can be turned off Navigation vessel s current lat lon on the SOUNDTRAC screen this lat lon indicates the position marker s location vessel s SOG and COG active waypoint if...

Page 20: ...ter settings You can always press AUTO on the ECHO screen to remove any manual settings you ve made and return to the standard automatic configuration Cruising or fishing Automatic mode has two variations autocruising and autofishing autocruising optimizes the gain and clutter for tracking the bottom with less emphasis on any fish in the water autofishing optimizes the gain and clutter for searchi...

Page 21: ...ll screen 200 kHz displayed on a full screen 50 kHz on the left half of the screen and 200 on the right 200 kHz on the left half of the screen and 50 on the right There are two reasons why you might want to operate with your preferred frequency on the right side of the screen the zoom function magnifies the frequency on the right side Table 2 50 kHz vs 200 kHz 50 kHz 200 kHz Longer wavelength goes...

Page 22: ...frequencies in use are shown pictorially on the tab just below the FREQUENCY key When you change the frequency echoes already on the screen don t change they just move off the screen as they are replaced by echoes at the new frequency A dual frequency screen is split into two halves separated by a gray vertical line in the center Adjusts the frequency of the data displayed currently showing dual f...

Page 23: ... water seen You can choose from any of six depth scales for the magnified echoes These scales cover the same depths regardless of the scale of the original echoes The available scales are 15 30 60 120 240 and 480 when using units of feet Similar scales are available for the other depth units NOTE When zoom is in use only a single frequency is displayed Unzoomed echoes are on the right and the same...

Page 24: ...seabed and displays it on the lower left side of the screen normal echoes are displayed on the right side If the seabed moves out of the lower half of the screen for example if you travel into deeper water the 491 automatically repositions the bottom back into the lower half You ll see the zoomed image jump up or down from time to time as the 491 keeps the seabed displayed on the screen Using bott...

Page 25: ...t and displayed in the center of the screen You can see any fish near the bottom as with the bottom lock zoom and also examine the seabed s composition Using marker zoom When you select marker zoom you can choose the part of the water column to magnify The top edge of the magnified area is determined by the location of the depth marker To move ...

Page 26: ...u have cycled through all the zoom modes Setup functions Several setup functions are available to configure the echo sounder to your preferences CAUTION Be careful not to change any of the functions that are not described here unless you first read about them and understand their functions in the Northstar 491 Installation Manual Accessing the setup screen To access the ECHO SOUNDER SETUP screen p...

Page 27: ... setup screen 957 958 echo sounder setup screen Entering numerical values When changing the numerical value of the echo sounder s settings you can either press the CURSOR PAD to increase or decrease the value or use the KEYPAD to manually type in the desired value ...

Page 28: ...the temperature graph 1 Press the CURSOR PAD up or down to highlight the line containing the temperature scale 2 Press EDIT 3 Press the CURSOR PAD left or right to highlight C or F as desired then press ACCEPT 4 To change the endpoints of the scale press EDIT DATA 5 Enter the lower temperature limit using the KEYPAD 6 Press the CURSOR PAD to the right to highlight the upper limit and enter its val...

Page 29: ...ange The line Fish controls fish waypoints The line Therm controls temperature waypoints The line Mark Wpts controls waypoints marked by the user 2 Press EDIT 3 Press the CURSOR PAD left or right to highlight the desired number of waypoints to be saved 4 Press ACCEPT Speed and temperature sensor setup These functions are set during installation The calibrations should not be changed unless they ar...

Page 30: ... an option Speed calibration is entered as a percentage from 99 to 99 to make the speed read lower or higher than the uncalibrated value NOTE You must enable the speed setting under the Data menu on the ECHO SOUNDER screen see Setting the ECHO screen data starting on page 5 44 Temperature is calibrated by entering the temperature error as a number of degrees from 99 to 99 This offset is added to t...

Page 31: ... 28 Adjusting the clutter weak echoes 3 29 Adjusting the signal level weak echoes 3 30 Suppressing interference the noise limiter 3 31 Selecting the color bar hue and white level 3 32 This chapter explains the different ways you can adjust the echo sounder s picture including gain clutter the noise limiter white level and signal level ...

Page 32: ...anges from green to yellow Changing the hue or adjusting the signal level has no effect on the operating mode the tabs stay green Adjusting the gain strong echoes The gain setting selects the echo strengths that will be displayed using the top color in the color bar the strongest echoes Any echoes stronger than this selection will use the same color and no detail can be distinguished The gain is u...

Page 33: ... adjust the gain to obtain the best looking picture also watch the real echoes on the right side of the screen 4 Press RETURN to go back to the ECHO screen When you adjust the gain the color of the AUTO and PICTURE key tabs changes from green to yellow since the echo sounder is no longer in automatic mode Also a yellow tab appears under the DEPTH key indicating that the range setting is no longer ...

Page 34: ... press CLUTTER 3 Press the CURSOR PAD left or right to adjust the clutter or use the KEYPAD to manually enter the desired clutter value The available settings range from zero minimum to nine maximum Clutter window 4 Press RETURN to go back to the ECHO screen When you adjust the clutter the color of the AUTO and PICTURE key tabs changes from green to yellow since the echo sounder is no longer in fu...

Page 35: ...the same color 4 Press RETURN to go back to the ECHO screen Suppressing interference the noise limiter The noise limiter reduces the adverse effects of interference from other nearby echo sounders or electronic devices however the noise limiter may cause some very weak echoes to be made smaller or entirely eliminated since it removes any echoes that aren t sustained for at least two pings To turn ...

Page 36: ...tes of colors to represent the echo strengths from weakest to strongest Color bar 1 is the one most often used Color bar 2 is similar but with fewer colors to aid visibil ity under some conditions Color bars 3 8 are additional variations with different background colors Color bar 9 uses only a single color ranging from very dark to very bright Selecting the color bar hue To select the color bar 1 ...

Page 37: ...tting the white level You can replace any single color in the color bar with white On the HUE AND WHITE LEVEL window press the CURSOR PAD up or down to select that color To remove the white level and return to using all the colors on the color bar press the CURSOR PAD down until the white level moves off the color bar ...

Page 38: ...Page 34 491 Echo Sounder Operations Manual ...

Page 39: ...ewing the depth marker 4 36 Changing the depth settings 4 36 Setting the picture s shift starting depth 4 37 Setting the picture s range 4 38 This chapter explains how to override the system s auto matic depth settings and manually control the depth range shown on the screen and how to use the depth marker ...

Page 40: ...wn press the CURSOR PAD 3 Press RETURN to go back to the ECHO screen The depth box can also be adjusted by pressing MORE then pressing DATA Viewing the depth marker You can use the depth marker to accurately measure the depth of an echo To display the depth marker a dashed horizontal line and move this marker up or down press the CURSOR PAD down Attached to the depth marker line is a small black b...

Page 41: ...The shift setting adjusts the depth of the top edge of echoes pictured on the ECHO screen You can shift the entire pic ture up and down to start at the surface of the water or any other depth you choose To shift the picture 1 At the ECHO screen press DEPTH 2 Press SHIFT Shift window The SET SHIFT window shows a picture of the current shift settings and range The large white number in the black box...

Page 42: ...depth at which the echo sounder picture starts relative to the surface of the water 3 Press the CURSOR PAD up or down to shift the picture or use the KEYPAD to enter the digits of the desired depth If you type the number on the keypad wait a moment for the system to enter the number 4 Press RETURN to go back to the ECHO screen Adjusting shift changes the AUTO and DEPTH key tabs from green to yello...

Page 43: ...ative to the depth of the water The white number in the black box indicates the current range setting 3 Press the CURSOR PAD up or down to adjust the range or use the KEYPAD to manually enter the desired depth 4 Press RETURN to go back to the ECHO screen Adjusting the range changes the AUTO and DEPTH sta tus tab from green to yellow since the echo sounder is no longer in fully automatic mode ...

Page 44: ...Page 40 491 Echo Sounder Operations Manual ...

Page 45: ...e MORE key 5 42 Changing the advance speed 5 42 Displaying the A scope 5 43 Setting the ECHO screen data 5 44 Introducing snapshots 5 46 Setting up the video image overlay 5 49 This chapter describes all of the functions of the MORE key except for the alarms which are covered in Chapter 7 ...

Page 46: ...which the vertical scan lines move from right to left across the ECHO screen There are five advance speeds plus the STOP setting which freezes the advance of all data 2 1 two identical lines of data are displayed for every ping 1 1 one line of data is displayed for every ping 1 2 one line of data is displayed for every two pings 1 4 one line of data is displayed for every four pings 1 8 one line o...

Page 47: ... scope The echo sounder s A scope lets you look closely at present or past echoes by horizontally magnifying the echo from each ping To display the A scope 1 Press MORE 2 Press A SCOPE to turn the A Scope on or off A scope on single frequency echo screen The A scope appears on the far right hand side of the screen magnifying the water column designated by the position marker ...

Page 48: ...screen A scope on dual frequency echo screen Using the A scope with zoom mode If you re in zoom mode the A scope magnifies the unzoomed echoes on the right side of the screen Setting the ECHO screen data Screen data refers to numeric and graphic information displayed on top of the echoes including bottom depth water temperature and speed through the water STW To access these functions press MORE a...

Page 49: ...ess the CURSOR PAD up and down to set the position of the box at the left edge of the screen Temperature box Press TEMP BOX key to set the size and location of the box displaying the water temperature white numbers on a black background Press this key again to cycle through the available sizes off small medium large and huge Press the CURSOR PAD up and down to set the position of the box at the le...

Page 50: ...You can store up to 10 different snapshots of echo sounder screens for later review These can be stored on floppy disk one snapshot per floppy or zip disk for permanent archiving The snapshot feature is not available in the 957 958 or 6000i Taking and storing a snapshot To take a snapshot press MORE and then SNAPSHOT The echo sounder screen will be recorded at that instant To store the snapshot yo...

Page 51: ...he desired slot and press ERASE You ll be asked to confirm erasure by pressing YES The slot will be erased to make room for a new snapshot Archiving and restoring a snapshot with a floppy disk If your 961 962 has a floppy disk drive installed you can save one snapshot on each disk To save a stored snapshot onto a floppy disk from the SNAPSHOT screen press the CURSOR PAD to highlight the desired sl...

Page 52: ...ears displaying the 10 slots available on the zip disk Archiving snapshots screen 961 962 only Press the CURSOR PAD to select an empty slot on the disk and press OK The snapshot will be written to that slot on the zip disk To restore a snapshot from a zip disk to any empty slot from the SNAPSHOT screen press the CURSOR PAD to highlight the desired empty slot and press RESTORE A new screen appears ...

Page 53: ...e ECHO SOUNDER screen first press MORE then press VIDEO To change the transparency of the image while you re displaying the ECHO SOUNDER screen go to a different screen press the navigator s IN or OUT keys then return to the ECHO SOUNDER screen To change the size and placement of the image on the ECHO SOUNDER screen press the STAR key to display the navigator s VIDEO SETUP screen then follow the i...

Page 54: ...Page 50 491 Echo Sounder Operations Manual ...

Page 55: ...ng past echoes using SoundTrac Viewing past echoes using SoundTrac 6 52 Marking fish as waypoints 6 53 Navigating to a past echo position 6 53 This chapter describes how to use SoundTrac to view past echoes and how to navigate to any past echo positions ...

Page 56: ... speed accelerates to quickly scroll to the desired location SoundTrac screen The lat lon of the position marker is displayed at the bottom of the picture along with the time elapsed since the ping and the distance and bearing from your present position to the marker assuming the navigator is providing position data As you move the position marker left and right these coordinates change accordingl...

Page 57: ... press the CURSOR PAD to the left to move the position marker a dashed vertical line to the position of the echo 2 Press MARK A MARK waypoint immediately appears on the SOUNDTRAC screen and on the CHART screen Each marked echo becomes a waypoint named MARK stored in the waypoint database You can perform all of the usual waypoint functions including going directly to this waypoint adding it to the ...

Page 58: ...n data 1 On the SOUNDTRAC screen press the CURSOR PAD left or right to move the position marker to the desired posi tion then press GO TO The SELECTED CURSOR POSITION window appears 2 Press GO NOW The navigator creates a new temporary waypoint called COORD and returns to the SOUNDTRAC screen The COORD waypoint isn t stored in the waypoint database instead it s placed in the trip where it replaces ...

Page 59: ...g the alarms 7 56 Setting the fish alarm 7 56 Setting the bottom alarm 7 58 Setting the temperature alarm 7 60 This chapter describes the echo sounder s three alarm functions for fish bottom and temperature conditions the settings for each alarm and how to turn them on and off ...

Page 60: ... to echo sounding you can set three alarms fish alarm seabed bottom alarm temperature alarm NOTE You also can set the echo sounder to automatically save FISH or THERM waypoints when fish or a certain temperature are detected if the fish or temperature alarms are on To automatically save these waypoints see Number of special waypoints starting on page 2 14 Setting the fish alarm When the fish alarm...

Page 61: ...l from 1 to 15 as represented on the vertical color bar Echoes received that are strong enough to be within the crosshatched area on the vertical bar will cause an alarm weaker echoes will be ignored Setting the upper and lower marker depths The fish alarm will sound only when there are echoes inside or optionally outside the upper and lower markers These markers are displayed on the ECHO screens ...

Page 62: ...f you set these markers as outside markers the fish alarm sounds when fish are detected outside of these markers To set the markers to inside or outside at the FISH ALARM window press INSIDE OUTSIDE MKRS to toggle between the inside and outside settings The setting is displayed in the tab below the INSIDE OUTSIDE MKRS key Turning the fish alarm on and off To turn the alarm on at the FISH ALARM win...

Page 63: ...s depth the bottom alarm will sound To set the bottom level at the BOTTOM ALARM window press the CURSOR PAD up or down to set the depth or use the KEYPAD to manually enter the desired depth NOTE The 491 s depth indicator reflects the depth below your vessel s transducer not below the keel Ask your installer for the exact dis tance between your transducer and your keel This value must then always b...

Page 64: ...s and the alarm is reset Setting the temperature alarm When the temperature alarm is set an alarm sounds if the echo sounder detects that the water temperature has either risen above or dropped below the limit you ve set To set the temperature alarm 1 At the ECHO screen press MORE 2 Press ALARM 3 At the ALARM screen press TEMP ALARM The options on the TEMPERATURE ALARM window are on off upper and ...

Page 65: ...dow is highlighted in black 2 Press the CURSOR PAD up or down to set the temperature or use the KEYPAD to enter the desired temperature The temperature is displayed in the tab just below the SET LOWER LIMIT key Setting inside or outside temperature limits If you set the upper and lower temperature limits as inside limits the temperature alarm sounds when the temperature is detected within these li...

Page 66: ...Page 62 491 Echo Sounder Operations Manual The alarm When the temperature alarm sounds an alert message is displayed This message clears automatically after 20 seconds and the alarm is reset ...

Page 67: ...Manual Page 63 8Troubleshooting 491 Operations Troubleshooting the echo sounder 8 64 Checking the transducer 8 65 This chapter explains how to troubleshoot the echo sounder system before returning the equipment for factory service ...

Page 68: ...oftware Change the Advance Speed setting Configure NMEA Port 3 for the 491 See Section 1 of this manual for instructions 1 Check fuses and circuit breakers 2 Check power to 491 with a voltmeter 3 Verify that the data cable between the 961 962 and the 491 is properly wired and connected 491 is communicating with the navigator 1 Check transducer connector for the proper wiring 2 If wiring is correct...

Page 69: ...Configure the navigatorʼs Aux Port for the 491 See Section 1 of this manual for instructions 1 Check fuses and circuit breakers 2 Check power to 491 with a voltmeter 3 Verify that the data cable between the navigator and the 491 is properly wired and connected 491 is communicating with the navigator 1 Check transducer connector for the proper wiring 2 If wiring is correct check transducer impedanc...

Page 70: ...acturer directly for instructions Maintaining a transducer Do not expose a transducer to gasoline or clean it with strong solvents These can penetrate and degrade the housing If a transducer will be exposed to saltwater coat it with anti fouling paint to prevent sea growth To clean a transducer of sand debris or sea growth use a stiff brush or putty knife Use sanding sparingly as repeated sanding ...

Page 71: ... 200kHz 600W Alarms Fish seabed temperature Update rate 10 Hz maximum depends on water depth Auto modes Fishing and cruising Zoom modes Marker bottom bottom lock bottom lock center Features Mark present or past echo as waypoint 961 962 Go directly to any past echo A scope analyzes present or past echoes Navigation data shown at bottom of screen Demo mode for training ...

Page 72: ...rthstar 961 962 GPS Chart Navigator Northstar 957 958 6000i GPS Chart Navigator Northstar recommended or compatible transducer Other Transducer outputs protected against open circuit short circuit LED monitors input power and system status Physical characteristics Dimensions 8 78 x 7 22 x 2 35 inches Weight 2 pounds Environmental characteristics Temperature 25ºC to 60ºC operating temperature 55ºC ...

Page 73: ...single color Other Screen color palette Depth marker Position marker Zoom in out Shift Ranges 15 30 60 120 200 400 800 1000 2400 feet 5 10 20 40 80 150 300 500 800 meters Advance speed 1 8 to 2 1 Depth units Feet meters fathoms Installation calibrations Include speed temperature gain and transducer type Table 4 491 echo sounder features Continued ...

Page 74: ...Page 70 491 Echo Sounder Operations Manual ...

Page 75: ...bject or seabed is detected by a sound wave echo sounder An instrument that uses sound waves to measure the depth of a body of water or an object such as a school of fish below the water s surface fathom A depth measurement that equals six feet or 1 83 meters of water gain The amount of amplification given to the received echo before it is displayed GPS Global Positioning System hue The gradations...

Page 76: ...al mile is about 800 feet more than a statute mile One nm is equal to 1 minute of latitude ping A pulse transmitted from the 491 echo sounder Also the act of transmitting a pulse from the echo sounder pinging SOG Speed Over Ground The actual speed of your vessel relative to the earth not the water tide The vertical rise and fall of water caused by the gravitational pull of the moon and sun transdu...

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