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Text File | 1997-03-28 | 12KB | 240 lines

              NavPak-32  NMEA Interface

                 POSITION PLOTTING

  This interface uses data from a GPS to display your position on the
screen, plot tracks, plot radar targets and radar cursor, steer to a way
point, control an auto pilot,  display COG (course over ground),
SOG (speed over ground), and calculate current set and drift.

  The default port is COM-1, otherwise select the port from the dialog box.

   Using a 9 pin Comm port on the computer,  pin 2 (RXD) is connected
 to the GPS output, or signal side of a NMEA data bus,  pin 3 (TXD) is
 connected to the auto pilot input, when present, and pin 5 (SG) is the
 common signal ground for the computer, GPS, autopilot, and NMEA data bus,
 when present.  Using a 25 pin Comm port on the computer,  pin 2  is the
 TXD line, pin 3 is the RXD line, and pin 7 is the SG line.

  The 9 pin 'D' connector for the computer is a common item, and the connector
for the nav gear (GPS etc...) will be supplied with the data cable obtained
from the manufacturer.

  After connecting the wires, choose "Connect" in the "NMEA" menu, then choose
"Data" to verify that data is being received.   "Center Position" will center
your position on the screen using the data received from the serial port,
and "Auto Scroll" will center your position when it gets within about 50
pixels from the edge of the screen.

    When NavPak is started, some items on the "NMEA" menu will be grayed
and not selectable.   To activate these grayed items, select "Connect" in the
"NMEA" menu.   This does not require that the Comm port is connected.

 The "Write Way Point", "Read Way Point", "Range/Bearing to Way Point"
and "Track" dialog boxes can stay open without suspending the operation
of the program (modeless dialog boxs), All the other dialog boxes in
NavPak,  require that they be closed before performing other operations.

  When using electronic plotting on a bitmap, keep in mind that most
charts are based on very old surveys.  The relative positions of items
on the chart are probably accurate,  but sometimes the longitude of the
whole chart is off or an isolated hazard is incorrectly plotted.  When
sailing close-in you should plot bearings on the bitmap to verify the
NMEA position, and check for warnings on your paper charts.


                         WAYPOINTS

   When NavPak is started, a blank list is initialized which can contain
100 way points.  Waypoints can be written to this list, or it can be filled
in from a list previously saved on disk.  A way point can be identified by
double clicking near it, or by selecting the "Search" button in the
"Read Way Point" dialog box.  There is a continuous read out of the
distance and direction to each way point based on the position received
through a serial port from a GPS or Loran.

   Each way point contains an ID tag, note, time, date, track, speed,
and position.  The time and date are automatically set from the computer
clock.  The position can be set from a point digitized on the screen or
received through the serial port.  The ID tag is a five letter field, and
the note is a 15 letter field which you can fill in.

   Way point lists can be recorded from any combination of different scale
charts, so that a list can be displayed on a different chart from which it
was made.  A list can be opened under one name and saved under another.
When a list is opened or saved to a disk file, the entire block of 100
way points is transferred.  The file size is the same for all lists,
regardless of how many blank points there are in the list.

  The current waypoint flashes and can be changed from the
"Read Way Point", "Write Way Point", and the "Range/Bearing to Way Point"
dialog boxes.   When you select "Connect" from the "NMEA" menu, the current
waypoint will start flashing with or without a GPS connected to the Comm
port. The current waypoint will also start flashing when the
"Write Way Point" or "Read Way Point" dialog boxs are open.

  The "Range/Bearing to Way Point" dialog box displays the distance and
course to the current waypoint.  This dialog can stay open as a steering
aid without suspending the operation of the program (modeless dialog box).
All the other dialog boxes in NavPak, except the "Write Way Point",
"Read Way Point" and "Track"  dialog boxes, require that they be closed
before performing other operations.

  To activate the "Range/Bearing to Way Point", select "Connect" from the
"NMEA" menu.


                      TRACK RECORDING

   The Track recording function will automatically record your position
and time at set intervals, and will plot the track line and points on the
screen.  The track list in NavPak-32 contains 760 points.

   The interval of track point recording can be set using the "Track"
dialog box.  The default of one minute intervals will record points for
12 hours and 40 minutes.   The interval can be changed at any time.

   Each track point contains the time and position.  The time is
automatically set from the computer clock and the position is set from
data received from a GPS through the serial port.  A track point can be
identified by double clicking near it, or by selecting the "Search" button
in the "Track" dialog box after digitizing near it.

   Track point lists can be recorded from any combination of different scale
charts, so that a list can be displayed on a different chart from which it
was recorded.  A list can be opened under one name and saved under another.
When a list is opened or saved to a disk file, the entire block of 760
track points is transferred.  The file size is the same for all lists,
regardless of how many blank points there are in the list.

   Track recording  can be turned on and off using the buttons in the
"Track" dialog box.    If a track list is not loaded, turning on the track
function will start recording at point numer 1.  Turning on the track
funtion while a track list is loaded will resume recording after the last
point recorded in the list.  If all 760 points are recorded, a message box
will appear and the program will pause for acknowledgement.   The track
list can be saved and a new track list can be started by selecting:
"Clear Track List" in the "File" menu.


                        AUTO PILOT

    The AutoPilot function transmits the "APB" sentence per NMEA 0183 to
 interface with an auto pilot.  NavPak receives from a GPS and compass
 and transmits to an autopilot on the same comm port, with the software
 controlling overlapped messages.

   Using a 9 pin comm port on the computer, the receiving  pin 2 (RXD) is
 connected to the GPS,  the transmitting pin 3 (TXD) is connected to the
 auto pilot, and pin 5 (SG) is the common signal ground for the computer,
 GPS and autopilot.

    Using a 25 pin comm port on the computer,  pin 2 is the TXD line,
 pin 3 is the RXD line, and pin 7 is the SG line.

    The "Auto Pilot" dialog box in NavPak-32 is used to start and stop
 the sentence transmission to the auto pilot.   The dialog box also
 offers the following options:

      1. Steering towards the active waypoint.

      2. Steering a fixed course defined by a
         point digitized on the screen.

      3. Steering a fixed course defined by
         typing the course in the box provided.

    NavPak will determine present heading from the NMEA HDT sentence.
 If HDT is not present on the data bus, it will use the VHW sentence.
 If VHW is not present, then it will look for the HDG sentence.
 If none of those are available, then COG from the RMC sentence will
 be used as present heading.   To get accurate results from COG requires
 DGPS.

                 STEER TOWARDS ACTIVE WAYPOINT
   When steering towards the active waypoint, the course will be corrected
 for leeway and drift.    While in this mode of operation, the
 "Range/Bearing to Way Point"  dialog will give a continuously updated
 indication of progress.  This mode will probably not work without
 differential GPS (DGPS).


                STEER TOWARDS A DIGITIZED POINT
   Steering towards a digitized point offers the advantage of quick
 course changes.  When using this option, digitize a point on the screen,
 then open the "Auto Pilot" dialog box to reset the course. This defines
 a fixed course which will not be affected by leeway or drift.


                    STEER NUMERICAL COURSE
   Entering a course by typing it into the edit box defines a fixed course
 which will not be affected by leeway or drift.


                        RADAR INTERFACE

   NavPak-32 will receive information from radar units which are ARPA (Automatic
 Radar Plotting Aids) equipped to transmit NMEA data.   The radar functions do
 not need to be turned on or off,  when the nesessary data is present at the Com
 port,  NavPak will use it.   The standard data sentences provided by the
 NMEA specification for radar interface are:

   1.  TLL, Tracked target latitude and longitude.
   2.  TTM, Tracked target message.
   3.  RSD, Radar system data.

   NavPak will plot up to 100 tracked targets as red squares, using the TLL
 sentence.  If TLL is not present, then NavPak will calculate the target lat/lon
 using the range and bearing from the TTM sentence.   If the radar is set to
 Relative bearing units, then an accurate heading is required.   NavPak will
 use VHW, HDT, or HDG for heading.   Where these are not present, then COG will be
 used from the RMC sentence.   COG is used as a last resort since it may differ
 slightly from the actual heading, depending on leeway.   The "Data" dialog box
 should be checked to see if a heading is being received from VHW, HDT, or HDG,
 otherwise check the COG in the "Title Bar" to see if it looks reasonable.

   If the radar is set to True bearing units, then a heading is not required.
 In either case, an accurate position must be present from RMC, GGA, or GLL.
 NavPak uses a great circle formula to calculate target lat/lon from range and
 bearing for accuracy at high latitudes and long distances.

   At any time, if you double click the left mouse button near a radar
 target, a dialog box will appear which displays all the information about the
 target from the TTM sentence.   The radar cursor position will be plotted as a
 square with a cross from the location given in the RSD sentence.

 Using the TTM sentence, NavPak will display the following:
   1.  Target number.
   2.  Target distance from own ship.
   3.  Bearing from own ship (T=True, R= Relative).
   4.  Target speed.
   5.  Target course, degrees (T=True, R=Relative).
   6.  Distance to closest-point-of-approach (NM).
   7.  Time to closest-point-of-approach (minutes).
   8.  Target name.
   9.  Target status (L=Lost, Q=Query, T=Tracking).
  10.  R=Reference target used to determine own
       ship position or velocity.
  11.  Acquisition (A=Auto, M=Manual).

    NavPak will plot the targets and radar cursor position on the chart with the radar
 operating in course-up, heading-up, and north-up modes.   Speed and distance units
 can be set to nautical miles, kilometers or statute miles.

                     CURRENT SET and DRIFT

    NavPak will calculate current set and drift by comparing GPS fixes with heading
 and speed obtained from a NMEA compatible compass and log or speedometer.

    NavPak-32 will look for the NMEA VHW sentence which gives speed and heading through
 the water.   If VHW is not present, it will look for VLW to get distance travelled
 from the log, and HDT, or HDG to get the present heading.   During an averaging
 interval, the heading and speed or distance is recorded every 2 seconds.   At the end
 of the averaging interval, the speed and headings are averaged and compared with GPS
 fixes, using a great circle formula for accuracy at high latitudes.   The averaging
 interval can be set from 0.1 to 12.0 minutes.   The resulting set and drift will be
 the combined effects of current and leeway.    To get accurate set and drift requires
 DGPS.

/***********************  end NMEA.txt  ************************/