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 ************************/