The Instruments

The GPS instrument

The GPS is the most complex but also most versatile instrument of the NemaTalker application.

It can be used in 3 modes: manual, auto and navigate. The latter 2 modes require an input file containing standard NMEA waypoint sentences ($GPWPL).

The value of many input fields can be changed with the up-down buttons located at the right side of the input boxes, or typed directly into the input boxes. Note that decimals are ignored when using up/down buttons.

The value of fields with a different background color (in the picture below yellow, but maybe different depending on your personal windows settings) cannot be changed directly, but are manipulated programmatically.

Note that not all fields are used in all sentences, and that some panels are only visible depending on selected options. For example the Bearing/Range/XTE/Steer panel will only be visible in Auto and Navigate mode.

Operation

To start there must be at least a valid starting position represented by the latitude/longitude fields. First time users may find the way of inputting data in these fields a bit awkward, but will once used to it appreciate the "error proof" entry!

Valid values should also be entered into the Cockpit console fields.

After clicking the "Start" button the GPS simulation will start, and NMEA sentences will be sent to the selected serial port of the PC, until the "Hold" or "Exit and Save" buttons are pressed. Note that only NMEA sentences will be send that are checked in the "NMEA sentences" panel.

When the "Open Log file" option from the main "File" menu was selected, the sentences will also be written to the user selected file.

To view the sentences being sent select the "View trace window" option from the main menu.

Clicking the "Hold" button will suspend operation, until "Cont." is clicked. "Exit and Save" will exit the instrument and save all current values in the windows registry, so that next time the instrument is used it can continue where it stopped.

A short description of each field will follow below.

Latitude/Longitude
Current position. To change the value the instrument must be stopped. To change: place the mouse cursor left of the leftmost digit, the display will look like this: " ___° __._____' _". Then just start typing the latitude or longitude, the cursor will advance automatically and the value will be automatically formatted. Note that you always have to type exactly 10 digits plus N,S,W or E.
Example: 53° 21.56' N must be entered as 0532156000N.
During a simulation session the position will be automatically updated to the newly calculated current position, taking course and speed into account.

Course
The current course. 0-360. Value is taken from the Cockpit.

Velocity
The current speed. 0-1000 Knots or KM/hr. Value is taken from the Cockpit. Knots or KM/hr depending on generic settings.

Altitude
The current antenna altitude. 0-10000 meters. Value is taken from the Cockpit.

UTC offset
The value to be added to the UTC time. Value is taken from the generic settings (set up menu). Note that the value can be set plus or minus.

UTC time/UTC date
Value taken from the PC clock, corrected with UTC offset. From version 1.3.3 UTC time is given in hundreths of seconds (hh:mm:ss.ss), both in the instrument and in the relevant sentences.

Magnetic Variation
Value taken from the generic parameter settings.

PDOP/HDOP/VDOP
Dilution of precision parameters. 0-9. Used in $GPGSA sentence.

Geoidal Separation
Height above/below reference datum. GNS sentences.

Satellites
Number of satellites in view. 0-12.

GPS fix quality
Fake GPS or DGPS fix. When DGPS, differential data is added to the GGA sentence

Satellites used
Tick the satellites you want to see in the $GPGSA sentence. GPS satellites are numbered 1 to 32, satellites in the WAAS system are numbered 33 to 64. Automatic numbering depending on the option you choose.

NMEA sentences panel
Tick the sentences you want to send, select the interval (0-59) and if you want a checksum calculated and added to the sentences or not.

Auto mode
Tick this box if you want the simulator to run a predefined route taken from an input text file containing $GPWPL sentences. Such a file could look like this:

$GPWPL,5126.253,N,00334.389,E,1-121021*54
$GPWPL,5125.000,N,00333.283,E,2-121025*55
$GPWPL,5124.925,N,00327.862,E,3-121027*54
$GPWPL,5123.610,N,00323.304,E,4-121031*55
$GPWPL,5122.306,N,00311.994,E,5-121043*50
$GPWPL,5121.742,N,00311.544,E,6-121104*57

The checksum is not necessary. NemaTalker does not check it. You can obtain the file from a route planning program or create it manually with a text editor like Windows Notepad.

You can select the file after clicking "Start" with the Auto mode option checked. During the session you can alter altitude and velocity in the Cockpit, but not the course, because bearing and range are calculated by the program.

Navigate mode
Tick this box if you want to simulate the run of a predefined route, but want to be in full control by using all Cockpit controls. Note the difference to the Auto mode! Besides the bearing and range to the next waypoint, the simulator shows also the Cross Track Error (XTE) and the direction to steer to correct the error!

The Navigate mode also needs an input text file containing $GPWPL sentences, similar to the file explained above under Auto mode.

In Navigate mode you can use the arrow keys of your keyboard to steer (left-right arrow keys) and to control speed (up-down arrow keys). The mouse cursor must be somewhere in one of the instrument fields to enable this.

The GPS instrument from NMEA version 2.30 and later

NMEA version 2.30
NMEA changed the specifications for many sentences considerably in version 2.30 and 3.01. Since we still have to deal with equipment (both hard- end software) that do not yet support these newer versions, NemaTalker (from NemaTalker version 1.1.0 and higher) is capable of simulation both the NMEA 2.20 and 2.30/3.01 version by setting a parameter in the Generic Settings.

This setting will influence the format of a number of settings in the GPS instrument. When the setting is for NMEA version 2.3/3.01 the GPS instrument window will look like this:

You will notice that a number of drop down boxes have appeared next to most sentences in the sentence selection frame. Here you may select the new field "Mode Indicator".

For GLL, RMC, VTG, XTE and RMB the meaning of the Mode Indicator is, according to the NMEA specifications, as follows:

A = Autonomous mode
D = Differential mode
E = Estimated (DR) mode
M = Manual Input mode
S = Simulator mode
N = Data not valid

Note that setting the Mode Indicator also influences the value of the Status field. The Status field will be set to "A" (data valid) for Mode Indicators A and D, and to "V" (data invalid) for all other values of the Mode Indicator.

For GGA there is no Mode Indicator field, but the Quality Indicator field is used as follows:

0 = Fix not available or invalid
1 = GPS SPS mode, fix valid
2 = Differential GPS, SPS mode, fix valid
3 = GPS PPS mode, fix valid
4 = Real Time Kinematic. Sat system used in RTK mode with fixed integers.
5 = Float RTK. Sat system used in RTK mode with floating integers.
6 = Estimated (DR) mode
7 = Manual Input mode
8 = Simulator mode