Setting up Navi GT31 for speed sailing
by Tom Chalko

There are 2 methods of setting-up Settings in GT31:
- manual: using buttons in GT31 itself
- using NavilinkII : this method enables you to backup/share/restore complete set of GT31 settings

1. MEMORY CARD
a. LOG SWITCH should be ON-FIX. There is no point to log any data when there is no position fix.
b. NMEA ITEMS should be set to "SBN 1 SEC". Binary logging ensures maximum possible trackpoint accuracy and maximum data reliability. If you want the most complete data set from your sailing session, use SBN (binary) format logging.
c. MIN SPEED should be set somewhere between 2 and 5, unless you are specifically interested to log data when you are stationary.
d. FORMAT CARD. Read readme.txt file to find which cards are compatible with G31. Format the card a few times in a row. If you succeed each time, you can rely on the SD card that you use. Formating is the most drastic writing operation you can perform on your card and hence it is the ultimate test of your SD card compatibility.
e. FILE SIZE should be set to CARD SIZE. This means that you do not put any limit on file size.

2. SETTINGS
a. USERNAME. It is important to set your name, so that data files are properly identified.
b. SPEED AVG TIME = 10 seconds if you am for 10s runs, or 21s if you aim for 500m world record.
c. SPEED GENIE = between 20-to-30 knots (37-55 km/h if UNITS are set to m). SPEED GENIE anticipates what you want to see. It automatically changes SPEED screens, resets MAX and MAXAVG speeds so that you do not need to push any buttons when you sail. SPEED GENIE is only active in the SPEED menu. The 1-99 value in SPEED GENIE is a speed threshold for activation of the real-time speed measurement screen. NEW: Speed Genie automatically saves 14 MAX speeds and 10 best n-second averages (n is set in 2b). You can RESTORE any value and make a photo showing your speed achievement. When SPEED GENIE in on (set to non-sero value) GT31 internal chipset parameters are optimized for maximum possible accuracy and reproducibility in speed measurement.

Speed Genie Sound: Beginning with version 1218T (Christmas 2008), GT31 Speed Genie provides bio-feedback sound indicators: acceleration, max n-second average speed increase and also audio-announces the max avg speed result. Sounds are designed for GT31 mounted inside the helmet to provide sailors audio-feedback so that they can focus their eyes on sailing to improve safety and maximize speeds. Set (ALERT>SPEED GENIE>ON) to enable Speed Genie helmet bio-feedback sounds. The bio-feedback is delayed about ~2 seconds, due to the limited bandwidth of the GT31 Doppler system.

c1. Bio-feedback: when Speed Genie speed threshold is exceeded and the display shows speed:
a. Short Buzzer Sound when MAX N-second AVERAGE grows
b. Long Buzzer Sound when speed grows (acceleration indicator for high speeds)
c. combined Long Buz/Short buz sound, when c1a and c1b are true

c2. When Speed Genie displays result on screen it produces
* Festive sound pattern if the Personal Best average speed was achieved.
* 3 digits of the N-second max average speed result encoded in the sound as follows (code is similar to Roman numerals, except for 4 and 8):
1 = . 2 = .. 3 = ... 4 = .... 5 = _ 6 = _. 7 = _.. 8 = .._ 9 = ._ 0 = __
where
. = ~100ms short sound
_ = ~300ms long sound
__= ~600msec for zero (not truncated)
silence separator between digits: ~300msec silence

The numeric speed result is repeated 3 times and then every 2 minutes when Speed Genie results are displayed on the screen. More info in Readme.txt file

3. DATA LOGGER
Data Logger (internal memory GPS logging arranged in a circular buffer that automatically overwrites the oldest data when full) has the capacity for ~7 hours of logging GPS data every second with full precision. It allows Navi GT31 to be used without SD card. Set INTERVAL=1s, MIN SPEED=5.

You may wish to CLEAR the data logger content before each sailing session. Before executing CLEAR please SAVE the content to SD card. Using this method regularly ensures that you will have a backup of all your GPS data on your SD card. Of course you can back-up SD card content to your computer.

4. BACKLIGHT
If you need maximum battery life, make sure that you set SETTINGS>BACKLIGHT>OFF, because light consumes a large proportion of GT31 power. GT31 with BACKLIGHT=OFF has been tested to work at full accuracy and with SD card for 25 hours continuously.

5. MISC
Version 1003 and above allows User to choose screens for Speed Genie result display.
Following screens can be enabled (ON) or disabled (OFF)
1. MAX/NS MAX - screen showing MAX run speed and best N-second average
2. HISTORY MAX - screen thak compares current run with history best run
3. MAX SPEED - MAX speed of the run in LARGE font
4. NS MAX AVG - in LARGE FONT
When more than one screen is enabled (ON) they will appear in a "rolling" sequence.
When you have only 1 second to look at all Speed Genie results, enable only screen 1.

6. OTHER settings
Other settings are almost irrelevant for speed sailing. You may consider setting UNITS (NM or m), BACKLIGHT etc... in SETTINGS to suit you.

Power up your GT31. If you have set up the SPEED GENIE as in C2b - GT31 will enter the SPEED menu automatically after it established connections with GPS satellites. SD card Logging activity is confirmed by flashing log indicator. Move GT31 with speed above the threshold you set in step C1c and check it.

You may wish to clear the "history" of your best speed runs, to keep track of your best daily achievements. There are 2 "histories': MAX SPEED and 10S MAX AVG speed and they can be cleared independently. 10 is an example value here and can be changed in step C2b. You may wish to keep MAX SPEED history and CLEAR only 10S MAX AVG history for example. Sequence of keys to clear each "history" is SPEED>MAX SPEED>CLEAR and SPEED>10S MAX AVG>CLEAR

If you do not clear history, your best speed results since last CLEAR will be stored, so you can store your lifetime achievements and be able to show them to friends at any time.

When you use two GT31 to measure the same speed you should take care to put their GPS antennae as close to one another as possible. Since GT31 antenna is located next to the joystick - this means that when two GT31 are insterted into an aquapack - joysticks should be close to one another. This means that one of the GT31 screens will be "upside down". If you want to read both screens right way up, use MISC>SCREEN FLIP settings to flip the "upside down" screen for easy reading.

When you neglect to place two GPS antennae close together, each GPS will read different speeds every time when there is rotation of the GT31 pair and/or the distance between them changes. For example when antennae are 10cm apart, and your arm that holds them rotates 30 degrees in 1 second, the speed difference measured by 2 units can be as large as 0.1 knot during your hand stroke. The speed difference depends on the direction of rotation and is typically smaller than 0.1 knots, but it is obvious that taking care of keeping both GT31 antennae as close as possible is clearly a step to improve the reproducibility of speed measurement by enabling two GT31 to cross-verify and re-confirm readings of one another.

The best way to ensure the highest possible reproducibility and repetability of speed measurement during sailing is to place both GT31 units inside the helmet. Not only GT31 antennae can be close together, but also rotation stroke magnitudes are restricted, because human head is subject to continous bio-feedback from the body to minimize its motion, even if the rest of the body vibrates in bumpy chop.

GT31 used in a helmet experiences the best possible satellite signal from the maximum possible number of satellites, which contributes to increasing reproducibility and reliability of speed measurement.