This screen is used to set up various general parameters in the SPEEDBOX, including low speed clamping of the speed signal, direction detection and the local time offset from GMT.
In normal operation, the SPEEDBOX uses both GPS and inertial sensors. However, there are some circumstances in which GPS-only operation may be preferred:
- When the SPEEDBOX base unit cannot be mounted in the correct orientation, and hence the inertial measurements will confound the results.
- Where there are pitching movements that may potentially confound the combined speed algorithm, such as on a small boat in rough seas, the GPS-only option may give better results.
When operating in GPS only mode, no parameters which require accelerometer readings are calculated or output. As such combined speed and accelerometer readings are not available from any outputs. Note that the SPEEDBOX-INS cannot be set to operate in GPS only mode, since the INS mechanism fundamentally requires inertial data to operate.
Combined Speed Parameters
A low speed clamp can be applied to the speed signal; this will remove the tendency to get very low speed readings when the vehicle is actually stationary. This is very useful for applications where the vehicle will not be moving for prolonged periods of time but the distance measurement is important. Without clamping the speed there will be a very gradual accumulation of error in the integrated distance during stationary periods.
Entering a value of zero will disable this option.
Entering a non-zero value will cause the speed clamp to be applied when the speed drops below the entered clamp value. The speed clamp will have the following effect:
- The speed will be output as zero below the clamp threshold.
- The integrated distance will use the clamped speed, and hence not increment when the speed is below the clamp threshold
- The heading will be held constant at the last non-clamped heading value when the speed is below the clamp threshold
- The gradient will be held constant at the last non-clamped gradient value when the speed is below the clamp threshold
Time offset from GMT
This value can be set to the local time offset from GMT. It is re-transmitted in the Race Technology GMT time message and the NMEA GPZDA message. This enables the user local time to be correctly interpreted from these messages, and may be useful, for example, if the time is used for file timestamping. Currently only integer hours are supported.