Why do I need forward/reverse input signal when using dead reckoning?

[jg_spitfire]

Hi, in the docs says that for DR we need:

1.-speed

2.-forward/reverse signal

But why do we need the forward/reverse signal?, the GNSS has an IMU, could a negative acceleration give us an idea of moving in reverse?, also the speed from CAN bus, a negative speed could indicate the reverse, maybe I am missing something

thanks

[Tesla DeLorean]

Because Speed is typically a series of pulses from an interrupter, like a hall sensor or led/disc, and is directionless.

This might be on the gearbox output shaft, and the direction from the reverse-light switch.

It's not to say that other methods aren't viable, just that this is quite prolific for DR application. Generally the IMU determines the orientation of itself in the vehicle, and the bias, ratio, and drifts of various inputs with respect to each out.

[GNSS-LeaT]

Just to add to Tesla Delorean's answer, when first started the system does not know the orientation of the IMU relative to the vehicle frame. The reverse indicator is a hint to the DR algorithm as to the orientation. This is especially useful when the system is first started, and a good GNSS PVT solution has not been achieved. As Tesla Delorean stated, the speed pulse from the vehicle typically does not indicate direction.

The CAN bus Vehicle Speed PID (0x0D) also does not indicate direction just magnitude. Usually a transmission message indicating the gear position is used to detect whether or not the vehicle is in reverse.