The SS300 was our original ultra low power, ultra small Doppler speed measurement radar. It could measure from about 1.3 mph to 105 mph.
We were then approached by a customer that required speed measurement below the 1.3 mph lower speed limit of the SS300. We developed the SS300U (Ultra low speed) version that shifted the speed measurement range lower to measure from 0.25 mph to 16 mph.
Subsequently, another customer wanted to measure even lower speeds and also mount the radar close to trains (i.e. a large target that may overload the radar receiver). We developed the SS300ULG (ultra low speed, Low Gain) that could measure from 0.13 mph to 11.2 mph. This product has lower gain in the radar receiver so that it would not overload when mounted up close to large targets. It also had other features that improved the measurement quality, ability to specify filtering (with a time constant) to minimize the jumping of output speeds (due to different parts of the train being measured) and also ability to decimate output rate of the speed.
The SS300 and SS300U have been superseded by the next generation SS400 and SS400U which feature even better performance and wider speed measurement ranges.
However, the SS300ULG is still in production and the recommended product if you want to measure very low speed up close to large targets (e.g. slow moving trains).
