Why are the H'bird units listed as SI, or DI. I am a Lowrance guy myself but helping a young man that I have mentored when it comes to fishing and he is looking at getting the Helix 9 SI GPS KVD set up for a boat he is purchasing. Wondering why the unit name has only SI, will it only do Side Imaging, Normal Sonar, and GPS and not do down imaging or will it do the down imaging also?