Unfortunately the grid ratio can only be an integer. If you want your innermost domain to be 1.5 km, I would recommend having a more coarse parent domain. So, for e.g., you could set parent_grid_ratio = 1, 3 and dx/dy = 4500, 1500.
Thank you for your response,Unfortunately, it is not possible. However, I don't understand why it is not possible when the inner domain input is always interpolated irrespective of being an integer and a fraction. In future versions, it would be fantastic if users had the ability to set fractional grid ratios.