Our goal is to ensure that we can read every tag 100 percent of the time on both incoming and outgoing trolleys moving through a dock door. Which approach would you recommend: installing additional fixed readers or putting more tags on the trolleys?
It's difficult to answer the question without knowing your particular layout, the products on the trolleys, the trolleys' orientation and other factors that might impact read rates. But I would think that placing tags on the four sides of each trolley should guarantee their ability to read every tag every time. This would be a much cheaper solution than purchasing additional interrogators.
I would also suggest you consider the types of tags you buy, and their placement, carefully. Confidex makes a tag that hangs off a trolley, keeping it away from metal, thereby enabling the tag to be read consistently. Container Centralen has used this hanging tag, and I believe it requires only one per trolley (see Container Centralen Says It's Ready to Roll Out RFID in Europe).
—Mark Roberti, Editor, RFID Journal