# Deriving Situation-Adaptive Policy for Container Stacking in an Automated Container Terminal
## 2 Operations in the Storage Yard of an Automated Container Terminal
These long-distance movements of containers in opposite directions easily lead to ASC interferences. One way of minimizing such interference is to have the containers tactically relayed through cooperation between the two ASCs. As an example, an inbound container stacked near the seaside end of the block can be moved first to a certain intermediate location by the seaside ASC and then to its final destination HP at the landslide by the landslide ASC. The first movement of this relay operation is called _repositioning_. Note that the stacking locations for the containers to be repositioned should also be determined by the stacking policy. Given a stacking policy, container handling in a storage block is typically performed in the following way. Whenever an ASC finishes its current job, it selects the most urgent job from the job queue that contains all the jobs requested for the next horizon of length, say thirty minutes. The jobs in the queue include those to be undertaken according to the loading/unloading schedules and those requested from the ETs that have already arrived at the landslide HPs but have not been serviced yet. The ETs expected to arrive at the block during the next horizon are not counted because their arrival time is highly unpredictable. If the current ASC is the landslide ASC, the most urgent job would be the ET job with the longest waiting time. If the current ASC is the seaside ASC, the most urgent job would be either a loading or an unloading job with the earliest deadline according to their schedules. When the job selected is a stacking job, the stacking policy examines all the available slots in the block and recommends the best one as the stacking location. An available slot can be found at the top of every stack unless it has already reached the allowed maximum tier. However, the stack should not belong to the bay where the other ASC is currently working. When the job selected is a retrieval job, no reference to the stacking policy is necessary unless there are other containers above the target container. If there are some containers above, they all must be relocated one after another to the locations recommended by the stacking policy. When the retrieval job selected requires a travel distance longer than a given threshold, the target container should be repositioned to the location recommended by the stacking policy before it can be sent to its destination HP by the other ASC. The efficiency of ASC operation in a block can be assessed by measuring the AGV delay and the ET waiting time, where the former is counted far more important than the latter. A good stacking policy makes the ASC operation efficient by reducing the interference, rehandling, and the overall container handling time, which makes it possible for the ASCs to provide the ASVs and ETs with faster services. However, it takes quite a long period of observation to see how good a stacking policy is. Whether or not the stacking locations recommended are good can be seen better when the containers are retrieved out of the block than when they come into the block. Since the average dwell time of containers in container terminals is often longer than a week, the AGV delay and ET waiting time should be measured for a long period during which time enough containers are retrieved, under the condition that the containers keep coming in and going out fairly constantly during that period of time.