What is the reason for the fence aft of the static ports on some airplanes and helicopters?
In yet another example of unique helicopter design complexity, issues can arise when it comes to mounting static ports on the side of the fuselage where they may fall within the rotor wash. Ideally, static ports should be mounted in an area of undisturbed air, but other design considerations sometimes make this impossible. Additionally, helicopters can fly in directions other than forward, including in the direction of the static port itself. To counter pressure differences from sideslip, many helicopters use a second port on the opposite side. To adjust for rotor wash interference and ensure that the instruments read correctly in climbs and descents, small structures may be placed around static ports to speed up air flow with convex surfaces or slow them down with dams (what you refer to as fences or steps).
Although there is no picture provided, I think this may be what you are asking about:

For more information, you may look here:
https://books.google.com/books?id=rxryAgAAQBAJ&amp;pg=PA409&amp;lpg=PA409&amp;dq=helicopter+static+port&amp;source=bl&amp;ots=p-H9klAkUl&amp;sig=ACfU3U2Yda5qCljafTMg97TSlRH_EpC13g&amp;hl=en&amp;sa=X&amp;ved=2ahUKEwj7paLgkLzqAhUIWa0KHZseD2cQ6AEwG3oECAgQAQ#v=onepage&amp;q=helicopter%20static%20port&amp;f=false