text: Drop Zones
  Figure 6-11. The 15-to-1 mask clearance ratio.
  6-105. If any of the GMRS markings fall within a 15-to-1 mask clearance ratio on the approach end of
  the drop zone, and it is not feasible to adjust the selected PI, you can place an Army code letter (H, E, A, T)
  or a far (marker) panel on the trailing edge of the DZ. However, before doing so, you must coordinate this
  during either the DZST or aircrew mission briefing. Using a code letter will distinguish this DZ from other
  DZs in the area. The code letter is located at the end of the DZ or where the pilot can see it best, and
  aligned with the corner and approach panel, parallel to drop heading.
  6-106. During daylight airdrops, the marker panels should be slanted at a 45-degree angle from the
  surface toward the aircraft approach path to increase the aircrew and jumpmaster's ability to see them. If
  security permits, smoke (other than red) may be displayed at the release point or corner marker to assist in
  DZ acquisition.
  ARMY VERBALLY INITIATED RELEASE SYSTEM
  6-107. The Army VIRS method establishes the release point on the DZ through radio communications. If
  tactically feasible, a code letter can mark the RP. However, the aircraft initiates the drop on verbal
  command from the ground.
  6-108. Emplace a standard Army code letter with VS-17 panels (for daytime operations) at the release
  point. Position the base panel of this code letter exactly on the RP. Use code letter H, E, A, or T. Make the
  letter at least two panels high by one panel wide (Figure 6-12, page 6-38).