These upgrades take the form of long 'whip' aerials or other large aerial set-ups. A vehicle with improved comms can communicate with other Imperial Guard formations, such as nearby reserves or artillery support. To represent this, the improved comms allow the Imperial Guard player to re-roll one Reserves roll per turn, and to re-roll the dice when checking to see if an enemy unit or obstacle is hit by a preliminary bombardment.
Improved Comms
Source: Codex: Imperial Guard (1999), p. 9