Generally IMTU the Imperial arms are the Navy and Marines with the army being a planetary thing.
The doctrine is generally marines attack, army defends but there is a special case when the Imperium are on the offensive where arny units come in directly after ground has been taken by the marines. This is where the Imperial army comes in. The sub-sector capitals maintain a planetary force but also Imperial Army units who are designated as priary backup to the marines. These units are also used in police action and rebellion situations where the marines would be overkill.
The IA is maintained at TL12 for consistency.
The core unit of the IA are regiments and sub-sector capitals will generally maintain divisions of these troops containing four regiments.
Squad: Grav + 2 x squads of 4
Platoon: 4 x Squads, 1 x Platoon HQ
Company: 4 x Platoons, 1 x Company HQ
Battalion: 4 x Companies, 1 x Battalion HQ
Regiment: 4 x Battalions, 1 x Regiment HQ, 1 x Training Battalion
c. 6,250+ men and 400 gravs
nb the training battalion remains at base.
A key element of the IA is maintaining an Electronic Intelligence Battle Grid (EIG). Each squad has intelligence gathering sensors and each tier of HQ has an element dedicated to collating the electronic intelligence from each combat element and also gathering more of their own via radar, ladar, drones etc.
nb C&C = Command & Comms, S&R = supply and repair (includes vehicle salvage), IF = Indirect Fire
1 x Command Squad (C&C / EIG)
1 x Command Platoon (1 x C&C Squad, 1 x EIG Squad, 1 x S&R squad, 1 x Medical Team)
1 x IF section (2 weapons)
1 x Command Company (1 x C&C Platoon, 1 x EIG Platoon, 1 x S&R Platoon, 1 x Medical Squad)
1 x IF Battery (4 weapons)
1 x Command Battalion (1 x C&C Company, 1 x EIG Company, 1 x S&R Company, 1 x Medical Platoon
1 x IF Battalion (16 weapons)
1 x Recon Company
1 x Engineer Company
(Recon = grav belt and stealth suit infiltrator unit, Engineers = battle dress bunker assault unit)
– at TL 10 hover tanks & IFVs
– at TL 11 flying tanks & IFVs
– at TL 12+ more like gunships
A quote from the Traveller forum seems appropriate:
Standard Kit is combat armor, plus a Gauss Rifle and under body RAM GL, one fire team has a PGMP-12 replacement, the other has a Gauss SAW. Two members of each fire team carries a Plasma LAW, the final two carry extra rounds for the Gauss SAW.