Missile hit chance

A guided missile in Broken Arrow hits or misses on a roll: the game multiplies the missile's accuracy by the target's ECM, by a factor for every decoy burning and, for some seekers, by a penalty for the shooter's stress. From the game's code Every guided missile's accuracy is 100% in the current data, so the odds come from the target's countermeasures and the shooter's stress. From the game's data Each rise in the number of burning decoys after the first roll makes the missile roll again, which is why flares dropped one at a time beat a burst. From the game's code

The hit roll

The game's code computes it like this: From the game's code

hit chance = Accuracy × ECM × ((1 − FlareResistance) × DecoyMultiplier) ^ decoys × Stress

In plain words: accuracy, times ECM, times one factor per burning decoy, times stress where it applies. From the game's code

A missile has no other accuracy stat. From the game's code The game's tutorial says the same: "When out of decoys, a plane's chance of being directly hit is 100% minus its ECM value." From the game's own text On a unit page, the ECM tooltip puts this in plain numbers: against ECM ×0.85, with no decoys burning, an incoming missile hits directly 85% of the time. Worked out by this site

Decoys and re-rolls

A missile that has not yet missed is "on course".

• The first roll counts every decoy burning at that moment, up to 100. It happens in the same step in which the missile registers its target. From the game's code
• Each rise in the count re-rolls. When the target's number of burning decoys climbs above the number the missile last saw, a missile still on course rolls again. From the game's code
• A re-roll counts one decoy, however many just landed, and applies ECM again. From the game's code
• The remembered count follows the decoys down. It is refreshed on every check, so it falls as decoys burn out. A count that holds level gives no re-roll. From the game's code
• A miss is final. Once a missile misses, it stays missed. From the game's code

So a missile hits only if it passes its first roll and every re-roll after it: its chance is all those rolls multiplied together. Worked out by this site

One at a time beats a burst

A burst raises the count once, so it earns one re-roll, and that re-roll counts a single decoy. From the game's code Decoys dropped one at a time, spaced by the cooldown, raise the count with every drop while the earlier ones still burn. From the game's code Once the oldest decoys start burning out, it depends on the timing: on a helicopter every drop still raises the count, while on a plane only the first three drops are certain to (see Automatic decoys). From the game's code

Take an AH-64E with its default ECM of ×0.85, fired on with no decoys burning. Two decoys dropped together after the first roll leave the missile a 65.0% chance, no better than one decoy. Dropped apart, the same two cut it to 49.7%. Worked out by this site

Worked examples

These figures hold for every missile with accuracy 1.0 and flare resistance 0, which is every guided anti-air round except the Hydra Fletcher. From the game's data They leave out the shooter's stress. Worked out by this site "Fired clean" means no decoys were burning at the missile's first roll.

An AH-64E (decoys ×0.9), with its default ECM and with its ×0.75 ECM option (LAIRCM): Worked out by this site

Decoys burning at the first roll meet ECM once; each decoy dropped later brings ECM in again, so the last row is lower than the second. Worked out by this site

A jet with ECM ×0.8 and decoys ×0.85: Worked out by this site

The calculator below uses the same rule as the unit pages and the duel: Worked out by this site

Automatic decoys

Every unit with decoys drops them on its own by default, and the player can turn this off. From the game's code

• The delay. While a guided missile has the unit as its target, an under-attack timer counts up. It resets as soon as nothing is incoming. From the game's code
• The drops. Once the timer reaches 0.5 s on a helicopter or 1.5 s on a plane, the unit drops one decoy each time its decoy cooldown allows. From the game's code The cooldown is 0.6 s on every helicopter and 1.0 s on every plane in the data. From the game's data
• The stock. It stops when nothing is incoming or none are left. From the game's code Helicopters carry 10; planes carry 25, or 50 on some ground-attack planes. From the game's data
• The burn. Each decoy burns for its decoy duration. From the game's code That is 2 s on every helicopter and 3 s on every plane in the data. From the game's data

What that means for the rolls:

• The first roll sees none of them. The missile makes its first roll in the step it registers its target, so its own decoys count only through re-rolls. From the game's code
• A helicopter re-rolls on every drop, at 0.5, 1.1, 1.7 s and so on after the missile targets it. Its burning count builds to 1, 2, 3, 4; after that the oldest decoy burns out just before each new drop, so the count dips to 3 and rises again. Every drop is a rise. From the game's code
• A plane certainly re-rolls at 1.5, 2.5 and 3.5 s. From the game's code From the fourth drop, an old decoy expires in about the same frame as the new one lands, so whether the count dips and rises again depends on frame timing. That is not known. Not known: our best reading The duel counts no re-roll for those later drops. Worked out by this site
• Flight time limits it. A missile gets only the re-rolls that come before its impact. From the game's code One that reaches a plane 2 s after targeting it gets one re-roll at most. Worked out by this site
• A second missile that makes its first roll while earlier decoys still burn counts them at that roll. From the game's code

The automatic drops already come one at a time, the pattern that earns the most re-rolls. Worked out by this site

Each helicopter's ECM and Decoys in the current data:

ECM on helicopters and armoured vehicles

ECM applies at the first roll and again at every re-roll. From the game's code

A helicopter's ECM comes from its ability. From the game's code In the data it runs from ×0.75 to ×0.95, and ×0.75, the LAIRCM figure, is the strongest. From the game's data Units with ×0.75 helicopter ECM (the list marks those that get it only from an optional loadout):

145pts

Mi-8AMTSh-VNAlso Mi-8AMTSh-VN Sapsan

Defense Forces

0kin front

80best pen

260km/hspeed

With a loadout option

120pts

Mi-8AMTSh

VDV Brigade

0kin front

80best pen

260km/hspeed

With a loadout option

140pts

Mi-35M

VDV Brigade

0kin front

950best pen

265km/hspeed

With a loadout option

320pts

AH-64E Guardian

Special Operations Forces

0kin front

900best pen

275km/hspeed

With a loadout option

120pts

MH-47G SOAR

Special Operations Forces

0kin front

20best pen

290km/hspeed

With a loadout option

Shtora and Hardhat are ECM at ×0.7. From the game's data Some tanks carry them, and so do fighting vehicles such as the BMP-3 and the M2A2 Bradley; none of these units has decoys. From the game's data So a missile fired at one gets a single roll: 70%, or less if a stressed shooter is guiding it. Worked out by this site Units with Shtora:

65pts

BMP-3

Mechanized Infantry

100kin front

600best pen

70km/hspeed

With a loadout option

65pts

BMP-3F

Coastal Troops

100kin front

600best pen

70km/hspeed

With a loadout option

170pts

T-90

Guard Tank Brigade

650kin front

750best pen

60km/hspeed

200pts

T-80U

Guard Tank Brigade

650kin front

750best pen

80km/hspeed

With a loadout option

Active protection (Active protection) is separate and has no roll. A ready system stops any round it can intercept that enters a box twice the target's size, at one charge per stop, and charges come back only by resupply. From the game's code Stops are at least the system's cooldown apart, which is 6 s on every APS in the data. From the game's data More in penetration and armour.

Stress: seekers the shooter guides

Two seeker types need the shooter to keep guiding the missile: terminal guidance and semi-active. From the game's code For those, the shooter's stress (its suppression) lowers the hit chance. From the game's code Every other seeker, fire-and-forget infrared missiles included, ignores the shooter's stress. From the game's code

How stress builds up, and how far it can lower the chance, is not known to this site. Not known: our best reading The duel therefore leaves suppression out and counts stress as 1. Worked out by this site Such a missile also dies if its shooter stops supporting it (how a missile flies). From the game's code

A lured missile can still hurt

A missile pulled off course can still explode on its proximity fuse for partial damage, the game's tutorial says. From the game's own text The plane-launched AMRAAM's fuse is 12 data metres with a 15 data-metre blast (24 m and 30 m as the game shows distances); the ground AMRAAM-ER's are 21 and 26 data metres (42 m and 52 m). From the game's data

Still unknown

• Flare resistance looks backwards. As the formula is written, resistance 1.0 makes the chance 0 whenever a decoy burns. From the game's code Two rounds have it: the Hydra Fletcher, an anti-air APKWS rocket, and the Khrizantema thermobaric missile. From the game's data That is the reverse of what the name suggests, and what happens in play is not known. Not known: our best reading
• Whether ECM depends on the seeker. No ECM ability names a seeker type, so our best reading is that LAIRCM works against radar missiles as well as infrared ones. Not known: our best reading The duel applies ECM against every seeker. Worked out by this site
• Two ECM systems on one unit. How the game combines them is not known. Not known: our best reading The duel uses the stronger. Worked out by this site