ECM — Electronic Countermeasures Definition (.ECM)

FA_2.LIB contains 30 .ECM files. Each defines the electronic warfare suite for one aircraft type (e.g. F15.ECM, AV8.ECM, B52.ECM). BRF plain text (NOT a Win32 PE DLL); very small — F15.ECM decompresses to 352 bytes. Loaded at runtime by the FA weapon/sensor system.

Tools

fx

fx ecm info   <file.ECM>               # human-readable field dump
fx ecm unpack <file.ECM> [-o out.txt]  # editable text
fx ecm pack   <in.txt>   -o out.ECM    # write back (byte-identical)

File Layout

Plain text; BRF syntax (see BRF.md). Hex values use $XX.

Full F15.ECM example:

[brent's_relocatable_format]
    byte 9              ; type identifier (9 = ECM)
    ptr si_names
    word 0              ; quantity (0 = built-in; N = pod carrying capacity)
    byte $0             ; category (0 = built-in suite, 1 = external pod)
    word $1f0           ; ECM power level (0=none, $f0=partial, $1f0=full active jamming)
    byte 30             ; effectiveness A  (variable — chaff)
    byte 35             ; band constant 1  (fixed across all ECM files)
    byte 95             ; band constant 2  (fixed)
    byte 24             ; band constant 3  (fixed)
    byte 30             ; effectiveness B  (variable — flare)
    byte 35             ; band constant 4  (fixed)
    byte 159            ; band constant 5  (fixed)
    byte 31             ; band constant 6  (fixed)
    byte 30             ; effectiveness C (+0x12) = radar Pk reduction (variable)
    word 100            ; overall ECM strength (100 = full; 0 = none)
    byte 0
    byte 0
    byte 40             ; secondary effectiveness (+0x17) = IR Pk reduction (variable)
    word 100            ; secondary strength
    byte 0
:si_names
    string ""           ; short name (empty for all ECM files)
    string ""           ; long name
    string "F15.ECM"    ; filename (self-reference)
    end

Pod vs. Built-In

Category	word (qty)	byte (type)	Examples
Built-in suite	0	$0	F14, F15, F22, A10, B52, EA6, MIG21
External pod	N	$1	ALE40 (qty 100), ALQ72 (qty 224), ALQ167 (qty 310)

ECM Power Levels

word	Decimal	Meaning	Examples
$0	0	No active jamming	MIG21, ALE40 (chaff/flare only)
$f0	240	Radar-only active jammer	ALQ72
$1f0	496	Radar + IR jammer	F14, F15, F22, B52, EA6, ALQ167

Power field is a bitmask (confirmed via @HARDFindJammer@4 at 0x00452EA0):

Bit	Hex	Meaning
4	0x010	Radar jammer active
5–7	0x0E0	Unused — always set alongside bit 4 in active-radar ECM files; no function tests these bits individually
8	0x100	IR jammer active

@HARDFindJammer@4 iterates hardpoints of type 9 (ECM) and rejects pods where the seeker type does not match the active jammer bit: IR seekers need 0x100, radar seekers need 0x10. A pod with $f0 = 0000 1111 0000 has bit 4 set (radar) but not bit 8 (no IR). A pod with $0 passes neither check and functions as a passive dispenser only.

Bits 5–7 confirmed unused: exhaustive scan of all AND/TEST instructions in the ECM/jammer code range (0x452000–0x454000) found zero tests against these bits of the power word. They are set in all active-radar ECM files ($f0 = bits 4–7; $1f0 adds bit 8) but the engine dispatches exclusively on bits 4 and 8.

Effectiveness Bytes — resolved

The 9-byte block (BRF offsets +0x0A–+0x12) contains three variable effectiveness bytes interleaved with six fixed jamming-geometry bytes forming two groups of three (radar at +0x0B/+0x0C/+0x0D; IR at +0x0F/+0x10/+0x11). All roles confirmed via Ghidra (HARDCanLoad and related callers):

Aircraft	eff_A (+0x0A)	eff_B (+0x0E)	eff_C (+0x12)	+0x17 (IR Pk)	ECM power
EA6 (dedicated EW)	99	99	80	80	$1f0
F22, A10, F14, B52	50	50	30–50	30–50	$1f0
F15	30	30	30	30	$1f0
MIG21	20	20	0	0	$0
ALE40 (chaff/flare)	30	30	0	0	$0
ALQ72 (radar jammer)	0	0	30	0	$f0

• +0x0A = chaff effectiveness — confirmed via _DAMAGEDoHit@12; non-zero means aircraft has chaff dispensers.
• +0x0E = flare effectiveness — confirmed via _DAMAGEDoHit@12; non-zero means aircraft has flare dispensers.
• +0x12 = radar jamming effectiveness — confirmed via _PROJHitChance@28 (0x004c3380), read when the shooter has a radar seeker (missile+0xb4 == 3): Pk_final = (100 − eff_C) × Pk_base / 100.
• +0x17 = IR jamming effectiveness — same function reads ecm_ptr+0x17 for IR seekers (missile+0xb4 == 2).

_DAMAGEDoHit@12 (0x0040f970) case 9 reads the dispenser bytes independently across three hit-probability bands when the player is hit:

// 0–24%: active jammer hit — full lock break
if ((*(ushort *)(entity + 0x08) & 0x110) != 0) { *(ecm_struct + 4) = 0; }
// 25–64%: chaff dispenser hit — radar lock disruption
if (entity[0x0A] != 0) { *(ecm_struct + 6) = 0; }
// 65–99%: flare dispenser hit — IR lock disruption
if (entity[0x0E] != 0) { *(ecm_struct + 7) = 0; }

ALE40 has eff_A=30 and eff_B=30 (both dispensers present); ALQ72 has eff_A=0 and eff_B=0 (active jammer only, no passive dispensers). ✓

Jammer Geometry Bytes — confirmed

The six fixed bytes form two groups used by the jammer placement / seeker-defeat calculation:

Offset	Value	Group	Role
+0x0B	35	Radar	Effectiveness-scaling parameter — passed directly to PROJRetargetMissilesOnDevice / MPLaunchDevice
+0x0C	95	Radar	Jammer azimuth half-angle — passed as 95 << 8 (fixed-point) to MakeObjRotationMatrix param_2
+0x0D	24	Radar	Jammer elevation half-angle — passed as 24 << 8 (fixed-point) to MakeObjRotationMatrix param_3
+0x0F	35	IR	Effectiveness-scaling parameter — same role as +0x0B
+0x10	159	IR	Jammer azimuth half-angle — passed as 159 << 8 to MakeObjRotationMatrix param_2
+0x11	31	IR	Jammer elevation half-angle — passed as 31 << 8 to MakeObjRotationMatrix param_3

The radar group is selected when cStack_29 == 3 (radar seeker); the IR group when cStack_29 == 2 (IR seeker). Band codes 0x0C (radar) and 0x0D (IR) are passed to GRAPHICAddDevice as the jamming-band identifier.

MakeObjRotationMatrix confirmed (from PROJLaunchDevice decompile): takes (output_buf, azimuth_halfangle<<8, elevation_halfangle<<8, vertical_tilt). It computes the seeker's angular-bounds block (5 dwords) for cone-overlap testing. When vertical tilt is 0 (all game calls), it initialises symmetric ±azimuth / ±elevation bounds then calls sincos (sin/cos) and FUN_004cf2d0 (rotation). The larger the half-angle byte, the wider the jammer coverage arc — IR (159/31) is much wider in azimuth than radar (95/24).

Hypothesis disproven: the original theory that these bytes encode RWR radar-band frequencies (L, S, C, X, Ka) is incorrect. They are jammer effectiveness and beam-geometry parameters, consistent across all 30 ECM files because they describe the jammer hardware characteristics, not aircraft-specific threat coverage.

File Inventory

File	Notes
A6.ECM	Intruder
A7.ECM	Corsair II
A10.ECM	Thunderbolt II
AC130.ECM	Spectre gunship
ALE40.ECM	Expendable countermeasures dispenser
ALQ167.ECM	ECM pod
ALQ72.ECM	ECM pod
AV8.ECM	Harrier (limited ECM)
B52.ECM	Stratofortress
EA6.ECM	Prowler (dedicated EW aircraft)
F4.ECM	Phantom II
F8.ECM	Crusader
F14.ECM	Tomcat
F15.ECM	Eagle
F18.ECM	Hornet
F22.ECM	Raptor
F104.ECM	Starfighter
KA50.ECM	Hokum
MIG21.ECM	Fishbed
MIG27.ECM	Flogger
MIG29.ECM	Fulcrum
MIG31.ECM	Foxhound
SEAHAR.ECM	Sea Harrier
SU24.ECM	Fencer
SU27.ECM	Flanker
SU37.ECM	Terminator
TU26.ECM	Backfire
TU95.ECM	Bear
TU160.ECM	Blackjack
YAK141.ECM	Freestyle

All 30 live in FA_2.LIB.

Related

Formats: BRF — family grammar and the aircraft definitions that reference ECM suites; SEE — seeker/sensor definitions whose performance ECM degrades; JT — weapon seekers that ECM jams.