MISSIONS.DAT

contains counters and race info on the ongoing acts. At the start of the game (beginner level) research in your home zone and terror in a random zone is initiated. After that each new month you will get two new missions, one of which is a terror mission (beginner level).

Structure of a mission entry

Each mission has 4 words. With 7 acts on 12 zones these are 672 bytes. E.g. infiltration(3) in Europe(4) would have the offset 3*8+4*(7*8)=0xF8.

offset	meaning
0	ufo-counter on the ufos within the mission (FFFF, the mission is not active)
2	sub-ufo-counter used for some missions
4	time-counter, 30 minutes to next ufo
6	alien race of mission

The race number can be looked up in ENGLISH.DAT, entries 466-476:

 0	Sectoid
 1	Snakeman
 2	Ethereal
 3	Muton
 4	Floater
 5	Celatid
 6	Silacoid
 7	Chryssalid
 8	Reaper
 9	Sectopod
10	Cyberdisc

Timetable of a mission

When a mission starts, the ufo-counter is set to 0 and the time-counter to some act-depending time. The time-count is reduced every 30 minutes. When 0, an ufo will appear, the ufo-counter (or the sub-counter on some mission) will be increased and a new random time-counter for the new ufo will be set.

Note on the timing

Battlescape missions do not affect the scheduling of future UFOs.

UFO appearances are established with a base time period to which a random amount of time derived from the base time is added. The effect is that a UFO arrival will be scheduled 50-150% of the base time. When the mission is created, the first UFO in the mission is spawned when the timer reaches zero. Then the mission entry details are set for the next the next UFO in line and the timer begins counting down again.

Shooting down a UFO will immediately add 24~224 hours (1~9 days) to the timer for the next UFO in that mission. Tycho (talk)

Research

ufo-c	time-c	ufo-type 
0/0	150h	s.Scout  (start of game Research mission is 2.5h*.  This is for later ones)
1/0	130h	m.Scout
2/0	150h	l.Scout
2/1	150h	l.Scout

Harvest

ufo-c	time-c	ufo-type
0/0	 130h	s.Scout   
1/0	 130h	s.Scout
2/0	 100h	m.Scout
3/0	 130h	l.Scout
4/0	 150h	l.Scout
5/0	  50h	Harvester
6/0	12.5h	Harvester
7/0	  25h	Battleship

Abduction

ufo-c	time-c	ufo-type
0/0	250h	s.Scout
1/0	300h	m.Scout
2/0	150h	l.Scout
3/0	150h	Abductor
4/0	  1h	Abductor
4/1	  1h	Abductor

Infiltration

ufo-c	time-c	ufo-type
0/0	275h	s.Scout
1/0	250h	m.Scout
2/0	230h	m.Scout
3/0	150h	l.Scout
4/0	  1h	l.Scout
5/0	  1h	Terrorship
6/0  	  1h	Supply
7/0	  1h	Battleship
7/1  	  1h	Battleship

Base

ufo-c	time-c	ufo-type
0/0	1w	s.Scout
1/0	1w	m.Scout
2/0	1w	l.Scout
3/0	1.5h*	Supply   
3/1	1.5h*	Supply   
4/0	0.5h*	Battleship

Terror

ufo-c	time-c	ufo-type
0	2.5h*	m.Scout
1	1w	l.Scout
2	1w	Terrorship
3	1w	Terrorship

Landing sites of Terrorships are terror missions, not ground assault missions. Your radars don't want to track a Terrorship after it drops off its cargo.

sub-ufo-c is empirically always 0 for Terror acts.

Retaliation

ufo-c	time-c	ufo-type
0/0	50h*    s.Scout
1/0	?       m.Scout
1/1	32h*	m.Scout (add 146.5h* if 1/0 shot down at first turning point)
2/0	52h*    l.Scout (add 3d if 1/1 shot down at first turning point)
2/1	?	l.Scout
2/2	3d	l.Scout (Not always...finally confirmed it could happen, no idea what factors control this one's existence.)
3/0	2d      Battleship
3/1	2d	Battleship

This mission only relates to the aliens attempting to find an unknown XCOM base in a zone. Once the location of a base is known to the aliens, another routine in the daily activities section generates the battleship that will attack the base.

A retaliation mission is possibly generated after a successful interception. The zone the mission is set will be either the location of the interception or the zone of the base from which the interceptor was launched. The random chance of a mission being generated and the chance of which zone is selected is affected by the difficulty level of the game.

After a certain point in research or after about 9 months of game time, retaliation missions are automatically generated for each zone that contains an XCOM base at the beginning of each game month.