Definitive Sensor Rules

Date: Sun, 3 Aug 1997 19:20:32 -0700
From: bmac@astro.ucla.edu (Bruce Alan Macintosh)
Subject: Definitive Sensor Rules

Introduction

As part of the FFS2 project, I offered to help with better passive sensor rules, based on realistic simulations of detector properties, background at different wavelength, and brightness of a target spacecraft at different wavelength. Target signature was modelled with three main components: reflected starlight, thermal emission from the hull, and thermal
emission from waste-heat radiators; the radiators were based on the assumptions that went into FFS2's powerplant rules (please - no debate about power plant radiator realism!) and were usually the dominant component, though reflected sunlight is also very high for non-black starships.

I may write a long article about the results later, but the main result was that even very small sensors can achieve initial detection - though not necessarily a position accurate enough for fire control - at very long ranges; even up to millions of km for a TL-8 5-m diameter sensor (a P1 in T4 terms) for a 100-ton non-military target.

Based on this I set out to write more realistic sensor rules. Improvements include a straightforward system for calculating initial detection ranges for any target (based on some suggestions by Anders, mostly). This seems especially useful to roleplayers - it's nice to have rules that define precisely how far away (for example) at Type S can detect a powered-off lifeboat parked next to an asteroid. it's also useful for heavyduty military gearheads - it answers questions like "can a big military sensor array detect a starship refuelling at a comet in the Kuiper belt".

The rules also make a distinction between detecting a target and achieving a fire control solution, and adding a much wider range of possible sensors (it always bugged me that any medium-sized FFS ship could fit the best possible sensor.)

Unfortunately, while I know a lot about visible/IR sensors, I know very little about radar - in the absence of a radar expert, Dave and I put together active sensor rules through the ancient Vilani technique of "Making It Up." In particular we tried to adjust things so that active sensors were roughly game-balanced with passive sensors (though I guess that this somewhat overrates active sensors.)

Also unfortunately, the rules as they appear in FFS2 are somewhat incomplete;
they include the details of how big these new sensors are, but not much detail on how to actually use these sensors in combat or role-playing. So, as a preview and general service, I figured I'd post my (draft) sensor rules here. I'll also include rules for converting FFS/QSDS/SSDS sensors and ships to the new system. FFS2 includes much more detail for designing ships with sensors - including very large ones - optimized for this new system, new ECM options, etc., and is recommended for people wanting to take full advantage of these rules.

I've broken the rules up into three parts for this posting: Basic (Part 2), Intermediate (Part 4), and Advanced (Part 5), with Basic covering all you need or simple spacecraft operations in a free space, intermediate adding more terrain options and a different treatment of active sensors, and advanced adding some even-more-complicated terrain rules for exotic solar systems. The rating/conversion rules get their own section (Part 3.)

If any Traveller web site maintainers are interested in this, I'd be willing to see it posted on a (non-IG) web site. If IG is interested in this, I'd be willing to negotiate turning it into a JTAS article or a rules section for a future book (like "Imperial Squadrons", or NAH.)

The rules aren't particularly tied to any combat system. They are oriented towards role-playing with a referee keeping enemy ships hidden, but could be used for two-player combat with a defending player keeping their ships secret or both players using dummy counters a la BR.

[Some designer's notes are intersperesed in square brackets like this.]
[These are drafts. Not all the exotic situations have been modelled in detail; some numbers are estimated and will change. I'm also willing to tweak models for game balance and would *love* to hear from people using these in role-playing situations or combat.]

Part 2: Basic Rules

The target detection attempts are resolved by evaluating the SIGNAL of each attempt using the formula below, and then comparing the signal to the success chart (Table 1) to determine the difficulty of the sensor operator's task. The SIGNAL must be >=0.0 for there to be any chance of detection; if the signal is sufficiently high detection is automatic.

Very important: only one passive and one active (if active sensors are on) sensor can be used per sensing ship per target per turn, even if the sensing ship has multiple sensors. [This is a game-balance decision; because the T4 task system is so coarse and even Impossible tasks so easy, it would otherwise always be better to put two cheap sensors on a ship than one expensive one. The intermediate rules do allow a ship to use multiple sensors for different purposes (like scanning different arcs.)]

Table 1: Detection Task Difficulty

SIGNAL Difficulty

<0	target cannot be detected under any circumstances.
0.0	Impossible	(TNE: Impossible)
0.5	Staggering	(TNE: Formidable)
1.0	Average	(TNE: Average)
>=1.5	Automatic detection

SIGNAL is calculated as follows:

SIGNAL = SENSITIVITY- RANGE + SIGNATURE +(MODIFIERS)

SENSITIVITY is the sensitivity rating of the sensor attempting the detection (determined in the design process.)

SIGNATURE is the target's signature, also determined during the design process but sometimes modified for specific conditions or actions. Active sensors use the targets active (radar) signature. Starships are rated with two different passive signatures. For the basic system, use only the target's emitted (infrared) signature.

MODIFIERS will adjust the target's signature based on its actions.

RANGE is the range factor, taken from the range chart (Table 2) - note that these range factors are chosen to match what I was told the T4.1 range bands would be.

Table 2: Range factors

Range:	RANGE
km	BL Hexes	T4 name	T4.1 name	term
<=500			regional	8
<=5,000	0		continental	9
<=50,000	1-2	VS	planetary	10
<=500,000	3-16	S	far orbit	11
<=5,000,000	17-160	M and L		12
<=50,000,000	161-1600			13
<=500,000,000 (3 AU)			interplanetary	14
<=5,000,000,000 (30 AU)			outsystem	15
<=50,000,000,000 (300 AU)			"oort"	16
<=500,000,000,000 (3000 AU)				17

Finally, the target's signature is modified by the following actions or conditions:

Condition	Active Sig. (radar)	Passive Sig. (emitted/IR)
Non-maneuvering	0.0	-0.5
Shutdown	0.0	-1.0
Using active sensors	+1.0	+1.0
Surprised	+0.5	+1.0
same hex as planet or asteroid	-1.0	-0.5
in atmosphere	0.0	-0.5
landed	-2.0	-1.0

Explanations:

Shutdown: all power (except life support and passive sensors) shut down - may not fire weapons or maneuver until powerplant restarted (which normally requires one full turn.)

Using active sensors: ships using any using any active sensor any active sensor must annouce this to all ships with functional active or passive sensors within the same solar system. [Yes, that's correct, within the solar system; there's an intermediate rule that defines precisely how far you can see an active sensor, but the answer is a long, long, way. For a real-world example, military over-the-horizon ICBM early warning radars (about a sensetivity 10 or 11) can be detected by an Arcebo-sized radio telescope (about a sensitivity 14 PEMS) several parsecs away (range factor 19.)]

Surprised: Targets that are unaware of the presence of any enemy ship (ref's discretion) are significantly easier to detect. [Ships are normally assumed to manage their thermal radiators to radiate most heat away from suspected enemy craft - which is impossible if you don't know there are any enemy craft out there.]

Planet: targets within the same 30,000km BL hex as a planet or asteroid are assumed to be using it for cover wherever possible.

In the basic system, once a target is detected, the referee should inform the sensor operator of that fact and the target's signature, and place the ship's counter on the map (if you're using one). In the basic system ships remain detected forever (or until they move far enough away to reduce their SIGNAL to -1.0). After one turn of detection, the referee can inform the sensor operator of the tonnage and basic configuration of the target; after two turns, the class.

Fire control:

For slightly more detail, at the instant of weapons fire, the firing ship must obtain a succesful FIRE CONTROL LOCK on the target. The same procedure as above is used, but the SIGNAL is reduced by -1.5 - firing on a target is much more difficult than merely detecting it. All other modifiers apply. Optionally, targets without a fire control lock may be fired on, but the fire task is increased by 3 difficulty levels.

Examples:

At interplanetary (500,000,000km) range detection the passive SIGNAL is

	14	+	0	-	14	=	0
	sensitivity		signature		range		SIGNAL

and detection is an Impossible task. The active SIGNAL is 12 + 0 - 14 = -2 and the target cannot be detected with active sensors.

If the range were 5,000,000km, the passive signal would be 14 + 0 - 12 = 2 and detection would be automatic.

If the target had shut down its power plant at 5,000,000 km, the passive signal would be 14 + 0 - 1.0 - 12 = 1 (average task.)

If the scout ship was then attempting to obtain a fire control lock at a range of 500,000km, the signal would be 14 - 1.5 + 0 - 1.0 - 11 = 0.5 (formidable task) - but the trader would be unable to return fire (since its power plant is shut down.)

Part 3: Rating ships.

FFS2 also distinguishes between sensors that are optimized for detecting new targets ("scanners") and those optimized for fire-control ("trackers") but sensors converted from FFS are considered to be dual-mode tracker/scanners (as are most high-TL sensors in FFS2), except for LADARs.

Sensor ratings:

Passive sensors from FFS/QSDS/SSDS are converted to the new system using Table 3:

Table 3: Passive Sensor Conversion Table

FFS range(hexes) or T4 rating	Sensitivity
0.01 - 0.1	13
1-2	13.5
3-4	14
5-6	14.5
7-8	15.0

Active sensors use Table 4:

Table4: Active Sensor Conversion Table

FFS range(hexes)or T4 rating	Sensitivity
0.01-0.1	11.5
1-7	12.0
8-16	12.5

LADAR sensors use Table 5 - but LADARs are considered "Trackers", and can only be used for fire control locks or for maintaining contact with previously detected targets.

Table 5: LADAR Sensor Conversion Table

FFS range(hexes) or T4 rating	Sensitivity
0.01-0.1	12.0
1-7	12.5
8-16	13.0

SIGNATURES:

The base radar signature is given by the following table:

surface area	radar signature	(hull size)
0.1-9m^2	-0.5	<1 ton
10-999m^2	0	1-200 tons
1000-99999m^2	+0.5	200-90,000 tons
100000-9999999m^2	+1	100,000 tons+

The hull size column is provided for T4 designs whose surface area is not given in the USP - use the hull size column instead. EMM decreases radar signature by 0.5.

The passive emitted (or infrared) signature is calculated based on a ship's power plant output. (For T4 ships that don't list the power plant output, use (Power Plant Rating * Tons / 2).)

Power	signature
0.000-0.009 MW	-2.5
0.01-0.09 MW	-2.
0.1-0.9 MW	-1.5
1 -9 MW	-1
10 MW	-0.5
100 MW	0
1000 MW	0.5
10000 MW	1
100000 MW	1.5
1000000 MW	2

Ships with Stealth reduce their emitted signature by 0.5. Ships with EMM reduce their signature by 1.0

(as an Advanced rule, increase the power by 0.0001 MW per m2 of surface area to include heat loading due to absorbed sunlight - this is normally negligible except for ships with power plants shut down.)

(also as an advanced rule, rather than using the generic -0.5 non-manuevering or -1.0 shutdown bonus in combat, designers may rate the emitted signature at different power levels (such as with all systems except life support shut down, or with power-using weapons not firing.))

The passive reflected (visible) signature, used in the Intermediate and Advanced rules, is calculated based on a ship's surface area:

surface area	reflected signature	hull size
1-9m^2	-2	<1 ton
10-99m^2	-1.5	1-95 tons
100-999m^2	-1	100-200 tons
1000-9999m^2	-0.5	300-3000 tons
10000-99999m^2	0	4000-100,000 tons
100000-999999m^2	0.5	200,000 - 1,000,000 tons
1000000-9999999m^2	1.0

(The hull size table is again provided for lazy T4 owners.)

Ships with TL10+ EMM reduce their reflected signature by 0.5

These base signatures are for normal TL10+ starships, which are assumed to have "chameleon" hulls that can change color and pattern. This capability is normally used for adverstising and thermal management, but in a combat situation the hull is adjusted to be as black as possible (typically 99% black for a civilian hull.)

Starships without this color-changing coating - TL9- ships, or (optionally) some higher TL cut-rate civilian ships - increase their reflected signature by +1. TL8-9 ships with Stealth ignore this penalty.

Example:

If the ship was equipped with EMM the emitted signature would be reduced to -1.0 (and the active and reflected to -0.5 and -2.0 respectively.)

If the ship was then running without manuevering, with the powerplant running at 50 MW, the emitted signature would be reduced still further to -1.5.

Even with the power plant completely shut off, the effective power is still 0.3 MW, for an emitted signature (including masking) of -2.5 The scout has a T4 sensor rating of A2 P3, which converts to a new sensor rating of A12 P14

A designer would record this on the ship record as follows:

Active Sig:	(-0.5)
Passive Sig (emit.refl):	(-1/-2)
	(-1.5/-2 at 50 MW power)
	(-2.5/-2 when shut down)

Or (the short form): Signature (act/emit/ref): -1/-1/-2

Part 4: Intermediate rules:

Revised Table 1: detection task difficulties:

SIGNAL	active detection task	passive detection task
<0	(target cannot be detected under any circumstances)
0	Impossible	Impossible
0.5	Average	Staggering (TNE: Formidable)
1.0	(automatic detection)	Average
1.5	Easy
2.0	(automatic detection.)

The second is a range table with fractional range bands (in-between the T4 range bands):

Revised table 2: Range factors

Range:		RANGE
km		BL Hexes	T4 name	T4.1 name	term
<=500				regional	8
<=1,600					8.5
<=5,000				continental	9
<=16,000		0			9.5
<=50,000		1-2		planetary	10
<=160,000		3-5	VS		10.5
<=500,000		6-16	S	far orbit	11
<=5,000,000		17-50	M		12
<=16,000,000		51-160	L		12.5
<=50,000,000		161-500			13
<=160,000,000	1 AU	501-1600			13.5
<=500,000,000	3 AU			interplanetary	14
<=1,600,000,000	10 AU				14.5
<=5,000,000,000	30 AU			outsystem	15
<=16,000,000,000	100 AU				15.5
<=50,000,000,000	300 AU			oort	16
<=500,000,000,000	3000 AU				17

The third change is that all ships have two different passive signatures, their emitted (infrared) signature, and their reflected (visible) signature. A passive sensor detection attempt must be evaluated against whichever of these provides the higher total signal. Normally this will be the emitted signature.

The fourth is that detected targets no longer remain automatically detected but must be reaquired each turn; however, previously-detected targets get a 1.5 increase to their signal, which generally makes repeat detection automatic. (Note that this modifier does NOT apply to fire control locks.)

Finally, the intermediate rules use a vastly expanded table of terrain and condition modifiers. These can be broken down broadly into effects that modify the target's signature and effects that modify the sensor's sensitivity:

Modifiers to signature

Condition	Active Sig. (radar)	Passive Sig. (emitted/IR)	Passive Sig. (reflected/vis)
Target actions:
Non-maneuver ing	+0.0	-0.5	0.0
Shutdown	+0.0	-1.0	0.0
Firing beam weapons	+0.5	+1.0	+0.5
Launching msls/SC	+0.5	+0.0	+0.5
Using active sensors	+1.0	+1.0	+1.0
Evading	-0.5	-0.5	-0.5
using HEPlaR 1-2G	+0.0	+0.5	+0.0
using HEPlaR 3-20G	+0.0	+1.0	+0.0
using HEPlaR 21G+	+0.0	+1.5	+0.0
Surprise modifiers:
Surprised	+0.5	+1.0	+0.5
Alert	+0.0	+0.5	+0.0
Battle Stations	+0.0	+0.0	+0.0
Terrian modifiers:
same hex as planet or asteroid	-1.0	-1.0	-0.5
in shadow	-0.0	-0.0	-2.0
landed	-2.0	-1.0	-0.5
landed and camoflaged	-2.0	-1.0	-1.0
near large GG	-0.5	-0.5	-0.0
Atmosphere modifiers
In atmosphere 6+	0.0	-0.5	-0.0
In atmos 8-9	0.0	-1.0	-0.5
In atmos A+	-0.5	-1.5	-1.0
upper GG atmosphere	-1.0	-2.0	-1.5
lower GG atmosphere	-1.5	-2.5	-2.5
deep GG atmosphere	-2.5	-4.5	-4.5
General modifiers (not for FC locks):
Target was detected last turn	+1.5	+1.5	+1.5
Target was detected within last 10 turns	+0.5	+0.5	+0.5

Non-maneuvering: no use of maneuver drive (except micro-evasion) during past turn.

Shutdown: all power (except life support and passive sensors) shut down - may not fire weapons or maneuver until powerplant restarted (which normally requires one full turn.)

Firing beam weapons opens covers exposing (hot) weapon focal plane arrays and reducing stealthing. Firing missiles or launching/recovering small craft similarly increases signatures

Using active sensors: ships using any using any active sensor any active sensor must annouce this to all ships with functional active or passive sensors within a range equal to the active sensor range + passive sensor range minus 5 (which is normally the whole solar system and occasionally the whole subsector.)

Surprised: Targets that are unaware of the presence of any enemy ship (ref's discretion) are significantly easier to detect. See the surprise rules in BR for details - ships can either be completely surprised, on alert status (some battle stations manned but no hostiles detected) or at battle stations with an enemy detected (no modifiers.)

Evasion: succesful BL/BR evasion attempt

HEPlaR (and Fusion rockets) have a large signature penalty.

Planet: targets within the same 30,000km BL hex as a planet or asteroid are assumed to be using it for cover wherever possible.

Ships in shadow (generally any ship in the same hex as a planet or asteroid may opt to be in shadow) have a much lower reflected (visible-light) sig.

Landed ships are any ship that takes one turn to land on a planet or asteroid surface; they are assumed to set down to take advantage of terrain, and to dump waste heat into the object rather than radiate it. Modifier is not cumulative with "same hex".

Landed ships may be camoflaged - this requires one person-hour per 100 dTons of ship, is a Formidable task (Ship Tactics or Sensor), and requires camoflage equipment (MCR 0.01 and 0.1 tons per 100 dTons of ship, comes free with EMM or any other masking.) Ships lose their camoflage advantage if they fire weapons or (obviously) maneuver. Clearing the camoflage requires 10 person-minutes per 100 dTons of ship.

Being within an atmosphere (or a gas giant atmosphere) reduces detectability and sensor sensitivity. These modifiers are cumulative with "landed" or "same hex" as applicable.

Ships in a gas giant atmosphere can be at one of three depths - upper, lower, and deep. Moving from one to another requires one turn.

Sensitivity modifiers:

Condition	Active Sense. (radar)	Passive Sense. (emitted/IR)	Passive Sense. (reflected/vis)
In atmosphere 6+	0.0	-0.0	-0.5
In atmos 8-9	0.0	-0.5	-1.0
In atmos A+	-0.5	-1.0	-1.5
upper GG atmosphere	-1.0	-2.0	-1.5
lower GG atmosphere	-1.5	-2.5	-2.5
deep GG atmosphere	-2.5	-4.5	-4.5

Intermediate example:

Part 5: Advanced

Condition	Active Sig. (radar)	Passive Sig. (emitted/IR)	Passive Sig. (reflected/vis)
Non-maneuver ing	+0.0	-0.5	0.0
Shutdown	+0.0	-1.0	0.0
Firing beam weapons	+0.5	+1.0	+0.5
Launching msls/SC	+0.5	+0.0	+0.5
Using active sensors	+1.0	+1.0	+1.0
Evading	-0.5	-0.5	-0.5
using HEPlaR 1-2G	+0.0	+0.5	+0.0
using HEPlaR 3-20G	+0.0	+1.0	+0.0
using HEPlaR 21G+	+0.0	+1.5	+0.0
Chem rocket 1-10G	+0.0	+0.5	+0.0
Chem rocket 11+G	+0.0	+1.0	+0.0
Surprised	+0.5	+1.0	+0.5
Alert	+0.0	+0.5	+0.0
Agressive baffling vs one target	+0.0	-1.0	+0.0
Agressive baffling vs. all others	+0.0	+0.5	+0.0
same hex as planet or asteroid	-1.0	-1.0	-0.5
in shadow	-0.0	0.0	-2.0
landed	-2.0	-1.0	-0.5
landed and camoflaged	-2.0	-1.0	-1.0
near large GG	-0.5	-0.0	-0.0
In atmosphere 6+	0.0	-0.5	-0.0
In atmos 8-9	0.0	-1.0	-0.5
In atmos A+	-0.5	-1.5	-1.0
upper GG atmosphere	-1.0	-2.0	-1.5
lower GG atmosphere	-1.5	-2.5	-2.5
deep GG atmosphere	-2.5	-4.5	-4.5
within 30 degrees of star	0.5	-0.5	-0.5
Target was detected last turn (N/A to FC)	+1.5	+1.5	+1.5
Target was detected within last 10 turns (N/A to FC)	+0.5	+0.5	+0.5

Target location:

Inner zone	-0.5	+0.0	+1.0
Habitable zone	+0.0	+0.0	+0.0
Outer zone	+0.0	+0.0	-1.0
>100 AU	+0.0	+0.0	-2.0
>1000 AU	+0.0	+0.0	-3.0
>10000 AU or deep space	+0.0	+0.0	-4.0

Ships that have detected an enemy target can employ "agressive baffling", turing their radiators away from that single target but increasing their signature elsewhere.
Being in the same arc as the star (in the Habitable zone or closer) relative to the sensing ship decrease detectability. (BL/BR players should designate one direction to be the "sun" direction and apply this bonus to any target in the same 60 degree arc.)

Sensitivity modifiers:

Condition	Active Sense. (radar)	Passive Sense. (emitted/IR)	Passive Sense. (reflected/vis)
In atmosphere 6+	0.0	-0.0	-0.5
In atmos 8-9	0.0	-0.5	-1.0
In atmos A+	-0.5	-1.0	-1.5
upper GG atmosphere	-1.0	-2.0	-1.5
lower GG atmosphere	-1.5	-2.5	-2.5
deep GG atmosphere	-2.5	-4.5	-4.5
Sensor scanning single 30 deg arc	+0.5	+0.5	+0.5
Sensor scanning single hex	+1.0	+1.0	+1.0
Sensor location:
Inner zone	+0.0	-0.5	-0.5
Habitable zone	+0.0	+0.0	+0.0
Outer zone	+0.0	+1.0	+1.0
Dust level (inner or habitable zone only):
Normal	+0.0	+0.0	+0.0
None	+0.0	+0.5	+0.5
Light	+0.0	+0.0	+0.5
Heavy	+0.0	-0.5	-0.5
Extreme	-0.5	-1.0	-1.5

Single arc: restricting a sensor to a single 30-degree arc improves sensitivity. Restricting scans to a single hex (including the ship's own) improves it still more. Note that ships may use multiple sensors to each scan a different arc or hex.

Location/Dust: sensitivity is reduced in the inner portions of a solar system due to scattered light from zodiacal dust. Some systems (referees discretion until I write the rules...) may have light or no such dust, particularly older stars and/or stars with no planetoid belts. Young stars or stars with several planetoid belts may posses heavy dust; extremely young systems with thick protoplanetary disks qualify for the "extreme" modifier.

Example:

Scan duration:

Turn Length	Sense Mod	Notes
1-60 seconds	-0.5	personal combat turns
1-120 minutes	0.0	space combat turns
2-100 hours	+0.5	"system crossing" timescale