Date: Fri, 23 Oct 1998 17:03:44 -0700 (PDT) From: Anthony Jackson Reply-To: Anthony Jackson Subject: Sensors in GURPS I've been experimenting on and off with rewriting gurps sensors in a way which made more sense than what's in the standard rules; I'm not very sure of all my numbers, but I decided to post this so comments can be made -- so please, make comments ;). Much of the issue here was some straightforward reality checks -- I wanted to make it so you can see nearby stars, while spotting a person from a mile away is pretty difficult even if you know where to look. --0-2078917053-909187425=:31899 Content-Type: TEXT/plain; CHARSET=US-ASCII Content-Description: sensor.rules.gurps I can't make too many promises about the accuracy of these tables, they are largely an attempt to generate sane results from the game system, particularly in making sure that nearby stars aren't invisible, while a person a mile away is hard to see. Ranges are based on the standard range tables, but extended to ranges which might be interesting in space. 1) Converting Sensor Stats for these rules: If scan is 0-20, do not adjust. If scan is 20-36, halve and add 10 (round normally). If scan is 37+, divide by 3 and add 16 (round normally). Note that these are atmospheric scan values -- GT lists space scan values, which are 6 higher. Subtract the +6 for space before converting. For reference, eyes have scan 10. The relevant GT sensors are: Cockpit/10: passive (29/2+10)=25, active(33/2+10)=27, radscan(29/2+10)=25 Cockpit/12: passive (31/2+10)=26, active(34/2+10)=27, radscan(31/2+10)=26 Basic/10: passive (32/2+10)=26, active(35/2+10)=28, radscan(31/2+10)=26 Basic/12: passive (32/2+10)=26, active(36/2+10)=28, radscan(35/2+10)=28 Command/10: passive (33/2+10)=27, active(36/2+10)=28, radscan(32/2+10)=26 Command/12: passive (35/2+10)=28, active(37/3+16)=28, radscan(38/3+16)=29 2) Determining 'signature' * Visual signature is equal to size modifier, plus chameleon surface. In space, the black paint used in stealth coatings acts as a chameleon surface -- subtract (TL-4) for basic, (TL-2) for radical. Basic black paint reduces signature by 3. A reflective or white surface adds 2. * Radar signature uses the normal computations and _halves_ them -- thus, base value is (size/2), subtract (TL-4)/2 for basic, (TL-4) for radical. For convenience, assume Lidar uses the Radar signature. Note that a ship currently _using_ active sensors has a signature of (scan-TL) and can be detected by a radscanner. * IR signature uses either the size modifier of the vehicle, _or_ a function of power consumption and expected waste heat; this number is usually significantly greater (and is, in fact, probably higher than the numbers I use). In any case, emissions cloaking reduces this, by (TL-4)/2 for basic, (TL-4) for radical. The major power users on a ship are as follows: Artificial Gravity: 1 MW per 50 spaces or fraction thereof. Jump Drive: 10 MW per unit of J-drive. Not usually very relevant, but important if a ship exits jump and then goes cold. Manuever Drive: 1 MW per 10 tons thrust. Weapons Fire: 16 MW for TL 10 laser, 18 for TL 12; 400 MW for bay PAW/meson, 19000 for spinal PAW/meson. Add up power consumption: signature is +8 for 1 MW, +9 for 2 MW, +10 for 5 MW, and +3 per *10. In situations where a ship's signature _changes_, IR signature will drop by 1 per 10 minutes (2/turn); it will rise essentially immediately (<1 turn). Add +1 to IR signature per TL below 10 -- lower tech engines are less efficient (these numbers assume about 10% heat emissions). A human has a base emitted signature of +0. Example: a TL 10 merchant (size +8) has around 600 tons thrust (60 MW) and 4 MW for artificial gravity. Signature is +13; drops to +9 if not using thrust, +8 if all major power shut off. Example 2: a TL 10 400tn raider (size +9) has 6000 tons thrust (600 MW) and 8 MW for artificial gravity, but radical EM cloaking. Signature is +10 with drive active, +4 with artificial gravity online, +3 with all major power shut off. * Natural Objects: generally speaking, the size modifier of a natural object should be the negative of a range modifier equal to its diameter, +2 (natural objects are generally spherical). Normal modifiers apply; signature will usually be within a point of the size modifier. A star has a visual signature of 76-1.2*absolute magnitude (70 for sol). 3) Signature of 'Events': Certain short-term events are reasonably likely to be spotted, most notably weapons fire and jumping. An energy weapon firing has a signature based on its power output -- use 10* its normal power requirement and compute based on that. Specifically, this works out to +12 for lasers, +16 for bay weapons, +21 for spinal weapons. Nuclear weapons have a signature of +14 for 0.001 kT, and +3 per *10. A starship jumping insystem has a signature of 17 + 3log10(# of J); this works out to 5+(size*1.5), +1 for J-3 or higher, and lasts for a minute (giving +2 detection). 4) Detection Process: If this is initial detection, simply add up range, signature, and all relevant modifiers; if the total is less than zero double the result. Use this as a modifier for a sensor operations roll. If this is _not_ initial detection, use the above process, but you can usually assume that the bonus for field of view will be +5. Special case: resolving the shape of an object is very different from detection, at least in space -- it is, in fact, sufficiently different to warrant a completely different detection process. For resolving the shape, apply _only_ the following modifiers: Time (maximum +2), Focus (maximum +2), Target Size, Range, Intervening Atmosphere, -2 if using thermal sensors or lidar, -6 if using imaging radar. Special case: objects which are quite near each other. Treat as per resolving the shape of an object, but use the size modifier for the distance between the objects. What you roll will determine what information you get: Success by 0-2: detection. For optical sensors, gives direction and relative brightness. For radar sensors, gives range and apparent signature. For thermal sensors, gives direction and relative brightness. For shape resolution, gives size and general shape. Success by 3-5: recognition For optical sensors, as above plus exact color. For radar sensors, as above plus exact velocity. For thermal sensors, as above plus temperature. For shape resolution, gives sufficient information to identify a class of vehicle or entity. Battle damage will be visible, etc. Success by 6+: identification For optical/thermal sensors, spectral analysis may be performed, and a time-based brightness curve generated. For objects which have known spectral lines, velocity to/away (to within 1 km/sec) may be determined. For radar sensors, a time-based brightness profile may be generated, allowing determinations about rotation and some sorts of shape computation. for shape resolution, gives sufficient information to identify a particular vehicle or person. 4) Determine Range. This is an extended table of range modifiers, optimized for space. Range Mod Range Mod Range Mod 1000 miles -35 1500 miles -36 2000 miles -37 3000 miles -38 4500 miles -39 7000 miles -40 1 hex(10k mile) -41 2 hexes -43 3 hexes -44 5 hexes -45 7 hexes -46 10 hexes -47 15 hexes -48 1 light-second -49 2 ls -51 3 ls -52 5 ls -53 7 ls -54 10 ls -55 15 ls -56 20 ls -57 30 ls -58 0.1 AU -59 0.15 AU -60 0.2 AU -61 0.3 AU -62 0.5 AU -63 0.7 AU -64 1 AU -65 1.5 AU -66 2 AU -67 3 AU -68 4.5 AU -69 7 AU -70 10 AU -71 15 AU -72 20 AU -73 30 AU -74 45 AU -75 70 AU -76 100 AU -77 150 AU -78 200 AU -79 300 AU -80 450 AU -81 700 AU (.01 ly) -82 1000 AU -83 1500 AU -84 2000 AU -85 3000 AU -86 .1 light-year -87 .15 ly -88 .2 ly -89 .3 ly -90 .45 ly -91 .7 ly -92 1 ly (.3 pc) -93 1.5 ly (.5 pc) -94 2 ly (.7 pc) -95 1 pc -96 2 pc -98 3 pc -99 5 pc -100 7 pc -101 10 pc -102 15 pc -103 Initial Detection Roll: Add up all modifiers; if < 0, _double_ (this is a hack -- an expert operator (skill 18) shouldn't have 10x the detection range of a moderate skill (skill 12) operator. This reduces it to about 3x, but the expert will be much more reliable). Roll electronics/sensor operation at this modifier. Basic Table of Modifiers: Modifier Visual Radar Thermal Turn Length (the human eye is limited to a +2 bonus). Use the scan time or the duration of the event being observed, whichever is _less_. 1 second -2 -2 -2 2 seconds -1 -1 -1 4 seconds +0 +0 +0 15 seconds +1 +1 +1 1 minute +2 +2 +2 4 minutes +3 +3 +3 15 minutes +4 +4 +4 1 hour +5 +5 +5 Limited Focal Area (the human eye is limited to a +3 bonus) 360 degree arc -2 -2 -2 180 degree arc (peripheral vision) -1 -1 -1 120 degree arc (normal vision) +0 +0 +0 60 degree arc +1 +1 +1 30 degree arc +2 +2 +2 12 degree arc(1/5 range) +3 +3 +3 6 degree arc (1/10 range) +4 +4 +4 3 degree arc (1/20 range) +5 +5 +5 Location of target (use the best; not applicable to shape resolution) -On ground, daytime +0 -2 -2(2) -On ground, nighttime +0(1) -2 -2(3) -On ground, nighttime/vacuum +0(1) +0 +0(3) -Near ground, daytime +2 +2 -2(2) -Near ground, nighttime +2(1) +2 +0(3) -Near ground, nighttime/vacuum +2(1) +4 +2(3) (1) assumes some form of night vision apparatus. Otherwise, apply normal darkness modifiers. (2) if has an emitted signature, give +1 if = size-1, +2 if (size), +4 if (size+1), +6 if greater than size+1; this is in addition to using emitted signature rather than size modifier. Reasonably likely to be applicable to spacecraft. For reference, this gives a +2 for spotting humans. (3) as above, but add +1 to emitted signature when computing bonus. -Silhoutte against space, nighttime +14(*) +8 +8 * assumes a truly dark sky. Near a city will be much worse. -Silhoutte against space, daytime +2 +8 +2 -Silhoutte against space, vacuum +14 +12 +12 -near planet or asteroid -3 -3 -3 -silhoutte against primary * -8 * Use the 'shape resolution' case, described above, but with a +2 bonus for anti-chameleon. You can only see an object against a star by seeing black spot, and that's _much_ harder than seeing an illuminated object against space. -solar corona -6 -6 -6 Note that lighting modifiers will usually negate the vision mod, and are reasonably likely to negate the IR modifier as well. -target within inner zone +0 -3 +0 Target Behavior (only applies to initial detection) Target moving against backdrop +2 +2 +2 Means 'moving by normal resolution of sensor in one turn'. To determine this, take (range mod)-(size mod of distance)+scan. For even the lowest-grade space sensors this means that an object moving at orbital velocity around a planet an AU away appears to be moving, so this modifier usually applies to detecting ships. Significant velocity towards/away +0 +2 +0 Assumes a doppler radar of some kind. Intervening atmosphere (if -10 or worse vision is totally blocked) Thin (per 5 miles, or to space) -0 -0 -1 Moderate (as above) -1 -0* -2 Dense (as above) -2 -1* -4 *Add additional -1 if going into space, due to layers in the upper atmosphere. Clouds (per 50 yards) -5 -0 -1 Gas Giant Atmosphere (choose X) -x -x/2 -x Lighting Brightness of Primary (X=Sig-70) +x(1) +0 +0(2) Nearness of Primary (X=range+67) +x(1) +0 +0(2) Shadow of Planet -12 +0 +0(2) (1) only if silhoutted against space. (2) modifier applies to signature based on size, but not signature based on power consumption; max -6 if object is warm enough for humans to survive. Simplifications for space combat: In general, for space combat purposes, you will have a +12 for IR/radar in space, a +4 for turn length, a -2 for 360 scan, and a +2 for a moving object (negating this is quite hard, though not impossible), for a total of +16 to scan; this gives a TL 10 basic bridge a 42 passive, a TL 12 command bridge a 44. This allows a ship which is mostly powered down to evade detection at reasonable ranges, but is significantly better range than what GT normally gives. Example: For a human to spot alpha centaurus (signature +71, range -97), at night: +10 scan, +71 signature, -97 range, +14 terrain, -1 atmosphere, total -3, doubled to -6. Probably not going to spot it randomly. However, scan a 30 degree arc (+2) and spend 15 seconds (+1) and you've got a pretty good chance. Example 2: for a merchant (scan 26 passive, net 42) to detect a pirate with no drives, but running artificial gravity (signature +4) at half the jump limit (2 light-seconds, range -51): total -5, doubled to -10, virtually zero chance. The pirate, meanwhile, has a TL 10 command bridge with scan 27, net 43, target signature +13, net +5 -- no problem. If the pirate kicks in his drives signature jumps to +10, for a +1 to scan -- depends on the alertness of the operator.