When to use Passives, When to use Actives

[Borealis4x (OP)]

I'm still unsure as to whether to add Passive Sensors or Active Sensors (or both) to a ship.

For instance, do capital ships that can't really hide anyway really need passive sensors when they have at least 3 different tiers of powerful actives? Do you want to put actives on vulnerable scout fighters to boost their efficiency, or passives to keep them hidden? What about ELINT scouts that you want to be concealed? Are there situations where you would only use one type of passive over the other?

[tobijon]

Passive sensors usually have a higher range than active sensors when detecting large ships, so they are still useful on capital ships, but that's your choice. Active sensors are for when you want to shoot something, so in the case of fighters, the usual approach is to have some with weapons and some with active sensors to give them targets. Passive sensors are useless on fighters as you can't use them to fire, and you don't generally use fighters unless you already know the enemy is there. Don't forget that active sensors can be detected so stealth ships should not have them.

[liveware]

I personally like to add at least one size smallest thermal and/or EM sensors and/or active sensors to every ship I build (in that order), other mission-related design constraints notwithstanding. That way if a ship gets ambushed I can at least get some idea as to what killed it.

Regarding actives vs passives, passives cannot be detected by enemy passive sensors. Actives can be picked up by enemy EM passive sensors at very long range, which means an enemy may be able to see your big active sensors from outside of active sensor range. Actives also have a hard upper range limit of about 150 million-ish km (if memory serves me properly) while passives have no effective upper range limit and are entirely dependent on the strength of the emission source. 150 million km is not a very large distance in Aurora terms.

Actives are an absolute requirement for targeting any sort of weapon system however, so you will always need some around to fight things. Beam ships can get away with relatively small active sensors as beam combat ranges are very short, typically well under 1m km. Missile ships can make use of much longer ranged active sensors. I personally prefer to design carriers with missile bombers instead of missile cruisers but missile cruisers are certainly strong combatants and are worth exploring.

Some other considerations are:

1. Colonies and large engines produce large thermal signatures which can be detected at long range by relatively small passives
2. Shields, large active sensors, and colonies produce large EM signatures which can be detected at long range by relatively small passives

A stated design goal by Steve for the current implementation of active/passive sensors has been to encourage the deployment of smaller sensor picket ships instead of a single large sensor platform as the single large sensor platform approach had become a dominant strategy in VB Aurora.

To illustrate some interesting uses of passives, below is my stealth scout design that I've been using for a while. At ranges greater than about 1m km, it is undetectable by some noteworthy spoilers (Precursors), and thus is very useful for scouting out systems in which they may potentially be laying in wait.

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SS-2 class Stealth Scout      24 tons       1 Crew       8.8 BP       TCS 0    TH 0    EM 0
1019 km/s      Armour 1-0       Shields 0-0       HTK 1      Sensors 1/1/0/0      DCR 0      PPV 0
Maint Life 24.66 Years     MSP 20    AFR 5%    IFR 0.1%    1YR 0    5YR 1    Max Repair 12 MSP
Lieutenant Commander    Control Rating 1   
Intended Deployment Time: 8 months    Morale Check Required   

UNRC Magneto-plasma Drive  EP0.48 (1)    Power 0.5    Fuel Use 29.58%    Signature 0.240    Explosion 3%
Fuel Capacity 1,000 Litres    Range 25.8 billion km (293 days at full power)

UNRC M2XS Thermal Emission Detector (1)     Sensitivity 1.1     Detect Sig Strength 1000:  8.3m km
UNRC M2XS Electromagnetic Wave Detector (1)     Sensitivity 1.1     Detect Sig Strength 1000:  8.3m km

This design is classed as a Fighter for production, combat and planetary interaction


[sadoeconomist]

I'm still pretty new so I've been trying to work this out myself lately as well, and I've come to some conclusions.  If you're trying to use sensors optimally on ships in your main fleet, I think you should ideally have a dedicated ship type for each type of sensor you want to use - both passive types and one for each resolution of active sensor you think is relevant.  Sure, you can put small backup sensors on large ships for just in case they happen to get separated so they can fire their weapons, and you might want to put a lot of redundant small active sensors on jump point assault ships, but small sensors don't actually do anything for you if they're always in the same location as a larger one of the same type.

Ships in an efficiently built fleet will be limited to certain maximum tonnages because of jump drives and shipyard capacity and various other considerations.  Because of that, I think instead of having two equal-sized large sensors on two ships, it's almost always going to be smarter to have one sensor that's twice as big on one ship and a different type of sensor that's twice as big on another, or more defensive capability instead of the second sensor.  And then if you want more redundancy, build more ships.  I've seen a lot of ship designs with both kinds of passive sensor in equal amounts on them and if you're doing that, think more about the ship's role, what it's searching for, what it needs to defend against, and whether it wants more thermal range or EM range.

In addition to the big primary sensors in the fleet itself, to extend the sensor capabilities of your fleet, you should have pickets in formation around it at set distances and angles.  With pickets, IMO the main thing you're trying to do is to keep your fleet from getting painted by an active sensor, so EM is maybe the most important capability for them, you want to try to spot small enemy scout ships with high-R sensors before they can get into range.  You'll also probably still be covered by the high-R sensors of the fleet, and high-R sensors have such high EM emissions that they'll make your pickets easy to find, so they should have low-R actives.  Thermals I think are less important on defense - the only threats to your fleet that 1.  don't radiate a lot of EM 2.  have a small enough ship size to avoid high-R sensors and 3.  also would show up on thermals would be fast small ships with short-ranged weapons, and detecting those very far out from your main fleet doesn't do too much for you since they can't do damage without closing the distance anyway.  Speaking of fast squadrons of small ships, those are probably what you should have to quickly react to any enemy scouts your pickets find.

Out beyond your pickets I think there are two more classes of scouts you'll need to have - scouts with active sensors and stealth scouts with pure passive sensors like liveware's SS-2 up there.  Because missile fire controls have 3x the range of active sensors, you need forward scouts to target enemy fleets so you can launch missiles from your main fleet at their maximum range, without giving the enemy fleet the opportunity to respond in kind.  The types of sensors that are useful on scouts are conveniently the opposite of the ones you want on pickets - you need long-range high-R active sensors (that should probably stay turned off most of the time since they will give your position away to EM sensors) to target large enemy ships for your fleet.  And you need thermals to passively find good targets that may not be radiating lots of EM - commercial vessels, for example, or pure beam warships.  For scout ships with active sensors to survive, they need to only turn their actives on long enough for the missile engagement to complete, then they need to shut their sensors off and reposition at high speed.  They'd probably benefit a lot from cloaking.  You can also use long-distance missiles with active sensor buoy second stages fired at a waypoint to light up enemy fleets that you have on passives.  Scouts with only passive sensors, though, can do their job without ever necessarily giving away their positions, and so they can be extremely stealthy and small and get right up close to the enemy.  You can also use passive sensor buoys that can be dropped by a minelayer or fired at a waypoint as the second stage of a long-range missile.  These are the things your pickets with R1 active sensors need to sweep for, they are almost impossible to spot any other way.

Then you have other classes of ships for which sensors are important in your rear areas, where hostile aliens may arrive through dormant jump points even if all your borders are guarded.  You've got to protect your colonies, jump points, and commercial ships in similar ways to a fleet.  Your DSTSs are very strong EM/thermal sensors but like all sensors they run into diminishing returns, so you probably want to put networks of sensor buoys all around your systems like you have pickets around a fleet, to cover the approaches to your colonies all the way through their orbits.  The same sensor type considerations apply here as well - for defensive purposes, you want EM buoys rather than thermals to detect long range active sensors and missile fire controls.  Thermals are more for finding targets if you have, for example, huge missile platforms orbiting your colonies.  Instead of/in addition to buoys, you can also have long-endurance patrol craft with looped orders to repeat routes around outer systems and then overhaul at your colony.  As for active sensors, your STOs have small R1 actives so they can fire, but you need to put defense stations in orbit around your colonies for them to have high-powered actives.  Pound for pound, stations are stronger than ships, and colonies can base fighters in relative safety on the surface, so you may want to use a combination of mostly stations and fighters rather than ships to defend systems.

The difference between sensor protection for military fleets and commercial ships is that you can try to avoid getting picked up on passives if you're escorting a commercial convoy through a risky area - commercial ships are limited to low active sensor strength and engine power so the thing that'll detect them at long range is very high-R active sensors, which are clear as day on EM.  So good EM sensors are IMO the most important sensor type for commercial convoy escorts, and you probably want to have an advance scout travel the convoy route ahead of them to look for danger and drop buoys.  Truly massive commercial ships, stations, and tugs might show up on thermals from across a system no matter what you do, though, so you might need to give them a full combat fleet escort if you're towing a multi-million ton habitat station through a combat zone like a madman.

Then with jump point assault and defense the main thing you need with sensors is just redundancy, everybody will be coming through within the range of small R1 sensors but your ships are going to get shot at a lot, you don't want an expensive HS50 sensor anywhere in the area, passives won't do a lot for you, you just need to not have literally all of your active sensors shot out on all your ships.  The same thing applies to colony invasions, almost any R1 sensor can see the entire battle area but lots of beams are going to be going back and forth.

[Platys51]

My fleet doctrine is 5t passives of both kinds on all warships which helps them detect activity in case I need to go dark or forget to turn on actives. Also used to not paint stuff using actives as that used to be considered a threat. 5t passive thermal on survey vessels to detect colonies or ships close by. Larger passives, usually 50 or 250t are reserved for command vessels or 50t for battleships, as they are large enough to not care about a bit of extra tonnage much.

Actives... 50t resolution 1 active on all warships. Passives are usually good enough to keep track of enemy fleet should command vessel be destroyed and my fleet doctrine is beam focused, so range is enough. Main thing is you need to detect missiles coming in to be able to fire on them no matter what. Command vessel usually has again, 250 or 50t active. I sometimes use 150t size for this too as its really just meant to round up everything there's to know about enemy as I approach. Passives do all the initial detection woprk.

[Jorgen_CAB]

I few things about sensors and how to use them to good effect.

A 50t EM passive sensor will detect a resolution 120 active sensor at roughly the same distance the active sensor will see you if they are built using the same sensitivity technology. A 100t EM passive sensor is equal with a resolution 20 active sensor in range. A 150t EM passive is equal with a resolution 5 active sensor and finally at 250t EM passive you can detect a resolution 1 active at the same distances.

If you look at the above numbers I would always install a 75-100t EM passive on all capital ships, it is a perfectly good life insurance policy. You then will have to figure how important it is to put bigger EM passives on smaller ships to detect enemy active before you yourself is detected. As you need a bigger sensor the smaller the platform they become increasingly more expensive to use on smaller platforms in terms of mission tonnage. Smaller platforms should likely be deploying more specialized ships and travel in larger groups, smaller platforms also are cheaper and so don't need the same redundancy that you want with larger more expensive platforms.

What you constitute as a large platform or small one are obviously hard to tell... but in general a ship at above 7-8k tons probably could afford a 75t EM passive sensor.

Thermal sensors are very different as they are to detect enemy engine emissions or incoming missiles. In my opinion thermal sensors are not really required for most capital warships and I usually only give them a very small one... usually about 25-50t so they have at least a rudimentary way to detect thermal threats very close to their location. Thermal sensors are for dedicated scout platforms which is something all capital ships should carry in hangars so they can deploy them tactically. Of course the same thing goes for EM passives as well but every capital ship need to know if they are detected with an active sensor on their own.

The amount of active sensors and their capabilities on capital ships should entirely be based on their role and the weapons systems they carry. I rarely put long range active sensors on ship, just enough to act as backup to their reconnaissance crafts and they should match the same resolution as their missile fire controls or resolution 1 for beam and point defense weapons. It is quite usual for me to only include active sensor coverage for missile targeting for medium range and below missile capabilities in capital ships. For longer ranged capabilities I would rely entirely on recon and scout craft/ships to do what they are designed to do.

What I would NEVER do is build any capital ship to be a completely blind platform... in my opinion that is a very dangerous strategy because if your dedicated sensor platform is destroyed or their equipment temporarily disabled you can essentially blind an entire battlegroup and you also become rather limited in flexibility in terms of tactical maneuvering. In an environment of a scouting and reconnaissance war you might be forced to separate your force more than you first anticipated. Since I quite often play with multi-faction games where I run multiple factions combat is a bit more multi-faceted in tactical considerations of maneuver of military forces. If you only fight against the AI it might not be that problematic.

As have said above every fleet or battlegroup need to operate in layers with their scouting platforms and ships. Every ship that you design need to have a role in what they are meant to do and your amount of research you can afford to put into multiple platforms and numbers build have to be the weighted against the level of threat that you face. If you have dedicated patrol ships that mainly are loaded with sensors you are then limited to how many of those you have and can build. If you have hangars in most of your escorts then they can be deployed alone or in small groups to perform that missions as well as their primary escort mission reducing the need for allot of dedicated scouting platforms, I often find a combination of these to work best, sometimes a dedicated platform is what you want or need, sometimes you just want numbers to cover a greater area quickly.

The smaller the ship the more dedicated they should be... most fighter sized crafts should be dedicated to one type of sensor or if active one resolution. You then combine fighters into squadrons to cover each other and their needs for effective scouting squadrons.

In general I rather expand a ships hangar rather than add more specialized sensors to them. A hangar is way more dynamic in what sensors and how many and where you can deploy them. But at the same time the ship itself need some sensors for self defense but only the bare necessity as otherwise the cost in resource will become way too big, sensors are one of the most expensive components on a ship given their size.

Another hugely favorable thing about smaller sensors distributed over more ships and platforms is one of research and refit opportunity. It is way quicker to adapt to new technology and you also will gain new sensor technology faster as you spend less research on actual sensor components.

So... as a conclusion... I rarely install large dedicated sensors to capital warships and I see little use in large sensor ships in Aurora C# in favor of more spread out dedicated platforms. If I have some truly large ships I do think it is if fine to add decently large passive sensors as they still will be insignificant to the ships size anyway (unless they require additional research doing so). A variety of smaller sensor platforms from 1000t or below are the way to go... small to medium ships 1-7k tons should be evaluated on a case by case basis and larger ships only need rudimentary sensors with hangars covering their need in the scouting and detecting of things in general. Don't skip on at least some basic sensors on your ships be them passive or active unless they are 1000 tons or less.

[nuclearslurpee]

A 50t EM passive sensor will detect a resolution 120 active sensor at roughly the same distance the active sensor will see you if they are built using the same sensitivity technology. A 100t EM passive sensor is equal with a resolution 20 active sensor in range. A 150t EM passive is equal with a resolution 5 active sensor and finally at 250t EM passive you can detect a resolution 1 active at the same distances.

If you look at the above numbers I would always install a 75-100t EM passive on all capital ships, it is a perfectly good life insurance policy. You then will have to figure how important it is to put bigger EM passives on smaller ships to detect enemy active before you yourself is detected. As you need a bigger sensor the smaller the platform they become increasingly more expensive to use on smaller platforms in terms of mission tonnage. Smaller platforms should likely be deploying more specialized ships and travel in larger groups, smaller platforms also are cheaper and so don't need the same redundancy that you want with larger more expensive platforms.

What you constitute as a large platform or small one are obviously hard to tell... but in general a ship at above 7-8k tons probably could afford a 75t EM passive sensor.

To add onto this:

The precise formula you can use to determine what mass of EM sensor you will need to detect another ship's active sensor before it can detect you is:

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EM sensor mass in HS >= 16 / pi / Resolution^(1/3) * (active_sensitivity / passive_sensitivity)
Here, both sensitivities are EM sensitivities of the active (i.e. other ship's) sensor and your own passive EM sensor respectively. Resolution of course is in HS. I'll happily provide the derivation if anyone asks for it.

So for example, if we assume both races have the same EM sensitivity tech level, then using Jorgen's examples:

• If the other ship has a sensor resolution of 120 HS, the required sensor mass comes out to 1.03 HS; in game this would be rounded up to 1.1 HS which is 55 tons, a bit over the quoted value.
• If the other ship has a sensor resolution of 20 HS, the required sensor mass comes out to 1.88 HS; in game this would be rounded up to 1.9 HS (probably 2 HS as most players like to have a round number, or 100 tons as Jorgen says).
• If the other ship has a sensor resolution of 5 HS, the required sensor mass comes out to 2.98 HS; in game this would be rounded up to 3 HS (150 tons).
• Finally, if the other ship has a sensor resolution of only 1 HS, the required sensor mass comes out to simply 16 / pi or 5.09 HS; rounded up to 6 HS in game (300 tons).

As the highest EM sensitivity possible is 75, any product of EM sensor mass and sensitivity which is greater than 382 will always detect any possible active sensor outside of the latter's range at least by a slight margin. The first tech level at which this is possible is EM Sensitivity 8 at which point an EM sensor of size 48 (maximum possible size is 50 HS) will accomplish this. Of course, if you are going up against an opponent with EM Sensitivity 75 with such a low tech yourself, sensor range is probably the least of your problems...

More generally, since the EM Sensitivity tech increases by roughly ~1/3 at every tech level (the largest relative jump is 37.5% from 8 to 11, all others are 33% or smaller), an EM sensor of about size 8 will detect any active sensor up one tech level better outside of its range, and a sensor of size 11 or 12 will detect any active sensor up to two tech levels better outside of its own range. Beyond this size, bigger is still better for gathering information but the benefits are not as pronounced.

For thermal sensors there is not a clear-cut answer for what size is ideal, but they are certainly necessary to spot an enemy who has his actives turned off. Noting that the minimum thermal signature that can be detected is just 1 (conveniently, any ship at the 1,000 ton mark will have this thermal signature when idling its engines), the minimum range at which any thermal contact can be detected by a thermal sensor is (250,000 km) * SQRT(sensitivity * size). It might make sense for example to choose a minimum range that exceeds your active sensor range, so that you can use thermal sensors to sneak up on a target and then turn actives on to shoot them

[Jorgen_CAB]

You also can use my simple xml to see this so you don't have to run the hoops of using the formula...


https://www.dropbox.com/s/3wq4hn1smxj9tsi/SensorCalculator.xlsx?dl=0


I also agree that for thermal sensors it is very difficult to give rally good answers. Being able to detect the thermal sensors from idling ships might be a good way to judge the effectiveness of some types of thermal. Of course detecting ships traveling at speed is also important or incoming missiles enough far out for you to react to them if you after all are engaged without your knowledge.

So you have to figure out this by evaluating the values of these scenarios and what size of thermal sensors you need and where around your fleet you need to station your thermal scouts.