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When a missile is engaged in a GPS free tracking loop how is its internal orientation estimated with respect to ground?

I'm assuming this orientation is needed because the force of gravity will be impacting the missiles flight path and this needs to be compensated for, am I wrong? Do they all use IMU? The IMU I have looked at all have bad performance and the cost of extreme precision IMU is greater than the smaller missiles so that can't be the solution.

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    $\begingroup$ What kind of missiles are you referring to? Weapons, scientific or hobbyists? If the latter two, please specify some type/types. Among weapons cost is definitely not an issue... $\endgroup$
    – Jpe61
    Apr 6 at 6:07
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    $\begingroup$ 1. highly likely this is classified or under NDA (what I know is, so no answer from me) 2. this is probably off-topic aviation.meta.stackexchange.com/q/4292/1467 $\endgroup$
    – Federico
    Apr 6 at 6:53
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    $\begingroup$ Simple, the missile knows where it is... ;-) $\endgroup$
    – B-K
    Apr 6 at 12:18
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    $\begingroup$ a question for you, asker: what do you think GPS does? Orientation directly from GPS is uncommon, but I know it's been done in multi-antenna systems. Velocity directly from GPS is a bit of a hack (you can derive it directly from Doppler shift). Really, it's meant for position. $\endgroup$
    – Erin Anne
    Apr 6 at 20:30
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    $\begingroup$ Before asking "how", ask "if" or "whether". Given high-speed aerodynamics, rocket propulsion, possible rotation of the missile, the multi-G accelerations of e.g. fighter aircraft trying to avoid the missile, gravity may be one of the lesser and least troublesome forces acting on it. $\endgroup$ Apr 6 at 20:52

4 Answers 4

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As per comments, this question is referring to guided weapons such as Stinger (IR guidance).

Guided missiles that acquire their target via signal return, such as infrared, radar or laser designation (Homing Guidance, see section 16.4.2) have absolutely no need to "know" their orientation in space. A target lock is established pre or post launch and then maintained by missile sensors, and the sensor signal is then used to calculate necessary maneuvers to steer towards the target.

Any information regarding where in space and in what alignment in reference to earth the missile actually is, is useless: only the position with regards to the target is of importance, as reaching the target is a matter of actions within the frame of reference of the missile, not the earth: the target exist in certain three dimensional(* point in space in relation to the missile sensor. The guidance system updates this "situational image" at certain intervals (several times a second) and the difference between images is used to calculate the rate of change and then the necessary steering inputs.

Adding sensors and algorithms relating to spatial situation with regard to earth's frame of reference add unnecessary complexity (and thus extra fail modes) to this kind of weapons. Within the maneuverability envelope of these missiles the "direction" of gravity is not of crucial importance.

*)To be precise, whether 3-, or just 2-dimensional situation image is actually necessary is an interesting question, as the missiles of this sort are not "throttle-able", so they speed on at full power until target is destroyed or fuel is depleted. Actual or relative speed are therefore of no importance. However, some missiles do not need to impact the target to detonate, so they must have some way to determine the sufficient proximity to the target to arm and explode.

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    $\begingroup$ The fuse is usually completely separate from the guidance system. It just detects an object in proximity at a certain threshold range, not its full 3D position. $\endgroup$
    – Therac
    Apr 6 at 9:59
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    $\begingroup$ @Jpe61 - "some way to determine the sufficient proximity to the target to arm and explode." Yes. That is called, wait for it...a "Proximity fuse". In the case of the AIM-9, it senses when it might be going "past" the target, and explodes. Hopefully punching many holes in it. $\endgroup$
    – WPNSGuy
    Apr 6 at 9:59
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    $\begingroup$ Yes, I am familiar with the concept of proximity fuse, but what exactly is the magic by which the said proximity fuse determines the proximity of the object @WPNSGuy $\endgroup$
    – Jpe61
    Apr 6 at 10:53
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    $\begingroup$ Proximity fuzes go back to WWII; the operation of those early units has long been declassified. Wikipedia has a good article -- en.wikipedia.org/wiki/Proximity_fuze $\endgroup$
    – Zeiss Ikon
    Apr 6 at 11:07
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    $\begingroup$ @FourierFlux, A simple PID feedback loop is way more than enough. It does not matter whether the target is moving in the sights because the missile is falling with gravity, getting blown aside by wind or the target is moving. Just steer until the target stays still. You only need navigation system for long range. $\endgroup$
    – Jan Hudec
    Apr 7 at 15:38
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Modern missiles pretty much all determine their orientation via some sort of IMU. Note that you do not need super-expensive RLG-IMU, but you might get away with cheaper MEMS-based IMUs (*1). However (and I cannot stress this enough), it all depends on your specification:

  • If you know your attitude from your IMU, you can better control your missile as you now are able use feedback-control (*2). Therefore your control-loops can minimize some error. It is also able to compensate for gravity (as you observed) or can actually follow the reference-value of the guidance loop.
  • You don't even necessarily need a velocity sensor (such as a GPS-unit or a pitot-tube), because you can at least roughly determine your velocity over the accelerometers, or via the known velocity of your missile at the end of the burn or via velocity dependent system dynamics. Kalman Filters are often used in determining the exact system states.

But again, it depends on your specifications. With very tight specifications, you generally need to achieve very high performance in your control loops, which is only possible if you have very good (e.g. expensive) sensor available. If you need to hit the third screw from the left of your target, then you need very precise and expensive sensors. If your warhead is powerful enough to obliterate the target in a 2km radius, you can figuratively just close your eyes and aim in the general direction.

The take-home message is (especially for closed loop control of any flying vehicle) is: Performance and robustness of a control loop are interchangable. The more performance you demand of a control loop, the less robust against sensor noise, external disturbances, system dynamic uncertainties etc. it will be. There are plenty of ways to overcome limitations of missing sensors etc. in order to do something cheaper, however it is up to the designer to check if the product is still be able to perform its task as intended.

Foonotes:

(*1): Expensive and cheap in the military context are a bit different. A cheap MEMS IMU might still cost several thousands of USD. (*2): Of course you do not need an IMU to close a control loop. Early missile designs directly coupled the angle-to-target (as determined by the seeker) with corresponding fin-deflection. However the addition of an IMU and the corresponding data streams offers more data for possible control loops.

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To expand on the answer given by @Jpe61, homing missiles use a technique called 'Proportional Navigation'. It is a surprisingly simple algorithm in which the missile attempts to keep the target on a constant bearing. As long as the missile is reducing the distance to the target this guarantees a hit.

This technique can be implemented using a PID controller where the error signal is the deviation from a constant bearing (detected on a photocell in the case of an IR seeker). The PID output then drives the control fin motors.

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  • $\begingroup$ Exactly. It does. not even matter which way is up as losing altitude would cause the missile to drop below target (hence the target becomes above the missile) and thus automatically corrects the missile's attitude by steering onto target. Complicated IMUs are used more on cruise missiles and ICBMs that fly long ranges. $\endgroup$
    – slebetman
    Apr 8 at 0:42
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Active Skyflash quoting the relevant part where it describes how the missile tracks the target:

http://www.ausairpower.net/skyflash-slammer.html

The Active Skyflash seeker feeds the missile's Integrated Power and Control Unit (ICPU) with serial signals proportional to target line-of-sight difference from the missile's boresight in yaw and pitch, and with a signal proportional to target range. The missile is steered by cruciform wings, a pair each generating yaw and pitch inputs, with one pair differentially controlled to provide roll stabilisation. The wings are actuated by solenoid controlled hydraulics, the solenoids driven by DC amplifiers fed from digital to analogue converters (D/A).

Basically, the missile boresight, orientation, and where the seeker head is telling it the target is.

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