The AIM-120 'AMRAAM' - History, Design, Performance & Discussion

Machinery of War Discussion Aircraft
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Damn I thought ~10months was enough time for you to pick a RF book and throughly read it and understand it, my bad. But I guess just copy pasting wiki paragraphs without a clue of what’s going on in a signal still does it right?

Here’s a tip, look up what the doppler bandwidth is in the f domain then why HPRF has an ambiguous range and an unambiguous velocity velocity and LPRF unambiguous range and ambiguous velocity. This( if you understand) is enough to show you that if you do doppler filtering, HPRF is better for high closure targets.

After this look up what frequency modulation is and how its used on pulsed signals. This( again, if you understand) will show you that range can be solved even when the range is greater than the max ambiguous range in a HPRF signal. Thus you can detect things at long ranges without compromising high closure targets.

I have no other choice if you don’t understand Peak power and AVG power( or even knew these 2 existed). It was the easiest way to break it down for you, and you don’t grasp the simple analogy.

Copypasting wiki again? I reiterate, read about FM and how range measurement is done with it in HPRF.

No you don’t because you keep parroting

Conversely, a high PRR/PRF can enhance target discrimination of nearer objects such as a periscope or fast moving missile leading to practices of employing low PRRs for search radar, and very high PRFs for fire control radars,
Did you read that? Can you comprehend it?
A radar system determines range through the time delay between pulse transmission and reception. For accurate range determination, especially over great distances a pulse must be transmitted and reflected before the next pulse is trnsmitted. Low PRF is for range detection.

then

If a target is on a beam aspect to the missile( or Vc is just near the missile velocity) then there’s no reason to have a huge doppler bandwidth.
For example if the seeker is on HPRF, the doppler bandwidth is 3000 m/s( ±1500 m/s) and it has 64 doppler cells, thats 47m/s each cell. And a max unambiguous range of 2km with 1 range cell. The seeker also has MPRF that has a bandwidth of 800m/s(±400 m/s )and same amount of doppler cells, thats 12.5 m/s per cell. Due to being MPRF it has an longer (as useful) maximum ambiguous range of 15km and 32 range gates, thus 468 m range cell.

Now if there’s a target(Vt=600m/s) head on to the missile(Vm=400m/s), Vc=1000 m/s, you can’t track it in MPRF as the max unambiguous velocity is 400m/s before it starts overlapping( in the frequency domain) and showing as a target thats going away. Thus you need to track it in HPRF which can track targets going ±1500 from the missiles TAS.
If the target is now near beaming the missile(Vm=400m/s), and the Vc drops to 476m/s. Lets say the MLC(“notch”) is at ±60m/s. If you continue in HPRF, with the doppler cell of 47m/s, the target will be in the second doppler cell but the MLC will be there aswell. Thus lock is lost. IF you switch to MPRF, the target will be in the 7th doppler cell and the MLC in the 4th doppler cell. Track is kept.. Chaff can be filtered better in MPRF if you think more.

On the second case, if we put the target has an altitude of 500m above the ground, the distance Target-missile is 700m, and the angle of the missile flightpath to the ground is 45°. The distance LoS ground-missile is 1407m. The target is within the HPRF range gate, but the ground clutter is on it aswell SO track is lost in the doppler and range spectrum. ON MPRF, the target is on the 2nd range cell and the ground clutter appears on the 4thrange cell. SO using HPRF we got a track in the range domain and in the doppler spectrum. Again, with chaff taking time to bloom, it can be out of the range cell. If the target effectively gets its signal in the MLC, Ex Vc=440m/s, but the geometry above is kept, track can still be done. Range and Doppler are BOTH analyzed in MPRF compared to HPRF where typically only doppler is analyzed. FM in HPRF will leave big range cells compared to MPRF.
This is why MPRF is important. IF YOU UNDERSTOOD THIS
Values are just examples. MPRF in one radar can be HPRF in another one and viceversa.

No, as stated previously. Besides simplification, the main radar has a greater gain that CW illumunators. Longer range is one imporvement, The rest were stated above.

Ironic. You don’t know the difference between a Continuous Wave and Continued Illumunation.

CW radar. You cannot see … and are dumbfounded how high PRF

This is my favorite. A CW signal has PRFs. TOP KEK

See above, the dude has always confused waveform with illumination. I still remember the AWG9 thread lmao. He also confuses carrier frequency and the PRF.

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256

Cracking read, cheers. MPRF always made my brain hurt haha, this has helped.

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I have a question for those who have played with the files more than I have and just know more about the topic:
How much does this influence the SARH illumination?

"illuminationTransmitter": {
    "power": 200.0,
    "antenna": {
      "angleHalfSens": 10.0,
      "sideLobesSensitivity": -32.0
    }

From what I can tell, radars that are able to guide SARH missiles have this piece of code in the config files, and most if not all have the exact same values. Is this what I would expect it to be (parameters for the SARH illuminator)? Do these influence the code? Does "angleHalfSens actually represent the beam width of the illumination signal, and does sideLobeSensitivity actually stand for the power level of the first side lobe?

If they do, wouldn’t bringing the beam width down, at least for aircraft that lack a CW illuminator and use main radar for intermittent illumination, massively help with keeping the missile on an intended target? As an example, the N019 does not have a separate illuminator, so the beam width would be 3.5° (I don’t know how much the change in frequency, if there even is any, would affect the beam width).
Also, what is usually the beam width for CW illuminators on aircraft like the F-4E

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That code was added as to all radars (that guide SARH missiles) at the exact same time as the mini RWR rework a couple of months ago (the one that made it so you got a lock warning if you were flying near someone who was locked).

So there is a non-zero chance that the code exists only to make the RWR react properly, and doesn’t actually impact missile performance (I don’t remember anyone noticing a change in missile behaviour after that code was added).

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The F-18’s radar most certainly dropped the CW illumination device early on primarily to increase MTBF of the radar… and because it offered no improvement in performance for guiding the Sparrow.

They state the “feature was eliminated” indicating that they had it originally, but deleted it for the reasons mentioned.
https://cdn.discordapp.com/attachments/398367636213334018/1151510905423659008/image.png
https://apps.dtic.mil/sti/tr/pdf/ADA142103.pdf

It took me less than a minute to find this on google after looking up; “F-18” “Continuous wave”. It was like the second result.

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We are talking about targeting. AMRAAM does not go in Med PRF in the terminal by itself. It must go High PRF as normal radar missiles.

Please learn to carefully read and comprehend what you find on the internet before posting THIS IS ONLY IN REFERNCE TO COMMAND LINK GUIDANCE capable aircraft.

It literally says AMRAAM moding at the top. It also is not talking about the missile in going ACTIVE in the terminal. But setting up the missile before and forcing it into a Med PRF configuration and supporting it after launch…LMFAO

Its a modification in allowing in which the launching aircraft can support it with med PRF and force the AMRAAM too as well.

The AMRAAM does not do this by itself or can. YOU NEED A COMMAND LINK GUIDANCE capable launch platform to force the missile into Med PRF and must support it.

I am sure there is some advantage for COMMAND LINK GUIDANCE in forcing the AMRAAAM into Med PRF. Pretty cool but still not what they naturally do.

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Good job ignoring everything else here.

Command link guidance isn’t necessary to “force the missile into MPRF”. It does this on it’s own, depending on what the target is doing. If conditions are favorable it can begin going active at further ranges and will decide to do so with HPRF in most cases, but this isn’t totally accurate… still better than tracking the data given via sidelobes from launch aircraft radar. Once it is within MPRF parameters / distances it will switch and guide towards target… who often at this time is defensive and MPRF is more favorable against this type of target.

In fact, the ranges it goes active are something selected via target size parameters prior to launch and not thereafter.

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The verbiage is clear.

The radar of the launching aircraft establishes message communication with the missile and initiates the Command Link messages.

Following launch, Time to missile Med PRF is then selected and displayed.

The time to Med PRF aka TMPRF is a countdown (YOU SELECT) in which you tell the missile when turn its radar on and use medium PRF.

The pilot tells the missiles when to go active. There wouldn’t be a countdown selection and a display.

LMFAO. You have no clue what you are talking about & you can barely read.

No… TMPRF is a preselected countdown after launch that tells the missile to go active no matter what the conditions.

You can’t even articulate when a manual is speaking in regard to a special guidance technique that certain aircraft offer to the missile or when its speaking about basic, standard Aim-120 functions.

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All that does is help update the missile to know when it should activate and begin searching for target. The verbiage is quite clear, and somehow you’ve misunderstood it in a couple of ways.

It states;
“For Normal and RHOJ launches the aircraft system initiates the Command Link messages automatically for the selected target.”

After which, it states;
“Following launch the MIS information for the selected target and the TMPRF (Time to Missile Medium PRF selected) are displayed. The TMPRF is the count down from launch to the point where the missile will acquire the target and the missile Radar will switch to Medium PRF mode to lock onto the target.”

As you can see, it quite clearly says it is being fired in normal or radar home on jam launches.
It clearly states “the missile radar will switch” indicating that the launch aircraft had nothing to do with this aside from letting the missile know it is “x seconds from impact” and to begin searching with it’s own radar to acquire the target. Of course, it does this in MPRF because that’s the optimal guidance mode for it in most scenarios.

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This is too hilarious. I am going to keep smoking you on this point.

Command Link Guidance issues a countdown from launch to point where the missile is forced to turn its radar on and use Med PRF.

Hence the name Command Link Guidance!

No…Genius. Does it say the missile’s radars will switch on based on range? Read it again. Take your time…

It says based on a selected COUNTDOWN initiated after launch by the Command Link Guidance.

Command Link Guidance overrides the Aim-120s standard guidance, it’s radar and PRF emission. That is why it is called COMMAND LINK GUIDANCE

Command link guidance initiates following (after) a launch. Why???

Because a fighter’s radar establishes message communication with the Aim-120 at launch (not before while it sits on the rail. Therefore, the Command link guidance must wait for the radar to establish communication first.

Additionally, it clearly states command link guidance is not for visual launches. Its main purpose is in maximization of range and for Aim-120. Even directing the missile to turn its radar at ranges outside of active targeting and use Med PRF to guide rather than target. It’s still switches back to HPRF at terminal.

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Again, no. You’re wrong.

The radar stops sending command-inertial information and the missile reaches an autonomous point. It guides itself in a interleaved HPRF and MPRF fashion providing it’s own inertial updates at a certain point. This is called “Husky”. When it is within active terminal homing range it switches to MPRF and guides to target.
https://cdn.discordapp.com/attachments/955829235493273680/1151749371705163886/image.png
Source: AIM-120 AMRAAM - Hughes (Weapons System Series by Ezio Bonsignore)

This is the key point here;
https://cdn.discordapp.com/attachments/955829235493273680/1151750513327603753/image.png

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Yes, there are Continuous Wave (CW) signals that have pulse repetitions (PRF)

WAIT…You thought you got me??? LFMAOOOOOO !!

Interrupted Continuous-Wave is a CW signal that has pulse repetitions… You want to know what it’s also known as? HIGH PRF

High PRF is a form of continuous wave frequency. LMFAOOOO

Yeah. so, moving forward. Don’t take it the wrong way. But, I am not interested in anything have to say. Its irrelevant to me. Sorry! I read that last part though!

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He did. Any “CW” signal has no pulse repetition frequency. That is why HPRF to guide stuff such as Sparrows is usually referred to as “CWI” or “Continuous wave, interrupted”. It’s such a high frequency that it can act as a sufficiently continuous emission for guidance of SARH missiles.

You confused normal CW transmitters output as the same thing as CWI from a HPRF output.

And again, good job ignoring the rest.

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Husky is HPRF-active range. Yes… but Pitbull is the more commonly known terminology which means MPRF active range for AIM-120. This is because MPRF is the much more common mode it chooses to do terminal homing. HPRF is for a very specific purpose, and it can switch between the two as necessary to better track the target. It just so happens that under most “normal” launch conditions as mentioned the AMRAAM does terminal homing in MPRF.

Also, the primary source from Hughes I shared earlier clearly shows that the missile is in complete autonomous guidance prior to going active. It also states the missile chooses when to go active. This destroys any argument you had wherein you claimed the launch aircrafts radar tells the missile when to go active or in what mode.

Hopefully your language can improve so this discussion doesn’t devolve further.

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I’ve never claimed to be a tech moderator, nor do I have plans on becoming one. I put in an application at the time they were requesting them but I was clearly not selected.

Also, I’ve never claimed radars use light emission. You’ve done nothing to support your argument and this is a public forum wherein you’ve demonstrated that you don’t have a proper understanding of how these things work. I’m not even saying anything from personal opinion right now, I’m just sharing sources.

The sources clearly say the missile chooses which mode to lock in, and has access to HPRF or MPRF. The Tornado manual clearly makes it known that the “normal” launch parameters has it choosing MPRF for terminal homing.
https://cdn.discordapp.com/attachments/398367636213334018/1151765408219676692/image.png

As you can see, the missile no longer relies on ANY information from the launch aircraft and operates only on it’s own inertial information until it goes active on the target.

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AMRAAM goes terminal in Medium PRF.

That isn’t something I found online. It’s an aircraft manual from the RAF archive.

The Tornado F.3s radar does not support MPRF. It uses FMICW High PRF.

No. The aircraft does not tell the AMRAAM to go active. AMRAAM decides when it is going to go active. TTMPRF is the aircraft weapon system’s estimation of when that might be:

Screenshot_20230914-072114
Screenshot_20230914-072114849×1168 138 KB

Screenshot_20230914-071737
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Notice how it is all based on estimations and calculations. For example:

Following release the TTMPRF value counts down the predicted time to the point the missile will stop responding to command link messages.

When the avionic system calculates that the missile has entered its terminal phase

I.e. the aircraft is not telling the AMRAAM what to do. It is estimating what AMRAAM is currently doing based on the known target / launch information.

Time To Medium PRF is the amount of time remaining before the weapon system thinks the AMRAAM will activate its Medium PRF seeker, lock onto the target, and start terminal guidance.

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The brevity codes, and my source explicitly state that the AMRAAM uses MPRF solely for locking onto target for terminal homing.

The source is directly from Hughes, the manufacturer of the missile. It’s a primary source.
Quit spamming up my thread.

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In normal operations yes.

You originally posted COMMAND LINK GUIDANCE.

That is not standard feature of the AMRAAAM

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Yes, it is. The command link is the receiver on the rear of the missile that allows it to receive updated inertial coordinates for target so that it can adjust it’s trajectory to better meet it. This is necessary until the missile is in the autonomous region (the point where it can provide it’s own updates and guide itself).

This is why the initial portion of the flight is called “command-inertial” phase.

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That is a fundamental part of AMRAAM:

  1. You provide AMRAAM with the target information (speed, heading, distance, etc.)
  2. You fire the AMRAAM
  3. AMRAAM uses inertial navigation to fly towards the predicted intercept point
  4. As the target maneuvers you send AMRAAM command guidance signals which tell it the new speed / range / heading of the target.
  5. AMRAAM uses the command link information to update it’s predicted intercept point
  6. Repeat steps 3 - 5 a bunch of times
  7. Once it is a certain (classified) distance from the target AMRAAM activates its MPRF active radar seeker and starts searching for the target
  8. AMRAAM finds and locks onto the target
  9. AMRAAM now knows exactly where the target is and guides towards it
  10. BOOM (hopefully)
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