How do AAM datalinks work?
July 25, 2011 2:46 PM   Subscribe

How does a datalink work on an air-to-air missile with a range of, say, 70km?

Just curious what kind of technology would be used to ensure a reliable datalink between missile and aircraft, given the stresses and distance involved.

What kind of data would be sent -- packet data? Just curious about how that works. I was just reading about a missile that used active homing during the terminal stage but prior to that could use a datalink with the aircraft that fired it.
posted by circular to Science & Nature (10 answers total) 1 user marked this as a favorite
I suspect that this is to some extent classified.
posted by silby at 2:49 PM on July 25, 2011

So I don't know. But I suspect it's simply analog. Even some recent drone camera links were "hacked" because the downlink is unencrypted analog video.

Some missiles have used video feeds with remote control, but I expect that the speed of the missile and latency of any link makes it impractical for any manual operation except for shortly after launch.

Also I'm curious why you even ask since every AA missile I've ever heard of is autonomous. Not that I'm an expert or anything, but to my previous point, they're next to impossible to control and pilots are pretty damn busy as is.
posted by GuyZero at 2:57 PM on July 25, 2011

Best answer: JTIDS.

Pretty sure it isn't analog at all.

You can guide the missile from an AWACS and JTIDS is used. I think these are all TDMA systems. Link 16 is another related concept.

I'm assuming it uses the same system if directed by the firing aircraft. It would make no sense to have two different systems for the same task.
posted by Ironmouth at 3:05 PM on July 25, 2011

Also I'm curious why you even ask since every AA missile I've ever heard of is autonomous. Not that I'm an expert or anything, but to my previous point, they're next to impossible to control and pilots are pretty damn busy as is.

The pilot does not do the guidance. However, the missile uses data from the radar set on the firing aircraft or an AWACS aircraft. The missile will get data from the aircraft's radar and obtain mid-course updates, letting it know changes in the target aircraft's position and speed.

These are highly complex systems and are not video links like on the drones.
posted by Ironmouth at 3:10 PM on July 25, 2011

This information may be a bit dated, but (From Shaw, Fighter Combat):

Command guidance may be likened to classic remote control. During missile flight the positions of both the target and the missile are monitored at the launching platform, and commands are sent to the missile to fly a course that will result in target interception. Tracking of target and missile is usually accomplished by radar, through electro-optics (television), or by sight. Of these three methods, only radar generally provides target/missile range information sufficiently accurate to allow computing of a lead-intercept trajectory for the missile, but since two tracking radars are usually required, this technique largely has been limited to SAM systems. Without range data the missile is ordinary guided along the LOS between the target and the launcher. This technique, known as "command to LOS," can be accomplished with no range information at all and is applicable to visual and electro-optical system as well as to radar and combination systems.

The guidance instructions to the missile are generally transmitted by radio data link, which is susceptible to jamming, as are most radar trackers. Trailing wires (wires connecting the missile and the launch platform) have been used for transmitting guidance commands with much success in several short-range air-to-surface and surface-to-surface applications. Such a system is highly resistant to jamming, and was employed by the first AAM. This was a German X-4, designed and tested late in World War II for use by the Me 262 and Fw 190. The X-4 was a command-to-LOS trailing-wire system that was controlled manually by the launching pilot along the visual LOS to the target aircraft. Apparently it was never used operationally.

Beam-rider guidance is somewhat similar to command-to-LOS guidance, except that the missile guidance system is designed to seek and follow the center of the guidance beam automatically, without specific correction instructions from the launching platform. The guidance beam may be provided by a target-tracking radar, by electro-optics, or by a visual system. Like radar-enhanced command guidance systems, radar beam-rider systems are not limited to daylight, good-weather conditions but they are more susceptible to electronic countermeasures than are electro-optical and visual trackers.

posted by Comrade_robot at 3:21 PM on July 25, 2011

Not a datalink expert, but just keep in mind that its much much easier to transmit things through the air in a straight line (i.e. with no buildings or mountains in between) than what we think of with cell phones and other datalinks where the signals have to reach into buildings, underground, around mountains, etc...
posted by jourman2 at 3:42 PM on July 25, 2011

Response by poster: Thanks for all the info. Got awesome infographic results from a Google Images search for JTIDS, too!
posted by circular at 4:41 PM on July 25, 2011

One more thing....

Missile doesn't necessarily need to send anything back. It can be tracked via radar (as presumably the target is) and tweaked en-route. Receiving antennas can look backwards toward the launch platform and ignore stuff coming from the front.

Not sure if AAMRAM did a data link with the aircraft or not.

Most of the stuff I worked on at Raytheon (Standard missile, Hawk, Sea Sparrow) got pre-launch info and used self-contained terminal guidance from seekers on the missile at the fun end of things.

Sea Sparrow was pointed in the general direction pre-launch, unless it was a vertical launch missile. Then, the VLS got heading instructions just before launch. I don't think it had any downlink. Might have had a beacon, though. (I worked on the VLS, not the missile.)

There's enough info out there now to build your own, so it shouldn't take long to get your question a detailed answer.
posted by FauxScot at 5:25 PM on July 25, 2011

Best answer: There are essentially two different types of datalinks. The first would be similar to satellite television signals - an antenna disperses the signal in a wide arc, blanketing the area and some percentage of that signal gets picked up by the receiver on the other end. Prone to interception, but is generally encrypted to prevent the Bad Guys from getting the information. You can also use channel hopping (Have Quick) to prevent interception. There are also anti-jamming and anti-spoofing techniques available to ensure the data is genuine.

There are narrow beam transmission methods, also, which may or may not be line-of-sight. These require steerable antennas at both ends which will require accurate position data if you're on a moving platform like an aircraft. Much harder to intercept, since there's not as much "leakage" around the recipient. I think there are both radio and optical systems that use this technique.

There are civilian uses for this technology, too. If you've ever watched DirectTV on a JetBlue flight, you have used the wonders of aircraft datalink systems. There are also commercial systems available, both for airlines and general aviation, that send out real-time weather information to the cockpit. Iridium and Inmarsat are being used more and more for Air Traffic Control over the oceans, where HF signals can't easily reach.

For commercial data systems (and I've worked on these, too), the information is packetized and checksummed to verify the data integrity. The big point to remember for these systems (like aviation weather) is that the data is not very critical - you will not be making a life-or-death decision in the next 30 seconds depending on the data you receive. So, if you miss one packet of data, it's ok to wait for the next one. If you never get it at all, that might even be ok (but annoying). These systems tend to rely on redundancy of data transmission to ensure the information is received. For example, a particular weather station may only update itself once an hour, but that information is transmitted every five minutes.
posted by backseatpilot at 4:48 AM on July 26, 2011

Best answer: I have worked intimately with a datalink-guided air-to-air missile. The information you are looking for (ie: basic principles, communications protocol, frequency and reliability) is classified, and you are unlikely to get the answers you are hoping for. Not to be dismissive, but most of the speculation here is wrong. I looked for public domain info to put you on the right course (no pun intended) but I couldn't find any. The Wikipedia page on the AIM-120 has as good information as anything else under the heading Interception Course Stage.
posted by Simon Barclay at 7:25 PM on July 26, 2011

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