Thread by @John_A_Ridge

Sigh, looks like I need to explain cruise missile guidance and route planning again. The word "turns" is somewhat misleading in this context and the phrase "demanding on the guidance system" lacks technical meaning. A 🧵with some context and explanations. 1/18

https://twitter.com/shashj/status/1602627281331126275

Below is a previous short 🧵I did on the subject to clear up similar misconceptions. 2/18

https://twitter.com/John_A_Ridge/status/1584667253936918528?s=20&t=zcV0pmPrEEzQZci4qR9kxg

Most modern cruise missiles (CMs) do not simply follow a straight flight path from their launch platform to their target. Rather, they follow pre-defined mission-specific flight paths. Historically, a specific route was necessary to ensure precise guidance of the munition. 3/18

All inertial navigation systems (INS), which are the basis of most CM's guidance systems, suffer from position error/drift that increases quadratically with flight time due to measurement error in its sensors. It is necessary to have some way to correct this error. 4/18

This was first achieved via terrain contour mapping (TERCOM), which uses a radar altimeter to continually compute the relative terrain elevation beneath the missile and compare it to a pre-loaded map. These TERCOM position fixes are then used to periodically update the INS. 5/18

Thus, a specific pre-planned flight path is needed to generate a corresponding terrain elevation map. This concept would be further developed into what is known as digital scene mapping area correlator (DSMAC) guidance. 6/18

Like TERCOM, DSMAC provides position fixes to update the INS. However, rather than measuring terrain elevation, DSMAC measures terrain features. This is usually implemented as an optical camera that continually images the terrain beneath the missile. 7/18

These images are processed and converted into binary contrast images that are then compared to a corresponding pre-loaded binary terrain feature map to compute a position fix. These reference maps are usually generated from imagery collected by reconnaissance satellites. 8/18

Though usually implemented in the optical band, infrared and radio frequency implementations do exist as well. Additionally, some modern DSMAC packages only provide terminal guidance near the target, relying on GNSS and TERCOM for mid-course position fixes instead. 9/18

Finally we get to global navigation satellite system (GNSS) guidance. GNSS uses timing and position signals received from a satellite constellation in Earth orbit (e.g. GPS, GLONASS, Galileo, or BeiDou) to generate a position fix. 10/18

It is only with the advent of GNSS that it became possible to provide accurate CM guidance without relying on a pre-defined mission-specific flight path. Route planning still has enormous practical significance however. 11/18

Besides GNSS, most CMs still use TERCOM and/or DSMAC to enhance precision and ensure their ability to operate in a GNSS-denied environment. TERCOM in particular enables a CM to safely cruise at lower altitudes allowing to more easily fly under the horizon of hostile radar. 12/18

Moreover, the ability to follow to a mission-specific flight path is itself a significant capability. A non-linear flight path degrades the ability of hostile air defenses (AD) to predict the missile's approach vector and enables avoidance of known hostile AD sites. 13/18

Additionally, it enables individual missiles to approach a target from very different directions, making it easier to overwhelm and saturate hostile AD assets. For this reason, many CMs feature GNSS waypoints to enable mission-specific routes without TERCOM/DSMAC. 14/18

Reports that 🇷🇺 CMs went from making "2 turns" to 40 at present are likely best interpreted as indications of initially sloppy route planning with very simple fight paths. This squares with the notion that 🇷🇺 grossly underestimated 🇺🇦's AD network at the invasion's outset. 15/18

I would also push back on the idea that more complex flight paths are "more demanding" on the guidance system. Aside from memory issues storing maps/waypoints for more complex and thus longer flight paths, I don't see why there would necessarily be an impact on performance. 16/18

If the published ranges for these CMs are even remotely accurate, then I would be very surprised if 🇷🇺 is having storage issues. Their design ranges vastly exceed the ranges they currently being used at. 17/18

I suspect that the main impact may be on the time and labor needed to generate and simulate the flight paths for the individual missiles before a strike. This @the_ins_ru piece has some fantastic details concerning the 🇷🇺 route planning process. 18/18
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