US-made GBU-39 SDB Glide Bomb is Beating Russian Jamming In Ukraine

16 GBU-39 Small Diameter Bombs hang from the right wing of an A-10 Thunderbolt II, for a testing mission, at Nellis Air Force Base, Nevada, April 20, 2023. (Image Credit: US Air Force photo by William R. Lewis).

The GBU-39 small-diameter bombs have proved resilient to jamming, with nearly 90 percent hitting their targets.

US-supplied GBU-39/B SDB (Small Diameter Bomb) is being proven largely “resilient” to Russia’s EW (Electronic Warfare) “jamming,” according to reports. However, the “effectiveness” of “satellite-guided munitions” like the “Excalibur” artillery round and the HIMARS (High-Mobility Artillery Rocket System)-launched version of the GBU-39, the GLSDB (Ground-Launched Small Diameter Bomb), has “plummeted.”

This is according to interactions with Ukrainian military officials and “confidential assessments” accessed by The Washington Post (WaPo). Developed by Boeing, it has a GPS-aided inertial navigation system, where the DiamondBack-shaped wings deploy after being released. It is also fitted onto the warhead section of a HIMARS rocket, becoming the GLSDB. Ukraine was reported to have received the GLSDB some time early in 2024.

‘GLSDB, Excalibur Failed, GBU-39 Succeeding’

“Russian jamming technology (has prompted) Ukrainian military officials” to limit, or “shun” their use “altogether,” while seeking “upgrades” from the Pentagon and “arms manufacturers.” “US-made guided munitions provided to Ukraine typically were successful when introduced, but often became less so as Russian forces adapted,” WaPo added.

After the Excalibur’s “reputation” as a “one shot, one target” weapon was “disproved,” the Ukrainians “reported the issue six months ago” and “Washington simply stopped providing the shells because of the high failure rate.” GLSDB and Excalibur found only “initial success” until the Russians, “adept” in EW, “adapted.”

Specifically, “GPS jamming” was found to be the “culprit”.

“But the modified weapons, known as ground-launched small-diameter bombs, or GLSDB, proved ineffective compared to those launched from airplanes,” the report added. “But, the GBU-39 small-diameter bomb, has proved resilient to jamming, according to the confidential documents. Nearly 90 percent of dropped bombs struck their target.”

Russian Reports of GBU-39 Use

A photo also appeared on May 25, 2024, of GBU-39s reportedly under a MiG-29. According to some Russian Telegram channels it the SDBs were loaded under the wing of a Sukhoi Su-27. But it first featured in the RuMoD’s (Russian Ministry of Defense) daily press update on March 2.

The Tsentr (Center) Group reported shooting down “six HIMARS rockets and two GBU-39 guided aerial bombs.” While it is not clear if the GBU-39 was launched as the GLSDB or in an air-launched mode, it is most likely the air-launched version, given the usually specific nature of Russian battlefield updates. This will be illustrated subsequently.

It also claimed “repelling counterattacks,” undertaking “50 strikes” with “bomber aircraft,” eliminating “525 military personnel,” “six tanks, one Bradley IFV, two armored combat vehicles, two cars, three D-30 howitzers, and a field ammunition supply point,” on that day. (The purpose of this section was to establish that RuMoD typically mentions a series of air, land and naval actions in a single daily update).

On May 10, the Russian MOD reported shooting down “30 GLSDB,” among “three ATACMS missiles, 17 AASM-Hammer guided bombs and a JDAM.” Russian Telegram channel ‘Milinfolive’ first reported the GLSDB’s use on February 15, by posting photographs of the weapon’s wing fragments and tails. It was used near Kremmenaya. (Here, the RuMoD has specifically mentioned the GLSDB, meaning the March 2 update claiming GBU-39 shoot down must have been the aerial-dropped version).

“These aerial bombs, after being separated from the missile part of the M26 missile, begin to glide towards the target, in the process of which they become a very difficult target for air defense systems due to their small size,” Milinfolive said. It can be assumed that the particular missile hit its target, as had it been shot down, the channel would have specified as such.

How Does EW Work in this Case?

One of the reasons why the GBU-39 is effective when launched from the air and not when fired from the HIMARS rocket as the GLSDB, is because of the parabolic flight path followed by the tube rocket. The rocket is detected as it travels upwards, possibly by Russian S-300 or Buk/Buk-M3 AD SAM radars.

Given the networked nature of Russia’s IADS (Integrated Air Defense System), the S-300 battery must be passing on the HIMARS rocket detection to the Buk system. Since Ukraine began using the AGM-88 HARM and the ADM-160 MALD, Russian AD crews also might be assuming a larger SEAD/DEAD intent behind HIMARS fires. Thus Buk and S-300 operators must be coordinating and trading priority targets in a manner where their radars do not remain on for a long time.

But, from a fighter-bomber (like the Ukrainian Su-27, as seen in the photo), it is released several kilometers before the frontline, when it travels the remaining distance ‘gliding’ to the target. Usually, fighters follow a low-high-low profile, where they fly at very low altitudes, close to the ground, pull up, release the bomb and then turn back.

Not being propelled with a rocket motor that has infrared signature or an active electromagnetic emission from the nose to be detected by AD missiles’ RF or IR seekers, makes the GBU-39 difficult to intercept. The WaPo report repeatedly mentions the munitions’ dependence on GPS guidance, and the Russian lead in jamming those signals. “Russian jamming signals are sent up from the ground and form a cone-shaped area. Any guided munition – or aircraft – passing through is at risk of interference,” WaPo added.

Now, there have been reports about Russian lead in EW predating even Ukraine since Syria. Naturally, Moscow can be assumed to have refined its systems like the Krasukha and the LEER even further. Thus, the SATNAV (Satellite Navigation) being the only signal linking the bomb and the launch platform, makes it easy to sever the connection, causing the bombs to miss.

“Jamming happens when huge amounts of energy are broadcast into an area, overwhelming a device’s signal. Russia has used the tactic on Ukrainian radios, drones and even GPS-guidable Excalibur 155 millimeter artillery munitions,” Reuters added.

About Parth Satam
Parth Satam's career spans a decade and a half between two dailies and two defense publications. He believes war, as a human activity, has causes and results that go far beyond which missile and jet flies the fastest. He therefore loves analyzing military affairs at their intersection with foreign policy, economics, technology, society and history. The body of his work spans the entire breadth from defense aerospace, tactics, military doctrine and theory, personnel issues, West Asian, Eurasian affairs, the energy sector and Space.