How did ‘war reserve’ mode make Soviet S-300 stealthy to US F-35?

Two months ago we wrote about an interesting incident of the American F-35 stealth fighter pilot who flew near Ukraine last year. As Craig Andrle recounts, his role was to fly his F-35 over Ukraine and the plane’s radar to scan into Ukraine.

F-35's total system performance was a serious mistake - Kendall
Photo credit: USAF

In this way, Andrle recalls, not only was NATO’s combat capability maintained during the war, but also the information gathered was provided to the Ukrainian command. But one story of his captures our attention more than his daily activities.

He recalls that on one of his flights, ground control in Germany informed him where in Belarus a specific Russian S-300PMU-1 Soviet-designed air defense system was located. Andrle was asked to use the F-35’s radar as it flew past the Ukrainian-Polish-Belarusian border to confirm the location of the system.

Andrle was greatly surprised when he did not detect the air defense system at all, which, according to other intelligence, had been there until hours before. It later emerged that the S-300PMU-1 was in ‘war reserve mode’.

What is ‘war reserve mode’?

The “war reserve” mode is a feature of the S-300PMU-1 air defense system that allows it to operate in a limited capacity even if some of its components are damaged or destroyed during a conflict. This mode is designed to ensure that the system can continue to function even if it takes damage from enemy fire or other sources.

When the S-300PMU-1 is in “war reserve” mode, it will automatically switch to spare components and systems if the main ones are damaged or destroyed. For example, if a radar antenna is destroyed, the system will switch to a backup antenna to continue tracking targets. This helps ensure that the system can continue to operate effectively even under enemy attacks.

When the S-300 is attacked

“War reserve mode” is typically used in situations where the S-300PMU-1 is under attack or at risk of being attacked. By switching to this mode, the system can continue to provide air defense cover even if it takes damage. This can be particularly important in a conflict situation where the ability to defend against enemy air attacks can be critical to the success of military operations.

S-300PMU-1 missile defense system
Photo credit: Wikipedia

The “war reserve” mode of the S-300PMU-1 air defense system is a feature that allows the system to conserve its resources and extend its operational life. In this mode, the system is partially or completely turned off, leaving only the essential components active. This reduces wear and tear on the system as well as saves fuel and other resources. But despite this, the system continues to provide air defense cover, but only when it is most necessary.


Taking into account everything said above, we can conclude that ‘war reserve mode’ gives advantages to the air defense battery at specific times. But where there are advantages, there are also disadvantages. It is the same in this case.

One major drawback is that the system is less responsive and less able to detect and respond to threats when in this mode. This can make it vulnerable to attack, especially if the enemy can launch a surprise attack or overwhelm the system with a large number of targets.

AN-APG-81-AESA - F-35's radar detects a Russian Su-57, but within a certain range
Photo credit: Northrop Grumman

Another potential disadvantage is that the system may take longer to reactivate and become fully operational again after being in “war reserve mode”. This can be a problem if the system needs to be quickly deployed in response to a threat, as it may not be able to react in time to prevent an attack.

In general, the decision to use “war reserve mode” should be based on a careful assessment of the risks and benefits involved. Although it can be an effective way to conserve resources and extend system life, it should only be used when the benefits outweigh the potential drawbacks and risks.

Why didn’t the F-35 detect the S-300PMU-1?

We need to dig deeper into the explanations. Here’s what happened on the day in question to Andrle and his F-35. He flew the established route. He was tasked with confirming the geolocation of the S-300PMU-1 in Belarus.

Andrle has therefore activated his fighter’s radar to actively scan targets. In a similar mode, the F-35 sees everything. When in this mode the F-35’s radar receives all active signals that bounce off and return to the F-35.

But at the same time, the S-300PMU-1 has turned off active scanning mode and switched to passive scanning mode. I.e. the Russian air defense system does not look for an enemy, it only listens. When the S-300 radar listens, it does not return signals, i.e. signals do not bounce. Thus, the F-35 radar cannot currently detect that it has a passively activated air defense system, as it does not receive active signals from it.

There will now be six air-to-air missiles in the F-35 'belly'
Photo credit: Aviation Week

Normal and combat mode

The S-300PMU-1 air defense system has several modes of operation, including “normal mode”, “combat mode” and “war reserve mode”. “Normal mode” is used in peacetime and is intended for regular training and system maintenance.

“Combat mode” is used during military operations, when the system is actively engaged in the detection and engagement of targets.


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