How does Soyuz detect the ground to fire its retro rocket at the end stage?

Is there any paper/reference describing the full entry descent and guidance algorithm and sensor suite used in Soyuz?

If Soyuz uses the sonar principle (just guessing), then one can't use Soyuz to land on the Moon.

If Soyuz uses a laser altimeter, then at such low altitude the time to return will be too small. I wonder if a laser altimeter can detect such resolution. Even if it does one will be pushing the sensor to its limits. Not a good idea.

If Soyuz uses a stereo camera, then one can't land on night.

The Luna and Surveyor series used a single extended ground contact sensor for engine cutoff. Soyuz does not definitely have it.

So what is used in Soyuz? Is there any sensor which will be better than what Soyuz uses?

  • $\begingroup$ Wow, great question! I've just asked How does a gamma ray altimeter work? What kind of sensor is used? $\endgroup$
    – uhoh
    Commented Apr 26, 2018 at 7:41
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    $\begingroup$ "If soyuz uses stereo camera. Then one cant land on night." In that case, putting a light next to the camera wouldn't exactly be rocket science. *baddum-tsh* $\endgroup$ Commented Apr 26, 2018 at 10:58
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    $\begingroup$ @DavidRicherby A camera/light housing that a) faces downwards, probably through the heat shield and b) stays transparent and intact through reentry would be rocket science, though. $\endgroup$
    – ceejayoz
    Commented Apr 26, 2018 at 19:02
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    $\begingroup$ @ceejayoz That’s an excellent point. I was just pointing out that not working at night couldn’t be the reason for rejecting a camera-based solution. $\endgroup$ Commented Apr 26, 2018 at 19:23

4 Answers 4


Earlier Voskhod models used a retractable metal probe, but it was unreliable, especially in windy conditions. So in Soyuz, as the other answers have pointed out, a gamma-ray source is pointed at the ground and the backscatter is measured. The instrument ("Kaktus"), was developed in the early 60s.


The advantage of using a gamma ray source rather than, for example, a radio altimeter, is that gamma rays can pass through less dense materials that the spacecraft coudn't land on. For example foilage or someone's outhouse in the Kazakh Steppe.

With the rockets fireing at only about 1.5m of altitude, it is critical that the altimeter reliably measures the distance to solid ground.

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    $\begingroup$ "Hello comrade! Sorry about your outhouse, but I am a Soviet citizen like you, who has just descended from space and must find a telephone to call Moscow!" $\endgroup$ Commented Sep 29, 2020 at 7:17

Soyuz uses gamma rays altimeter "Cactus", which starts soft landing engines just 1 meter before ground.

Translation from russian wikipedia:

The altimeter uses a source of gamma radiation (usually - Со 60, Сs 137 isotopes). The receiver detects backscattering, reflected from the atoms inside the underlying surface. Gamma ray altimeters are used at low altitudes (meters, tens of meters from the surface). The main application is for a soft landing system for descent vehicles of spacecraft.
In particular, in the Soyuz spacecraft a gamma-ray altimeter (Cactus) is installed at the bottom of the descent vehicle and the place of its installation is marked with a radiation hazard sign.

I even read somewhere (can't find a source now) that this was the single element, which Chinese engineers were unable to create for Shenzhou spacecraft and they just purchased those altimeters from Russia

  • $\begingroup$ A website link in Russian is not very useful to the general community on Space.SE, please consider finding an English version to link (given it's on Wikipedia, that should be quite easy) $\endgroup$
    – Edlothiad
    Commented Apr 26, 2018 at 5:46
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    $\begingroup$ Can't find english source, will translate $\endgroup$ Commented Apr 26, 2018 at 7:07

Just to add a better source: From the Soyuz Crew Operations Manual

The АКСП consists of barostatic and time mechanisms and the Гамма-лучевой высотомер (ГЛВ) (Gamma Ray Altimeter). The barostatic and time mechanisms operating according to their settings issue commands for sequential parachute deployment and for the execution of pre-landing operations in the CA. The АКСП units are located at the CA Module bottom. The purpose of the ГЛВ Altimeter is to issue the landing event signal at the altitude of 15 m (ТСЭ-4 “ПОСАДКА” (Landing) light goes ON) and the command for the ДМП thruster ignition at the altitude of 0.8 m.


At the descent final portion following the ГЛВ Altimeter commands: - at the altitude of 15 m the audio signal is switched On and the “ПОСАДКА”(Landing) light illuminates (ТСЭ-4); - at the altitude of 0.8 m 4 ДМП thrusters are fired; - manual ОП rope jettison inhibit is removed; - The система дыхательной вентиляции (ДВ) (Breathing Ventilation System) is put to operation. When landing onto the firm ground the Commander jettisons only one parachute rope. When the space-craft lands onto the water surface he jettisons the parachute completely.


They use Cesium-137, which produces low-level radiation, for the landing altimeter.

Soyuz hazards (spaceref.com PDF)

Cesium Altimeter- is a very LOW LEVEL radiation hazard. STAY CLEAR--15 foot radius from bottom. There is no hazard on sides or top. Remove Hatch Opening Tool by reaching around from the side of the capsule bottom If Soyuz capsule is upright, there is no hazard.

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    $\begingroup$ That is much too short for a good answer. $\endgroup$
    – Uwe
    Commented Apr 26, 2018 at 8:15
  • $\begingroup$ Information about the radiation hazard does not explain how altitude is detected. $\endgroup$
    – Uwe
    Commented Apr 29, 2018 at 19:42

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