Can astronauts/cosmonauts performing spacewalks at the International Space Station communicate directly with radio stations on the ground (even if only as a back-up)? Or do they need to relay their signals through the ISS?

  • $\begingroup$ would Suit → TDRS → Ground be indirect, even though it bypassed the ISS? $\endgroup$ – uhoh Nov 9 '18 at 5:52
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    $\begingroup$ @uhoh: I would count that as indirect, but an acceptable answer. $\endgroup$ – DrSheldon Nov 9 '18 at 7:35
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    $\begingroup$ Now that the EMU Systems Workbook is online we can confirm that the EVA suits use UHF radios. ia802802.us.archive.org/14/items/EMUSystemsWorkbookJSC19450B/… $\endgroup$ – Organic Marble Jun 9 at 17:00
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    $\begingroup$ @OrganicMarble excellent, thanks! 0.11 W at 414..2 MHz and an isotropic antenna, now all that's needed is sufficient coffee... $\endgroup$ – uhoh Jun 10 at 2:33
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    $\begingroup$ I'm pretty sure this never happened w/ shuttle or ISS EVAs but I am not sure that it couldn't have. $\endgroup$ – Organic Marble Jun 10 at 3:15

Can astronauts on an EVA communicate directly with ground stations?

I don't see why not, unless it breaks some laws. With a simple vertical whip antenna or small Yagi and 10 W transmitter, you could do this with a portable UHF transceiver.

OrganicMarble's comment links to Extravehicular Mobility Unit Systems Training Workbook; EMU SYS 21002 JSC-19450 Rev. B and on page 3-33 (pdf page 117) it says:

Table 3-7. SSER specifications

Item                 Parameter                Specification
-------------        ---------------------    -------------------------
System Range         EMU to EMU                75+ meters (246+ feet)
                     Range: Orbiter to EMU    160+ meters (525+ feet)
                     Range: ISS to EMU         80+ meters (262+ feet)
                     Transmit power           0.11 watts
Transceivers         Primary frequency        414.2 MHz
                     Alternate frequency      417.1 MHz  

These distances are specifications so the capabilities under extraordinary situations will be different.

It goes on to explain that the antenna is "omnidirectional" but I doubt it has a perfectly spherical radiation pattern. Nonetheless let's assume it transmits its power in $4 \pi$ Sr. The astronaut may have to orient themselves closer to head-nadir than feet-nadir to get line of sight from the top of the backpack to the ground, and that orientation may change during the pass.

Figure 3-12. SSER and EMU antenna location

The EMU antenna is a low profile, omnidirectional UHF antenna mounted on top of the SSER with Velcro (Figure 3-12). Its internal construction is a single hollow section that acts as a resonating cavity for radio signals in the frequency range of those used by SSCS.


  • 400 MHz, $\lambda = $ 0.75 m
  • 1000 km (for say 1 minute of line-of-sight communications)
  • 10 kHz bandwidth (for voice without compression)
  • transmit: 100 mW isotropic, gain 0 dBi
  • receive: isotropic (area of a 10 cm diameter "dish antenna") gain -8 dBi

I used $\sim \left( \frac{\pi d}{\lambda} \right)^2$ from here to get the receive antenna gain for uplink. This won't be a problem because we can assume that the transmitter on the ground is a hundred times (or more) more powerful than the suit transmitter.

Intensity received on the ground will be 100 mW divided by $4 \pi \times 10^{12}$ square meters, or about $1 \times 10^{-14}$ watts per square meter. A room-temperature front-end receiver will have an NEP of $k_B T \Delta f$ = $4 \times 10^{-17}$ watts which is way lower than the received power per square meter, so a simple amateur radio whip antenna (vertical 1/4 wave) or a few element Yagi is all you need.

This is consistent with Suitsat and Kedr stories, amateur radio operators should be able to pick these up. Amateur radio satellites are in this power ballpark and are also routinely received by simple systems. You may need a motor to track the motion if you use a directional antenna. As mentioned above, uplink would be no problem since you can use hundreds of watts compared to the 100 mW transmit power of the suit.

Of course a vertical antenna has a null overhead, so if the ISS passes directly overhead you should tilt it a bit.

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    $\begingroup$ Nice answer. I forgot about the 93-minute orbital period. The communication window would be rather brief. $\endgroup$ – DrSheldon Jun 10 at 8:29
  • $\begingroup$ @DrSheldon ya Ham radio operators can do QRP for very long distances on Earth, but they use HF which stays between the Earth's surface and the ionosphere so that it bends around the planet. This has to be line-of-sight $\endgroup$ – uhoh Jun 10 at 8:34

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