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In this July 2018 tweet by Cdr. Scott Kelley, with the north face of Mt. Everest over his shoulder, he mentions his "Emergency" watch.

I've searched and then found the Hodinkee.com article Hands-On: The Breitling Emergency, Or The Safest We’ve Ever Been With A Watch.

Last week, we welcomed astronaut Scott Kelly back to Planet Earth following his 342-day mission in space by taking a look at some of his watches – both in flight and on Earth. And many of you expressed some interest in what appeared to be a larger-than-life 51 mm wristwatch. It is in fact very real – developed specifically for the most perilous missions. Today, we go hands-on with the Breitling Emergency (not Kelly’s mind you).

That link in the block quote is also worth checking out.

Reading further in that article, it says

A follow up to the 1995 Emergency, this version (originally introduced as the Emergency II, although the "II" has been dropped) the watch presents an improved personal locator beacon (PLB) capable of transmitting its coordinates on two separate frequencies. The decision to create a dual-frequency version of the original Emergency is a result of the decision, by emergency beacon monitoring agency Cospas-Sarsat, to phase out its satellite monitoring at the long-standard 121.5 MHz aviation emergency frequency – which was also used by the first Breitling Emergency watch. (Cospas-Sarsat is a satellite-based search and rescue system, which has been around since 1982; Cospas is an acronym for "Space System for the Search of Vessels in Distress" in Russian, and Sarsat stands for "Search and Rescue Satellite-Aided Tracking.")

The latest beacon now carries a digital signal on the 406 MHz frequency via Cospas-Sarsat's worldwide satellite system, on top of an analog signal on the 121.5 MHz frequency, used by search and rescue (SAR) teams to home in on victims at land, air or sea. Cospas-Sarsat originally monitored both frequencies. The FAA in the United States began requiring 121.5 MHz emergency beacons on civilian aircraft in 1973, and though the frequency is still monitored (which is why the Emergency transmits on both) 406 MHz is both longer range and gives better accuracy.

To pick up signals from a watch, I would suspect that you'd need to be in LEO, and fairly close-by. The problem there is that it takes quite a large constellation of satellites to provide continuous coverage of all points on Earth.

Question: So I'd like to ask which satellites can hear emergency signals from Scott Kelley's watch? How many of them are there, what's the altitude, and how quickly will they detect a signal from any random spot on Earth and then route the message somewhere where action can be taken?

Scott Kelley's tweet

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    $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$ – Russell Borogove Sep 18 '18 at 22:36
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    $\begingroup$ Yes, I am asking this seriously. I would like to ask which satellites can hear emergency signals that are from transmitters so small and low power that they can fit into a wristwatch! Also, I needed to link to this question here. $\endgroup$ – uhoh Apr 23 at 4:08
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    $\begingroup$ @uhoh Are you asking about how the satellites are build (blueprints)? Because otherwise the question already contains the answer. $\endgroup$ – Polygnome Apr 29 at 8:07
  • $\begingroup$ @Polygnome No the answer is not contained in the question. If you know the answer, which would identify the satellites, then please post it! ("Cospas-Sarsat's worldwide satellite system" is not helpful as an answer, that just begs the next question (what satellites are in "Cospas-Sarsat's worldwide satellite system" that can hear...) because some satellites in that system might not be able to, and there may be other satellites also capable of receiving these signals not included in that system.) $\endgroup$ – uhoh Apr 29 at 9:17
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The space segment of the Cospas-Sarsat system has two components, SAR signal repeaters (SARR) and SAR signal processors (SARP).

The Sarsat instruments [...], receive these messages. They are downlinked to the distress terminals — known as Local User Terminals (LUT) — distributed around the globe. The message are then processed and distress alerts generated. The Mission Control Centres (MCC) receive those alerts and relay them to the appropriate Rescue Control Centre (RCC), which begins the actual search and rescue operation.

The SARR instrument receives 406 MHz beacon transmission bursts and retransmits them immediately on the dedicated 1544.5 MHz downlink to the LUTs. Because there isn't a memory within SARR this service only works in local mode, i.e. the beacon and LUT must be in simultaneous view of the satellite. All processing is performed by the LUT. Each transmission is detected and the Doppler (change in wavelength caused by the motion of the satellite, with respect to the beacon) information calculated and a beacon position is then determined.

SARP receives and processes emergency signals from the 406 MHz beacons. It determines the identification, frequency and time of the signal. These pre-processed data are then fed, in real time, to the SARR instrument for immediate transmission, plus simultaneously stored in the memory for additional later transfer to the LUTs. This process can locate beacons within an accuracy of 5 km. A low-power 121.5 MHz homing signal included in most 406 MHz beacons helps rescuers determine the final location.

(Source)

These instruments are secondary payloads on other missions, and belong to three different groups of satellites:

Low-Earth Orbiting Search And Rescue (LEOSAR) Satellite

  • NOAA Polar Orbiting Environmental Satellites (POES) – known as ‘SARSAT’
  • ESA/EUMETSAT Polar Orbiting Meteorological Satellites (MetOp) – known as “SARSAT”

Geostationary Orbiting Search And Rescue (GEOSAR) Satellites

  • NOAA Geostationary Orbiting Environmental Satellites (GOES)
  • ISRO Indian National Satellite (INSAT)
  • ESA Metosat Second Generation (MSG)

Medium-altitude Earth Orbiting Search and Rescue (MEOSAR) Satellites

  • United States’ Global Positioning System Satellites (GPS)
  • Russia’s Globalnaya Navigazionnaya Sputnikovaya Sistema (GLONASS) Satellites
  • European Space Agency GALILEO Global Positioning Satellites

A list of the currently active payloads is provided by Cospas-Sarsat under Current Space Segment Status and SAR Payloads.

Note that the GEOSAR satellites are not used to determine position, since they can not use the doppler shift, but are still valuable tools for quick responses:

The GEO satellites are not able to use Doppler location processing since they have no relative motion between them and the emergency beacons. Therefore, they are not able to determine a location for a beacon. They can, however, provide immediate alerts. This is a valuable tool for SAR personnel since it allows them to begin their initial verification of the alert using the National 406 MHz Beacon Registration Database. Often this detective work yields a general location of the vessel or aircraft in distress and SAR assets can be readied or dispatched to that general area.

But the huge advantage of the GEOSARs seems to stem from the fact that some beacons can indeed track their location via GPS and transmit that as well:

There is a significant advantage with a GEOSAR satellite detection of a beacon with encoded location. Here’s how it works: emergency beacons determine their location using a GPS receiver that is either integrated into the beacon (called a location protocol beacon) or fed by an external GPS receiver. This accurate location information (generally around a football field in size for positional accuracy) is then encoded into the 406 MHz signal that is transmitted by the beacon.

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  • $\begingroup$ Wow that's roughly 50 active satellites at the moment! Does each satellite have both a SARR and SARP? I'm guessing that it's the former that does the receiving? $\endgroup$ – uhoh Apr 29 at 16:02
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    $\begingroup$ @uhoh I have added a short description of each module. $\endgroup$ – Polygnome Apr 29 at 16:48
  • $\begingroup$ You are certain Scott Kelley's watch can transmit all the way to satellites in GEO? That's your answer? $\endgroup$ – uhoh Apr 29 at 18:41
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    $\begingroup$ @uhoh Are you implying Cospas-Sarsat are lying about how the system works? $\endgroup$ – Polygnome Apr 29 at 19:23
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    $\begingroup$ @uhoh The power requirements for a PLB are described in §2.3.2 of the spec: cospas-sarsat.int/images/stories/SystemDocs/Current/…, so there is actually not that much variation between manufacturers possible. $\endgroup$ – Polygnome Apr 30 at 11:10
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The answer is given within the question, the emergency signals are received by the the International Cospas-Sarsat Programme satellites.

From wikipedia:

Space segment

The Cospas-Sarsat system space segment consists of SARR and/or SARP instruments aboard:

Five satellites in polar low-altitude Earth orbit called LEOSARs
Seven satellites in geostationary Earth orbit called GEOSARs
Over 35 satellites in medium-altitude Earth orbit called MEOSARs

From the webside of the watch:

Technical data Beacon Watch

Standard applied COSPAS-SARSAT

C/S beacon classification PLB

Category 2 Transmission frequencies 121.5 MHz / 406.040 MHz

Transmission power ≥ 30 mW @ 121.5 MHz / ≥ 3.2 W @ 406.040 MHz

Transmitter autonomy ≥ 18 hours at –20°C / ≥ 24 hours at +20°C

Operating temperature (beacon) –20°C to +55°C

Power supply Beacon: 1 x Li-ion 4.2 V, 1.1 Ah / Watch: 1 x silver oxide 1.5 V

An article about the watch.

I will extend this answer within the next days.

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    $\begingroup$ @uhoh you can take a look here hovedredningssentralen.no/wp-content/uploads/2017/04/… this is only a few of the satelites that locates emergency beacons. $\endgroup$ – trond hansen Apr 29 at 9:49
  • $\begingroup$ I'm looking for the satellites themselves. Names of programs, frequencies etc. are all superfluous. Thanks. $\endgroup$ – uhoh Apr 29 at 14:31
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To the question

Which satellites can hear emergency signals from Scott Kelley's watch?

I'll supplement @Polygnome's answer with a literal answer to the question as asked using details from this link cited there. This presumes that even the satellites in GEO, roughy 60 times farther away than LEO can receive signals from a wristwatch. For more on that see Can I transmit signals to geostationary orbit from a wristwatch?.

Currently (30-Apr-2019) about fifty possible satellites, but of course it is far fewer than that at any given time and location:

LEOSAR:

Sarsat:    five spacecraft      (7, 10, 11, 12, 13) 

MEOSAR:

Galileo:   nineteen spacecraft, nearly all of them
GPS:       nineteen spacecraft, nearly all of them

GEOSAR:

GOES:      two spacecraft       (15, 16) 
MSG:       three spacecraft     (1, 3, 4) 
Misc:      three spacecraft     (Louch-5A, INSAT-3D, INSAT-3DR)

According to Transition to MEOSAR (White Paper), it seems like the focus is on the spacecraft constellations in MEO. GEO provides no Doppler shift and therefore no information about location within the hemisphere, and LEO would provide spotty coverage unless a very large number of satellites are so equipped.

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