# How do you send out SOS in space if there is no absolute coordinate?

Imagine your spaceship encountered a gravitational anomaly and discovered that you are lost in space, your computer tells you that you are still inside the Milky Way galaxy but it cannot find the Solar system. I think in order to triangulate the spaceship current position we need at least 3 other spaceships in the proximity otherwise parallax error gets big, is there a way to transmit a signal so that only 1 station no matter how far away can easily pinpoint the source of the SOS signal?

Edit: the crews were being kept in cryogenic tube but... Hold a sec I thought I'm asking for sending out SOS signal and my officers are still investigating the anomaly which somehow fried the ship's only blackbox.

• Just to clarify, if you use astronomy to determine you are in our galaxy you will also automatically have all you need to calculate the location in our galaxy. So are you using some kind of space-time distance estimate of the jump to determine you cannot be outside our galaxy? Also what are you using to communicate? Because if you can send a message to Earth, whatever is blocking you from seeing where you are must be transparent to your broadcast communications, so you should be able to use that for directions to others using similar communications, including Earth and any possible colonies. – Ville Niemi Mar 6 '20 at 7:05
• If you are using radio waves, do you have any reason to think the signal will reach Earth in time to be relevant? – Ville Niemi Mar 6 '20 at 7:22
• Using radiowaves (or any photon based transmission) means your SOS will take an incredibly long time to reach anywhere. It takes an year for the signal to travel one light year (hence the name), and that's not going to be much use as an SOS. – StephenG Mar 6 '20 at 7:47
• If there is no FTL tech, how did you get lost? What exactly did the "gravitational anomaly" do to you? – Michael Richardson Mar 6 '20 at 15:47
• If you have no FTL, then unless you're in the solar system and fairly near Earth, you're not getting rescued in your lifetime. – pjc50 Mar 6 '20 at 16:20

Don't forget the Pioneer plaque

The radial pattern on the left of the plaque shows 15 lines emanating from the same origin. Fourteen of the lines have corresponding long binary numbers, which stand for the periods of pulsars, using the hydrogen spin-flip transition frequency as the unit. Since these periods will change over time, the epoch of the launch can be calculated from these values. The lengths of the lines show the relative distances of the pulsars to the Sun. A tick mark at the end of each line gives the Z coordinate perpendicular to the galactic plane.

You can use pulsars to mark your position, or alternatively Cepheid variables, since they act as standard candles for measuring distances.

A Cepheid variable (/ˈsɛfiːɪd, ˈsiːfiːɪd/) is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances.

• "they act as standard candles for measuring distances." You can use standard candles (i.e. was and a small fire on top) to measure distance? I'm probably missing out on a space thing called candle. – Blueriver Mar 6 '20 at 22:18
• – Malcolm Mar 6 '20 at 23:15
• Is a pulsar detectable everywhere, or only near the cone swept by the line of its magnetic poles? – Anton Sherwood Mar 8 '20 at 18:41

# The question is inconsistent.

If you know that you are in the Milky Way, and not in Andromeda or whereever, then you have navigational data. Either you know not even that much, or you know much better where you are.

A starship would not have star charts which are dots on paper. There would be databases with size, color/spectrum and orbit, and then it is "merely" a question of pattern matching to find where you are. The orbit data even allows you to correct for an unknown elapsed time.

If that fails, that is a clear sign of weirdness. You might not even be in the same universe any more.

• Maybe the computer can pinpoint the nearest supermassive blackhole, sagittarius A* by the broad spectrums of radiowave and probably xray and its mass. ;D – user6760 Mar 6 '20 at 6:48

you are still inside the Milky Way galaxy

Then you aren't lost. You have an absolute position to the center of the galaxy, and that's easy enough to figure out, assuming that you can see the stars. If you can't, that's pretty bad, but hopefully you can, and if you can see the stars, it's merely a matter of identifying the constellations and nebulas and notable star systems, and so forth, until you can figure out, based on what stellar landmarks you can see, where you are relative to the center of the Milky Way. Then just transmit that.

• Constellations don't work because they're created by our perspective. Landmarks in general are a good idea though. – Ryan_L Mar 6 '20 at 5:35
• @Ryan_L: If our star db is good enough, we can attempt a fit on the local constellations to find out where we are. – Joshua Mar 6 '20 at 16:29
• The Green Bank Observatory visitor's center has an exhibit where you look through a small window, and see tiny LED lights, arranged in a familiar constellation against a black background. Then, you walk around the corner and see through a different window that you were looking into box, maybe fifteen or so feet deep, in which some of the LED "stars" are much closer to the first window than others. The picture looks completely different when seen from the side – Solomon Slow Mar 6 '20 at 20:24
• @SolomonSlow Presumably, it would not be a human looking out a window that is making the measurements. A computer would be analyzing the local starfield trying to find a match within the 3D database using apparent position as well spectroscopic analysis of starlight. – Harabeck Mar 6 '20 at 21:03
• @Harabeck, I was supporting what Ryan_L said. IMO, the idea of a star ship navigator using the word "constellation" is like the idea of a department-of-highways administrator who talks about "horseless carriages." We call some group of stars a "constellation" when they all appear close to each other in our sky, but from the point of view of the star ship, stars belonging to that same constellation could appear completely opposite each other in the star ship's "sky." – Solomon Slow Mar 6 '20 at 22:07

You know what? Let's get abstract!

It all depends on the nature of signal detection and signal propagation.

You know what you said about triangulation? That's required if your detection method returns a direction but not a distance. For instance, when a submarine detects a sound from "heading 281", they don't necessarily know how far away that noise is. So they run a bit in a perpendicular direction, then stop - and use the difference in headings to roughly triangulate where the sound came from.

But if a submarine 'pings' its target, it not only gets the bearing, but the distance as well - based on the time it takes the sound to echo off the target and return back to the originating pinger. Triangulation is not required there, because the information being returned includes both angle and distance.

So, let's start taking a look at options.

Signals have a direction but no distance. For instance, someone emits an electromagnetic signal and someone else detects it. Or they emit a FTL version of that signal. The receiver is getting an angle to which the signal corresponds. So how does one indicate their position? Easy: just include within the information on the distances to two nearby celestial objects. After all, the receiver could trace back where the signal came from, and there'd only be two possible points on that line with a distance X from object Y. If they give two such distances, that narrows it down to one specific point. Aka, a distress call would be, "I'm 1.98 floobars away from Omicron Alpha 7, and 1.99 floobars away from Omicron Alpha 8."

Signals have a direction and a distance. For instance, someone emits a signal that has a half-life depending on distance. The receiver gets both an angle and a direction - basically, the same information as a submarine ping. In that case, there's no need to transmit location information at all. (Just like a submarine doesn't need anything more than a simple 'ping' sound to get what it needs: the signal itself is good enough.)

Signals have a distance but no direction. Aka, an ansiable network that has a half-life depending on how far away the signal came from - but there's no telling which direction the signal came from. This is when triangulation comes into play. In that case, the best solution is to have three detectors working in concert, using the distances of each to plot a point in 3D space where the signal came from. In that case, there's no need to transmit location information at all - you'd be relying on the detector to do the legwork of triangulating your position.

Signals have neither a direction nor a distance. Aka, an ansiable network which just 'is' - it's like a bulletin board where posted messages just appear and you have no clue where they came from or how far away the poster was. This is when things get tricky. In that case, you have to perform the triangulation yourself. If you list your current distance away from 3 known stellar objects, that will be enough to triangulate yourself to anyone knowing the position of those objects. In which case, a distress signal is you listing your position in regards to three stars/etc.

If you cannot see any stellar landmarks for whatever reason, your only hope is to send repeated SOS messages at regular intervals. A single station can eventually work out your position by scanning for the signal with a directional antenna. They would scan first with a wide field of view antenna to get a rough direction. Then they would scan in that rough direction with antennas with successively narrower fields of view until the direction is precise enough for their comfort.

Then, they can get a reasonable measurement of the distance by comparing the strength of the signal they're receiving with an estimated guess about how strong your transmitter is. The weaker the signal, the further away you are. They can know exactly how far away you are if you include how strong your transmitter is in the message, but it shouldn't be essential.

Alternatively, if you can send repeated SOS messages over the course of months or years, they can measure your distance fairly precisely by measuring the parallax between where on their night sky you are on Day 1 and Day 100.

• If I am unable to identify constellations, then the SOS won’t be received for decades, probably centuries. – WGroleau Mar 6 '20 at 16:47
• @WGroleau or maybe your ship has no windows and its cameras have malfunctioned. You could be in low earth orbit and not be able to tell. – Ryan_L Mar 6 '20 at 17:02
• Could be either one, but OP didn’t say that. If “your computer tells you you are in the Milky Way galaxy,” then it probably can tell you you’re not near earth. On the other hand, if it can identify the galaxy, it can probably identify the location. – WGroleau Mar 7 '20 at 13:33

As others have said: without knowing more about the situation they find themselves in, and how they got there, we can't establish how they'd communicate their location.

• Normally, the center of the galaxy is a known, fixed 'origin' point; the plane of the galaxy gives one of two known, fixed orientations; and the rotational direction of the galaxy differentiates between those two possible orientations. Once you have those things, you have a fixed coordinate system, and to orient yourself in those coordinates you need only your distance from the center, your distance above or below the rotational plane of the galaxy, and how far around the disk you are relative to, say, Earth.

• Normally, if any landmark stars could be identified by instruments or eyes, then you will know where you are in the galaxy. We have a 3D map of the milky way now, and that will only get better as time goes on: we wouldn't need to rely on "constellations", but rather on being able to see enough stars to identify them by their relative positions, brightnesses, and spectra. A mobile phone could do this, so you can assume that every person on the space ship has enough sensors and computing power immediately on their person to do it. Downside is, this requires line of sight to enough recognizable things, and if all external ship sensors are broken and nobody can go spacewalking, or if the ship is inside a cloud or other obscuring thing, then you don't get line of sight.

• Normally, you can rely on radio communication and its features. However, the Milky Way is 105,000+ lightyears across, so it's lightly that the tale will rely on handwavey FTL communications.

• Normally, communications are directional, so the receiver can at least use a directional antenna to get a rough idea of direction the signal's coming from. However, with FTL comms, direction might not be a factor.

• Normally, communications are affected by doppler shift, so the receiver can establish a relative velocity relative to the sender, which would tell things like how far away it is. However, with FTL comms, doppler shift might not be a factor.

• Normally, attenuation of a signal could give some idea of distance, but maybe not FTL comms.

• Normally, a long-range signal might be affected by gravitational lensing (if they were crazily lucky), in which case even a single receiver could triangulate to the source, since the signal might take multiple paths. However, FTL comms might not be lensable.

• Normally, a timestamp would allow the receiver to tell how far away the sender was, by how much time had elapsed in transit at light speed. However, with FTL comms, transit time might not depend on distance; and with FTL travel the timestamp is unlikely to match between source and receiver anyway.

• Regardless, every message sent should contain an internally accurate timestamp, correct for the source's time.

• Normally, anything periodic or any event that can be identified would help, if accompanied by a timestamp. Even orbital periods of local planets, etc. Four events are enough to triangulate a position (if the position doesn't drift much between them): two would cut it down to anywhere on a specific ring, and three cuts it down to just one of two points. However, this requires an ability to see events.

• Even if all that can be seen is a pulsar, pulsars vary very slightly and unpredictably in frequency, and that variance can be sent with the message, which would allow the pulsar to be identified by observers and, in combination with a timestamp, would allow distance from that pulsar to be established. However, since the source and receiver will be observing the pulsar from thousands of lightyears apart, their observations will not be synchronous: to use this for ranging, it will rely on a "tree-ring-like" stored record of pulsar variations over time (which you can easily build for all pulsars, once you've solved FTL comms and travel).

If the ship is inside a cloud, stuck in a wormhole, or otherwise screened from all incoming radiation, but still able to communicate using a handwavey faster-than-light system that's unaffected by physics, then I'm not convinced that locating the ship is possible.

Just use the Galactic Positioning System.

https://spectrum.ieee.org/tech-talk/aerospace/space-flight/what-if-gps-stood-for-galactic-positioning-system

Analogous to Global Positioning System, it uses Pulsars in place of satellites.

You need to spot a couple more beacons to compensate for the lack of a timestamp, but otherwise it works just like GPS.

Spherically! while a lot of our comms are built on high gain antenna arrays to maximize bandwidth etc, any SOS would be very helpful to emanate from center to Anyone within receiving range.

This does make an assumption that there are a series of receiving stations that would be looking around for any SOS or other signal...

We constantly point telescopes of all types at the sky, but think about how small a window we sweep throughout that looking...

HOWEVER, we then make it to spherical explosions that ripple through all of space-time and a detector that isn't Directional then can pick up. i.e. volumetric neutrino detectors that then can look at the vector a single ν wandered through (trace the line) OR, multiple detectors in a triangle that can then figure the signal origin (i.e. black hole and gravitational waves)

So, again, what is the signal/receiver "triangle" or even "pyramid" look like?

Use the signal itself to communicate your relative location to the recipient: when they pick it up, they get the direction that you're in by default, and if you include some information about the transmission amplitude, they can also get the distance, which is all they need. For an added bonus, include the transmission frequency, and they can get an estimate of your relative velocity as well.

That might not get them to a pinpoint, technology depending, but it'll get them close enough that you can use some local measures to tell them exactly where you are (say, by transmitting your distance from and perceived angles between some nearby stars to anybody who gets close).