To possibly change his programmed route, interrupt the mission, etc. How will poseidon know where he is? . The maneuvering missile communicates with GPS has some inertial system and TERCOM. Poseidon can only have a TERCOM equivalent. But he should have at least one-way communication options from some command center to the underwater missile.
The Poseidon is a dooms day weapon... most of the time when it has been launched WWIII has already started and is under way... all sides launching full scale nuclear attacks on each other... odds are there wont be satellites to send course corrections.
That is not to say the Poseidon is fixed to a single route... it can be fitted with its own sensors to listen for the active sonars needed to find it and it will know where it is and be able to find its way to its target. Do you have a GPS navigation system in your car? Set a location and start driving and it will calculate an ideal route based on its database of roads and speed limits. If you take a wrong turn it doesn't crash and stop working. It doesn't ask you for help. It recalculates a new route based on your cars current location and the direction it is facing. If you are near a tipping point where there are two or three possible ways of going to get to your location it might pick the fastest or the shortest direct route, but that might be through the heaviest traffic so you might continue to disobey it till it is forced to go another route.
The GPS system uses satellites to determine the current location of the car. Its maps are used to determine all the possible paths it could take and you enter the destination. With that information it can chart paths for you and rechart new paths if you deviate.
With Poseidon... it could use passive sonar and its own engine noise to map the terrain it is travelling over... together with inertial guidance that should be accurate enough... basically with a cruise missile intertial guidance over thousands of kms gradually gets less and less accurate over time... but 20 minutes in to the flight if you programme the missile to fly over a specific set of crossroads by a large bridge, then as it flys past it can determine its location... if it was supposed to fly over the bridge but when it arrives the bridge is 500m to its left then it can recalibrate its actual current location based on the location of the bridge... the bridge is at x and y coordinates and we are 500m north or south of it now... our position is now accurate so manouver back on the flight path and continue.
For an underwater vessel, you could have it follow underwater features and move from feature to feature determining its precise location each time with inertial navigation systems getting it from point to point... a bit like early pilots used to navigate across land masses by flying from land mark to land mark.
Why would Poseidon not have an inertial navigation system?
Poseidon is a doomsday weapon it needs to communicate with Moscow like the warhead of an SS-18 needs to... ie it does not.
A modified Poseidon being used as a drone needs communication, but it wont be on a one way trip... it might be sent out to map the ocean floor around the world so that when it is needed it will have modern current accurate maps to enhance its accuracy. Or it might be used in a role where endurance and speed are important because it should have plenty of both...
But, just yesterday, in another thread this is what you said about drones communicating underwater:
That is perfectly consistent... the Poseidon is not really a drone... it is an underwater cruise missile, or ballistic missiles warhead... no need for communication.
A drone used to hunt submarines however is more like that flying drone looking for terrorists... it uses a camera to look day and night for targets, but that only works if it transmits that video image continuously to the controller, who will be directing where the drone flys to find targets. All the time the drone is operating it is sending data on the video channel showing what the camera is seeing... all the time the drone is operating it is receiving signals from the drone control vehicle or building to tell it which direction to turn the camera and when to zoom in or out and when to change flight path to keep the target in view or to search new areas for targets. That means a datalink communication channel with the drone that is continuous.
Do it over Syria and you are fine because there is really not much ISIS can do... try to fly drones over the Crimea and their communications will be detected immediately and it will be destroyed.
A drone underwater has even greater need for communication, because there is an enormous variety of things in and on the water that make all sorts of noise and it is very much a game of cat and mouse... if you have a large number of drones searching an area for subs or torpedoes then to coordinate their actions they need to communicate.... triangulation only works when you have multiple known points detecting signals... plot the points where the detectors are and draw a line in the direction of detection... with three or more points detecting a target those lines should cross somewhere and then you have your target... or a target... but you need to analyse the sound to determine what it is... is it a torpedo in the water... is it elephant seals having sex... is it a spider crab fighting a spider crab for food or for sex... the drones might have a library of sounds, but the enemy also have a library of sounds so they might add noises to their subs to make them sound like something harmless, while launching decoys making all the right noises.
In a sense ASW already uses enormous numbers of drones... they are called dispensable sonobuoys... but right now they are relatively cheap but not super cheap means of narrowing the location of a target. Using drones instead will probably make them orders of magnitude more expensive, and in doing so make them targets for the enemy to actively hunt to protect their underwater activities...
The idea of a swarm of underwater drones to kill all the enemy subs is like the idea of a swarm of any drones doing anything... it is bunk.
So how are you ensuring safe, secure communication between underwater drones?
Keep their distances apart relatively small... which is the opposite of what you want to do in finding subs or mines, and perhaps have a tethered float bobbing above them on the surface for communication between drones and HQ and controlling vessels.
For defensive use, where for example you might be looking for mines where there is unlikely to be enemy subs then such drones make sense, but keep in mind small drones means less endurance, speed, and range than a sub or a ship and much smaller sensors.
A well equipped modern SSN could probably sweep an area much more effectively than a tiny drone, while a properly equipped ship could probably do better as well... and at the same time with the equipment to deal with anything they find... unlike a drone.
I don't hate drones... they certainly have their place, but they are a long way away from replacing everything just yet.
Jobs like travelling up and down underwater cables or pipes checking for damage or leaks is an ideal job for a drone for example...