Earth Based Telemetry for Guidance? Mars mission problems?

IDNeon

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Bear with me, being new to all this I have ideas but the questions are to help me understand the theory....I guess.

So spacecraft like Apollo and newer craft all rely upon Telemetry data from earth, confirming the ship's exact position and orbit, everything needed to do things right the first time, correct?

On board systems are synced to that and confirmed in case of Apollo with star readings, correct?

But the linchpin is the earth upload of Telemetry? The other systems are meant only as backup and may not be most accurate?

As for the Mars mission question, if the above were true, how much bandwidth does the Telemetry consume?

I heard that one of the recent probes to pass a kupier belt object took 2 years to send back just the bits of data for the pictures it took? Because at those distances the bandwidth is very small.

I thus wonder if the Telemetry data is considered prohibitively too much for what is feasible bandwidth even at distance of Mars, let alone outer planets?

Is this a major factor in why human spaceflight hasn't progressed past Lunar orbit?
 

N_Molson

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Live telemetry link is impossible... because speed of light or "c". It takes a few seconds for the Earth-Moon-Earth round trip, and up to 30 minutes for Mars. Its a constraint and there is no way around. So spacecraft need to have their own navigation devices, also in case of Apollo you have a comm blackout each time the Moon blocks the line of sight. Often you an inertial navigation system, that records all changes in acceleration and keep track of the velocity vector. Gyroscopes are used to keep track of orientation. And Apollo had a navigation computer (Apollo Guidance Computer). Future lunar missions will use various space probes as satellites relays, and this is also a role for the Gateway lunar station, being a comm "node", and a "mission control outpost", with astronauts having a flight control role. But still, the spacecraft have to be autonomous, its more like a confort thing.

Of course yes the farther away you are, the more powerful your signal has to be to achieve the same bandwith. Probes have a very limited power supply, so the solution is constant, very low bandwith transmissions. New Horizons has a computer with a decent memory that processes and stores the images and data taken by the sensors, then the "files" (data blocks) are sent byte-to-byte to Earth. Now New Horizons and Voyagers are extremes, things are better for Mars, which is never that far.
 

Urwumpe

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As for the Mars mission question, if the above were true, how much bandwidth does the Telemetry consume?

Not much - for the Mars Mariner 6 mission, programming the computer for the encounter took 4 hours of time. It was really slow. The communication protocol wasn't meant to be fast. While the decoder onboard operated with a 2400 Hz clockrate, it actually just allowed two words per second, to make sure all commands are properly received and decoded, even in bad reception.
 
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