Dopplar effect and Interplanetary travel

[tipsyMJT]

I was thinking today. Would the command center of a certain interplanetary mission need to tune in to a lower frequency because the ship would be moving away. If it's a neglible amount, at what speed would the Dopplar effect change anything?

[K^2]

The effect is there and it's measurable, but in terms of sending/receiving radio transmissions it is not really significant at the speeds of typical spacecraft. Channel selection is going to be something like 1/100. So you'd have to travel about 1% of the speed of light to make a difference. This might be closer to 1/1000 in the microwave ranges, but it's still much, much faster than any spacecraft ever launched.

[Bunsen]

It's there, and whether or not it matters depends very much on how the radios are set up. Some systems are quite sensitive to frequency error, others not so much. Higher speed systems are usually less picky. For example, the cubesat I'm working on right now has one downlink transmitter on approximately 435 MHz with a 10 kHz channel width, and for low Earth orbit the ground station has to tune plus or minus one full channel to compensate for Doppler shift. It also has a 2.4 GHz radio with a 280 kHz channel width, which doesn't give a damn.

Interplanetary missions see larger relative velocities, and often use rather low data rates to overcome the long distances and limited power availability. Doppler adjustment is designed into the system at about the same level as figuring out where to point the antennas, though, so it's not like it catches anybody by surprise.

[Thaniel]

SETI for instance compensate for the dopler created by the movement of earth in it´s orbit around the sun to confirm/rule out terrestial signals. I.e. if there is no shift, it´s not of celestial origin.

[SargeRho]

There was this funny incident with cassini-huygens, where huygens' transmitter's frequency was slightly off. They solved it by changing the relative speed between cassini and huygens.

[tipsyMJT]

Thank you everyone. I asked my physics techer and he looked at me like he'd never expect someone to ask that and he just pondered and said. It's much to small to even detect so in theory, yes, but in practicality it's not.

[Thaniel]

Well, you´re teacher is part right, and part wrong.

Check this out for reference: http://www2.jpl.nasa.gov/basics/bsf10-1.php It has some basic info about telecomunication in interplanetary space.

Under coherence dopler is mentioned, and how it´s measured and compensated for, among other disturbances in the force.... signal.

But like Bunsen says, the wider the channel width, the less the equipment cares about dopler.

[K^2]

It's much to small to even detect so in theory, yes, but in practicality it's not.

Much too small to detect? If your teacher actually said precisely that, I suggest you mention to him that police radar measures Doppler shift on a moving vehicle to determine its speed. Keeping in mind that police radars have been around since the 50's and they measure the shift caused by difference in velocity due to a few miles per hour. Ditto weather radar, which uses the same technology to detect movements of water droplets in the atmosphere.

It's a small effect, but if it's what you are actually looking to measure, small differences between two frequencies can be measured extremely precisely due to effect known as beat in acoustics. The reflected signal and source signal are mixed, and the mixed signal is picked up by the receiver tuned to pick up much lower frequencies. Measuring the actual frequency that is picked up lets you determine the velocity of the moving object.

[PakledHostage]

Much too small to detect? If your teacher actually said precisely that, I suggest you mention to him that police radar measures Doppler shift on a moving vehicle to determine its speed. Keeping in mind that police radars have been around since the 50's and they measure the shift caused by difference in velocity due to a few miles per hour. Ditto weather radar, which uses the same technology to detect movements of water droplets in the atmosphere.

The same is true of laser gyros. They measure Doppler shift in laser light to determine rate of rotation. Laser gyros are accurate enough that they can precisely measure minute variations in the Earth's rotation rate. The data is used, in part, to improve the accuracy of the GPS system.