Naked Science Forum

Non Life Sciences => Technology => Topic started by: Pierre Le Count on 02/11/2008 10:08:22

Title: How are tranmission times taken into account for satellites?
Post by: Pierre Le Count on 02/11/2008 10:08:22

Pierre Le Count asked the Naked Scientists:

How exactly is the time adjustment done for satellites (e.g the speed of the
satellite, distance above earth) do you you also need to know how long the
satellite has been on orbit.

Thanks
Pierre Le Count

What do you think?

Title: How are tranmission times taken into account for satellites?
Post by: graham.d on 02/11/2008 11:05:37

If a satellite needs to keep very accurate time it wil have to have a precise clock and this clock may need adjusting occasionally to make it even more accurately synchronised to an earth-bound clock of even greater precision. Is this what you mean? The GPS satellites have to do this for example. This is done by sending adjustment signals via a radio link (the jargon is an "uplink"). In principle it is like adjusting your wristwatch - the clock is changed forward or back as required by the amount needed to get it's received downlink time correct. The satellite position and velocity is known to great precision and allowance is made for Doppler shifts (even your handheld GPS device has to do this).

Title: How are tranmission times taken into account for satellites?
Post by: Bored chemist on 02/11/2008 14:23:45

The other thing about the GPS clocks is that they were set to run at the "wrong" speed so that, in orbit, where they have a high velocity but low gravity environment they run in synchrony with clocks here on earth. If you put an ordinary atomic clock in orbit it would run at the right rate for its environment, but it wouldn't keep time with a clock here on earth.

Title: How are tranmission times taken into account for satellites?
Post by: lyner on 03/11/2008 22:21:29

And the system is constantly being corrected by feeding back into it the calculated coordinates of known locations.

Title: How are tranmission times taken into account for satellites?
Post by: Pierre Le Count on 05/11/2008 22:21:07

Hi,
Thanks everyone for answering my question.
Pierre Le Count