Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Geosync orbit problem.

  1. Oct 26, 2005 #1
    A satellite in synchronous earth orbit has a life of 10 years. The maximum acceptable east or west drift in the longitude of the sat during its lifetime is 10 degrees. What is the margin of error in the radius of its orbit?
    I am having some trouble with the above problem. I reasoned that the maximum error would be 1 degree a year, and therefore 1/365 degree a day. Since the circumference of the orbit varies directly with the radius, I thought I could do:
    (delta r)/r = (1/365)(360),
    Since I know r for a synchronous sat, (42,000 km) from an earlier problem, I can get the numerical value of delta r. Unfortunately, this gives me 320 m as the error in the radius, and the book gives 210 m as the answer. A bit too large for round-off error, I think :smile:
    Can anyone help me on this?
  2. jcsd
  3. Oct 26, 2005 #2


    User Avatar
    Homework Helper

    Do not forget Kepler's third law. If Ro is the radius of the synchronous orbit and To is the time period around this orbit (1 day), and R and T are the same for the actual orbit,
    You get the relation between the error of R and the error of T by differentiation.
    DelR/ Ro = 2/3 *(T/To)^{-1/3}\ DelT/To
    The allowed error in T is 1/(365*360). (Well, not quite so, look after the accurate length of year.)
    As T=To = 1 day, the corresponding relative error of R is two-third of the relative error of T.
    Just multiply it by 2/3....
  4. Oct 29, 2005 #3
    Thanks, Ehild. That was a big help. I appreciate it.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook