A communication satellite orbiting around earth has an eccentricity of \(0.0014\). Find  the time period of the satellite, altitude at apogee and velocity at perigee, if the satellite has an altitude of \(400\,km\) at perigee from the surface of the earth.

A communication satellite orbiting around earth has an eccentricity of \(0.0014\). Find  the time period of the satellite, altitude at apogee and velocity at perigee, if the satellite has an altitude of \(400\,km\) at perigee from the surface of the earth.

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    Satellite orbitSatellite orbit

    \[\frac{{{r_{\max }}}}{{{r_{\min }}}} = \frac{{1 + e}}{{1 – e}} = \frac{{1 + 0.0014}}{{1 – 0.0014}} = 1.0028\]

    \[ \Rightarrow {r_{\max }} = {r_{\min }}\left( {1.0028} \right)\]

    \[{r_{\min }} = {\rm{radius}}\,{\rm{of}}\,{\rm{earth}}\,{\rm{ + }}\,{\rm{altitude}}\,{\rm{above}}\,{\rm{earth’s}}\,{\rm{surface}}\]

    \[ \Rightarrow {{\rm{r}}_{\min }} = 6.4 \times {10^6} + 0.4 \times {10^6} = 6.8 \times {10^6}\,m\]

    \[\Rightarrow {r_{\max }} = 6.8 \times {10^6}\left( {1.0028} \right) = 681904\,m = 6.81904 \times {10^6}\,m\]

    Therefore satellite’s altitude at apogee is

    \[6.81904 \times {10^6} – 6.4 \times {10^6} = 419040\,m = 419.04\,km\]

    Time period of the satellite is given as

    \[{T^2} = \frac{{4{\pi ^2}}}{{{k^2}}}{a^3}\]

    \[{k^2} = G{M_{earth}} = 3.986 \times {10^{14}}\,{m^3}/{s^2}\]

    \[ \Rightarrow a = {\rm{semi}}\,{\rm{major}}\,{\rm{axis}}\,{\rm{ = }}\frac{{{r_{\max }} + {r_{\min }}}}{2} = \frac{{6.81904 \times {{10}^6} + 6.8 \times {{10}^6}}}{2} = 6.80952 \times {10^6}\,m\]

    \[{T^2} = \frac{{4{\pi ^2}}}{{3.986 \times {{10}^{14}}}} \times {\left( {6.80952 \times {{10}^6}} \right)^3}\]

    \[ \Rightarrow T = 5592.242\,s = 1.5534\,hr\]

    \[{V_{perigee}} = {r_{\min }}\dot \theta \]

    \[\dot \theta = \frac{h}{{{{\left( {{r_{\min }}} \right)}^2}}}\]

    \[{h^2} = {r_{\min }}\left( {1 + e} \right){k^2}\]

    \[ \Rightarrow {h^2} = \left( {6.8 \times {{10}^6}} \right)\left( {1 + 0.0014} \right)3.986 \times {10^{14}}\]

    \[ \Rightarrow h = 5.20987 \times {10^{10}}\]

    Therefore,

    \[\dot \theta = \frac{{5.20987 \times {{10}^{10}}}}{{{{\left( {6.8 \times {{10}^6}} \right)}^2}}} = 0.0011267\,rad/s\]

    \[{V_{perigee}} = \left( {6.8 \times {{10}^6} \times 0.0011267} \right) = 7661.56\,m/s = 7.66156\,km/s\]

    techAir Answered on 9th March 2021.
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