STEP 8: FINDING THE R.A. DIFFERENCE

Da



 
Element Symbol Value
Epoch to 05:54:21.171 UTC December 16, 2007
Inclination i 51o.9970
R.A. of the Ascending Node aW0 251o.0219
Eccentricity e 0.0001492
Argument of Perigee w0 33o.8641
Mean Anomaly at TLE Epoch Mo 326o.2322
Mean Motion n 12.62256095 orbits / solar day
Propagation Time Dt 1.7677141 solar days
Mean Anomaly at Time t M(t) 1.37777389 radians (78o.940629)
True Anomaly at Time t n(t) 78o.95065818
Semi-major Axis a 7791.787473 km
Perigee Distance P 7790.624938 km
Geocentric Distance r(t) 7791.564499 km
Precessed R.A. of Asc. Node aW(t) 245o.6400244
Precessed Arg. of Perigee w(t) 37o.77767416


Although we now know where the satellite is with respect to its own orbit plane, we still do not know where the satellite is in relation to the Earth's center. To do this, we first need to establish a relationship between the satellite's orbit plane and the Earth's equatorial plane. Fortunately, we can use the (precessed) R.A. of the Ascending Node, the (precessed) Argument of Perigee, the orbit Inclination and the True Anomaly to find this relationship, i.e. the difference in the R.A. coordinate between the satellite's geocentric position at time t and its precessed R.A. of Ascending Node.

First, we need to determine the orbit's Argument of Latitude at Time t. This is simply the addition of the orbit's (precessed) Argument of Perigee and its True Anomaly:

m(t) = w(t) + n(t) - 360o { INTEGER [ ( w(t) + n(t) ) / 360o ] }

m(t) = 116o.7283323

The determined Argument of Latitude can be directly projected onto the Earth's equatorial plane to find the R.A. difference (Da) with the help of the following conditional equation:

Da = cos-1 { cosm(t) / [1-sin2isin2m(t)]1/2 }
for 0o < i < 90o AND 0o < m
(t) < 180o OR 90o < i < 180o AND 180o < m(t) < 360o

Da = 360o - cos-1 { cosm(t) / [1-sin2isin2m(t)]1/2 }
for 0o < i < 90o AND 180o < m
(t) < 360o OR 90o < i < 180o AND 0o < m(t) < 180o

 

Da = 129o.278845
 

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Step 8: Finding the R.A. Difference Was Last Modified On January 02, 2012