Expert: Joel Osborne - 7/13/2007

Question
What are the advatages and disadvantages of astronomical observation from a low earth orbitting obervatory(satellite)? Please Hurry!

Answer


Okay, comments recieved and I shall explain to you the difference and explain my answer.


Basically, there are many types of observations that we can possibly take to look at the position of a satelite, which we call astronomical observations.

First is called the two line elements, or TLE's for example i'll pull up the TLE's currently for the ISS:

Satellite: ISS
   Catalog Number: 25544
   Epoch time:      07208.63003017   =   yrday.fracday
   Element set:     900
   Inclination:       51.6322  deg
   RA of node:       254.8574  deg
   Eccentricity:     .0009105     
   Arg of perigee:   171.0255  deg
   Mean anomaly:     189.1073  deg
   Mean motion:   15.75945050  rev/day
   Decay rate:    2.00000E-04  rev/day^2
   Epoch rev:            1716
   Checksum:              265

which produces the following elements:
 TWO LINE MEAN ELEMENT SET
 
   ISS
   1 25544U 98067A   07208.63003017  .00020000  00000-0  20000-3 0  9006
   2 25544  51.6322 254.8574 0009105 171.0255 189.1073 15.75945050 17167
 
The first difference is location, location, location. This is accurate up to a certain degree, were not looking to rendezvous, maybe your just trying to look at when it might take a pass above you, this is what you would have to look for to see when to look up.

If were looking to rendezvous, were going to have to look a lot harder at location this is using some other elements, which takes a bit more of math to calculate, and its based on cartesian cordinates:
       M50 Cartesian                         M50 Keplerian  
   -----------------------------------       --------------------------------
    X    =        4095696.27                 A    =         6714568.69  meter
    Y    =        -928572.89  meter          E    =           .0009435
    Z    =        5235593.51                 I    =           51.30243
    XDOT =       1933.134913                 Wp   =          156.08589
    YDOT =       7458.968440  meter/sec      RA   =          254.29243  deg
    ZDOT =       -197.739693                 TA   =          295.74912
                                             MA   =          295.84648
                                             Ha   =            189.247  n.mi
                                             Hp   =            179.490

             M50 Cartesian                         J2K Cartesian  
   -----------------------------------       --------------------------------
    X    =       13437323.73                 X    =         4080342.12
    Y    =       -3046498.97  feet           Y    =         -882859.25  meter
    Z    =       17177144.08                 Z    =         5255452.66
    XDOT =       6342.306146                 XDOT =        1850.544523
    YDOT =      24471.681232  feet/sec       YDOT =        7480.124089  meter/sec
    ZDOT =       -648.752275                 ZDOT =        -188.547998

             TDR Cartesian                         TDR Cartesian  
   -----------------------------------       --------------------------------
    X    =      -13322385.28                 X    =        -4060663.03
    Y    =       -3126067.76  feet           Y    =         -952825.45  meter
    Z    =       17252213.03                 Z    =         5258474.53
    XDOT =       4826.017195                 XDOT =        1470.970041
    YDOT =     -23799.272928  feet/sec       YDOT =       -7254.018389  meter/sec
    ZDOT =       -613.087811                 ZDOT =        -186.869165
 

As we look at what they call the X dot Y dot and Z dot we've got an actual speed in M/S if i'm looking to hook up with the ISS I can throw those into what they call Clohessy-Wiltshire (CW) Equations or Hills equations.

Punching this out we can produce an exact time to hook up with the ISS, and you just have to put in this equation to get there, and i'll direct you to a web site that would help explain this better:

http://ccar.colorado.edu/asen5050/lecture12.pdf


So you can see what the difference is, let me know if you need anymore information regarding this difference.

Joel Osborne
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Thanks for the question.

Advantages of astronomical observations would be position of time and location, or more importantly, be where it is when we say so.

Disadvantages are that it takes a lot of math, gettting gps is just that much more quick.

Thanks, sorry for the delay.

Joel Osborne