[comp.sys.atari.st] norad

logajan@ns.network.com (John Logajan) (06/30/90)

pwp@iuvax.cs.indiana.edu (Paul Purdom) writes:
>Does anyone know the format for the orbitals.dat file for the norad program
>that was posted recently?

Sorry for the delay in responding, our area has had a news slow-down.


              Orbital Elements by  Ted Molczan, 4 Feb 1990

There are 3 lines of data for each object.  The first contains the name and
certain physical data.  The second and third are the orbital elements, in the
standard NORAD "2-line" format.

Line 1
------
Line 1 contains the name, dimensions and estimated standard magnitude, in the
following format:

Column
01-15  Name
17-20  Length, m  *
22-25  Width, m  **
27-30  Depth, m  ***
31-35  Standard magnitude (at 1000 km range, and 50 % illuminated)

*   If width and depth are zero, then the object is a sphere, and the length
    is its diameter. Objects with unknown dimensions have been assumed to be
    spherical, and a value of diameter has been "guesstimated".

**  If depth is zero, then the object is a cylinder, and width is its
    diameter.

*** The standard magnitude is an estimate based on the mean cross-sectional
    area derived from the dimensions. To estimate the magnitude at other
    ranges and illuminations, use the following formula:

    mag = stdmag - 15.8 + 2.51 * log10 (range * range / fracil)

    where : stdmag = standard magnitude as defined above

            range = distance from observer to satellite, km

            fracil = fraction of satellite illuminated, [ 0 <= fracil <= 1 ]


Line 2
------

Column
01-01  Line number 1 of NORAD elements
03-07  NORAD number
08-08  Class
10-11  International designation - year of launch
12-14  International designation - number of launch since start of year
15-17  International designation - code for specific piece from this launch
19-20  Epoch - year
21-32  Epoch - day of the year and fraction, UTC
34-43  One half of first derivative of mean motion with respect to time,
       rev/day**2
45-52  One sixth of second derivative of mean motion with respect to time,
       rev/day**3 (leading decimal point assumed, not shown)
54-61  BSTAR drag term used by SGP4 (leading decimal point assumed, not shown)
63-63  Ephemeris type
65-68  Bulletin number
69-69  Modulo 10 checksum. Letters, blanks, periods = 0, minus sign = 1,
       plus sign = 2.


Line 3
------

Column
01-01  Line number 2 of NORAD elements
03-07  NORAD number
09-16  Inclination, degrees
18-25  Right ascension of the ascending node, degrees
27-33  Eccentricity  (leading decimal point assumed, not shown)
35-42  Argument of perigee, degrees
44-51  Mean anomaly, degrees
53-63  Mean motion, rev/day
64-68  Revolution number
69-69  Modulo 10 checksum. Same rules as for line2.


Header Lines
------------

The elements are preceeded by a header line, and followed by a trailer line,
as shown below.  These are included to provide start/end points to enable
programs to distinguish between the elements and any comments which may be
included.


startn2l

ALOUETTE 1       0.9  1.1  0.0  8.2
1 00424U 62B-A  1 90 25.21309753  .00000220  00000-0  25410-3 0  2561
2 00424  80.4628  67.0294 0022286 281.5113  78.3546 13.67284761363155

endn2l


Comments on Accuracy
--------------------

Most of the elements in the N2L-XXX.ZIP files originate with NORAD, and are
distributed by NASA.  I receive them from several sources, in hard-copy and
electronic form.  These sources are usually accurate, however, they do not all
include all of the original NORAD data.  For example, some sources omit the
second time derivative of the mean motion, BSTAR, bulletin number and
revolution number.  Except for BSTAR, these are not serious ommisions.  To
ensure compatibilty with SGP4 users, missing BSTAR data has been estimated
based on the mean motion, XN0 and the first time derivative of the mean
motion, XNDT20, using the following formula :


       BSTAR = XNDT20 / XN0 / C2 / 1.5

       where :  XN0 and XNDT20 have been converted to units of radians and
                minutes, as in SGP and SGP4

                C2 = value computed during SGP4 initialization


Values of BSTAR obtained in this way are usually within about 3% of the
original NORAD value.

The orbital decay terms produced by NORAD (and others) are often very
inaccurate.  In most cases, it is possible to make more accurate predictions
by using average values. Most of the elements in the N2L-XXX.ZIP files have
mean values for the first time derivative of the mean motion and BSTAR.  The
exceptions are satellites which make large manouevers or are greatly affected
by solar radiation pressure, which can more than offset drag.

The average drag terms are obtained as follows. Using two elsets separated in
epoch by about 7-10 days, the average value of the first time derivative of
the mean motion is computed:

                              mean motion 2 - mean motion 1
          average XNDT20 =    -----------------------------
                                  2 X (epoch2 - epoch1)


Then BSTAR is computed using the formula presented earlier.


For further information on the N2L-XXX.ZIP files, or on any other satellite
observation related topic, leave a message for me on the Canadian Space
Society's BBS, at 416-458-5907, 24 h/d, <=2400 B, 8N1.

                                                        - Ted Molczan

-- 
- John Logajan @ Network Systems; 7600 Boone Ave; Brooklyn Park, MN 55428
- logajan@ns.network.com, john@logajan.mn.org, 612-424-4888, Fax 424-2853