Re: Unknown values

Bjorn Gimle (
11 Aug 1995 19:30:54 GMT

Jeffrey Hunt wrote Re: my mail:

>Bjorn Gimle recently posted information on two decaying
>rocket debris objects:
>22325 Express 1 SL-12 r/b     0.76 m2
>23015 Raduga 31 SL-12 r/b     0.78 m2
>I assume the fractional number is the effective surface area
>in square meters.
>The OIG Satellite Situation Report Volume 35, Number 6,
>dated June 30, 1995 found on the
><> site lists an "RCS" for:
>22325 as 0.4566, and
>23015 as 0.8780
>Question #1.  Would Bjorn or others know what the
>correlation is between the two values, if any?  Since there
>is no linear releationship, maybe they are not related.  I
>could not find "RCS" defined in the report.

My values are the "identical" values from the July 31 report, 
just available on OIG. Since objects are tumbling, individual
measurements will vary, and although OIG should publish 
averages, and smooth them as months go by, even they will vary.
(Recently I checked the supposedly identical Russian EORSATs -
they varied from 14 to 137 sq.m. if I remember correctly)

>Question #2.  An additional question that has been bugging
>me for some time is the four elements found on line 0 of
>some elsets (Ted Molczan's and some others).  What are those
>four elements describing?  Again, I have never come across a
>description for what those values are for.
>Thanks in advance,   Jeff Hunt <>

Here is Ted Molczan's description, from his file N2LDESC.TXT
referenced in his headres :

                    Description of N2L-XXX.ZIP DATA

                        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:

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

*** 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

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,
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
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

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


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


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
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

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