Re: 2nd SDP4 failures case

Bruno Tilgner (Bruno_Tilgner@compuserve.com)
Thu, 3 Apr 1997 11:47:41 -0500

Rob Matson wrote on 31 March:

>UTC Date:  1/17/97      position in km
>
>UTC Time         X                   Y                   Z
>--------  ----------------   -----------------   -----------------
>14:40:00  12058.6968436073   -9088.05059434088   -1604.73341693802
>14:40:30  12207.3904182008   -9040.79898042420   -1596.37913955513
>14:41:00  12354.8551383471   -8992.63608004684   -1587.86366489113
>14:41:30  12501.0976325425   -8943.58238643458   -1579.19061502540
>14:42:00  12646.1245522158   -8893.65787621351   -1570.36352079508
>14:42:30  12789.9425652844   -8842.88202551640   -1561.38582463892
>(jump)
>14:43:00  13726.9960697113   -7532.85729295977   -1330.06349285878
>14:43:30  13862.6766127548   -7467.35366752016   -1318.48402930620
>14:44:00  13997.0964328723   -7401.16989068805   -1306.78421250686
>14:44:30  14130.2638901796   -7334.32287724387   -1294.96703273207
>14:45:00  14262.1872942258   -7266.82911673137   -1283.03540513679

>So run your code at the later time.  I'll bet you see the jump...  --Rob

>Note to Bruno Tilgner:  Could you try it also?  Yours should fail too.


I have tried it and came up with the following result:

 Date     TUC         X [km]            Y [km]            Z [km]
----------------------------------------------------------------------  
17Jan97 14:40:00 +12028.8956091222  -9088.1364111265  -1602.0573143201
17Jan97 14:40:30 +12177.8601432317  -9040.9382121643  -1593.8081437370
17Jan97 14:41:00 +12325.5930509350  -8992.8244469893  -1585.3972820529
17Jan97 14:41:30 +12472.1009731473  -8943.8157436790  -1576.8283610678
17Jan97 14:42:00 +12617.3905750208  -8893.9322093518  -1568.1049210008
17Jan97 14:42:30 +12761.4685394282  -8843.1934464125  -1559.2304133420
(no jump)
17Jan97 14:43:00 +12904.3415608982  -8791.6185682438  -1550.2082036091
17Jan97 14:43:30 +13046.0163400919  -8739.2262143205  -1541.0415740007
17Jan97 14:44:00 +13186.4995785737  -8686.0345648412  -1531.7337259669
17Jan97 14:44:30 +13325.7979740327  -8632.0613548325  -1522.2877826855
17Jan97 14:45:00 +13463.9182158746  -8577.3238877610  -1512.7067914538

I don't get the jump between 14:42:30 and 14:43:00. However, as you can see,
my X values are consistently larger by nearly 30 km whereas the Y and Z
values are within the limits one can expect from different compilers.
I use 6378.135 km for the Earth's equatorial radius, but I don't think this
explains the discrepancy. In order to eliminate this remote source of error
it would be preferable for all future comparisons to use Earth radii as
the unit of measure for X, Y and Z.

In this case one could also compare the velocities (in Earth radii/minute).

There are other constants in the code which could potentially be responsible
for different results. For instance in my code the Earth's flattening and the
J constants are those of WGS72. I have not yet tried what will happen if I
replace them with more recent values.

Bruno Tilgner
Bruno_Tilgner@compuserve.com