DRAG Function Format
Note: The date records in the drag function file are not Year
2000 compliant, since they use a 2-digit notation for the year.
Two FORTRAN77 routines (csnx.f, frdrag.f) for the evaluation of
drag functions are available, Two FORTRAN77 routines.
1. Form:
-----
DSIDP GFZ1.ORB.PRD DRAG FUNCTION A 990506
IRV SET 990506 EPHEM NO 545 SATELLITE 8001 MAXEPOCH 6
EPOCH -240.5 DRAG FRCO 0.0 857.0 38.2 NMAX 20
EPOCH -239.5 DRAG FRCO 0.0 837.6 37.2 NMAX 20
EPOCH -238.5 DRAG FRCO 0.0 855.2 36.9 NMAX 20
EPOCH -237.5 DRAG FRCO 0.0 868.2 37.8 NMAX 20
EPOCH -236.5 DRAG FRCO 0.0 830.6 36.0 NMAX 20
EPOCH -235.5 DRAG FRCO 0.0 825.2 36.1 NMAX 20
2. Explanations:
-------------
1. Record: DSIDP is the Data Set ID Parameter. The "A" is
a quality flag (A=good, B=not so good, C=poor, X=unknown).
"990506" is date of determination and dissemination of
the drag function.
2. Record: IRV set "990506" was disseminated at that date;
can be found in the DSIDP record of the IRV set. The ephemeris
number can be found in line 3 of each individual IRV. "8001"
is the satellit ID in the IRVs. MAXEPOCH is the number of EPOCH
records following. Usually MAXEPOCH will be 1. In times of high
solar and geomagnetic activity the number of EPOCH records is
increased, i.e. several drag functions over the prediction period
are generated.
3(+MAXEPOCH-1). Record: The EPOCH of the drag function is given in
JD2000 days and in the UTC time scale. The coefficients following
DRAG FRCO are the coefficients a, b and c of the drag function as
explained below. NMAX is the degree of the Fourier expansion
recommended. In case of several drag functions one has to pick the
valid set of coefficients for the specific epoch.
3. Usage:
------
The drag time bias for a given epoch is given by
drag TB = a + b*[-cosx + cos(2x)/4 - cos(3x)/9 ...
+- cos(NMAX*x)/(NMAX*NMAX)]
+ c*[-sinx + sin(2x)/2 - sin(3x)/3 ...
+- sin(NMAX*x)/NMAX]
where
x = 2*pi*(t-0.5)
with drag TB in ms and t in d since epoch.
4. Example:
--------
In order to compute the drag function for May, 8th, 1999, one uses
EPOCH -238.5 DRAG FRCO 0.0 855.2 36.9 NMAX 20,
i.e. a=0.0, b=855.2, c=36.9 and NMAX=20.
0:00 UTC = -238.50 JD2000 = 51306 MJD drag TB = 1365 ms
6:00 UTC = -238.25 drag TB = -147 ms
12:00 UTC = -238.00 drag TB = -702 ms
18:00 UTC = -237.75 drag TB = -203 ms
24:00 UTC = -237.50 drag TB = 1365 ms
In order to compute the drag function for May, 9th, 1999, one has to
use the next record
EPOCH -237.5 DRAG FRCO 0.0 868.2 37.8 NMAX 20,
i.e. apply a=0.0, b=868.2, c=37.8 and NMAX=20 to above formula.
