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vec2tle.new
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VEC2TLE Version 9421
--------------------
This file lists the updates made to Vector to Two-Line Elements (VEC2TLE)
for Version 9421. The previous release of this software was VEC2TLE
version 9331. This is a major update, with the new features as follows:
1. Addition of Mission Elapsed Time (MET) Reference
In addition to the standard Coordinated Universal Time (UTC)
reference, vectors and maneuver burn times may be optionally referenced
to Mission Elapsed Time (MET). Because MET is the time elapsed since
launch, the program requires a launch time. The "standard" used by
Dave Ransom's STSORBIT PLUS was adopted (i.e., STSPLUS.LTD) to avoid
duplication of this information. VEC2TLE has the ability not only to
use the information in STSPLUS.LTD, but it can also add and change
records in this file.
The ability to reference to MET is most useful for sets of pre-
launch nominal vectors, which are often available. These vectors are
are in the Earth-Fixed Greenwich coodrinate system and are generally
referenced to MET. With this information, nominal Keplerian TLEs can
be computed by providing VEC2TLE with the scheduled launch time. If
the launch time changes, VEC2TLE requires ONLY the new launch date and
time to recompute all the Keplerian TLEs from the MET-referenced EFG
vectors.
2. Improved Drag Parameter Estimation
VEC2TLE has an improved drag estimation algorithm implemented.
The default for the model currently presumes a "typical" spacecraft
ballistic coefficient and a "typical" atmospheric density profile.
Since both of the previous factors can vary widely from case-to-case,
VEC2TLE pays considerable attention to manually-entered drag (ndot/2
and/or Bstar) parameters. When drag parameters are manually entered,
VEC2TLE updates an internal multiplier for the specific satellite,
considering both the current multiplier value and the value the
multiplier should have been for the algorithm to produce the manually-
entered drag. The intent is that the internal drag algorithm be
adaptive to empirically-measured and/or theoretically-determined drag
drag values. The drag multiplier may also be manually changed by the
user.
3. Automatic Revolution Number Estimation
VEC2TLE has the capability to estimate the revolution number for
a state vector/Keplerian TLE. The estimation algorithm uses the
launch time and date information made available for MET reference
computations and the current orbital period of the spacecraft. It
should be stressed that this algorithm usually produces a reasonably
reliable estimate, but is not always accurate. The estimate is most
accurate within about a week of launch when the orbital altitude has
not changed significantly.
4. Date Computation
VEC2TLE may now convert standard date formats (MM-DD-YY) to and
from year/day-of-year (YY/DDD) date formats. (The year/day-of-year
format is standard for space operations use.) These features were
added as utilities to support other functions as well as convert
information from existing displays.