----+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
XEPHEM 2 500 920324000000. 920325000000.
----+----0----+----0----+----0----+----0----+----0----+----0----+----0----+----0
COLUMNS FORMAT DESCRIPTION DEFAULT VALUE & UNITS
1-6 A6 XEPHEM - Indicates that an external
ephemeris will be provided for use in
the measurement model routines for
determining the orbits. Note [1]
7-8 I2 Number of satellite ephemerides provided 1
on external ephemeris. The ID's for the
satellites are included in the header
record of the ephemeris file.
11-14 I4 Size of internal buffers used to hold the 300
satellite ephemeris information. Note [2]
15-16 I2 Unit number for the file that contains the 66
external ephemeris. This unit is read only
in IIE. IIS does not verify that the file
exists, nor does it verify that the file
actually contains the number of satellites
or time periods specified. IIE will terminate
if the information is incorrect.
25-44 D20.8 Start time for the external ephemeris in the
form: YYMMDDHHMMSS. Start time must be at
least three time steps prior to the first observation
point for the 10 point hermitian interpolation. NOTE [3]
45-59 D15.3 End time for the external ephemeris in the
form: YYMMDDHHMMSS. End time must be at least
five time steps after the last observation for
the 10 point hermitian interpolation. NOTE [3]
NOV 1994
9411.0
GLOBAL SET
XEPHEM (cont)
Note [1]: The external ephemeris contains cartesian elements for the satellites
in a true of date reference frame. A 10th order hermitian interpolator
is used to interpolate the satellite elements to the data times.
Satellite epoch parameters must still be provided in the Arc set on
the SATPAR, EPOCH, ELEMS1 and ELEMS2 card for each satellite in the
run. The measurement model uses the external ephemeris, but the
program still needs to perform integration for the variational
partials.
Note [2]: IIE uses a double buffering scheme where each buffer is
(6 * No. of satellites * Buffer size) 64 bit words. In general, the
larger the buffer the more efficient the run.
Note [3]: To determine the start time for the external ephemeris note
that the integrator evaluates the first point 5 integration steps
before the listed epoch time. The external ephemeris start time
must be at least 3 time steps prior to this point. So, if the
integration step size is Tstep, and the interval on the external
ephemeris is Textint, then the external ephemeris start time must
be less than Tstart - 5*Tstep - 3*Textint.
To determine the stop time for the external ephemeris note
that the integrator may go 2 integration steps beyond the epoch
stop time. The external ephemeris stop time must be at least 5
time steps after this point. So, if the integration step size is
Tstep, and the interval on the external ephemeris is Textint,
then the external ephemeris stop time must be greater than
Tstop + 2*Tstep + 5*Textint.
IF CARD OMITTED: Satellite ephemeris is obtained from integration of orbit.
NOV 1994
9411.0
GLOBAL SET
XEPHEM (cont)
EXTERNAL EPHEMERIS
The external ephemeris file is a binary file consisting of a header
record and any number of data records. The total number of data records
is optional, but it must be sufficient to at least cover the data times
in the GEODYN run (+3 steps before the first data point and 5 steps
after the last data point for the 10 point hermitian interpolation).
Following is a description of the header and data records.
HEADER RECORD. (Record Length, RECL = 6*nsat+1)
--------------
Word(s)
1 - Number of satellite ephemerides in this file (nsat)
2 - Start time of ephemeris data in UTC yymmddhhmmss.
3 - Time interval between ephemeris points. (Step size)
4 -> nsat+3 - Satellite ID's of the satellites in this file in the
order that the satellite ephemeris data is provided
in the data records. (i.e. the first satellite ID
must belong to the first set of six satellite elements
in the data record)
nsat+4 -> RECL - Set to zero. Not currently used.
DATA RECORDS. (Record Length, RECL = 6+nsat+1)
-------------
The time between data records must correspond exactly to the time
interval specified in word 3 of the header record. The satellite
elements provided in each data record must be in the same order as the
satellite ID's specified in the header record. The satellite elements
provided must be true of data cartesian elements.
Word(s)
1 - Time in seconds from start time specified in
the header record. For the first data record
this must be zero seconds.
2 -> 7 - Six satellite elements for satellite number 1
.
.
6*(nsat-1)+2 -> 6*nsat+1 - Six satellite elements for satellite number nsat