Re: accuracy of ISS / Venus transit track

From: Thomas Fly (
Date: Thu Jun 03 2004 - 11:34:37 EDT

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    Kevin Fetter wrote:
    > Using this location I got for 8:19:56 UTC
    > 5.3530 S  22.0301 W  and again 0 m above the WGS84 ellipsoid. Can't remember
    what model, it used for atmospheric refraction.
    > The iss is in the following state in relation to this site.
    > Azimuth (deg)    Elevation (deg)     Range (miles)
    >  65.603              9.955           839.205168
    These compare to my resuls:
    Greatest transit of Venus
    11210   8:19:44   8.4   65.9  906  -6.3542  -23.3779   8.3   0.2
    11222   8:19:56  10.0   65.8  839  -5.3198  -22.0455  10.0   0.2
    In this case, the transit spot is over the Atlantic, so sea-level certainly is
    Strangely, the computed values for azimuth, elevation, and range are in quite
    good agreement; Venus & the ISS are low in the ENE.  However, my "shadow spot"
    is 2.68 miles north of yours!
    Atmospheric ray-bending would have the effect of moving the shadow in the
    direction of the ISS; effectively equivalent to increasing the observer's
    elevation above sea-level by 75 meters, in this case (which, given a range of
    839 miles, would not be very significant).
    I'm currently using WGS72 constants in my program; however, the differences
    between it and WGS84 are quite small: 6378.135 km Earth radius in WGS72, and 2
    meters more, in WGS84 (Earth flattening is 1:298.26 versus 1:298.2572 in WGS84).
    I decided to check CalSKY for this time, which currently appears to be using
    this identical TLE:
    ISS             73.0 44.5 27.5  0.5 d  234
    1 25544U 98067A   04159.51921885  .00020000  00000-0  20000-3 0  9074
    2 25544  51.6331  15.0248 0005532 209.7910 150.2937 15.68809969316800
    Its results are in extremely close agreement with mine:
    Tuesday 8 June 2004
    Observer Site
    WGS84: Lon:  -22d02m43.8s  Lat:  -5d18m52.2s  Alt: 3m
    All times in UT
     8h19m56.15s ISS Crosses the disk of Venus. Separation: 0.00d  Position Angle:
    Angular Velocity: 10.4'/s.  Transit duration: 0.09s
    Angular diameter: 13.7"  size: 73.0m x 44.5m x 27.5m
    Satellite at az:  65.8d ENE  h: 10.0d  dist: 1351.2 km  mag=14.4m
    Satellite apparently moves to direction 336.6
    Centerline, Closest Point ->Map: Lon:+22d02m41s W  Lat:  -5d18m55s dist: 0.13 km
    az: 140.9d SE  Path direction:  50.9d NE  ground speed: 14.505 km/s  width: 1.1
    km  max. duration: 0.1 s
    This made me suspicious, so I tried putting your values into CalSKY- it appears
    your values are quite close for a transit of (the center of) the Sun!
    I doubt that the MCC will update their ISS ephemeris page today; I hope they'll
    update it tomorrow.  Until they do, one could use CalSKY to make comparisons
    (except that Arnold has apparently implemented GTOPO30 DEM, so CalSKY would use
    accurate elevation information- though the 3 meter elevation for the Altantic
    Ocean, here, is a little suspect ;-).
    Unfortunately, I don't have time to do any more myself, this morning.
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