Standard magnitudes for KH's

Rainer Kracht (0412188960-0001@t-online.de)
Sat, 9 Dec 95 17:09 +0100

Standard magnitudes for KH's
----------------------------

Ted Molczan has asked me:

>Given the moderately high eccentricity of the sun-synchronous KHs, did you
>find that the circular orbit approximation produced acceptable range 
>estimates, or did you have to use elements?

I have answered:

>I have used circular orbits derived from Russell's observations, I think
>that they give acceptable range estimates (from my experience with
>orbit determinations). At apogee the range will be overestimated and at
>perigee underestimated. Usually there will be more observations near
>apogee (if the satellite is still visible). So their might be a small
>trend to overestimate ranges (and brightnesses) with my method, if
>the orbit is not circular.


I have now analysed Russell's observations in more detail using an
average altitude ((perigee+apogee)/2) of 630 km for the KH's (84-122A,
87-90A, 88-99A):

All KH's

 altitude > 630 km           465   3.93 +/- 0.06   0.0266 +/- 0.0019
 altitude < 630 km           229   4.22 +/- 0.07   0.0288 +/- 0.0023
 -------------------------------------------------------------------
 all observations            694   4.05 +/- 0.05   0.0283 +/- 0.0015
 average of >630 and <630          4.08

Because there are only few observations of 87-90A and most observations
of 84-122A were made after the eccentricity of its orbit was decreased
in November 1990, I have analysed 88-99A in more detail:

KH 11-8 (1988-99A)

 altitude > 630 km           265   3.83 +/- 0.09   0.0252 +/- 0.0027
 altitude < 630 km            87   4.29 +/- 0.12   0.0273 +/- 0.0038
 -------------------------------------------------------------------
 all observations            352   3.98 +/- 0.07   0.0273 +/- 0.0022
 average of >630 and <630          4.06

The effect of over- and underestimating ranges and brightnesses is
clearly visible. Observations near apogee give a lower limit, observations
near perigee give an upper limit for standard magnitude. The average of
lower and upper limits is close to the standard magnitude derived from
all observations.

Rainer Kracht
0412188960-0001@t-online.de