Oscar 10 ALON/ALAT

Satellite operators frequently ask "what is the attitude of Oscar-10", or "can I have ALON/ALAT values for my computer program". Well, it looks like eleven year old AO-10 is coming in good again. Here's the analysis.

When the computer failed on Oscar-10 in 1986 we knew approximately what its orientation was. Since then of course there has been no telemetry, and the only "measurements" we can take are subjective interpretations of transponder performance and beacon wobblies.

At the time it finally failed, the spin axis/antenna direction lay exactly on the Sun's orbital arc (the Ecliptic). So because of the Sun's annual motion the solar panels were in turn fully lit then completely dark, twice a year. The Sun's angle to the panels just cycled up and down a sawtooth from -90 to 0 to +90 to 0 to -90. As you would expect, at zero illumination conditions the battery flattened, the beacon disappeared and there was no transponder. Exactly what we see to this day.

Also at the time it failed it was spinning at some 30 rpm. Due to tiny frictional effects this spin was gradually falling at a rate of 0.0035 rpm/orbit, so the current (1994) spin rate is very low.

Now a spinning satellite is just a gyroscope, and in the absence of any torques, should stay pointing in the same direction for all time. So one can extrapolate from the last known (1988) direction to the present day and beyond. The key phrase here is "absence of any torques"; if the spin rate is falling there must be some coaxial torque at least, and there is no reason not to expect some non-coaxial torques which would change the orientation a little.

So with the forgoing caveats, the table below shows the predicted attitude of Oscar-10 based on the last known 1988 data. The pattern is likely to be more or less correct, but by now there is probably a few week's slippage.

The only way to find out is to use the satellite and correlate experience with expectation. If users will take the trouble to let me have their observations we can together almost certainly refine this table. But I am not going to do it on my own.

It looks to me as though we are in for some near "normal" times on AO-10 for a couple of years. Bear it in mind during those irksome periods when Oscar-13's attitude is so unsociable.

DATE                  ALON   ALAT     SA  ILL %    SEL    SAZ  Arg P   RAAN
-------------------------------------------------------------------------------
1995 Jan 23 [Mon]      201    -19     16     96    -25    121    244    287  
1995 Feb 20 [Mon]      199    -19     44     71    -30    150    252    283  
1995 Mar 20 [Mon]      196    -19     72     29    -27    179    259    278  
1995 Apr 17 [Mon]      193    -18     79     18    -15    203    266    274  
1995 May 15 [Mon]      190    -18     52     60      0    223    274    270  
1995 Jun 12 [Mon]      188    -17     25     90     17    242    281    265  
1995 Jul 10 [Mon]      185    -17     -1    100     32    265    288    261  
1995 Aug  7 [Mon]      182    -16    -27     88     39    295    296    256  
1995 Sep  4 [Mon]      179    -15    -54     57     34    326    303    252  
1995 Oct  2 [Mon]      176    -14    -82     13     20    351    310    248  
1995 Oct 30 [Mon]      173    -12    -70     34      0      9    318    243  
1995 Nov 27 [Mon]      170    -11    -41     74    -20     27    325    239  
1995 Dec 25 [Mon]      167     -9    -13     97    -37     51    332    234  

DATE                  ALON   ALAT     SA  ILL %    SEL    SAZ  Arg P   RAAN
-------------------------------------------------------------------------------
1996 Jan 22 [Mon]      163     -8     15     96    -45     87    340    230  
1996 Feb 19 [Mon]      160     -6     43     72    -37    122    347    225  
1996 Mar 18 [Mon]      157     -4     71     31    -19    145    354    221  
1996 Apr 15 [Mon]      154     -3     80     16      2    161      2    217  
1996 May 13 [Mon]      150     -1     53     59     24    177      9    212  
1996 Jun 10 [Mon]      147      0     26     89     42    200     16    208  
1996 Jul  8 [Mon]      143      2      0    100     49    236     24    203  
1996 Aug  5 [Mon]      140      4    -26     89     40    271     31    199  
1996 Sep  2 [Mon]      136      6    -53     59     21    293     38    195  
1996 Sep 30 [Mon]      133      8    -80     16     -0    308     46    190  
1996 Oct 28 [Mon]      130     10    -71     32    -23    324     53    186  
1996 Nov 25 [Mon]      126     12    -43     72    -42    348     60    181  
1996 Dec 23 [Mon]      123     14    -14     96    -50     27     68    177  
-------------------------------------------------------------------------------

Key
---
 ALON deg  Attitude longitude in orbit plane coordinates (BLON)
 ALAT deg  Attitude  latitude in orbit plane coordinates (BLAT)
 SA   deg  Sun Angle
 ILL%      Percentage illumination = 100*COS(SA)
 SEL  deg  Sun's elevation above orbit plane
 SAZ  deg  Sun's azimuth in orbit plane coordinates  

 Orbit plane coordinates: longitude is measured clockwise from perigee; 
 latitude is measured up from the orbit plane  

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