Iridium, Iridium, Iridium

Walter Nissen (dk058@cleveland.Freenet.Edu)
Wed, 25 Nov 1998 11:24:54 -0500 (EST)

The Iridial constellation has grown and evolved in a number of ways.  An 
updated simplified table is presented below. 
It shows the 66 operational objects organized into 6 planes of 11 objects 
each.  The planes are separated by a nominal 30 degrees.  More precisely, 
5 of the 6 spacings are 31.6 degrees, these are the 5 between the planes 
proceeding "in the same direction"; and the 6th is 22 degrees, this is the 
spacing in the transitional region as the apparent motion flips from 
ascending to descending.  Any comments on whether this spacing enhances 
the robustness of the service? 
The Iridia tend to cluster at the poles, in a dynamic sense.  The spacings 
in the individual planes are highly uniform, as previously noted, with the 
consequence that successive Iridia pass by at highly regular 9-minute 
intervals.  But at any given time, at most one Iridium will be high in the 
sky from any point on Earth's equator; while at all times, numerous Iridia 
are high in the sky from any point near the poles. 
See my previous posts to SeeSat-L for some related details. 
A number of observers noted that bright events occurred "at the same time" 
from 2 different Iridia, and that one of the objects involved was 
Ir P3 B 51 = 25262 = 98-18A.  The simplified table places Ir P3 B in a 
distinguished position, a position intended to suggest the possibility of 
such double events.  Such standby objects orbit with substantially 
different periods, which causes them to appear to drift or slew with 
respect to the rigid structure of the undecagon (11-sided polygon) formed 
by the 11 operating objects in the plane.  That such double events 
actually occur suggests that the standby object Ir P3 B is maintaining 
attitude control as if it were operational.  If the software is 
sufficiently sophisticated, perhaps it is operational.  Any comments on 
that?  This may be a question that can be answered from monitoring by 
HearSat-L subscribers.  Meanwhile, praise is due the observers who 
reported the Ir P3 B events. 
What about the other standby craft?  Are they capable of generating double 
bright events?  Observation reports are very much welcome.  Even negative 
reports are of very high interest, because if numerous attempts have been 
made by careful observers without success for some of the standbys, this 
would imply that different standbys are in different attitudes, or 
possibly that some of the craft thought to be on standby have actually 
You can find such standbys in the simplified table by simply avoiding the 
objects in the rigid rectangular part of the table which contains the 66 
operating craft, and avoiding the S and T and U objects which are the ones 
which appear to have failed.  These failed objects are the ones which 
produce pretty spectacular flashing as they tumble. 
As you can tell by now, when I said above that the Iridia pass by at 
9-minute intervals, I was referring only to the 66 operational craft. 
The simplified table identifies the pairs of objects whose vulgar names 
have been confused.  Because of these confusions, it is vitally important 
to provide something beyond the vulgar name to allow others to identify 
the object.  Another vulgar name problem is that some people use Iridium, 
some Ir, some Irid and some my own suggested Irdm.  I don't know if I've 
seen it with Iridia, but sometimes hyphens intervene as in STS 95 and 
STS-95.  Also spacing within vulgar names is a problem.  Sometimes you see 
Iridium 9, sometimes Iridium  9, leaving room for the additional digit 
needed in most names.  Also capitalization.  Sometimes you see IRIDIUM, 
sometimes Iridium.  The catalog number is not subject to these problems 
(at least not if people avoid 25,544 to replace the conventional 25544). 
A distinguished, experienced observer, who shall remain nameless here to 
protect the guilty, posted to SeeSat-L a uniquely valuable observation of 
a tumbling Iridium without giving the catalog number or the COSPAR id.  As 
a result, at least 2 of us had a devil of a time finding that report. 
Jeff Hunt deserves much thanks for helping out. 
As a result of such problems, and a virtually total lack of progress in 
dealing with them, my WARNING on vulgar names must necessarily soon 
reappear on SeeSat-L. 
When Iridial glints are seen, even in daylight, at high altitude, the 
motion of the craft may be obvious, especially to the acute observers of 
SeeSat-L.  However, at lower altitudes, the object may well appear 
stationary, especially during the day or thru light cloud or haze. 
The P-numbers give the order of the objects in the undecagons.  Just after 
Ir P00 goes by, Ir P01 will go by on a virtually identical path slightly 
to the West.  If P01 passes low in the East, 9 minutes later P02 will pass 
along the same path, but slightly higher (West).  Eventually, P00 will 
follow P0A, which followed P09.  After the P1 plane goes over, the next 
plane will be P2, then P3.  The P-numbers are not ambiguous, and I have 
hope it will be possible to maintain this far into the future. 
Note the horizontal spacing in the simplified table very closely.  The 
three objects which are drifting out of their natal planes are shifted 
slightly to the left showing their change in RAAN. 
Any help to eliminate question marks from the table is much appreciated. 
Walter Nissen          
-81.8637, 41.3735, 256m elevation 
medium, media 
bacterium, bacteria 
colloquium, colloquia 
equilibrium, equilibria 
labium, labia 
millennium, millennia 
symposium, symposia 
quantum, quanta 
Iridium, Iridia 
And thank goodness for it, because otherwise we'd be stuck with an 
abomination, Iridiums. 
Here is the updated simplified table: 
    P00  64     P10  24*    P20  32     P30   5     P40   9*    P50  39 
    P01  65     P11  47     P21  33     P31   6     P41  10*    P51  80 
    P02  66     P12  48     P22  57     P32   7     P42  52     P52  17 
    P03  67     P13  49     P23  58     P33   8     P43  12     P53  15 
    P04  68     P14  26     P24  59     P34   4     P44  13     P54  81 
    P05  74     P15R  3     P25  60     P35  35     P45R 83     P55  38 
    P06  72     P16  22     P26  55     P36  61     P46  16     P56  41 
    P07  73     P17  23     P27  28     P37  19     P47  53     P57  43 
    P08  70     P18  76     P28  29     P38  34     P48  56     P58  18* 
    P09  62     P19  25     P29  30     P39  37     P49  50     P59  42 
    P0A  63     P1A  45     P2A  31     P3A  36     P4A  54     P5A  40 
    P0 B 75                             P3 B 51                 P5B? 77 
                                                                P5C? 82 
                P1 S 46*   P2 S 27                  P4 S 11     P5 S 20* 
                P1 T 71                             P4 T 14     P5 T 44 
                                                              P5 U 21 
        P1B? or P1U? 69                             P4 ?  2    P? B 79 
                                                    P4 ? 86 
                                                    P4 ? 85 
                                                    P4 ? 84 
*  šnumbers have been confused, resolution uncertain