Re: [amsat-bb] alon/alat in AO40 telemetry

[Reinhard Richter <r.richter@xxxxxxxx>]

Hello Wayne,

Monday, January 12, 2004, 6:41:00 AM, you wrote:

> Is the alon/alat in the AO40 telemetry manually updated by the command
> stations?

In July I have asked Stacey the same question, and I got a detailed reply
from him, which you find copied below.

73,
 Reinhard
 DJ1KM                            
--------------------------------------------
At 06:24 PM 2003-07-16 +0000, you wrote:

>Re mail Wednesday, July 16, 2003, 4:18:05 PM,
>
It is important to understand how the FEC blocks work. Currently, a FEC
block is just the last 256 bytes of a 512 byte A-block.  The first four 
lines of the A-block are not included in the FEC encoding.  P3T is able to 
re-create the first line from data in the 256 byte block.  However, the 
data in line 3 ( ALON, ALAT, HEIGHT, etc. ) is NOT included in the 256 
bytes of FEC information, so it cannot be recreated.  Thus, the 
"RECONSTRUCTED FROM...." is placed in this position.  When P3T transmits 
standard blocks, this information is present in line 3, but not in the FEC 
blocks.

All of this is subject to change, but at the moment, there is no easy way 
to get this information into the FEC block.
>
>thanks for info. I understand. Another question. I thought that values of
>ALON and ALAT which are displayed in the A-Block are written into the 
>telemetry
>manually by the command stations after having moved the attitude during 
>perigee.

Not quite correct.  These values are calculated by the IHU-1 and displayed 
in the telemetry, just as you see them.  The lon, lat, height are 
calculated by the IHU-1 based on the keplerian elements in the 
spacecraft.  These are quite accurate, though the display has a small 
integer "round-off" error.  The ALON/ALAT are calculated by the IHU-1 based 
on where the ground stations last told it that it was (a 3-dimensional 
vector, called SV), where it wants to go ( a 3-D vector called SSV), and 
its calculation of how much magnetorquing has moved SV towards SSV.  The 
mystery effect is also calculated by the IHU-1 and factored into the 
ALON/ALAT calculation. When the ground stations upload a new magnetorquing 
command, SV is automatically reset. Therefore, these values are accurate 
immediately after an new torque command, and they stay close for several 
orbits, but over time progressive errors build up and the display becomes 
less accurate.  There is no sensor feedback for this calculation.  It's 
based on deterministic equations.

>In orbit 1243 at the beginning there was ALON 5 and shortly before the end
>at MA248 there was ALON 2. How comes?

As above, a new torque command was sent during this time that updated the 
SV, based on ground calculation.  As soon as the SV is updated, the IHU-1 
recalculated ALON/ALAT and displayed the new values on the next MA change.

> From YACER I learned that
>determination of ALON and ALAT has to be done manually by the command
>stations and values are not written automatically into the telemetry.

The true determination of ALON/ALAT is done on the ground, but the SV 
vector is the IHU-1's "best guess" of ALON/ALAT.  The YACER docs probably 
need a slight update because of the addition of these data to line 3 of the 
A-blocks

----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org