To: rac, plucinsk, das, mwb@space.mit.edu, ybutt Subject: Possble New SCP4 Threshold Date: Thu, 04 Nov 1999 22:45:21 -0500 From: Shanil Virani Hello Gentlemen, Given my email from earlier tonight, I have gone through the P4 flux history for the length of the mission thus far with the goal of trying to reduce the currently specified threshold of 10^3 counts/s/cm**2/sr. If I take the radbelt duration to be that length of time specified by the OFLS system plus an additional 16ks of pad time (8ks on either side), then we can set the SCP4 threshold to **10^2** counts/cm**2/sr/s. However, I should point out that even with the above definition of rad belt duration, there would have been _one_ instance when we would have safed the spaceraft -- it would have occurred on Day 271 at ~0215Z on the exit portion of that rad belt transit. If my analysis from earlier tonight is correct though, then we should have safed the SIs anyway so this excursion is fine. Now, the reason why no one reported a CTI increase at that time is because ACIS was still in the NIL position -- we had not performed a SIM translation yet. Just as a reminder, you can find the ephin flux history plots that I've been using to establish thresholds at http://asc.harvard.edu/acis/radbelt where the file is called radhist-log.ps. At any rate, this new threshold would have minimized the recent ACIS CTI degradation had they been in place. This new threshold value also allows for a relatively large margin of error (ie we would not needlessly safed the spacecraft) since the P4 background far from the belts (and with the 16 ks pad time) is of order 10 counts/s/cm**2/sr. I suggest we lower the P4 threshold to ***10^2 counts/s/cm**2/sr***. Cheers, Shanil ------------------------------------------------------------------------ Shanil N. Virani svirani@head-cfa.harvard.edu Chandra X-ray Observatory Center Harvard-Smithsonian Center for Astrophysics http://hea-www.harvard.edu/~svirani 60 Garden Street, MS-70 FAX: 617-495-7356 Cambridge, MA 02138 USA PHONE: 617-496-7855 "It is not in the stars to hold our destiny but in ourselves." -- William Shakespeare ------------------------------------------------------------------------ To: Rob Cameron Subject: Re: threshold tradespace cc: plucinsk, odell@cosmos.msfc.nasa.gov, das, ybutt Date: Tue, 09 Nov 1999 15:16:01 -0500 From: Shanil Virani Hi Rob, > Because we have concern that a P4 threshold of 100 p/(cm2-s-sr) > may still be too high to prevent damage from the radbelt wings, > I have asked Shanil to examine the tradeoff between pad time and > threshold, with the assumption of 0 autonomous safings allowed. > Specifically, what pad do we need for P4 = 10? I had already previously flirted with the idea setting the threshold to 10 p/(cm2-s-sr) but did not suggest it because we would have had only one magnitude as a margin of error. That is, the background P4 rate outside of the magenetopause/belts is approximately 0.1 p/(cm2-s-sr). Nevertheless, if we wanted to use 10 p/(cm2-s-sr) as the P4 threshold, then we would need a pad time of about 12-13 ks. To date, there are two instances where we would have safed Chandra if we only had a pad time of 8 ks and used 10 p/(cm2-s-sr) as a threshold. Both these instances occurred on the exit leg. We really would have only needed an 11 ks pad time to have prevented safing actions from these two events. Furthermore, let me remind you that we currently do two 8ks CTI measurements after the belts so this proposed 13ks pad would be actually lower than what we are currently using. Secondly, as I just stated, both these events occurred on the exit leg so conceivably we could just an 8 ks pad on entry to the perigee transit and a 13 ks pad on exit. However, I think where these flares occur is a function of season so we may not want to do that. At any rate, the bottom line is that for a threshold of 10 p/(cm2-s-sr), you would need a pad of at least 11 ks -- my recommendation being at least 12/13 ks. Cheers, Shanil ------------------------------------------------------------------------- Shanil N. Virani svirani@head-cfa.harvard.edu Chandra X-ray Observatory Center Harvard-Smithsonian Center for Astrophysics http://hea-www.harvard.edu/~svirani 60 Garden Street, MS-70 FAX: 617-495-7356 Cambridge, MA 02138 USA PHONE: 617-496-7855 "The most pathetic person in the world is someone who has sight but has no vision." -- Helen Keller -------------------------------------------------------------------------