RHESSI Decimation/Event Download Status Record

Normally, every event in the RHESSI germanium detectors is recorded and sent to the ground. Events include energy deposits (mostly photons) up to the limits for spectroscopy (around 2.7 MeV in the front segments and 17 MeV in the rear segments), Upper Level Discriminator (ULD) events for which we only know that the energy is higher than these limits (most of these are cosmic-ray interactions), and "resets", which are events in which the charge-sensitive preamplifier is actively drained of accumulated charge. Charge accumulates due to photon and particle events and also due to leakage current across the detector.

In order to control the data rate into RHESSI's onboard solid-state recorder (SSR), events can be eliminated in several ways. Reset events can be disabled one detector segment or more at a time. Events can be commanded off entirely one segment or more at a time. Finally, events can be "decimated" one segment at a time. When decimation is active in a segment, it is controlled by two 4-bit quantities: the first sets an energy threshold below which counts will be removed, and the second establishes what fraction of the events will be retained. For example, if the decimation control value of a rear segment has the hexadecimal code a3, it means that there is an energy threshold of roughly a*50 = 500 keV below 3/4 events are deleted. All events above the threshold are downlinked.

Decimation usually occurs automatically in the front segments whenever the SSR is more than 50% full. The energy level and depth of decimation is a function of the exact SSR state and the attenuator state. The details are in this document. The attenuator state at any moment is available as a flag in the observing summary data available from the Solarsoft command line or the RHESSI GUI. Changes in attenuator strategy (e.g which attenuator states was the system allowed to choose from on a given day) are tabulated on the attenuator state page.

Decimation in the rear segments is commanded for particular fixed times rather than being dependent on spacecraft conditions. These commands to date are summarized below, although the instantaneous decimation state during an observation of interest can also be returned by the data analysis software.

Most users of RHESSI data will have no use for reset or ULD events. But for the front segments, the resets are useful for helping us identify and compensate for frequent data dropouts lasting a fraction of a second, which we believe are probably due to the interaction of heavy cosmic-ray nuclei in the detectors.

Each line in the table represents actions taken during a single satellite contact with the Berkeley Ground Station. Times are approximate times for the pass, so the moment of motion can be anywhere within about 10 minutes of the time shown. More precise times are available if needed -- see the contact information below.

The first table tracks changes in front segment decimation, the second in rear segment decimation. Abbreviations are as follows:

O = off (all events in that segment are off)

A = all (all events in that segment are on)

?? = a two digit hexadecimal number (e.g., 93) indicating a specific depth and energy threshold for the decimation (see description above)

r = resets from the segment are shut off

u = upper level discriminator (ULD; mostly cosmic ray) events from the segment are shut off

R = resets are allowed

U = ULD events are allowed

N = the state described is active during spacecraft night

H = The spacecraft is in a region of high magnetic latitude

[N+H]: = the state described is active during simultaneous conditions of spacecraft night and high magnetic latitude. There is an ascii table of when these conditions applied during the beginning of these operations.

D: = the state described is active during spacecraft day, or else during the condition not otherwise described (e.g. if a decimation condition is specified for H, then D means "not H").

B?,? = For day/night switching, indicates the approximate number of minutes after sunset and before sunrise, respectively, during which the "day" mode is allowed to overlap into the nighttime period

G?: = the detector under the grid indicated by ? (1-9) is in a different state, as shown

T? = For the front segments, a table number, describing the automated decimation based on the remaining solid state recorder (SSR) capacity and the attenuator state. The tables can be viewed by clicking on the entry ("T1", "T3", etc.) below.

** Due to incomplete commanding routines, at this time we believed we were commanding the rear decimation parameters to 83, but there was in fact no change.

This page is maintained by David Smith, who can be reached at dsmith (at) ssl.berkeley.edu