Secondary data is obtained from the four minutes of dark count observations which are stored in each orbit which is used for Princeton UV observations. This dark count observing period is chosen to correspond to what would normally not be good observing time,* so the recording of background information does not affect the observing efficiency of the satellite. The spacecraft is not slewed off-source for these observations; the fact that the IRU null and Princeton Fine Error Sensor (FES) null do not generally correspond to within one arc sec is assumed to provide adequate offset. The FES is disabled during the dark count interval, so the spacecraft is IRU stabilized and the star being observed during any particular series of Princeton orbits will not generally be on the slit (0.3 arc sec wide), even during those dark count intervals scheduled when the target star is not occulted by the earth. However, depending on the time since the IRU was updated and the orbital conditions (e.g., orientation with respect to the earth's magnetic field or position over the earth's surface), the star may drift across the slit, or have a portion of its image lying over the slit.
The first type of data has been extensively used and is the main topic of this memo. The second type of data will be used to study time variations and is just beginning to be used. In the near future, we hope to produce a plot of the residuals (Ncp - NT)i versus orbit number or time, where Ncp is the dark count observed in a given Princeton orbit, NT is the predicted dark count for the time at which dark counts were taken, and the subscript i refers to one of the six data tubes: U1, U2, U3, V1, V2, and V3.
The data from UCL intervals is reduced with a modified version of the main data reduction program used for Princeton telescope-spectrometer data. The contents of the registers are scaled and averaged into one minute intervals. The time (GMT) of each data point is used to determine which satellite orbit the data refers to and the time passed since the crossing of the ascending node for that orbit. The origin of a particular orbit is defined as the time of the crossing of the equator moving northward. For any given orbit, the GMT of the origin is determined from the "World Maps" published biweekly by GSFC (Lombard). The "SET" time, or spacecraft quarter-minute mark, corresponding to the node crossing is determined from the GMT-SET time relationship used in scheduling by the GSFC staff. This relationship is updated on a biweekly basis and is accurate to ±8 seconds civil time. The first SET time greater than that corresponding to the node crossing is the first of four successive points which are averaged and tagged as "minute one". The next four are labeled "minute two", etc. Ninety-five spacecraft minutes comprise one orbit. (1 S/C minute = 62.8 civil seconds = 4 SET's).
Each orbit is labeled by the longitude of the ascending node (LAN) crossing for the orbit. Due to the rotation of the earth, this parameter precesses 25.3° westward each orbit, so that after 65 days the satellite repeats its course over the surface of the earth (to less than 1° in longitude). Orbits characterized by common node crossings, to within 5°, are referred to as standard orbits. Since the UCL intervals are more or less randomly interleaved between Princeton observing sessions, more continuous dark count data is available for same standard orbits than others.
At this point in the data reduction, it is necessary to check for suitability of the available data as true dark count data. In particular, data for some tubes must be rejected for the same intervals where data on other tubes is acceptable. A series of tests is run for each tube, according to the algorithms listed in table 1.
The data remaining, after the various checks listed in table 1 have been performed, are examined for the possibility that a stellar signal is present: a star near the slit which may drift across the slit as the attitude control drifts during an orbit. The Copernicus guidance system consists of an inertial reference unit (IRU) which provides stabilization during spacecraft manueuvers, and a fine guidance system. The latter is a null-seeking device, which causes the star to be centered on the slit via error voltages used to control the spacecraft reaction wheels. The dark counts are recorded when the spacecraft is under IRU control, the IRU is frequently updated so the spacecraft pointing axis corresponds to the telescope optical axis, but the reference gyros are subject to a drift amounting to 2-6 arc sec per hour in all three axes. By comparing dark counts taken with no star near the slit with data taken with a star near the slit, it was found that the contamination of dark count data due to stellar photons can generally be recognized as a one to two minute increase in the counts on various tubes, followed by a slow decrease to normal dark count levels. Initially, data taken in orbits when the satellite was pointing near bright stars (used as targets for updating the IRU during UCL orbits) were not reduced. Currently, all UCL data is reduced and data containing possible contamination is removed by hand.
Following this final check of the raw data, values of Ni, the 14-second dark count on each tube, averaged over successive one-minute intervals (4 data points) in each orbit, are punched on cards, tagged as to orbit (LAN) and to time, in minutes, since the node crossing (TSN). The cards are compiled on tape, and all data common to 5° intervals in the LAN are averaged together again, minute by minute. The resulting set of backgrounds refer to the counts that would be recorded if the satellite were traveling in an orbit with values of LAN of 2.5°, 7.5°, 12.5°, etc. These hypothetical orbits are called "standard orbits", 72 of which are required to describe passage of the satellite over all portions of the earth's surface. 95 spacecraft minutes of data exist in each standard orbit (about 100 civil minutes). For each minute of each LAN interval, the residuals Rk defined as Sumj [Nj(LAN,TSN) - Navg(LAN,TSN)] are computed, where j is a particular orbit and k is the minute (TSN), and a standard error is computed. For tubes U1, U2, and U3, the residuals, Rk typically equal about 10% of the dark count values themselves when 4-5 values of Nj(LAN,TSN) are available for the same point on the surface of the earth. The residuals are examined by eye and when values larger than average are encountered, the data used in deriving the point in question are examined in detail. In virtually all cases, anomalies are traceable to some unanticipated data problem, except for those data which were recorded within 5-10° of the South Atlantic Anomaly. For this latter region, there may be time variations due to solar activity, but the probable source of the often large residuals is the fact that the averaging of orbits into 5° intervals is too coarse, and a 1° average must be used for the prediction of backgrounds. Points taken in these critical background regions of the orbits are labeled for modification at a later date. The values of TSN for which present predictions are suspect may be found in table 2. The standard orbits listed correspond to data in orbits with an LAN of 0-4.99° (std. orbit 1), 5-9.99° (std. Orbit 2), etc., where degrees are taken as increasing eastward, the direction corresponding to satellite motion.
Data for tubes V1, V2, and V3 are treated in a slightly different way than data for U1, U2, and U3, since the values of Rk described above were about 25% when raw data were averaged. By comparing data from similar orbits taken over a baseline of 4000 orbits, it was found that the background on V1, V2, and V3 increased monotonically since launch, but leveled out by orbit 4000. Consequently, before averaging the raw data, time correction factors were applied. Different factors were chosen for each tube to minimize the sum of the respective mean errors for the standard orbits (data averaged into 5° intervals). Table 3 lists the factors by which the dark count at orbit 4000 must be divided to derive the predicted particle background for a given orbit. The mean values of the particle counts as a function of time are accurate to better than 10% when the factors listed in table 3 are used, as far as the available data are concerned.
The dark count corrections are presently stored as a table on the public disk at the computer center, corrected to apply to orbit 4000. The data set containing the corrections is referred to as U. JENKINS. FULL. TABLE. NU04. The corrections are easily made for any given Princeton data point by computing the values of LAN and TSN for that point and reading the appropriate value from the table, using linear interpolation in time to obtain values for each quarter minute of data. The printout of a program for making dark count corrections is attached to this memo. The special data handling subroutines called by the program are explained in the memo entitled "Instructions for Information Retrieval from the Copernicus Disk Data Set" (June 1, 1973), by E. B. Jenkins, available on request. Note that the LAN is listed on our disks as increasing westward, and has a range of values ±180°. The algorithm for converting from LAN to standard orbit is included in the attached program (following statement 297.)
* The dark count intervals are scheduled to provide four minutes of data during, in decreasing order of preference, occultation by the dark earth, occultation by the sunny earth, pointing of the spacecraft within 30° of the sunlit limb of the earth, or pointing within 5° of the physical limb.
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