Hayabusa SPICE Data Issues ======================================================================== By Boris Semenov, NAIF/JPL, July 16, 2010 Introduction ------------ This document provides a brief discussion of the analysis of the Hayabusa SPICE kernels done in the course of preparation of the Hayabusa SPICE PDS data set, the issues with the data discovered by that analysis, and additional kernel files produced to address some of those issues. This document consists of four sections, each covering particular kernels created by a particular producer: -- SPK files created by the Hayabusa Joint Science Team (HJST) -- SPK file created by Dr. Olivier S. Barnouin-Jha -- CK files created by HJST -- SCLK file created by HJST SPK Files Created by HJST ------------------------- The set of SPK files produced by Hayabusa Joint Science Team (HJST) included 84 short-coverage SPKs named "hayabusa_YYYYMMDD_yyyymmdd_v01.bsp", covering from one to a few days, produced on a regular basis during operations at Itokawa, and one long-coverage SPK named "hayabusa_itokawarendezvous_v01.bsp", covering the whole period of operation at Itokawa, produced at the end of the operations at Itokawa. Both the short-coverage SPKs and the long-coverage SPK were created by packaging the spacecraft position and velocity relative to Itokawa determined on-board into Type 9 (Lagrange interpolation)/degree 1 SPK segments using the MKSPK program. During the archive preparation the contents of the short-coverage and long-coverage HJST SPKs were dumped using the SPICE Toolkit's SPY program and the dumps were compared to verify that the data stored in the files indeed came from the same source. The comparison showed that this was the case and that the total set of points in all short-coverage SPKs was a subset of points from the long-coverage SPK. Because of this the long-coverage SPK was considered as superseding the short-coverage SPKs and was the only HJST SPK used in further analysis. The positions dumped from the long-coverage SPK were plotted in a variety of ways, one of which was the plot Figure 1 showing the X, Y, and Z components of the Hayabusa position relative to Itokawa in the HAYABUSA_HP reference frame (+Z axis along the geometric direction from Itokawa to the Earth; +X axis as close as possible to the geometric direction from Itokawa to the Sun; +Y axis completing the right-handed frame): Figure 1Plotting the data demonstrated that the set of positions stored in the long-coverage HJST SPK included a few obvious outliers, such as the one seen on this zoom-in plot Figure 2: Figure 2
Moreover the plots showed that the data during some intervals did not make physical sense, with the trends in the Z coordinate indicating the spacecraft moving from a position about 30 km in front of Itokawa to a position about 30 km behind Itokawa and then "jumping" back to the front multiple times in a short period of time, as seen on this zoom-in plot Figure 3: Figure 3
All of these anomalies were documented in the comments added to the long-coverage HJST SPK. In addition to that the dumped data were "cleaned" to eliminate the outlier points and points during anomalous periods, split into chunks to prohibit interpolation over large gaps resulting from elimination of anomalous data, and repackaged using the same MKSPK setup parameters as those that were used to make the original long-coverage HJST SPK into a new SPK file named "hay_jaxa_050916_051119_v1n.bsp". SPK File Created by Dr. Olivier S. Barnouin-Jha ----------------------------------------------- The SPK file created by Dr. Olivier S. Barnouin-Jha was originally named "Sept-Nov_SPK.bsp". In order to be included in the archive it was renamed to "hay_osbj_050911_051118_v1.bsp". This file was created in October 2008 and covered the whole period of operation at Itokawa. It was created by packaging the discrete position data points determined from Hayabusa LIDAR and WAC data into a Type 9 (Lagrange interpolation)/degree 1 SPK using the MKSPK program. During the archive preparation the positions stored in the file were dumped using the SPY program and rotated to the HAYABUSA_HP frame. Then they were plotted in a variety of ways, one of which was the plot Figure 4 showing the X, Y, and Z components of the Hayabusa position relative to Itokawa in the HAYABUSA_HP reference frame: Figure 4
Plotting the data showed a number of gaps -- intervals during which the file contained no data points but still provided linear interpolation, likely resulting in grossly incorrect interpolated positions, -- such as the one seen on this zoom-in plot Figure 5: Figure 5
Moreover the plots showed that the data for October 28, 2005 did not correctly represent the activities that took place during operations, with all three position components suggesting that the spacecraft flew around Itokawa on a slightly inclined circular trajectory, as seen on this zoom-in plot Figure 6: Figure 6
Finally the velocity components for all points in the SPK were set to zero, prohibiting the use of the file in any application that requires velocities consistent (with positions) or at least realistic. All of these issues were documented in the comments added to Dr. Barnouin-Jha's SPK. In addition to that, the dumped data were "cleaned" to eliminate the points during the anomalous period on October 28, 2005, augmented with velocities computed using simple differencing of neighbor positions, split into chunks to prohibit interpolation over large gaps, and repackaged using the same MKSPK setup parameters as those that were used to make the original Dr. Barnouin-Jha's SPK into a new SPK file named "hay_osbj_050911_051118_v1n.bsp". CK Files Created by HJST ------------------------ The set of Hayabusa orientation CK files produced by HJST included 84 short-coverage CKs named "hayabusa_YYYYMMDD_yyyymmdd_v01.bc", covering from one to a few days, produced on a regular basis during operations at Itokawa, and two long-coverage CKs -- "hayabusa_itokawarendezvous_v01.bc" produced on November 22, 2005 and "20050902-20051126_att.bc" (renamed to "hayabusa_itokawarendezvous_v02.bc" for the archive) produced on November 29, 2005, -- each covering the whole period of operations at Itokawa. Both the short-coverage CKs and the long-coverage CKs were created by packaging the spacecraft orientation data from on-board star cameras into Type 3 CK segments using the MSOPCK program. During the archive preparation the contents of the short-coverage and long-coverage HJST CKs were dumped using a custom program and the dumps were compared to verify that the data stored in the files indeed came from the same source. The comparison showed that this was the case and that the total set of points in all short-coverage CKs and in the earlier long-coverage CK were subsets of points from the later long-coverage CK. Because of this the later long-coverage CK was considered as superseding both the short-coverage CKs and the earlier long-coverage CK and was the only HJST CK used in further analysis. The data points dumped from the later long-coverage CK were checked in a variety of ways for outlier points and consistency with the nominal pointing modes (implemented using dynamic frames). While these checks identified a few dozen physically impossible identity quaternions and quaternions with two perfect zero components, in general the data seemed to have been consistent with the pointing modes for major spacecraft activities -- asteroid observations, Earth communications, etc. Additional analysis was performed to check the spacing between data points to confirm that the rather long interpolation interval of 1800 seconds chosen for the CK made sense. This analysis showed that most of the time, except for a very few cases, the spacing between points was less that 128 seconds (as seen on the plot Figure 7), which is adequate to allow interpolation for a three axis stabilized, slowly rotating spacecraft: Figure 7
On the other hand this analysis showed that the data stored in the file came not from a single, fixed frequency "stream" (a set of packets generated every 128, 64, or 16 seconds) but from a combination of two or more asynchronous streams, as suggested by the diagonal lines on this zoom-in plot Figure 8: Figure 8
Further zoom-in to a millisecond level indicated that numerous points in the file were just one to a few milliseconds apart, as seen on this plot Figure 9: Figure 9
While such close spacing in itself doesn't always indicate a problem, in the case of Hayabusa it seemed to be an issue due to possible different on-board delays for the various asynchronous streams. Because of the possible different delays some of the close neighbor quaternions appear to be incorrectly time tagged, as seen in the two examples below in which the quaternions marked by (*) appear to be out of place compared to the trends established by their neighbors: 2005-11-03T05:23:09.58000 +0.73761440580137 2005-11-03T05:23:12.76700 +0.73742965636688 2005-11-03T05:23:20.76600 +0.73846154665125 (*) 2005-11-03T05:23:20.76700 +0.73705486031520 2005-11-03T05:23:25.57900 +0.73685107236940 2005-10-20T04:44:49.32500 +0.69323771051346 2005-10-20T04:46:36.41600 +0.68483801616792 2005-10-20T04:46:54.16500 +0.68375293310090 2005-10-20T04:46:54.16600 +0.68483801616792 (*) 2005-10-20T04:46:57.32200 +0.68363023030721 While no acceptable approach was found to correct this particular issue, a new CK file "hayabusa_itokawarendezvous_v02n.bc" excluding the non physically possible quaternions and containing made-up angular velocities to increase the file's usability was created for the archive using the dumped quaternions and the same MSOPCK setup parameters as those that were used to make the original long-coverage HJST CK. SCLK File Created by HJST ------------------------- The Hayabusa on-board clock correlation provided in the SCLK file "hayabusa.tsc" created by HJST was analyzed by comparing it against the reference, single linear segment correlation trend passing through the SCLK's first and last correlation points. This reference correlation trend was encapsulated into an auxiliary SCLK kernel with clock attributes identical to those of the HJST SCLK kernel and the following single correlation record: 3.35581810000000e+07 1.05712107715935e+08 0.031249185802 The correlation provided by the HJST and reference SCLK kernels were compared using a custom program that converted a set of Ephemeris Times (ETs) to SCLKs using the HJST SCLK kernel, then converted the SCLKs back to ETs using the reference SCLK kernel, and finally subtracted the resultant ETs from the input ETs to compute the difference between correlations. This plot Figure 10 illustrates the difference over the course of the mission, from launch through the end of operations at Itokawa: Figure 10
The plot above clearly indicates that the Hayabusa on-board clock ran at two different rates, apparent from two distinct up and down global trends, and switched between these rates a few times during the mission, probably after commands from the ground control center. It also indicates a problematic correlation area during the last third of the cruise to Itokawa, apparent from the up-and-down jumps of almost twenty seconds seen in this zoom-in plot Figure 11: Figure 11
Fortunately the correlation for the period of operations at Itokawa looks much more smooth, as seen on this zoom-in plot Figure 12: Figure 12
During this period the magnitudes of the "spikes", which are most likely artifacts of inaccurate input time correlation data and/or improper modeling in the correlation algorithm, are on the order of 0.2 seconds, as seen on this zoom-in plot Figure 13: Figure 13
No attempts to produce an improved version of the HJST SCLK kernel were made during the archive preparation process.