Data Set Overview : The Magellan (MGN) Radio Occultation (ROCC) Raw Data Archive (RDA) is a time-ordered collection of raw and partially processed data from radio occultation experiments conducted using the Magellan spacecraft while it orbited Venus.  The spacecraft radio system was used as the signal source. The spacecraft high-gain antenna (HGA) beamed those signals through the atmosphere of Venus toward Earth, where they were received at stations of the NASA Deep Space Network (DSN). Changes in the amplitude and frequency of the received signal can be related to the refracting and absorbing properties of the atmosphere as a function of raypath penetration depth. These parameters can then be inverted to give the temperature and pressure structure and the density of absorbing species in the atmosphere.  Transmit polarization along the boresight of the spacecraft antenna was linear at the S-band wavelength (13 cm, or 2298 MHz) and circular at the X-band wavelength (3.5 cm, or 8415 MHz). Signals could be received coherently in both right- and left-circular polarizations at both wavelengths when a 70-m antenna was used at the DSN station. When a 34-m antenna was used, only a single polarization at each wavelength could be received; and, in some cases, only one wavelength was captured.  During these experiments, frequency of the spacecraft transmitter was controlled by a signal radiated from the ground. The spacecraft oscillator locked to the 'uplink' signal, then generated 'downlink' signals coherently at both S- and X-band. Perturbations on the radio signal during TWO passes through the atmosphere must be considered during analysis of these data.  During several experiments, the spacecraft attitude was adjusted during the occultation to compensate for refractive bending in the Venus atmosphere. These 'limb-track' maneuvers allowed deepest penetration of the atmosphere and were usually scheduled when a 70-m ground antenna was available.  The first set of experiments (5-6 October 1991) and their results have been described in the literature [STEFFESETAL1994], [JENKINSETAL1994], [HINSON&JENKINS1995].  The data set includes Original Data Records (ODRs), which are digitized recordings of baseband receiver output, and Archival Tracking Data Files (ATDFs or TDFs), which are edited records of closed-loop receiver output and related parameters. The ATDF also contains coefficients that can be used to derive the frequency of the 'uplink' signal transmitted from the ground.  Typical users of the data set would derive the frequency and amplitude of the signal captured by the DSN receiving antenna from ODRs (or, for low resolution, from an ATDF). After correction for known geometrical effects, the residual frequency could be attributed to the radially variable index of refraction of the atmosphere. Reduction of the signal amplitude from the predicted level could be attributed partially to refractive spreading (which may be estimated from the frequency analysis) or to mispointing of the HGA (which can be estimated from spacecraft attitude data). The remaining attenuation is believed to result from absorption by atmospheric constituents, especially sulfuric acid. See [STEFFESETAL1994], [JENKINSETAL1994] and references cited therein for more information.   Parameters : Open loop data records (ODRs) contain 8-bit samples of receiver output. Each block of (nominally) 4000 data samples is accompanied a 166 byte header. The ODR is described by a Software Interface Specification (SIS), included as RSC11_11.TXT in the DOCUMENT directory of the data set archival volume.  When 70-m antennas were available, two X-band (3.5 cm wave- length) open-loop receivers were used, one capturing right- circularly polarized signals and the other capturing left- circularly polarized signals. These are identified in the data stream as Channels 1 and 3. Two S-band open-loop receivers were also used, yielding right- and left-circularly polarized output on Channels 2 and 4, respectively. When smaller antennas were used, no more than two receivers were used; the bands, polarizations, and channel assignments varied among experiments.  Each ATDF data record contains 117 parameters, stored in records of 288 bytes at rates up to 10 per second. The parameters include estimates of received signal strength and frequency as well as status information on components of the DSN system. The ATDF is described by a Software Interface Specification (SIS), included as TRK_2_25.TXT in the DOCUMENT directory of each ROCC archival volume.   Processing : Open loop data (ODRs) are not edited or otherwise processed before being incorporated into the ROCC RDA. A 32-byte label at the beginning of each ODR tape is not included in the data file; it gives the software version of the data taking equipment and is saved instead in the PDS label that accompanies the data file.  DSN Navigation Team performs editing on data during conversion from the Intermediate Data Record (IDR) to ATDF formats; this function serves mostly to remove anomalous (out of nominal range) values.   Data : Original Data Records (ODRs) were the primary data type for this raw data archive. ODRs were generated at stations of the NASA Deep Space Network (DSN) and were distributed on 6250 bpi tape. Those tapes have been copied to disk files, defined as PDS TABLE Objects, and archived on these CD-WO volumes. ODRs are stored on CD-WO volumes in this archival data set in chronological order; ancillary data to support analysis accompany ODRs in the CALIB, GEOMETRY, and DOCUMENT directories. The table FILELIST.TAB in the document directory shows relationships among ODR files and ancillary data.  A nominal ODR contained 24000 records, each record having 166 bytes of header data and 4000 bytes of 8-bit sample data. For the carefully planned experiments, each of four receiver channels (X-RCP, S-RCP, X-LCP, and S-RCP) was sampled at 50000 samples per second. A single file represented 8 minutes of observing time. For the ad hoc experiments (February-March 1992, January 1993, and October 1994) channel assignments and sampling parameters varied.  On MGN ROCC archival volumes ODR files have names of the form ydddhhmm.ODR where y gives the least significant digit of the year, ddd is the 3-digit day-of-year, hh is the 2-digit hour, and mm is the 2-digit minute on which data acquisition for the file began.  Each ODR data file is accompanied by a full PDS detached label with name ydddhhmm.LBL which completely describes the contents of the ODR file. The format and contents are also described in RSC11_11.TXT in the DOCUMENT directory of each archival volume.  In two instances (1992-068T20:43 and 1992-070T21:48) two DSN stations (DSS 15 and DSS 45) began recording occultation data at the same ydddhhmm; the file with the slightly earlier start time (8 seconds in the first case and 2 in the second) has had its 'mm' manually decremented by one so as to avoid duplicate file names.  Archival Tracking Data Files (ATDFs or TDFs) are considered to be primary raw data for many radio science experiments. In this case, they served a backup and ancillary role. ATDFs are generated by the Radio Metric Data Conditioning Team in the DSN. They were delivered either on computer-compatible tape or as electronic files. They have been copied to disk, defined as PDS TABLE objects, and archived on these CD-WO volumes.  A typical ATDF covered several days and had a volume of 35 MBytes. ATDF rates for these Magellan ROCC experiments were typically on the order of one sample every 60 seconds. Each sample was of 117 items, including measurements of signal amplitude and frequency from closed loop receivers, and was stored as a single record of 288 bytes. Sampling of the signals was limited to one polarization at S-band and one polarization at X-band. Time resolution was far coarser than from the ODR and no information on signal phase was retained. The ATDF, from this perspective, provided low-resolution backup to the ODR.  The ATDF also provided data on the uplink transmitting system, including constant and linear coefficients for a software driven oscillator that served as the uplink frequency reference. This information is needed for analysis of the ODR, especially when the uplink signal was 'ramped' -- used a linearly varying frequency.  Each ATDF has a file name of the form ydddeeeX.TDF where y is the least significant digit of the year and ddd is the day-of-year for the first ATDF record. The eee gives day-of-year for the last data record in the ATDF, and X is set to 'A' unless there is more than one ATDF with ydddeee in the data set. For the MGN ROCC raw data archive there were no duplicates of ydddeee among ATDFs.  Each ATDF is accompanied by a full PDS detached label with name ydddeeeA.LBL which completely describes the contents of the ATDF. The format and contents are also described in TRK_2_25.TXT in the DOCUMENT directory of each archival volume.   Ancillary Data : For each ODR, there is a set of ancillary files needed for analysis. These are stored in the CALIB and GEOMETRY directories, as appropriate. A table FILELIST.TAB in the DOCUMENT directory shows relationships among ODRs and ancillary files.   Uplink Frequency Data --------------------- The transmitted signal from the ground was used as the frequency reference for the spacecraft radio signal. During early experiments the uplink was 'ramped' -- the frequency varied linearly with time. Coefficients for linear uplink frequency functions are stored in the ATDF (in items 112-114; see TRK_2_25.TXT in the DOCUMENT directory). These were abstracted from the ATDF and stored in files of their own for the first set of experiments in 1991. They are stored as tables with names TUNnnnnn.TAB in the CALIB directory on the first CD-WO volume. Each TUNnnnnn.TAB file is accompanied by a detached PDS label with name TUNnnnnn.LBL.  For later experiments, the user must obtain the uplink coefficients directly from the ATDF in the TDF directory. For experiments in 1994 ramping was discontinued, and a constant frequency was transmitted. It is not known when the change occurred; users interested in working with data from 1992 and 1993 should at least plan to obtained uplink coefficients (and times) from ATDFs.   Downlink Frequency Files ------------------------ Also, for the 1991 experiments (only), downlink frequencies were predicted at both S- and X-band. These are stored as tables with names DSBnnnnn.TAB in the CALIB directory. The files give offsets from the expected carrier frequency at S-band (B:S) and X-band (B:X) after Doppler effects from relative motion and effects of Earth's atmosphere are taken into account. They do NOT include effects from the atmosphere of Venus, so subtracting the prediction from the observed frequency will give a first-order measure of the effects of the atmosphere in the frequency domain. Each DSBnnnnn.TAB file is accompanied by a PDS detached label DSBnnnnn.LBL. These prediction files appear only on the first CD-WO volume; users desiring them for experiments in 1992-1994 must calculate their own.   High-Gain Antenna (HGA) Radiation Pattern ----------------------------------------- The radiation pattern of the high-gain antenna (HGA) is described in a PDS document with both ASCII and extended PostScript components in the CALIB directory. The HGA pattern would not be needed if the HGA always pointed toward 'virtual' Earth. In ad hoc experiments, there was no limb track maneuver; and, when maneuvers were planned, they were simple linear offsets rather than true tracks of the image of Earth refracted through Venus' atmosphere. On 1994-221 the spacecraft executed its limb track maneuver two minutes early; so corrections based on the HGA patterns will be needed if the results are to be interpreted accurately. The HGA calibration document is described in the detached PDS label HGA_XXX.LBL in the CALIB directory; it is included on all CD-WO volumes.   Observing Geometry ------------------ Files describing the observing geometry are stored in the GEOMETRY directory. These include NAIF 'text' format SPK files and NAIF 'transfer' format ephemerides. Files describing spacecraft attitude are also included in the GEOMETRY directory, but are described in the next section.  NAIF (PDS Navigation and Ancillary Information) 'text' format SPK files are available for all observations through 1992-342. These contain the positions of Magellan, Venus, Earth, and the Sun in data modules that can be used readily with NAIF Toolkit software. Each 'text' SPK file has a name ending with the extension 'TSP'; each TSP data file is accompanied by a detached PDS minimal label having the same file name but ending with the 'LBL' extension.  NAIF 'transfer' format ephemeris files are available for the remaining observations (1992-355 through 1994). Files with names MGNyyddd.XSP contain the Magellan trajectory with respect to Venus beginning on year yy and day-of-year ddd. A single file PEPH8996.XSP contains the positions of Earth, Venus, and the Sun from 1989 to 1996. Each XSP file is accompanied by a detached PDS minimal label having the same file name, except for an 'LBL' extension.  Before using either 'text' or 'transfer' NAIF files, the user should convert ASCII carriage-return line-feed delimiters on each record to the default delimiter for text files on the local system, then use the NAIF utility SPACIT to convert the archival file to local binary.  Information on acquisition, installation, and use of the NAIF software can be found in TK_FTP.TXT in the DOCUMENT directory.   Spacecraft Attitude (Quaternions) --------------------------------- Data describing the attitude of the spacecraft during radio occultations is needed when the HGA was not pointed toward the image of Earth, as seen from the spacecraft. Limb track maneuvers were planned and executed for several experiments -- to improve pointing during experiments; attitude data defines the accuracy to which the image of Earth remained on the HGA boresight.  Attitude data are available in as many as five forms, though no more than three forms are included here for any single experiment.  For the first set of experiments (1991), telemetry of quaternion values is available in PDS tables with file names of the form DRFnnnnn.TAB. Filtered versions of the telemetry are available in files FILnnnnn.TAB. Reconstructions of antenna pointing are in tables with file names HGAnnnnn.TAB. In each case a detached PDS label completely describes the format and content of the table. One set of DRF, FIL, and HGA files is stored for each of the three Magellan orbits observed in 1991. These files appear in the GEOMETRY directory, but only on the first CD-WO volume.  For the remaining experiments during which a limb track maneuver was planned spacecraft attitude is described by either a table of quaternion values (QUAnnnnn.TAB file name), a text file of polynomial coefficients which can be used to generate quaternions (QCFnnnnn.TXT), or both. The QUA*.TAB files are each accompanied by a detached PDS label with file name QUA*.LBL which completely describes the content and format of the associated table. QCF*.TXT files are self-documenting. In some QCF*.TXT files multiple sets of coefficients have been archived; in these cases the experiment investigator had not determined at the time of archiving which set had actually been executed on the spacecraft.   Coordinate System : SPK ephemeris files and quaternion files were produced for the J2000 inertial reference frame. SPICE reader routines may be used to convert these to other coordinate systems.  Other data types are not dependent on definition of a coordinate system.   Software : Software for parsing, reducing, and analyzing data such as these has been developed at Stanford University and elsewhere. The Stanford software has not been prepared for general distribution and is not included with the archival data set.   Media/Format : The archival data set is written on CD-WO media using the Young Minds CD Studio authoring system. The CD-WO volumes conform to ISO 9660 standards.

Overview : At the time the raw data were archived relatively little analysis had been performed on the data, except for the 1991 occultations (1991-10-05 and 1991-10-06) [STEFFESETAL1994], [JENKINSETAL1994], [HINSON&JENKINS1995].   Review : This archival data set was distributed for peer review panel prior to its acceptance by the Planetary Data System (PDS). The review was conducted in accordance with PDS procedures.  Prior to creation of the final version of the archival data set, key elements of the archive were distributed for preliminary review. These included electronic versions of example PDS labels, CATALOG files, and Software Interface Specifications. These materials were distributed to PDS personnel, the experiment investigator, and others, as appropriate. An earlier version of the first CD-WO volume (MG_2201, DATA_SET_ID : 'MGN-V-1-ROCC-V1.0') was distributed for review in February 1995.  Data on the CD-WO volumes was checked using Stanford parsing and reduction software to ensure that it was both complete and an accurate replica of the data contained in the original tape files.   Data Coverage and Quality : The times covered by ODR tapes are given below. Notes on presumed quality of the data follow each list of ODR times. Note that ODRs from 1992-355 and 1992-356 are included here for completeness; but those data were not in the the archive.  DSS START STOP --- ----------------- ----------------- 43 1991-278T20:15:54 1991-278T20:23:54 43 1991-278T20:23:54 1991-278T20:24:24 43 1991-278T23:31:28 1991-278T23:39:27 43 1991-278T23:39:28 1991-278T23:40:27 43 1991-279T02:47:01 1991-279T02:55:01 43 1991-279T02:55:01 1991-279T02:56:00  No problems known with data above.  DSS START STOP --- ----------------- ----------------- 65 1992-048T08:14:59 1992-048T08:37:00 65 1992-048T11:32:31 1992-048T11:52:30 61 1992-049T10:21:19 1992-049T10:41:19 61 1992-049T13:36:54 1992-049T13:56:52 61 1992-050T09:10:12 1992-050T09:30:13 61 1992-050T12:25:48 1992-050T12:45:46 61 1992-051T07:59:10 1992-051T08:19:10 61 1992-051T11:14:46 1992-051T11:34:44 65 1992-052T10:02:46 1992-052T10:23:46 65 1992-052T13:18:23 1992-052T13:39:21 45 1992-067T21:52:06 1992-067T22:12:02 45 1992-068T01:07:58 1992-068T01:27:56 45 1992-068T04:23:54 1992-068T04:43:52 65 1992-068T11:32:39 1992-068T11:52:31 65 1992-068T14:11:51 1992-068T14:31:43 15 1992-068T17:27:42 1992-068T17:47:40 45 1992-068T20:43:43 1992-068T21:03:39 15 1992-068T20:43:51 1992-068T21:03:39 45 1992-068T23:59:41 1992-069T00:19:39 45 1992-069T03:15:43 1992-069T03:35:41 65 1992-069T13:03:03 1992-069T13:25:01 15 1992-069T16:20:02 1992-069T16:40:00 15 1992-069T19:36:11 1992-069T19:56:09 45 1992-069T22:52:25 1992-069T23:12:21 45 1992-070T02:08:37 1992-070T02:28:35 15 1992-070T15:14:25 1992-070T15:34:21 15 1992-070T18:31:15 1992-070T18:51:13 15 1992-070T21:48:46 1992-070T22:08:44 45 1992-070T21:48:48 1992-070T22:08:44 61 1992-071T10:38:08 1992-071T10:58:10 61 1992-071T13:53:54 1992-071T14:13:45 15 1992-071T14:09:58 1992-071T14:13:38  No limb track maneuvers were planned, so penetration depth into atmosphere was limited. No SPK file for 1992-052. ODR files were not checked for anomalies prior to archiving. DSS 61 used non-standard receiver filters (7500 Hz). These data were typically collected using DSP mode 4; X-band samples were obtained using ADCs 2-4 (the samples from ADC 1 are redundant and should not be used).  DSS START STOP --- ----------------- ----------------- 43 1992-342T03:21:45 1992-342T03:29:44 43 1992-342T03:29:45 1992-342T03:37:44 43 1992-342T03:37:45 1992-342T03:45:44 43 1992-342T03:39:45 1992-342T03:37:44 43 1992-342T03:45:45 1992-342T03:49:24 43 1992-342T06:36:09 1992-342T06:44:08 43 1992-342T06:44:09 1992-342T06:52:08 43 1992-342T06:52:09 1992-342T07:00:08 43 1992-342T07:00:09 1992-342T07:03:45  Effect of atmosphere was not included in calculation of receiver tuning predictions; X-band signal drifts out of passband during observation periods. Occultations were at apoapsis, so vertical resolution in the atmosphere was not as good as during 1991 experiments.  DSS START STOP --- ----------------- ----------------- 14 1992-355T21:57:00 1992-355T22:04:59 14 1992-355T22:05:00 1992-355T22:12:59 14 1992-355T22:13:00 1992-355T22:20:59 14 1992-355T22:29:00 1992-355T22:36:59 14 1992-355T22:37:00 1992-355T22:44:59 14 1992-355T22:45:00 1992-355T22:45:16 14 1992-356T01:04:17 1992-356T01:12:16 14 1992-356T01:12:17 1992-356T01:16:15 14 1992-356T01:16:16 1992-356T01:24:15 14 1992-356T01:24:16 1992-356T01:28:59 14 1992-356T01:29:08 1992-356T01:37:07 14 1992-356T01:37:08 1992-356T01:42:07 14 1992-356T01:42:09 1992-356T01:50:10 14 1992-356T01:50:11 1992-356T01:51:42 14 1992-356T01:54:20 1992-356T01:59:37  Recording system failed, and no quaternion files have been located. Data corresponding to the ODR files above were NOT included in the archive.  DSS START STOP --- ----------------- ----------------- 15 1993-013T20:31:30 1993-013T20:47:29 15 1993-013T20:47:30 1993-013T21:03:29 15 1993-013T21:03:30 1993-013T21:08:02 15 1993-013T21:08:25 1993-013T21:24:24 15 1993-013T21:24:25 1993-013T21:31:38  No ATDF; but experiments were conducted with constant uplink frequencies in 1994, so uplink frequency rates MAY not be needed here. No limb track maneuver means penetration depth in the atmosphere is not great. ODRs were not reviewed before archiving, so their quality is unknown.  DSS START STOP --- ----------------- ----------------- 14 1994-175T17:40:44 1994-175T17:51:39 14 1994-175T18:17:41 1994-175T18:33:40 14 1994-175T18:33:41 1994-175T18:34:22 14 1994-175T19:13:39 1994-175T19:15:12 14 1994-175T19:50:41 1994-175T20:01:39 14 1994-175T20:46:41 1994-175T20:57:39 14 1994-175T21:23:41 1994-175T21:34:39 14 1994-175T22:20:01 1994-175T22:36:00 14 1994-175T22:36:01 1994-175T22:36:35 14 1994-175T22:56:39 1994-175T23:07:39  Incorrect pointing of ground antenna (pointing was calculated for DSS 15, then used at DSS 14 without correcting for physical displacement between antennas; error was large enough that spacecraft was well off the S-band boresight and in the sidelobes at X-band). Also, incorrect spacecraft polarization (and transmitter) was specified. Incorrect entries in DSN keyword file meant ground sequencing was in error, including erroneous uplink transmit timing. The last occultation may be OK, but the others are degraded. By 1994 uplink transmissions did not use 'ramping'; so the ATDF should not be required for details of uplink radio frequencies.  DSS START STOP --- ----------------- ----------------- 43 1994-197T02:26:04 1994-197T02:37:59 43 1994-197T03:59:35 1994-197T04:11:32 43 1994-197T05:32:35 1994-197T05:44:32  No known problems with the ODRs above.  DSS START STOP --- ----------------- ----------------- 61 1994-221T13:37:09 1994-221T13:57:08 61 1994-221T13:57:09 1994-221T14:16:13 61 1994-221T15:10:11 1994-221T15:30:10 61 1994-221T15:30:11 1994-221T15:49:02 61 1994-221T16:43:10 1994-221T17:03:09 61 1994-221T17:03:10 1994-221T17:22:01 61 1994-221T18:16:13 1994-221T18:36:12 61 1994-221T18:36:13 1994-221T18:55:01 61 1994-221T19:49:10 1994-221T20:09:09 61 1994-221T20:09:10 1994-221T20:28:01  Spacecraft executed limb track maneuvers 2 minutes early. DSS 61 filters are non-standard. Otherwise, no known problems with ODR data above.  DSS START STOP --- ----------------- ----------------- 65 1994-274T12:55:41 1994-274T13:11:40 65 1994-274T13:11:41 1994-274T13:20:17 65 1994-274T13:23:47 1994-274T13:39:47 65 1994-274T13:39:48 1994-274T13:55:47 65 1994-274T13:55:48 1994-274T14:11:47 65 1994-274T14:19:38 1994-274T14:34:40 65 1994-274T16:42:57 1994-274T16:58:56 65 1994-274T16:58:57 1994-274T17:14:56 65 1994-274T17:14:57 1994-274T17:30:56 65 1994-274T17:30:57 1994-274T17:33:36 65 1994-274T17:34:09 1994-274T17:37:52 15 1994-275T17:46:04 1994-275T18:02:03 15 1994-275T18:02:04 1994-275T18:04:59 15 1994-275T20:50:00 1994-275T21:05:59 15 1994-275T21:06:00 1994-275T21:19:59 15 1994-279T20:16:00 1994-279T20:31:59 15 1994-279T20:32:00 1994-279T20:47:59 15 1994-281T18:26:05 1994-281T18:42:04 15 1994-281T18:42:05 1994-281T18:58:04 15 1994-281T21:31:00 1994-281T21:46:59 15 1994-281T21:47:00 1994-281T22:02:59 61 1994-283T10:27:00 1994-283T10:46:38 61 1994-283T10:46:58 1994-283T11:03:46 61 1994-283T13:32:11 1994-283T13:52:10 61 1994-283T13:52:11 1994-283T14:05:01 61 1994-283T16:37:11 1994-283T16:57:10 61 1994-283T16:57:11 1994-283T17:10:01 42 1994-284T08:03:12 1994-284T08:19:11 42 1994-284T08:19:12 1994-284T08:35:11 42 1994-284T08:35:12 1994-284T08:38:03 61 1994-284T11:05:28 1994-284T11:25:27 61 1994-284T11:25:28 1994-284T11:43:04 61 1994-284T14:06:56 1994-284T14:26:55 61 1994-284T14:26:56 1994-284T14:43:07 61 1994-284T15:42:10 1994-284T16:02:09 61 1994-284T16:02:10 1994-284T16:15:31 61 1994-284T17:14:10 1994-284T17:31:02 42 1994-284T21:54:41 1994-284T22:10:40 42 1994-284T22:10:41 1994-284T22:26:40 42 1994-284T22:26:41 1994-284T22:30:03 42 1994-284T23:27:10 1994-284T23:43:09 42 1994-284T23:43:10 1994-284T23:59:09 42 1994-284T23:59:10 1994-285T00:02:03 42 1994-285T00:59:39 1994-285T01:15:38 42 1994-285T01:15:39 1994-285T01:31:38 42 1994-285T01:31:39 1994-285T01:34:33 61 1994-285T09:55:28 1994-285T10:07:25 61 1994-285T10:37:25 1994-285T10:57:24 61 1994-285T10:57:25 1994-285T11:17:24 61 1994-285T11:17:25 1994-285T11:37:24  No limb track maneuvers, so atmospheric penetration expected to be limited. DSS 61 used non-standard filters. ODRs had not been checked for quality when archiving began; but no problems known a priori. Spacecraft communications with Earth ended on 1994-285.   Limitations : No ATDF is available for 1993-013. No trajectory data are available for 1992-052. Otherwise, the limitations in this data set follow from the quality of the execution, which is described above under Data Coverage and Quality.