Description of the Small Bodies Occultations bundle V4.0 Bundle Generation Date: 2024-04-10 Peer Review: March 4, 2024 Asteroid Review Discipline node: Small Bodies Node This data set is intended to include all reported timings of observed asteroid, planet, and planetary satellite occultation events, as well as dimensions and other data derived from those timings. The dataset includes observations made over a period of more than 50 years, and has grown rapidly in recent years. Most of these timings are otherwise unpublished. This version is complete through to July 2023, with partial observations through to December 2023. In relatively recent years light curve inversion 3D models (Shape Models) have become available for a large number of asteroids via the DAMIT and ISAM web services [https://astro.troja.mff.cuni.cz/projects/asteroids3D/ and http://isam.astro.amu.edu.pl/]. Wherever possible, the asteroidal occultation observations are matched to available Shape Models. The dataset has been comprehensively reviewed in its entirety over the 4 years from the last version (PDS4 V3.0) to remove errors, ensure consistency of data over the duration of the observations, incorporate current observing techniques, include Shape Model identifications, and include volume-equivalent diameters derived from fits to Shape Models. In addition, it now contains astrometric report details for each event. ==== Data ==== The data set is made up of 10 files Four files contain the complete set of observation data. Asteroid_2024Mar.psv AsteroidTimes_2024Mar.psv Planet_2024Mar.psv PlanetTimes_2024Mar.psv Asteroid_2024Mar.psv contains all data elements relating to an an asteroid occultation event as a whole, while AsteroidTimes_2024Mar.psv contains the details of the individual observations of an event. The files are linked by a sequence number in the Asteroid_2024Mar.psv file. Comets are included in these files. Planet_2024Mar.psv and PlanetTimes_2024Mar.psv contain observation of occultations by the major planets and their satellites. Two files are summary files, limited to the object size/shape derived for each individual observation AsteroidSummary_2024Mar.psv PlanetSummary_2024Mar.psv Four files contain the primary results derived from an analysis of the observations AsteroidAstrometry_2024Mar.psv PlanetAstrometry_2024Mar.psv AsteroidDiameters_2024Mar.psv DoubleStars_2024Mar.psv AsteroidAstrometry_2024Mar.psv lists the astrometric position of asteroids & comets, while PlanetAstrometry_2024Mar.psv contains astrometry for the major planets and their satellites. The data content is that set by the IAU Astrometry Data Exchange Standard (ADES) standard for small solar system objects, and includes all columns required for submission to the Minor Planet Center under observatory code 275. This includes the columns headed deltaRA and deltaDec, which are always set to 0. AsteroidDiameters_2024Mar.psv provides volume-equivalent diameter of the asteroid derived from the fit of all observations against an available shape model. DoubleStars_2024Mar.psv provides the double star solution(s) for double stars discovered in an occultation. One file contains images of the 'best' occultation observations. occultations.pdf This provides a graphic of all events with fit quality code 3 or 4. There are 385 images, which is almost 9% of all events. The scale of the images varies according to the object. If a satellite is involved, the scale is reduced so as to show both the primary object and the satellite. The file is located in the document directory. ================ Acknowledgments ================ This dataset contains over 30,000 observations of almost 10,000 events made by over 4,000 individuals from around the world, made over a period of more than 50 years. Observers have generally made these observations at their own expense, including occasions when they have traveled significant distances. Users of this database are requested to acknowledge their contributions with a statement like: We acknowledge the contributions of the 4000 observers who have provided the observations in the dataset. Most of those observers are affiliated with one of more of: * European Asteroid Occultation Network (EAON) * International Occultation Timing Association (IOTA) * International Occultation Timing Association - East Asia (IOTA-EA) * Trans-Tasman Occultation Alliance (The Occultation section of the Royal Astronomical Society of New Zealand) The occultation observations would not have been possible without the efforts of those who make, or have facilitated the making of (by way of software), predictions of occultation events over the duration of the observations in this dataset. Special mention must be made to the coordination software Occult Watcher which facilitates coordination of observers wherever they are located around the world. Final mention is made to the various software packages specifically designed to measure occultation events on video or CCD recordings. ============================== Occultations Analysis Program ============================== The observations are reduced using the software package Occult that is freely available for download at: http://www.lunar-occultations.com/iota/occult4.htm The basic methodology is given in 'Precise astrometry and diameters of asteroids from occultations - a data set of observations and their interpretation', Herald et al, MNRAS 499, 4570-4590 (2020) Processes for reporting observations have evolved over the duration of the dataset. For many years now observers submit an observation report that provides disappearance and reappearance timings (in UTC), observing location coordinates (WGS84) and elevation (Mean Sea Level), observing conditions and circumstances. The observations are reduced using the Besselian fundamental plane. The methodology is as follows. - The apparent position of the asteroid is computed at four 1-hour steps; the time of the 2nd step is rounded to 0.1 hrs, and is close to the mean time of the observed occultation events. These positions are combined with the apparent positions of the star computed for the same four times (corrected for stellar parallax, proper motion) to derive coordinates of the asteroid's shadow on the fundamental plane. Those coordinates are used to derive a cubic expression for the _motion_ of the asteroid's shadow on the fundamental plane. - For each reported time, the (x,y) coordinate of the observer on the fundamental plane is computed. The time of the observation is increased by the light travel time from the observer to the fundamental plane. - The reference time for the event is set as the unweighted mean time of all reported Disappearance or Reappearance event used to derive the position/shape of the shadow. All observer positions on the fundamental plane as at the observed time are transposed to their position on the fundamental plane as at the reference time, using the cubic expression for the motion of the asteroid's shadow. This creates a set of (x,y) observer coordinates on a reference frame that is fixed relative to the moving asteroid's shadow. - For fitting purposes, the origin of the (x,y) coordinates on the reference frame is set at the mean location of the (x,y) observer coordinates. Most commonly, the events used to define the origin of the reference frame are all Disappearance and Reappearance observations. However: * Events given a zero weight, and Miss events, are excluded; * If the star is a double star, only events involving the primary component are used to define the origin of the reference frame. * If a satellite is involved, the relative motion may be unknown. To this end, the reference frame (including the reference time) is defined using only events involving the satellite. This results in the position of the main body being transposed in that reference frame in accordance with its (presumably) dominant motion, to the mean time of the satellite events - thereby ensuring correct evaluation of the separation and position angle at the reference time. For a small number of satellites an ephemeris is available from the IMCCE Miriade system [https://ssp.imcce.fr/webservices/miriade/api/ephemsys/], and the 'known' relative motion of the satellite is incorporated. - Fitting a shape model, ellipse, double star solution, and satellite solution, occurs on this reference frame. - The axis of the occultation shadow corresponds to the center of the fitted ellipse, or the center of a fitted shape model. The astrometric position of the asteroid is set to be the position of the star as seen from an observer 'located' at the point of intersection on the fundamental plane, of that shadow axis. This location is converted to geocentric equatorial GCRS J2000 coordinates, with the time of observation being the reference time. - If the event involves a double star, there may be a single solution, two solutions, or four solutions (depending upon the number of observed chords). The solution provides a Separation and Position Angle on the Apparent reference frame. For reporting purposes the Position Angle is converted to the BCRS reference frame. The separation of almost all double stars detected in an occultation is smaller than the resolution of the Gaia catalogue; to provide an astrometric position relative to the Gaia star position, the photo-center of the double star components is used. - For Miss events, the listed time is generally (but, by omission, not always) set to be the time when the miss chord passes closest to the ellipse fitted to the event. =================================== Deriving data from the observations =================================== For each event in the dataset a quality code is provided. That code has the following definitions: 0 = 'No reliable position or size'. Such events have been assessed as being unreliable. 1 = 'Astrometry only. No reliable size'. Such events typically involve only one, a small number of closely spaced, observed chords. The event can be used to report astrometry, but nothing meaningful about the asteroid's size. 2 = 'Limits on size, but no shape'. The chords provide a poor coverage around the profile, but a diameter of the asteroid is determinable (often with a fit to a shape model) 3 = 'Reliable size. Can fit to shape models'. There is good coverage around much of the profile enabling a reliable determination of the asteroid's size - even without a shape model 4 = 'Resolution better than shape models'. The profile of the asteroid well covered with highly consistent results - even without a shape model. ======================================= Astrometric uncertainties - Error Code ======================================= The astrometry derived from occultations includes a Error Code from the error model used for the observations. The error model is somewhat complex, covering the range of practical situations that occur. Because the observation is of event time, the dominant sources of uncertainty are best resolved in the Along-path / Across-path directions, rather than Right Ascension/ Declination. Some example of issues that are dealt with are (i) single chord observations, where the Along-path uncertainty is dominated by uncertainty in the event times, whereas the Across-path uncertainty is associated with the diameter of the asteroid, and (ii) the presence of a near-by Miss chord to a single-chord observation can result in the across-path uncertainty being greatly reduced. For the final astrometric report, the Along-Path and Across-Path uncertainties are rotated to provide uncertainties Right Ascension and Declination, with an associated correlation coefficient. Parts of the Error Code use the following 4 parameters, which relate to the distance of certain chords on either side of the center of the asteroid. In these parameters, the side of the path is indicated as Plus, or Minus - with Plus referring to the north-side of the path. Hit refers to positive occultation events, while Miss refers to Miss events. The Hit parameters specify the distance of the northern-most and southern-most positive chords. The Miss parameters specify the distance of the closest Miss chord on either side of the path, with a value of +9 or -9 if there is no such chord on the particular side. The distances are expressed in units of the asteroid's assumed radius. The 4 parameters are: Well-located : PlusHit >= 0.3 _and_ MinusHit <= -0.3 Poorly-Located : (PlusHit - MinusHit) > 0.5, with either PlusHit < 0.3 or MinusHit > -0.3 Constrained : either PlusMiss < 1.3 or MinusMiss > -1.3 Unconstrained : none of the above Two additional parameters apply to the code (d) values (that is, to major planets and their large moons) - ScaleFactor_AlongPath & ScaleFactor_Across_Path The default values for both these factors is 1. If the absolute value of (PlusHit - MinusHit) <0.3: - the MEAN value is Absolute(PlusHit + MinusHit)/2, limited to being no greater than 0.8, and no less than 0.3 - ScaleFactor_AlongPath = 1/Sqrt(1 - MEAN*MEAN) - ScaleFactor_AcrossPath = 1/MEAN The Error Codes are as follows: a - the uncertainty is that from a least squares fit of an unrestricted ellipse to the observed chords. This provides the minimal value of the uncertainties. (a) is usually replaced by one of the following codes. However it will appear for occultations by the major planets and larger planetary satellites, when they have a known diameter and are known to be spherical. b - For asteroids only. If the astrometric position is set to match the _center_ of a shape model (center of mass solution) - the uncertainties for both the Along-Path and Across-Path directions are set at 2% of the assumed diameter. c - For asteroids, if (b) does not apply. If the observation has been fitted to a shape model, with the quality of that fit being either 'poor' or 'good' c1 - If the overall event quality is 'Reliable Size...' or 'Resolution better than Shape models': uncertainties for both the Along-Path and Across-Path directions are set at 4% of the assumed diameter. c2 - If the overall event quality is 'Limits on size, but no shape': uncertainties for both the Along-Path and Across-Path directions are set at 8% of the assumed diameter. d - for major planets & their large moons. This relies on they being essentially circular in profile, and their diameters being accurately known - to provide good astrometry in circumstances which would not apply to asteroids. d1 - If Well-located: uncertainties of (a), unchanged d2 - If Poorly-located: three times the uncertainties of (a) d3 - if Constrained: (i) If only 1 chord, or the spread of multiple chords [absolute value of (PlusHit-MinusHit)] is less than 5% of the assumed diameter: * if the displacement in the time-uncertainty-period is greater than the Along-Path uncertainty, the Along-Path and Across-Path uncertainties are set as that displacement multiplied by ScaleFactor_AlongPath, and ScaleFactor_AcrossPath, respectively. * otherwise the Along-Path and Across-Path uncertainties are the uncertainties from the fit, multiplied by ScaleFactor_AlongPath, and ScaleFactor_AcrossPath, respectively. (ii) if not (i), the Along-Path and Across-Path are the uncertainties from the fit, multiplied by ScaleFactor_AlongPath, and ScaleFactor_AcrossPath, respectively. d4 - if Not-constrained: (i) If only one chord, * the Along-Path uncertainty (but not the Across-Path uncertainty) is computed on the same basis as for 'd3 - if Constrained', (i), first dot point, but multiplied by ScaleFactor_AlongPath * the Across-Path uncertainty is: - if the chord is located more than 2 arc-seconds from the center of the object, the Across-Path uncertainty is computed on the same basis as for 'd3 - if Constrained', (i), first dot point - but multiplied by ScaleFactor_AlongPath - otherwise the Across-Path uncertainty is set as 25% of the object's assumed diameter. (ii) If more than one chord, * the Along-Path uncertainty is computed on the same basis as for 'd3 - if Constrained', (i),second dot point, but multiplied by ScaleFactor_AlongPath * if - the MeanLocation value is greater than 25% of its assumed radius, AND (PlusHit - MinusHit) >0.05, OR - the MeanLocation value is greater than 15% of its assumed radius, AND (PlusHit - MinusHit) >0.10 the Across-Path uncertainty is the uncertainty from the fit, multiplied by ScaleFactor_AcrossPath * otherwise the Across-Path uncertainty is set as 25% of the object's assumed diameter. e - if (b), (c) and (d) do not apply: for events where quality is better than 'Astrometry only', and either (i) the Major + Minor axes are included in the Least Squares solution, or (ii) the solution is for a circle. The uncertainties are separately considered in the Along-Path and Across-Path directions, and are set as the larger of the uncertainty from (a), and the uncertainty specified in the relevant one of e1 to e8: If the overall event quality is 'Limits on size, but no shape' e1 - Well-located: uncertainty must be at least 8% of assumed diameter e2 - Poorly-located: uncertainty must be at least 12% of assumed diameter e3 - Constrained: uncertainty must be at least 16% of assumed diameter e4 - Unconstrained: uncertainty must be at least 20% of assumed diameter If the overall event quality is 'Reliable size', or 'Resolution better than Shape models' e5 - Well-located: uncertainty must be at least 5% of assumed diameter e6 - Poorly-located: uncertainty must be at least 8% of assumed diameter e7 - Constrained: uncertainty must be at least 12% of assumed diameter e8 - Unconstrained: uncertainty must be at least 16% of assumed diameter f - if none of (b) to (e) apply: The Across-Path uncertainty set on following basis: f1 - Well-located: the uncertainty in the assumed diameter of the asteroid. f2 - Poorly-located: twice the uncertainty in the assumed diameter of the asteroid. f3 - Constrained: twice the uncertainty in the assumed diameter of the asteroid (same as f2). f4 - Unconstrained: 40% of the assumed diameter of the asteroid. For each of f1 to f4, the Along-path uncertainty set on following basis: For each positive chord: if chord length > 0.8 of assumed diameter - the Along-Path uncertainty for that chord is set at 5% of assumed diameter of the asteroid. else if chord length > 0.6 of assumed diameter - the Along-Path uncertainty for that chord is set at 10% of assumed diameter of the asteroid. Otherwise - the Along-Path uncertainty for that chord is set at 20% of assumed diameter of the asteroid. Along-Path uncertainty set as the mean of the individual Along-Path uncertainties, assessed in inverse quadrature (to give greater significance to smaller uncertainty values). ======================================================== Star identifiers and positions; gravitational deflection ======================================================== The star identifiers used in this dataset are (in order of priority) HIP (for Hipparcos2), Tycho2, UCAC4, USNO-B1 and NOMAD; 83 stars are identified by Jhhmmss.s-ddmmss, or Jhhmmss.ss-ddmmss.s. The identifiers are merely for identification purposes. The listed GCRS position of the star for the date of the event (used as the reference point for reporting astrometry) incorporates stellar parallax, proper motion and foreshortening. An occultation observation relates to the exact coincidence in the apparent position of the star and asteroid. Gravitational light deflection displaces the apparent position the star and asteroid differently; the deflection of the asteroid is always less than that for the star, in an amount dependent on the geocentric distance to the asteroid. As that distance depends upon any orbit solution derived using the observations, it is inappropriate to incorporate allowance for gravitational deflection outside of that orbit solution methodology. Hence the position of the star is not corrected for gravitational deflection by the Sun or any of the planets. The star positions for 99.8% of the events are from Gaia EDR3 (VizieR catalogue I/350). The following is a list of the 31 stars and associated events where the source of the star position is not Gaia EDR3. It can be assumed that there is a problem with the position of all of these stars, other than those which are too bright to be included in EDR3. The USNO Bright Star Catalog (UBSC, VizieR catalogue J/AJ/164/36) provides high accuracy recent epoch position for such stars. 9 stars from USNO Bright Star Catalog (2022). HIP 28380 = theta Aur : 2022 Apr 13 (2826) Ahti HIP 31681 = gamma Gem : 1991 Jan 13 (381) Myrrha HIP 37740 = kappa Gem : 1975 Jan 24 (433) Eros HIP 43103 = iota Cnc : 2007 Apr 18 (411) Xanthe HIP 49669 = alpha Leo : 1959 Jul 7 (P2M00) Venus, 2005 Oct 19 (166) Rhodope, 2015 May 24 (1669) Dagmar, 2016 Oct 13 (268) Adorea HIP 56647 = nu Leo : 1999 Mar 4 (748) Simeisa HIP 78821 = beta2 Sco : 1971 May 14 (P5M01) Io HIP 92855 = sigma Sgr : 1981 Nov 17 (P2M00) Venus HIP 112961 = lambda Aqr: 2014 Apr 16 (P2M00) Venus 21 stars from Hipparcos2 (VizieR catalogue I/311) HIP 13702: 1989 Feb 17 (P4M00) Mars HIP 20719: 1991 Dec 31 (50) Virginia HIP 28416: 1997 Jan 6 (363) Padua HIP 28558: 2012 Nov 24 (1309) Hyperborea HIP 34773: 2008 Feb 29 (2578) Saint-Exupery HIP 42917: 2018 Feb 15 (189) Phthia HIP 45688: 2023 Jan 12 (994) Otthild HIP 47258: 2022 Jun 7 (2476) Andersen HIP 47352: 1979 Dec 11 (9) Metis HIP 47706: 2000 Mar 25 (201) Penelope HIP 48618: 2003 Nov 5 (72) Feronia HIP 51222: 2012 Jan 2 (705) Erminia HIP 51380: 2002 Nov 28 (356) Liguria HIP 59807: 2007 Apr 20 (324) Bamberga HIP 67385: 1985 Apr 15 (275) Sapientia HIP 68025: 2015 Apr 15 (595) Polyxena HIP 68038: 2012 Apr 18 (252) Clementina HIP 71779: 2013 Mar 7 (329) Svea HIP 94925: 1991 Sep 11 (1564) Srbija HIP 110000: 2014 May 24 (33) Polyhymnia HIP 113058: 1997 Sep 19 (536) Merapi 2 stars from Gaia DR2 (VizieR catalogue I/345) UCAC4 363-138222: 2019 Mar 17 (201) Penelope UCAC4 521-018804: 2005 Sep 8 (814) Tauris 1 star from Gaia EDR3, with no proper motion available. The UCAC4 position (VizieR catalogue I/322A) at the UCAC4 epoch was used to derive and add proper motion values Tycho2 1299-00981-1: 2012 Oct 5 (232) Russia ============ Shape Models ============ The data set includes Shape model fits wherever possible. There can be up to 6 shape models for an event in this data set. Two sources of Shape Models have been used: * DAMIT [Database of Asteroid Models from Inversion Techniques] https://astro.troja.mff.cuni.cz/projects/asteroids3D/. [See also Durech et al. (2010), DAMIT: a database of asteroid models https://ui.adsabs.harvard.edu/abs/2010A%26A...513A..46D/abstract] * ISAM [Interactive Service for Asteroid Models] http://isam.astro.amu.edu.pl/ In 2023 the shape models in ISAM were renumbered, with model numbers less than 100 being duplicates of models in DAMIT. Models numbered greater than 100 are not present DAMIT. This data set only includes ISAM models with numbers greater than 100. A shape model quality setting is provided, with the following definitions: 0 = Not fitted. Too few chords to make a fit to the shape model. 1 = Bad occn data. The occultation observations are either inconsistent or unreliable, with no ready way of distinguishing between reliable and unreliable observations 2 = Model wrong. The shape model is clearly inconsistent with the observed chords 3 = Minimum Dia. Usually associated with a single chord observation, with that chord being matched to the largest dimension of the shape model to give its minimum diameter 4 = Diameter but no fit. The chords do not sensibly match the shape model, yet it is possible to derive minimum and maximum possible diameters. [NOTE: flag 1 - Model wrong - should be used unless there is a degree of confidence that the derived diameters are 'in the right ball-park'] 5 = Poor fit. The shape model is generally consistent with the observed chords - but has some broad deviations 6 = Good fit. The shape model has a high degree of correspondence with the observed chords. 7 = Not constrained. Is used when ever the chords are insufficient to place any reasonable constraints on the size of the asteroid. Typical situations are: chords with very large event uncertainties compared to the nominal diameter, chords which can be fitted to the shape model over a large range of diameters, and chords which are shorter than about 60% of the asteroid's nominal diameter, The Shape model fit does not redetermine the shape of the shape model. It is simply a matching of the shape model (with possible rotational variation) to obtain the best fit of the chords to the shape model. The diameter of the asteroid is obtained by deriving the length of the unitary scale of the shape model that gives the best visual fit to the occultation chords. ========================= Light curve availability ========================= Around 2400 asteroid occultation light curves from recent events are available at: https://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=B/occ The number of available light curves is expected to significantly grow with future events. ===================== Modification History ==================== The first version of this data set, introduced in 2003, included occultations only through to 1998. The update of 2004 not only added occultations through to March 1, 2004, but also provided a more systematic arrangement of the data. The data set has been updated annually since then. From 2019 to 2023 the dataset has been extensively reviewed and modernized to cater for more recent observational techniques, improve the error modeling, incorporate the results of shape model fitting, and to detect and correct data errors. The number of asteroid occultations included in each successive version is as follows: Year: Version: Number of occultations: 2003 V1.0 183 2004 V2.0 524 2005 V3.0 680 2006 V4.0 865 2007 V5.0 1055 2008 V6.0 1203 2009 V7.0 1417 2010 V8.0 1662 2011 V9.0 1935 2012 V10.0 2102 2013 V11.0 2275 2014 V12.0 2469 2015 V13.0 2717 2016 V14.0 2933 2017 PDS4 V1.0 3224 2018 PDS4 V2.0 3598 2019 PDS4 V3.0 4342 2024 PDS4 V4.0 9729 ----------------------- KNOWN ISSUES ----------------------- ================= Data set revision ================= The dataset underlying this archive has been revised from previous versions to identify and correct errors, review the reliability of observations, enhance quality assurance when adding new observations, develop a comprehensive error model for astrometric results, incorporate codes for modern observing techniques, include information about fitting to asteroid shape models, and change the description of a MISS event to 'No occultation detected'. The revision involved the individual review of each of the more than 8000 events. The revision updated a number of data fields, and added an extra data item. Two fields have transitional issues: * Signal-to-Noise. This is a new data element, with no data being available for most events before 2019 and a continuing non-availability for many new observations. The signal used in this measure is the depth of the occultation light drop compared to the noise in the full-light signal. The value of SNR is 0 [zero] whenever data is not available. It is generally not set when no occultation event is detected (a 'Miss' event). * Method of observation. Prior to the revision there were several categories relating to visual observations - reflective of typical observation techniques prior to 2000. Those have been replaced by a single 'Visual' category. In the revision the majority of old visual flags did not get translated into the new Visual category; it may validly be assumed that any observation where the observing method is 'Unspecified' was made visually; this is especially the case if there is a value for Personal Equation. ================ Site coordinates ================ The great majority of observations have been made in Europe, Japan, North America, Australia and New Zealand. The dataset includes a field for the geodetic datum. The field includes tags for WGS84, ED1950, NAD1927, Tokyo and GB1936 - but not AGD1966 nor NZ1949. The transition from national coordinates to WGS84 coordinates occurred over quite a few years. For many events - but especially the older events - this field is not set. Most site coordinates based on national geodetic datums occur before about the year 2000. After 2000 most coordinates are based on WGS84, or close equivalents. When a national geodetic datum has been used, the approximate maximum offsets from WGS84 are: * Europe (ED1950) 150m * Japan (Tokyo) 500m * North America (NAD1927) 150m * Great Britain (GB1936) 200m * Australia (AGD1966) 200m * New Zealand (NZ1949) 200m The effect of these offsets is two-fold: a. when fitting chords derived using different datums, relative chord displacements up to the above maximum datum offset might occur. However (apart from very rare trans-continental observations) this can only occur over a transitional period when coordinates from a region were being reported against differing datums. In practice (especially having regard to observational uncertainties at those times) any such 'error' in the chord placements is generally too small to be noticeable. Many such instances will have been identified in the review of the data set, with coordinates from 'Google Earth' being used where the observing site was clearly identifiable (eg an observatory). b. the derived astrometric position of the asteroid is potentially affected. For all but the Tokyo datum, reductions based on the WGS84 datum are fully adequate at the 0.1mas level; for the Tokyo datum 'errors' of up to about 0.3mas might arise. For this reason special effort has been made to ensure all site coordinates based on the Tokyo datum have that datum set in the dataset (all instances are before 2003). The dataset includes an event observed from the Kuiper Airborne Observatory [KAO](1978 May 29), an event observed from an F18 fighter jet (2000 Jan 10), and an event observed from the Stratospheric Observatory for Infrared Astronomy [SOFIA] (2015 June 29). For these separate entries exist for the D and R events, as the site coordinates differ as a result of the aircraft movement. The datum of the site coordinates for these observations is unknown (presumably NAD1927). The position of the KAO was uncertain by about 1km [THE DIAMETER OF PALLAS FROM ITS OCCULTATION OF SAO 85009 (1979) AJ 84 at pg.262]. For the SOFIA observation, the times and aircraft location were deduced from a conference presentation [citation not known]. An issue that has caused problems in the past is ensuring the sign of Longitude is correct. Current processing regimes are good at ensuring the sign is correct. The greatest risk of having the wrong sign occurs when the longitude is within about 10 degrees of the Greenwich meridian. While every effort has been made to validate past observations, it remains possible that a very small number of observations before about 2005 might have an undetected incorrect sign for an observer's longitude. An independent check of the validity of site coordinates has been undertaken by comparing the reported site altitude with the altitude of that site location from standard data sources. This identified a small but significant number of sites where the reported site coordinates were in error, sometimes by more than 1 degree. Where possible corrections were made - based on advice from the observer, or from secondary information in the observation record (in particular, the field OBS_NEARBY in the AsteroidTimes_2024Mar.psv and PlanetTimes_2024Mar.psv files. ============================ Observer names and locations ============================ The data set allows for the names of up to two observers, and a flag if there were more than two observers. There is a lack of overall consistency in observer names. The majority of names include a first name (or initial) together with a family name, whereas many old observations contain just the family name. In some cultures the family name is the first element of a name; such names are included with the family name last. Efforts have been made to identify and correct name errors, but undoubtedly errors remain. There an additional issue of whether a person has reported using their 'true' first name or a 'familiar' version [for example, Robert vs Bob, William vs Bill]. There is also the issue of cultural differences in how family names are constructed; generally, a multiple family name is limited to the principle family name component. For a number of events the name of the actual observer is not known - just the name of the observatory or observing group. The field for the observer location (OBS_NEARBY in the AsteroidTimes_2024Mar.psv and PlanetTimes_2024Mar.psv files) is frequently empty, as a result of variable data collection regimes over time and across the observing regions. Its prime purpose is to provide a low-level means of confirming the validity of the specified site coordinates. Similarly for the field OBS_COUNTRY, which gives a two- or three- letter country code (or potentially state codes - for Australia, Canada, and the USA). ================ Shape Model fits ================ Where a diameter is derived by fitting to a shape model, we have provided a 'volume-equivalent diameter'. That is, the diameter of a sphere having the same volume as the shape model. Also the fields SHAPE_SURFVOL_x in the files Asteroid_2024Mar.psv and AsteroidSummary_2024Mar.psv list, provides for each shape model the Surface-Diameter/Volume-Diameter ratio - the ratio of the Surface-equivalent diameter to the Volume-equivalent diameter of the shape model. The Surface Equivalent diameter is always greater than the volume-equivalent diameter. The fit to shape models is readily available in the AsteroidSummary_2024Mar.psv list. All fits have been made by way of a visual comparison of the occultation chords to the shape model. Shape models have been associated with all relevant events, irrespective of whether the occultation data was sufficient to derive a fit to the shape model. One category of shape model fit is 'Minimum diameter'. This typically involves an occultation with only one (or a small number of closely-spaced) chords - such that the correct location of the chord on the shape model is indeterminate. We have fitted the chord to the maximum extension of the asteroid to derive a minimum diameter of the asteroid consistent with that chord - provided the chord length is greater than about 60% of the expected diameter; otherwise the shape model is flagged as 'Unconstrained'. In the file Asteroid_2024Mar.psv, the derived diameters of the asteroid from fitting to shape models has been made solely on the basis of that event. The file AsteroidDiameters_2024Mar.psv provides a weighted mean diameter from all events where a diameter was determined from fitting to a shape model. This is provided for each shape model. Differences between observed chords and a shape model can be caused by any of: * inaccuracies in the occultation data * inadequacies in the shape model * the shape model being incorrect. This issue has two aspects: 1. When a shape model is determined there are generally two different solutions for the axis of rotation, with corresponding different shapes. Occultation observations potentially/frequently identify the 'correct' model; 2. It may be that all available shape models are incorrect. Where the observation was sufficient to center the shape model on the observed chords, the uncertainty in the astrometric position is set to 0.02 of the asteroid's diameter - to reflect the accuracy provided by the shape model, the uncertainty in the detail of the shape model, and the uncertainty in fitting the shape model to the observed chords. Ultimately occultations and shape models have an iterative relationship, with occultation chords providing a basis for improving shape models, and the subsequent fit of the occultation chords to improved shape models providing an improved diameter determination. =================== Asteroid satellites =================== The dataset includes a number of observations involving satellites of asteroids. In the reduction process the primary solution for such events is the Separation and Position Angle of the satellite from the primary body. Where possible and appropriate, astrometry for the asteroid is for the center of mass of the system. That location is identified on the basis of the relative volumes of the two bodies as derived using circular or elliptic fits to the occultation chords for the two bodies. There are several instances where the observed occultation was of the satellite only. In such cases the dataset includes a nominal entry (usually, but not always, based on a prediction) for the main body of the asteroid, with the Separation and Position Angle of the satellite being referenced to that nominal entry. Importantly: * no astrometry is reported for the 'artificial' main body location * the astrometric position of the satellite is derived using the measured separation and position angle of the satellite from the 'artificial' main body location - which when combined gives the location of the satellite relative to the star independent of the location of the main body. ==================== Accuracy/Reliability ==================== Early occultation observations were hampered by low prediction accuracies associated with the accuracy limitations inherent in star catalogues and asteroid ephemerides prior to the Hipparcos mission, compounded by the consequential effect of visual observers needing to monitor the relevant star for perhaps 10's of minutes (with associated problems of reliability). These issues have improved over time with: * Hipparcos {including Tycho2) - providing accurate positions for brighter stars, free of zonal errors * UCAC catalogues (2, 3 and 4) - providing accurate star positions on the Hipparcos reference frame, for stars down to magnitude 16 * Astrometry of asteroids being reduced against the Hipparcos reference frame (typically via UCAC), removing zonal error problems from asteroid astrometry * the move from visual to video observing techniques, which commenced in the late 1990's. * improving time reliability by way of video time insertion based on GPS time signals, which commenced in the early 2000's * Gaia astrometry, which essentially reduced prediction uncertainty to uncertainty in the asteroid's ephemeris This history is apparent in the dataset by way of the increase in the number of successfully observed events each year. To validate observations, the dataset generally includes a 'prediction' line with each event. This prediction line is not a basis for computation of O-C values; indeed, the location of the prediction line depends on the ephemeris used to generate that prediction - and this is variable throughout the dataset. Rather its purpose is merely to identify erroneous observations, through the level of (in)consistency with that prediction. In reviewing events prior 2008, prediction lines were generated using asteroid elements generated using the JPL Horizons system; comments in the records may refer to such prediction lines as a 'postdiction'. In later years prediction lines are generally derived from the actual prediction for that event. The confidence level in the results of each event is indicated by way of the 'Quality Code for fit'. The lowest level is: ' 0 = 'No reliable position or size' The quality of the observations is insufficient to allow a reliable fit to either the asteroid's diameter or position ' About 4% of the events in the dataset have this Quality code. Reasons include: * the observation is 'wrong' in the sense that it is greatly inconsistent with the prediction line. [Probable causes include long observing periods for visual observers, poor observing conditions, the difficulties with distinguishing between short events and atmospheric effects, and difficulties in detecting small magnitude changes (especially for visual observations).] * inconsistent observations for an event * single-chord events, where the observer only obtained a (reliable) time for one or other the D and R events. In the years prior to 2001 'false' observations ran between 10% and 50% of events, with the major causative factor being the combination of low prediction accuracy and visual observing for a long duration. Since 2008 the rate has been less than 4%, with the major causative factor being a partial observation (eg one event obscured by cloud) rather than an erroneous observation.