Data Set Overview
     =================
       The Giotto Radio Science Original Experiment Data Set was
       generated by ground equipment connected to the Canberra 64-m
       antenna of the NASA Deep Space Network (DSN) in Australia.  It
       includes two types of data -- an Archival Tracking Data File
       (ATDF) from closed loop receivers and Original Data Records
       (ODRs) from open loop receivers.  These should be considered
       'raw' data which much be processed before being applied to
       science problems.  The data in 'GIO-C-GRE-3-RDR-HALLEY-V1.0' have
       been partially processed and may be more suitable for immediate
       science application.
 
       ATDFs are files of radiometric data produced by the Network
       Operations Control Center (NOCC) Navigation Subsystem (NAV). They
       are derived from Intermediate Data Records by NAV and contain all
       radiometric measurements received from the DSN station including
       signal levels, antenna pointing angles, Doppler, range, and
       residuals.  Doppler data are often used to infer spacecraft
       radial motion relative to the tracking antenna -- in this case to
       determine the reduction in velocity when Giotto passed through
       the dust and gas surrounding Comet Halley.  Data values in ATDFs
       are reported at rates no higher than 10 per second.
 
       One ATDF is included in this data set.  It covers the time
       interval 1986-03-13T15:32:47 to 1986-03-14T05:10:00.
 
       ODRs are files of digitally sampled medium-bandwidth receiver
       output.  In conjunction with gain settings and receiver tuning
       information (contained in the ODR header) these files allow the
       investigator to derive the amplitude and frequency of the Giotto
       radio signal to highest accuracy with post-experiment processing.
 
       For the Giotto Radio Experiment an open-loop receiver bandwidth
       of 25 kHz was selected, and 8-bit sampling was performed on each
       of four channels.  The channel assignments were: (1) S-RCP, (2)
       S-LCP, (3) X-RCP, and (4) X-LCP where S-Band signals are at 13 cm
       wavelength, X-Band signals are at 3.6 cm wavelength, RCP denotes
       right circular polarization, and LCP denotes left circular
       polarization.  During the Giotto experiment only X-RCP was
       transmitted by the spacecraft, so channels (1) and (2) should
       contain only noise and channel (4) should contain, at most, a
       leakage signal.
 
       Two ODR files are included in this data set; each covers eight
       minutes.  Together they cover the Giotto Halley encounter. Their
       start times (UTC) are 1986-03-13T23:56:02 and 1986-03-14T00:04:02
 
       The measurements of frequency and frequency residual may be
       converted to changes in spacecraft velocity during the Halley
       encounter.  The changes in velocity may then be interpreted in
       terms of mass fluence (dust and gas) within Halley's coma
       [EDENHOFERETAL1986A], [EDENHOFERETAL1986B], [EDENHOFERETAL1986C],
       [EDENHOFERETAL1986D], [EDENHOFERETAL1987A].
 
       More detailed analyses of the dynamics of the spacecraft as
       inferred from the GRE data have been published by [BIRDETAL1988],
       [PAETZOLDETAL1988], [PAETZOLDETAL1990], [PAETZOLDETAL1991A],
       [PAETZOLDETAL1991B], and [PAETZOLDETAL1991C].
 
 
     Parameters
     ==========
       Each ATDF data record contains 117 parameters.  These 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, included as
       TRK_2_25.TXT in the DOCUMENT directory of the data set archival
       volume.  A complete list of parameters is given in Table 3 of
       that document.
 
       Open loop data records contain 8-bit samples of receiver output
       as described above.  Each block of 4000 data samples is
       accompanied an 80 byte header, a 300 byte trailer of Precision
       Power Monitor data, and a 10 byte trailer of operator-entered
       timing and frequency offsets.  The ODR is described by a Software
       Interface Specification, included as RSC_11_9.TXT in the DOCUMENT
       directory of the data set archival volume.  A complete list of
       header parameters is given in Figure RSC-11-9-1 in that document.
 
 
     Processing
     ==========
       DSN NAV performs editing on data during conversion from the IDR
       to ATDF formats; this function serves mostly to remove anomalous
       (out of nominal range) values.
 
       Open loop data are not edited or otherwise processed.

[EDENHOFERETAL1987B] reported that
       'The maximum frequency offset in the one-way data was 16.9 Hz.
       But the two-way data acquired before and after the encounter are
       consistent with a maximum of only 4.64 Hz.'
 
       Two explanations have been offered [EDENHOFERETAL1987B]. First is
       that the drag forces amplified the oscillator drift through
       acoustic, electrical, or thermal energy deposition.  In this
       case, the measurements differ from the actual values by only a
       scale factor.  The analysis presented by [EDENHOFERETAL1987B]
       adopts this assumption.
 
       The second explanation is that transients during the encounter
       (possibly associated with failure of the NMS experiment and
       switching of the TWT amplifier tubes) also induced a frequency
       offset in the on-board oscillator.  In this case, both an offset
       and a scale factor may be needed to correct the data.
 
       No applicable pre-launch tests were conducted on the radio system
       to allow choosing between these assumptions.
 
       There is no other information on quality of the data.  It is
       believe that operations during data acquisition were nominal and
       that these two data types represent the highest quality raw data
       that could be obtained under the circumstances.