From: NCF::DWILLIAMS "Dave Williams, NSSDC - (301) 441-4197" 24-OCT-1994 16:15:45.68 To: HILLS CC: Subj: PVO DSC -02D B44307-000A 78-051A-02D _______________________________________________________________________ M.I.T. Center for Space Research MIT-PV-A&R V.2 70 Vassar Street, Cambridge, MA 02139 December 1, 1988 From: Peter G. Ford, MIT M.I.T. Room 37-601, tel: (617) 253-6485 Subject: Pioneer Venus Radar Altimeter/Radiometer Data Set Format _________________________________________________________________________ This memo describes the format and contents of the altimeter/radiometer database generated from the Pioneer Venus radar mapper experiment. The database consists of a pair of files. On mag- netic tape, they are written using Level 3 ANSI standards, each user file preceded and followed by an ANSI label file. The first user file contains an ASCII version of this memorandum. On magnetic tape, it is written as 80-byte logical records, blocked into a series of 800-byte physical blocks. There are no separators between logical records, and each physical block therefore contains 10 lines of text. The file name recorded in the ANSI label is PVORAD.DOC. The second user file contains the altimeter/radiometer data itself, in the form of 144132 logical records, each of 160 ASCII bytes. Its ANSI file name is PVORAD.DATA. It begins with three header records, con- taining the following "self-defining" information: [1] The number of data fields, followed by 4-byte-long field names. The Fortran format is (I3,n(1X,A4)). These names are listed in the second column of table 1. The first 4 fields, the Pioneer Project- supplied S/C Date, UT, Orbit, and Time-from-Periapsis fields are not included either in the field count or names in the first header record. They are included in the format specification and undefined definitions in the second and third header records. [2] A Fortran "FORMAT" specification that may be used to read the remaining records. The individual format codes are listed in the third column of table 1. [3] A record containing "undefined" values for all record variables. It may be read according to the format string in record 2. If a data field in a subsequent data record has the same value as the corresponding value in this record, that data item is undefined, and should be ignored. The undefined values are listed in the fourth column of tables 1. [4] The remaining 144129 records contain the ORAD radar data itself. Appendix C contains an example of a Fortran 77 program that can list the data in this file. As supplied to NSSDC on 1/2'' magnetic tape, the logical records are packed 200 per 32000 byte physical tape blocks, with no embedded end- of-record indicators such as carriage-return or line-feed characters. The tape is recorded on 9 tracks with odd parity at 1600 bytes per inch. Each file ends with a single tape mark, and an extra tape mark is writ- ten after the last EOF2 label. The following examples assume that the program listed in Appendix C has been compiled into an executable named listorad. On IBM's OS/370, the data file would be read as follows: // EXEC PGM=LISTORAD //FT05F001 DD UNIT=TAPE,VOL=(2,SL),DISP=(OLD,KEEP),DCB=OPTCD=Q and on IBM's VM/CMS as FILEDEF 5 TAP1 SL 2 ( OPTCD Q LISTORAD A UNIX system might read it with the following commands: mt -f /dev/rmt0 fsf 4 dd if=/dev/rmt0 ibs=32000 cbs=160 conv=unblock | listorad On VMS, it would be read as follows: ASSIGN FOR005 MSA0:PVORAD.DATA RUN LISTORAD +----------------------------------------------------------------------+ | Table 1: Format of Altimeter/Radiometer Data Record | +---+-------+------+-----------+------+--------------------------------+ | | Field | F77 | Undefined | Data | Field | | | Name | Fmt | Value | Unit | Description | +---+-------+------+-----------+------+--------------------------------+ | 1 | Date | I8 | 0 | | Year and day of observation | +---+-------+------+-----------+------+--------------------------------+ | 2 | Time | I9 | 0 | msec | UT from midnight | +---+-------+------+-----------+------+--------------------------------+ | 3 | Orbit | I5 | 0 | | Orbit number | +---+-------+------+-----------+------+--------------------------------+ | 4 | Roll | I6 | 0 | sec | Time after periapsis (UT) | +---+-------+------+-----------+------+--------------------------------+ | 5 | RDAT | I8 | 99999999 | | Year and day of measurement | +---+-------+------+-----------+------+--------------------------------+ | 6 | RAUT | I9 | 999999999 | msec | UT of radar measurement | +---+-------+------+-----------+------+--------------------------------+ | 7 | BLAT | F7.3 | 999.999 | degN | Radiometer footprint latitude | +---+-------+------+-----------+------+--------------------------------+ | 8 | BLON | F7.3 | 999.999 | degE | Radiometer footprint longitude | +---+-------+------+-----------+------+--------------------------------+ | 9 | PCAL | F6.1 | 9999.9 | mV | Radiometer voltage reading | +---+-------+------+-----------+------+--------------------------------+ |10 | SCAL | F6.1 | 9999.9 | mV | Radiometer background reading | +---+-------+------+-----------+------+--------------------------------+ |11 | RBRT | F6.1 | 9999.9 | K | Planet Brightness temperature | +---+-------+------+-----------+------+--------------------------------+ |12 | RLAT | F7.3 | 999.999 | degN | Altimeter footprint latitude | +---+-------+------+-----------+------+--------------------------------+ |13 | RLON | F7.3 | 999.999 | degE | Altimeter footprint longitude | +---+-------+------+-----------+------+--------------------------------+ |14 | XLIM | F5.0 | 9999. | km | Cross-track alt footprint size | +---+-------+------+-----------+------+--------------------------------+ |15 | YLIM | F5.0 | 9999. | km | Along-track alt footprint size | +---+-------+------+-----------+------+--------------------------------+ |16 | RRAD | F8.3 | 9999.999 | km | Measured planetary radius | +---+-------+------+-----------+------+--------------------------------+ |17 | DRAD | F7.3 | 999.999 | km | Formal error in DRAD | +---+-------+------+-----------+------+--------------------------------+ |18 | SLOP | F7.3 | 999.999 | deg | RMS slope at meter scale | +---+-------+------+-----------+------+--------------------------------+ |19 | DSLO | F7.3 | 999.999 | deg | Formal error in SLOP | +---+-------+------+-----------+------+--------------------------------+ |20 | RRHO | F5.2 | 99.99 | | Fresnel reflectivity | +---+-------+------+-----------+------+--------------------------------+ |21 | DRHO | F5.2 | 99.99 | | Formal error in RRHO | +---+-------+------+-----------+------+--------------------------------+ |22 | RCOR | F5.2 | 99.99 | | Correction to RRHO | +---+-------+------+-----------+------+--------------------------------+ |23 | RASL | F5.2 | 99.99 | | Formal RRAD - SLOP correlation | +---+-------+------+-----------+------+--------------------------------+ |24 | RARH | F5.2 | 99.99 | | Formal RRAD - RRHO correlation | +---+-------+------+-----------+------+--------------------------------+ |25 | SLRH | F5.2 | 99.99 | | Formal SLOP - RRHO correlation | +---+-------+------+-----------+------+--------------------------------+ DESCRIPTION OF DATA FIELDS Date: The year and day-of-year of the observation, as supplied by the Pioneer project. The year occupies the first 5 bytes, the day the remaining 3. This field may be used to correlate this radar observation record with data from other Pioneer Venus instru- ments, but the RDAT field should be used for radar mapping pur- poses. Time: The time of the observation, in milliseconds from midnight UT, as supplied by the Pioneer project. As with the Date field, this should only be used for comparison purposes. The accurate radar observation time is contained in the RAUT field, described below. Orbit: The Pioneer Venus orbit number. The spacecraft maintained a nearly 24-hour orbit. Radar data was taken from orbit 3 on De- cember 7th, 1978 through orbit 834 on March 19th, 1981. Roll: The time from periapsis, in seconds, supplied by the Pioneer project. Negative values represent pre-periapsis measurements. These time fields are specified in precisely 12 second intervals from the periapsis time derived from S/C doppler tracking. The 12-second interval was chosen to closely approximate the S/C spin period. Radar data was taken once per S/C rotation, and therefore not at exactly 12-second intervals. In addition, the radar altimeter was able to refine the measurement of UT of periapsis. Each ra- dar observation was therefore assigned to a particular 12-second interval by counting S/C revolutions before or after the true periapsis, assigning the last radar measurement before periapsis to the Roll field with value 0. RDAT: The year and day-of-year of the observation. The year occupies the first 5 bytes, the day the remaining 3. RAUT: The UT of the first main bang of the radar altimeter, i.e. the time that the first altimeter pulse left the transmitter, meas- ured in milliseconds from midnight UT. BLAT: The latitude[2] of the center of the antenna beam projected on the planet during the radiometric mode. This is the latitude of the radiometric brightness measurement reported in the PCAL, SCAL, and RBRT fields. All latitudes and longitudes are expr- essed in degrees. Latitudes are positive in the northern hemi- sphere, negative in the southern. Longitudes are always posi- tive, in the range 0 through 360. Longitudes increase eastward of the prime meridian, and the (retrograde) planetary rotation is from east to west. The body-fixed 1985 IAU coordinate system (VBF85) is used. See appendix B and the bibliography for more details. BLON: The longitude of the radiometric brightness measurement reported in the PCAL, SCAL, and RBRT fields. PCAL: The voltage reading from the radar receiver during the ra- diometry period, when the transmitter was turned off and the an- tenna pointed within about 5 degrees of the plane defined by the spacecraft spin-axis and the nadir. SCAL: The voltage reading from the radar receiver during the radiometry-background period, when the transmitter was turned off and the antenna pointed within about 5 degrees of the plane defined by the spacecraft spin-axis and the zenith. RBRT: The microwave brightness temperature of the planet, in degrees Kelvin, derived from PCAL and SCAL. The data set was divided into 1o latitude intervals. For each interval, the average SCAL reading was taken to represent a measurement of the 3K cold sky. (A number of measurements were made in which the Sun appeared in the antenna beam during the SCAL readings, and these were therefore discarded.) The average PCAL values from lowland regions were interpreted as measuring an average temperature of 735K with an emissivity of 88%. RBRT is therefore obtained from and by the linear relationship 735 * 0.88 * (PCAL - ) + 3 * ( - PCAL) RBRT = -------------------------------------------------- - RLAT: The latitude of the average radar altimeter footprint for this observation. Because of the delay and doppler filtering applied by the on-board data processing electronics, the footprint is not necessarily centered about the sub-orbital point (S/C na- dir), nor is it symmetric with respect to the antenna axis. In addition, some observations are, in fact, averaged over up to 4 measurements taken at varying doppler frequency offsets, corresponding to 4 separated footprints spaced along the S/C ground track. RLON: The longitude of the average radar altimeter footprint. XLIM: The average cross-track dimension of the altimeter footprint, in km. This is determined by the delay-resolution of the radar re- ceiver. YLIM: The average along-track dimension of the altimeter footprint, in km. Above about 500 km S/C altitude, this is also determined by the delay-resolution of the radar receiver. Below this alti- tude, it is determined by the frequency resolution. The high altitude footprints are therefore circular, the low altitude ones are approximately elliptical, with YLIM <= XLIM. RRAD: The planetary radius in kilometers, measured by the radar altim- eter. A correction has been made for atmospheric delay (see Kliore et al., 1985). DRAD: The formal error in RRAD, in km, derived from the statistics of the time-sampled altimetry echo-power profile. All other fields that are described here as averages are weighted by this formal error when averaging over multi-doppler-offset measurements. SLOP: The r.m.s. average surface slope at meter scale, in degrees, from the altimeter measurement, derived by analyzing the time- sampled profile according to Hagfors' Law of near-normal- incidence scattering from a quasi-specular surface, where the specific radar cross-section, sigma0(t) varies with the scatter- ing angle "t" according to C 4 2 -3/2 sigma0(t) = R * - * [ cos (t) + C * sin (t) ] 2 where R is the Fresnel reflectivity (the RRHO variable, see below), and the Hagfors constant C is 1/sqrt(SLOP). DSLO: The formal error in SLOP, in degrees, derived during the process of fitting the returned time-sampled radar echo to a set of Hag- fors Law templates. RRHO: The Fresnel reflectivity of the surface. Represented as "R" in the Hagfors equation, above. This is related to the (complex) surface bulk dielectric constant "e" via the expression | sqrt(e) - 1 |2 Rf = |-------------| | sqrt(e) + 1 | where the difference between Rf and R is explained in the RCOR entry, below. DRHO: The formal error in RRHO, derived during the process of fitting the returned time-sampled radar echo to a set of theoretical templates derived from Hagfors Law. RCOR: A correction to RRHO to account for that fraction of the in- cident radar beam that is reflected by sub-wavelength-sized scatterers. This component was estimated, where possible, from the ORAD side-looking radar imaging mode, and modeled by the phenomenological formula: 3/2 sigmaD(t) = g * a * Rf * cos (t) where g = 2.69 is a fitted geometrical factor, and R = (1-a) Rf. "a" is the fraction of the surface covered by diffusely-scatter- ing material, and Rf represents the "true" Fresnel reflectivity. RCOR is equal to Rf - R, i.e. it must be added to RRHO to obtain the true Fresnel reflectivity. This correction is only avail- able for that portion of the altimetric dataset this is also covered by side-looking imaging data, i.e. from about 15'S to 45'N latitude. RASL: The formal correlation coefficient between RRAD and SLOP, derived while fitting the observed echo-power profile to Hagfors Law derived templates. RARH: The formal correlation coefficient between RRAD and RRHO. SLRH: The formal correlation coefficient between SLOP and RRHO. ________________________________________________________________________ A. REFERENCES o T. Hagfors, Radio Sci., 5, 189 (1970). o G.H. Pettengill, D.F. Horwood, C.H. Keller, "Pioneer Venus Orbiter Radar Mapper: Design and Operation", IEEE Trans. Geosci. Remote Sens- ing, GE-18, No. 1, January 1980. o G.H. Pettengill, E. Eliason, P.G. Ford, G.B. Loriot, H. Masursky, G.E. McGill, "Pioneer Venus Radar Results: Altimetry and Surface Pro- perties", J. Geophys. Res., 85, 8261 (1980). o H. Masursky, E. Eliason, P.G. Ford, G.E. McGill, G.H. Pettengill, G.G. Schaber, G. Schubert, "Pioneer Venus Radar Results: Geology from Images and Altimetry", J. Geophys. Res., 85, A13, 8232 (1980). o G.H. Pettengill, P.G. Ford, S. Nozette, "Venus: Global Surface Radar Reflectivity", Science, 217, 640 (1982). o P.G. Ford, G.H. Pettengill, "Venus: Global Surface Radio Emissivity", Science, 220, 1379 (1983). o A.J. Kliore, V.I. Moroz, G.M. Kesting, "The Venus International Reference Atmosphere", Advances in Space Research, 5, 11 (1985), Per- gamon Press. COSPAR Report JPL-D-2216. o G.H. Pettengill, P.G. Ford, B.D. Chapman, "Venus: Surface Electromag- netic Properties", J. Geophys. Res., 93, B12, 14881 (1988). ________________________________________________________________________ B. VENUS COORDINATE SYSTEMS The latitudes and longitudes in this data set are expressed in the Venus body fixed system adopted by the IAU in 1985 and used by the Magellan Project. It is related to other coordinate systems via a series of time-dependent rotation matrices. These coordinate systems are as fol- lows: PVO80 Venus body fixed (Pioneer Venus) VME50 Venus equator of 1950 EMO50 Earth ecliptic of 1950 EME50 Earth equator of 1950 EME00 Earth equator of J2000 EMO00 Earth ecliptic of J2000 VME00 Venus equator of J2000 (JPL/IAU) VBF85 Venus body fixed (Magellan) (IAU of 1985) [1] The Pioneer Venus Coordinate System (PVO80) Latitude of Venus pole . . . . . . . . . 88.50737 deg Longitude of Venus pole . . . . . . . . . 31.48165 deg Rotation period . . . . . . . . . . . . .243.0 days Latitude of Venus ascending node. . . . . -1.39873 deg Longitude of Venus ascending node . . . . 51.91788 deg Longitude of prime meridian in 1964.0 . .164.6089 deg The angles and ascending node are defined in the EMO50 coordinate sys- tem. The following transformation matrices operate on right-handed cartesian 3-vectors. [2] PVO80 -> VME50 Rotation | cos(d) -sin(d) 0 | V(VME50) = E * V(PVO80) = | sin(d) cos(d) 0 | * V(PVO80) | 0 0 1 | d = 164.6089 - (JD-1950)*360/243.0 [3] VME50 -> EMO50 Rotation V(EMO50) = F * V(VME50) | 0.616606488128 -0.786958046198 0.0222142369303 | F = | 0.78689300063 0.616939511419 0.0136031176373 | | -0.0244099233564 0.00909245696085 0.999660683866 | [4] EMO50 -> EME50 Rotation V(EME50) = B**-1 * V(EMO50) | 1.0 0.0 0.0 | B**-1 = | 0.0 0.9174369451139180 -0.3978812030494049 | | 0.0 0.3978812030494049 0.9174369451139180 | [5] EME50 -> EME00 Rotation V(EME00) = A * V(EME50) | 0.9999256794956877 -0.0111814832204662 -0.0048590038153592 | A = | 0.0111814832391717 0.9999374848933135 -0.0000271625947142 | | 0.0048590037723143 -0.0000271702937440 0.9999881946023742 | [6] EME00 -> VME00 Rotation V(VME00) = C**-1 * V(EME00) | 0.99889808 0.04693211 0.0 | C**-1 = | -0.04325546 0.92064453 0.38799822 | | 0.01820958 -0.38757068 0.92166012 | [7] VME00 -> VBF85 Rotation | cos(w) sin(w) 0 | V(VBF85) = D**-1 * V(VME00) = | -sin(w) cos(w) 0 | * V(VME00) | 0 0 1 | w = 160.39-1.4813291*(JD-2000) [8] Resulting PVO80 -> VBF85 Rotation Cartesian 3-vectors are transformed by the product of the individual rotation matrices, i.e. D**-1(w) * C**-1 * A * B**-1 * F * E(d) which depends on time through (w) and (d). At the epoch of 1980.0, i.e. approximately in the middle of the Pioneer Venus data taking period, | 0.999990805 0.001520115 -0.004009573 | V(VBF85) = | -0.001530001 0.999995801 -0.002462105 | * V(PVO80) | 0.004005809 0.002468222 0.999988929 | ________________________________________________________________________ C. FORTRAN 77 PROGRAM TO READ ALTIMETER/RADIOMETER DATA FILE= c LENGTH = maximum record length in bytes c MAXVAR = maximum number of variables per record integer LENGTH, MAXVAR parameter ( LENGTH=160, MAXVAR=50 ) real*4 rbuf(MAXVAR), dbuf(MAXVAR) integer*4 ibuf(MAXVAR), idbuf(MAXVAR) character*4 name(MAXVAR) character*1 frmt(LENGTH) data name(1) / "Date" / data name(2) / "ScUT" / data name(3) / "Norb" / data name(4) / "Roll" / data nint / 0 / c read field names read (5,"(I3,100(1X,A4))",iostat=ios,end=40,err=40) x nitem, (name(i+4),i=1,nitem) c read field format read (5,"(1000A1)",iostat=ios,end=40,err=40) x (frmt(i),i=1,LENGTH) c Compute nint = number of integer variables in each record. c It is assumed that all integers precede all floats. do 10 i=1,LENGTH 10 if (frmt(i) .eq. "i" .or. frmt(i) .eq. "I") x nint = nint + 1 c read undefined field values read (5,frmt,iostat=ios,end=40,err=40) x (idbuf(i),i=1,nint), (dbuf(i),i=nint+1,nitem+4) do 20 nrec = 1, 100000 c read a data record read (5,frmt,iostat=ios,end=30,err=40) x (ibuf(i),i=1,nint), (rbuf(i),i=nint+1,nitem+4) if (nrec .gt. 1) write (6,"(1x)") c transform seconds +- periapsis into roll number ibuf(4) = ibuf(4)/12 c display field values do 20 n = 1,nitem+4 if ((n .gt. 4 .and. n .le. nint x .and. ibuf(n) .eq. idbuf(n)) x .or. (n .gt. nint .and. rbuf(n) .eq. dbuf(n))) x then write (6,"(1X,I6,'.',I2,'.. ',A4,' = ?')") x nrec, n, name(n) else if (n .le. nint) then write (6,"(1X,I6,'.',I2,'.. ',A4,' = ',I9)") x nrec, n, name(n), ibuf(n) else write (6,"(1X,I6,'.',I2,'.. ',A4,' = ',F12.5)") x nrec, n, name(n), rbuf(n) end if 20 continue 30 stop c error return 40 write (0,"('read error ',i3)") ios stop 1 end ________________________________________________________________________