[Provided by D. Williams, Oct. 24,1994.] [References restored May 7, 1996 by C. Ng] Entered into NSSDC Technical Reference File (TRF) as B44306-000A. Data Sets: 78-051A-01A, 78-051A-02B, 78-051A-07A, 78-051A-11A, 78-051A-12A, 78-051A-13A, 78-051A-15B, 78-051A-17A, 78-051A-18C Pioneer Venus: Radar Measurement (UADS-LFD File) ----------------------------------------------------------------- Pioneer Venus Orbiter Radar Mapper Documentation of UADS/NSSDC Data Entries Peter G Ford Massachusetts Institute of Technology 1. Introduction The Radar Mapper experiment (ORAD) consists of a 14 inch directional S- band antenna which rotates in a plane perpendicular to the spin axis of the spacecraft, an 18 watt transmitter, and a receiver and associated control circuits. A detailed description of the instrument appears elsewhere.[1] The sensitivity of the system limits data taking to altitudes below 4700 km, corresponding to a period of about 25 minutes either side of periapsis. Within this interval, the instrument is pre-programmed to switch between several operating modes determined by the altitude of the spacecraft above the surface. Before each periapsis passage, uplink commands are transmitted to the instrument to allow it to estimate this altitude at all times during data taking. The instrument operates in one of three distinct modes: Altitude Mode 4700km - 700km Altimetry (single frequency) Below 700km Altimetry (4 frequency) and single-sided Imaging Below 700km Altimetry (4 frequency) and double-sided Imaging Altimetric data is taken in all modes during that portion of each 12 second spacecraft roll in which the ORAD antenna points to the nadir. A series of radar pulses is transmitted, each modulated by phase inversion according to a pseudo-random code. The receiver detects the reflected pulses and de-convolves the modulation using the same code starting at 64 equally spaced time intervals. The 64 receiver outputs ('range cells') define a range window in which the peakof the radar echo may be located. The width of the time 'window' varies with altitude: above 1515 km, it is 384 microseconds wide, and below that altitude it is 256 microseconds wide. When operating in the double-sided imaging mode, the contents of the first 55 range cells are reported to the downlink telemetry system. In the other modes, the instrument detects the range cell containing the peak return value and reports the contents of 17 cells beginning 7 cells before the peak. Below 700 km, 4 altimetric measurements are taken during each spacecraft roll. Uplink commands select whether these are to be made with the receiver tuned to the same frequency throughout, or to be offset by +3260, +9320, -9320 and -3260 Hz for the 4 measurements. Without the offsets, the four surface footprints almost lie on top of each other. When the offsets are enabled, the footprints shift away from the nadir and lie in a line along the sub-orbital track. Unfortunately, the signal-to-noise ratio is severely degraded at the 9320 Hz offsets and, for this reason, all data since orbit 295 have been taken with the offset feature disabled. Single- or double-sided Imaging data are taken during those portions of each roll in which the radar antenna views the planet at an angle of between 30 and 60 degrees from the nadir. Unmodulated pulses are transmitted and the receiver performs a fourier transform in frequency at 8 delays to synthesize an 8 x 8 cell map of the surface to one or both side(s) of the spacecraft. Design and sensitivity considerations limit useful imaging data to altitudes below 550 km. Because of the quantity and restricted interest in imaging data, there is currently no plan to include it in UADS or NSSDC entries. It may be obtained upon application to the principal investigator, Professor Gordon Pettengill, at M.I.T. 2. Low Frequency Data Because the Orbiter Radar Mapper data are related to a non-rotating (crust-fixed) coordinate system, they have been entered into UADS in a somewhat different manner to those of other orbiter experiments. The Low Frequency file (LFD) contains five ORAD data variables per 12 second UADS epoch. Rather than interpolating each variable to what it might have been if measured at that time, the data is entered roll-by- roll. The user should not assume that the data was taken within the roll defined by the LFD variable SCUT, although this is often the case when close to periapsis. The choice of which 12 second LFD record receives which ORAD roll data is determined by periapsis time, but this time is NOT that reported in the LFD header. ORAD analysis is very sen- sitive to periapsis time so that it is a corrected value that is used when assigning ORAD roll 0 (the roll with RIP UT immediately prior to periapsis) to the central LFD record. The five ORAD LFD variables are average values over the surface foot- print, or, at altitudes below 700 km, averages over 4 footprints. The size of a footprint varies with altitude; the following table gives some representative dimensions: Spacecraft Altitude Footprint Size (km) (km, across x along track) 200 23 x 7 300 28 x 11 500 36 x 18 1000 49 x 39 2000 80 x 61 4000 101 x 101 3. Data Analysis As described above, each altimetry measurement yields either 17 or 55 time-sampled values of radar echo power. These are read from the ORAD EDR files and are compared with a series of sample 'templates' con- structed from a model[2] of radar scattering at small angles of incidence. In this model, the radar cross section per unit surface area at angles of incidence and observation is given by 4 2 -3/2 ( RHO C/2 ) ( COS T + C SIN T ) where RHO is the Fresnel reflection coefficient for normal incidence. The parameter C is inversely related to the steepness of the surface undulations; for C larger than about 10, the average slope (in radians) over a scale length of the order of the radar wavelength (17 cm) is approximately given by SQRT(1/C). This subject is treated in greater detail in a recent paper by the Pioneer Venus Radar Group.[3] The theoretical templates have been computed for many values of space- craft altitude, receiver offset frequency, and C parameter, and they also model the effects of the on-board deconvolution of the pseudo- random noise code. Partial derivatives of each template with respect to delay, offset frequency, and C are also available, so that statistical errors may be estimated during the fitting procedure. Each echo profile yields a value of RHO, C, and location within the altimeter time window. Using the spacecraft ephemeris obtained from ground tracking, and a knowledge of the precise time of the radar measurement, a value is derived for the mean planetary radius of the footprint. 4. LFD Variables All ORAD LFD variables are floating point quantities. Data taken below 700 km are averaged over the 4 footprints, weighting each value by its estimated statistical error. If individual values are suspect, they are dropped from the average. In particular, ALL data taken at frequency offsets of +9320 and -9320 Hz have been deleted because of their low signal-to-noise. RLAT the Venus crust-fixed latitude of the center of the radar footprint(s) expressed in degrees. The coordinate system is that adopted by the IAU (and is also used in the SEDR ephemeris files). RLON the Venus crust-fixed longitude of the footprint(s) expressed in degrees (0 to +360, increasing eastwards counter to the direction of planetary rotation). RRAD the average radius of the footprint(s) in kilometers relative to a mean planetary radius of 6051.2 km. Positive RRAD values denote elevated regions, and vice versa. SLOP the root mean square meter-scale surface slope, in degrees, of the footprint(s). It is derived from the fitted value of the C parame- ter by SLOP = 360 * SQRT ( 1 / C ) / PI RRHO the average value of the Fresnel reflection coefficient for normal incidence of the footprint(s). This is a dimensionless quantity (less than unity) related to the surface dielectric constant e by RHO = SQRT | ( 1 - e ) / ( 1 + e ) | 5. Composite Data Files To avoid the necessity of extracting all LFD data variables when plot- ting ORAD data, a single composite data file has been submitted to UADS. The file is designed to be handled as a single entity. It can be read directly into core memory on many large virtual storage computer systems (with 5 megabytes or more of addressable memory) and contains chain pointers to aid in on-line graphical representation of all or part of the total ORAD database. The file consists of 80 byte logical records and is arranged in eight sections; the first five sections are of fixed length and each fits exactly into an integral number of logical records. The remaining three sections are of varying length. Each begins at the start of a logical record, and the last record of each section is padded on the right with hexadecimal zeroes. The sections have the following contents: Periapsis 1000 double precision floating point fields containing the periapsis times of orbits 1-1000 expressed in units of milliseconds counting from midnight Dec 30/31st of the previous year. If no ORAD data is available for the orbit, the field will contain a zero value. The first of these fields does not contain a time value. No ORAD data was taken on orbit 1. Instead, the first 4 bytes of the field (i.e. of the file) contain the number of altimeter data values con- tained in the varying length sections of the file, expressed as a 32 bit binary integer. Semi-Major 1000 double-precision floating point fields containing the semi- major axes of the osculating orbital ellipse at periapsis for orbits 1-1000, expressed in kilometers. Eccentricities 1000 double-precision floating point fields containing the eccen- tricity of the osculating orbital ellipse at periapsis for orbits 1-1000. Data-Source 1000 16-bit binary integers indicating the source of ORAD data for orbits 1-1000. The codes are as follows: 0: no data available 1: quick-look data 2: first processing of EDR/SEDR tapes 3: re-processing of EDR/SEDR tapes Data-Editing 1000 16-bit binary integers indicating the number of times that ORAD data for orbits 1-1000 has been processed by the interactive CYTHP editing system. Altimetry seven arrays of 32-bit fields. The dimension of each array is given by the integer field located in the first 4 bytes of the first record of the file. There is no padding between the arrays. The first six arrays contain floating point variables: 1: Crust-fixed latitudes (RLAT) 2: Crust-fixed longitudes (RLON) 3: Planetary radius relative to 6051.2 km (RRAD) 4: Hagfors scattering law parameter (C) 5: Fresnel reflection coefficient (RRHO) 6: Spacecraft radial velocity (km/sec) and the seventh array contains a pair of binary integers in each 32 bit field: Bits 0-19: Orbit number Bits 20-31: Roll number relative to periapsis + 128 Latitude an array of 32 bit integers that index the data arrays in the sixth section of the file, used to reference the data in that section by increasing value of RLAT. Thus if, in a Fortran program, the data arrays are read into variables RLAT(N), RLON(N), etc, and the lati- tude pointers into ILAT(N), the sequence RLAT(ILAT(I)) will inrcease monotonically with increasing I value. Longitude an array of 32 bit integers that index the data arrays in the sixth section of the file, used to reference the data in that section by increasing value of RLON. Thus if, in a Fortran program, the data arrays are read into variables RLAT(N), RLON(N), etc, and the long- itude pointers into ILON(N), the sequence RLON(ILON(I)) will inrcease monotonically with increasing I value. 6. Error Analysis An analysis of purely statistical errors shows that values of SLOP and RRHO obtained from the profile fitting procedure are usually highly correlated, while RRAD is only slightly correlated with either. At an altitude of 200 km, the statistical error in RRAD is of the order of 30 meters, while the errors in SLOP and RRHO approach 10%. At 4000 km altitude, the error in RRAD varies between 100 and 300 meters (it varies with SLOP), while errors in SLOP and RRHO are on the order of 30%. These formal errors are very much smaller than the following known sys- tematic errors: 6.1. Estimation of on-board clock time pulses This chiefly affects RRAD according to ERR(RRAD) = RADVEL * ERR(RIP) where ERR(RIP) is an inaccuracy in the measurement of the Roll Index Pulse time, and RADVEL is the current spacecraft radial velo- city with respect to Venus. RRAD has been corrected for gross RIP timing errors but a residual error of several milliseconds remains leading to errors in RRAD of the order of 100 meters. Gross errors in RIP time also affect RRHO because they lead to a miscalculation of the direction in which the radar antenna is pointing. Although only weakly directional (3 db loss at 15 degrees off axis), a large error in RIP determination affects the computation of this loss and therefore affects the analysis of the total echo power. 6.2. Ephemeris errors These have been estimated by comparing RRAD values of overlaping footprints measured during separate orbits. Of greatest concern is the uncertainty in time of periapsis passage as this affects all RRAD values of a given orbit in the same manner as RIP timing. This error decreases with altitude, vanishing at periapsis. Errors in other orbital elements have less effect on RRAD. We estimate the residual effect of all ephemeris errors on RRAD as 50 meters at periapsis, increasing to 150 meters at 4700 km spacecraft altitude. 6.3. Anomalous surface properties When the composition of the surface footprint differs sufficiently from the 'model' surface used to compute the theoretical radar tem- plates, we expect erroneous results. The three most probable causes of error in our model would come from footprints that con- tain either A) a large smooth area tilted in the direction of the spacecraft, or B) large changes in relief, or C) very rough material on the meter scale, or some combination of the three. Such a failure of the scattering model can lead to very large errors in RRAD, SLOP, and RRHO. Obvious failures are deleted automatically by the fitting algorithms, and most of the remaining suspect values have been analyzed by hand. References: [1] Pettengill, G.H., Horwood, D.F., and Keller, C.H., Pioneer Venus Orbiter Radar Mapper: Design and Operation, IEEE Trans. on Geosci. and Remote Sens., GE 18, 28-32, 1980. [2] Hagfors, T., Remote probing of the moon by infrared and microwave emissions and by radar, Radio Science, 5, 219- 227, 1970. [3] Pettengill, G.H., et al., Pioneer Venus radar results: altimetry and surface properties, J. Geophys. Res., 85 A13, 8261-8270, 1980.