Data Set Overview :  This dataset consists of data products derived from data collected by Inertial Measurement Units (IMUs) on the two Mars Exploration Rovers (MERs) during their entries into the martian atmosphere. The DATA/ directory contains one data file for each spacecraft (MER1.TAB and MER2.TAB). Each time file contains time ordered records of measured acceleration, derived velocity, derived position, derived atmospheric properties and other derived quantities.  The MER-2 rover, which was launched on the MER-A mission to Gusev Crater on 10 June 2003, was later renamed Spirit. The MER-1 rover, which was launched on the MER-B mission to Meridiani Planum on 7 July 2003, was later renamed Opportunity.  Parameters :  Each data file contains the following items.  Time (Spacecraft Clock Time, seconds) SCLK_TIME Axial acceleration (m s^(-2)) AXIAL_ACCELERATION Normal acceleration (m s^(-2)) NORMAL_ACCELERATION X-component of acceleration (m s^(-2)) X_ACCELERATION Y-component of acceleration (m s^(-2)) Y_ACCELERATION Z-component of acceleration (m s^(-2)) Z_ACCELERATION X-component of velocity (m/s) X_VELOCITY Y-component of velocity (m/s) Y_VELOCITY Z-component of velocity (m/s) Z_VELOCITY Speed of spacecraft relative to atmosphere (m/s) VREL X-component of position (m) X_POSITION Y-component of position (m) Y_POSITION Z-component of position (m) Z_POSITION Radial distance (m) RADIAL_DISTANCE Areocentric latitude (degrees north) LATITUDE Areocentric longitude (degrees east) LONGITUDE Axial force coefficient, CA (dimensionless) CA Normal force coefficient, CN (dimensionless) CN Angle of attack (degrees) ALPHA Mach number (dimensionless) MA Knudsen number (dimensionless) KN Density (kg m^(-3)) RHO Pressure (Pa) PRESS Temperature (K) TEMP  Each data file also contains 1-sigma uncertainties for those items. Units for the uncertainties in item XXX are the same as for item XXX. Those names, which are derived from the above names, are as follows:  SIGMA_SCLK_TIME SIGMA_AXIAL_ACCELERATION SIGMA_NORMAL_ACCELERATION SIGMA_X_ACCELERATION SIGMA_Y_ACCELERATION SIGMA_Z_ACCELERATION SIGMA_X_VELOCITY SIGMA_Y_VELOCITY SIGMA_Z_VELOCITY SIGMA_VREL SIGMA_X_POSITION SIGMA_Y_POSITION SIGMA_Z_POSITION SIGMA_RADIAL_DISTANCE SIGMA_LATITUDE SIGMA_LONGITUDE SIGMA_CA SIGMA_CN SIGMA_ALPHA SIGMA_MA SIGMA_KN SIGMA_RHO SIGMA_PRESS SIGMA_TEMP  Timing :  All data products in this dataset are derived from PDS volume MERIMU_1001 [KASSETAL2004], which uses Spacecraft Clock Time (SCLK time) as its time reference. For consistency, all times in this dataset are also SCLK times.  Coordinate Systems :  All data products in this dataset are derived from 4 Hz backshell IMU measurements in DATA/XTRANSFORMED.TAB in PDS volume MERIMU_1001 [KASSETAL2004], where X is either 1 or 2. These acceleration measurements are given in an XYZ cartesian frame fixed relative to the spacecraft. This spacecraft frame is neither inertial nor fixed relative to Mars. The spacecraft is axisymmetric about the Z axis.  The axis to which axial and normal accelerations are referenced is defined in the data processing section of this file.  Position, velocity and acceleration are given in an XYZ cartesian frame whose origin is at the center of mass of Mars. The +Z axis passes through the north pole and the +X axis passes through the equator at zero degrees east areocentric longitude at a specified time. The +Y axis completes a right-handed set. The specified time is SCLK_TIME : 128270000.000 for MER1 and SCLK_TIME : 126460000.000 for MER2. Position is also given as radial distance, areocentric latitude, and areocentric longitude. This is the standard spherical polar coordinate system found in mathematical textbooks. Longitude increases to the east. Altitude is defined as radial distance minus the radial distance from the center of mass of Mars to the relevant landing site. Altitude is not referenced to any equipotential surface.  Data Processing :  Data processing is also described in [WITHERS&SMITH2006A] and [WITHERS&SMITH2006B].  Four data products from DATA/XTRANSFORMED.TAB in PDS volume MERIMU_1001 [KASSETAL2004], where X is either 1 or 2, were used: SPACECRAFT_CLOCK_START_COUNT, BACKSHELL_IMU_ACCEL_X, BACKSHELL_IMU_ACCEL_Y and BACKSHELL_IMU_ACCEL_Z. Data after backshell separation were discarded. Backshell separation occurred when the terminal descent radar indicated that the spacecraft was about 10 m above the ground. It is reasonable to assume that the position and velocity at landing are the same as the reconstructed position and velocity at backshell separation due to the magnitude of the errors in reconstructed position and velocity. This is relevant for the determination of the entry states.  Backshell separation was identified as the time of the last non-zero value of BACKSHELL_IMU_ACCEL_X, BACKSHELL_IMU_ACCEL_Y, or BACKSHELL_IMU_ACCEL_Z. Some lines of data are duplicates (for example, SPACECRAFT_CLOCK_START_COUNT:128277624.125 for MER-1). The extra lines of data were discarded. The nominal sampling rate is 4 Hz and times usually end on .125, .375, .625, or .875 seconds. However, some sections of the data have a higher sampling rate of 8 Hz (for example, SPACECRAFT_CLOCK_START_COUNT:128278390.000 for MER-1). In order to ensure a uniform timestep, such sections were reduced to a 4 Hz sampling rate by discarding any lines of data where the time did not end .125, .375, .625, or .875 seconds.  Two lines of data are missing from 1TRANSFORMED.TAB, corresponding to SPACECRAFT_CLOCK_START_COUNT:128278462.875 and SPACECRAFT_CLOCK_START_COUNT:128278463.125. Values of BACKSHELL_IMU_ACCEL_X, BACKSHELL_IMU_ACCEL_Y and BACKSHELL_IMU_ACCEL_Z at these times were replaced by linear interpolation using neighbouring data points. Two missing lines of data from 2TRANSFORMED.TAB, corresponding to SPACECRAFT_CLOCK_START_COUNT:126462354.625 and SPACECRAFT_CLOCK_START_COUNT:126462355.875, were replaced in the same way.  Data prior to the time of atmospheric entry were discarded. At this stage in the processing, each data series begins at the time of atmospheric entry and ends at backshell separation, which occurs after parachute deployment. Some of the analysis described in [WITHERS&SMITH2006A, WITHERS&SMITH2006B] used these data series, including the work on determining the entry states. The data products archived in this PDS volume were generated from truncated versions of these data series. Data after parachute deployment were discarded to form these truncated data series. These low-altitude data were discarded because the assumptions made about spacecraft attitude for the trajectory reconstruction are significantly less reliable after parachute deployment than before.  The MER-1 data series from atmospheric entry to parachute deployment begins at SPACECRAFT_CLOCK_START_COUNT:128278194.625 and ends at SPACECRAFT_CLOCK_START_COUNT:128278444.625. The MER-2 data series from atmospheric entry to parachute deployment begins at SPACECRAFT_CLOCK_START_COUNT:126462085.625 and ends at SPACECRAFT_CLOCK_START_COUNT:126462336.375.  SCLK_TIME equals the processed SPACECRAFT_CLOCK_START_COUNT values. AXIAL_ACCELERATION equals -1 times the processed BACKSHELL_IMU_ACCEL_Z values. The factor of -1 was introduced for convenience. It ensures that AXIAL_ACCELERATION is positive, rather than negative, at peak aerodynamic deceleration. AXIAL_ACCELERATION can be positive, zero, or negative. NORMAL_ACCELERATION equals the square root of the sum of the squares of the processed BACKSHELL_IMU_ACCEL_X and BACKSHELL_IMU_ACCEL_Y values. NORMAL_ACCELERATION can be positive or zero, but cannot be negative. 1-sigma uncertainties in these quantities are SIGMA_SCLK_TIME, SIGMA_AXIAL_ACCELERATION and SIGMA_NORMAL_ACCELERATION.  Next, the trajectory was reconstructed. The total acceleration acting on the spacecraft is the sum of aerodynamic and gravitational accelerations. It is necessary to transform the aerodynamic accelerations from a frame defined with respect to the spacecraft to a frame defined with respect to Mars. It was assumed that the aerodynamic acceleration consists of AXIAL_ACCELERATION only, that the aerodynamic acceleration vector is parallel to the velocity of the spacecraft with respect to the atmosphere, and that the atmosphere rotates at the same angular velocity as the solid body of Mars. Winds are thus neglected. The spacecraft rotates about the axis of symmetry on a timescale that is short compared to the timescale for changes in accelerations normal to this axis. Therefore it is reasonable to neglect NORMAL_ACCELERATION in the trajectory reconstruction. The three components of the transformed AXIAL_ACCELERATION are X_ACCELERATION, Y_ACCELERATION and Z_ACCELERATION. 1-sigma uncertainties in these quantities are SIGMA_X_ACCELERATION, SIGMA_Y_ACCELERATION and SIGMA_Z_ACCELERATION.  Gravitational accelerations were calculated as follows:  g : grad(U) (1)  U : GM/r ( 1 + (Rref/r)^2 C20 P20(cos theta) ) (2)  P20(x) : sqrt(5) 1/2 (3x^2 - 1) (3)  where theta : colatitude GM : 4.282E14 m^3 s^(-2) Rref : 3394.2 km C20 : -8.5791E-4 [TYLERETAL2000]  Entry states were determined as discussed in [WITHERS&SMITH2006A] and [WITHERS&SMITH2006B].  The time, position, and velocity for MER-1 at atmospheric entry are: SCLK_TIME : 128278194.625 s RADIAL_DISTANCE : 3522200 m LATITUDE : -2.9 degrees LONGITUDE : 340.9 degrees east Speed relative to Mars-centered inertial frame : 5700 m/s Flight-path angle : 11.5 degrees (angle below horizontal of velocity vector) Flight-path azimuth : 86.5 degrees (angle east of north of velocity vector) X_POSITION : 3410501.1 m Y_POSITION : 861754.30 m Z_POSITION : -178198.42 m X_VELOCITY : -2449.4191 m/s Y_VELOCITY : 5131.4622 m/s Z_VELOCITY : 398.04798 m/s  The time, position, and velocity for MER-2 at atmospheric entry are: SCLK_TIME : 126462085.625 s RADIAL_DISTANCE : 3522200 m LATITUDE : -17.7 degrees LONGITUDE : 161.8 degrees east Speed relative to Mars-centered inertial frame : 5630 m/s Flight-path angle : 11.5 degrees (angle below horizontal of velocity vector) Flight-path azimuth : 79.0 degrees (angle east of north of velocity vector) X_POSITION : -3307198.9 m Y_POSITION : 567076.35 m Z_POSITION : -1070865.2 m X_VELOCITY : -176.76616 m/s Y_VELOCITY : -5464.3396 m/s Z_VELOCITY : 1344.1153 m/s  After the completion of this work, alternative entry states were determined independently using SPICE kernels archived elsewhere at the PDS. Software used in this independent determination is archived in the EXTRAS directory of this PDS volume as file meraltedlandtimes.pro. According to documentation associated with those SPICE kernels, the UTC times of entry for MER-1 and MER-2 were 2004-01-25T04:48:41.720 and 2004-01-04T04:19:51.720, respectively.  According to the SPICE kernels, the position and velocity of MER-1 at the time of entry were: SCLK_TIME : 128278194.142 s RADIAL_DISTANCE : 3522201.3 m LATITUDE : -2.9091901 degrees LONGITUDE : 340.90205 degrees east Speed relative to Mars-centered inertial frame : 5698.6452 m/s Flight-path angle : 11.469960 degrees Flight-path azimuth : 86.503350 degrees  According to the SPICE kernels, the position and velocity of MER-2 at the time of entry were: SCLK_TIME : 126462085.048 RADIAL_DISTANCE : 3522202.1 m LATITUDE : -17.749196 degrees LONGITUDE : 161.77632 degrees east Speed relative to Mars-centered inertial frame : 5627.5267 m/s Flight-path angle : 11.494375 degrees Flight-path azimuth : 79.025420 degrees  These alternative entry states, which are very similar to those obtained by [WITHERS&SMITH2006A, WITHERS&SMITH2006B], were not used to produce these datasets.  After determination of the entry states, as described in [WITHERS&SMITH2006A, WITHERS&SMITH2006B], the equations of motion were integrated forwards in time from the initial position and velocity using the total acceleration, which produced a time series of position and velocity (X_POSITION, Y_POSITION, Z_POSITION, RADIAL_DISTANCE, LATITUDE, LONGITUDE, X_VELOCITY, Y_VELOCITY, Z_VELOCITY). This reconstructed trajectory extends from the time of atmospheric entry to parachute deployment.  The derived velocity values are archived as X_VELOCITY, Y_VELOCITY and Z_VELOCITY. 1-sigma uncertainties in these quantities are SIGMA_X_VELOCITY, SIGMA_Y_VELOCITY and SIGMA_Z_VELOCITY. The derived position values are archived as X_POSITION, Y_POSITION and Z_POSITION. 1-sigma uncertainties in these quantities are SIGMA_X_POSITION, SIGMA_Y_POSITION and SIGMA_Z_POSITION. The derived position values are also archived in a spherical polar coordinate system as RADIAL_DISTANCE, LATITUDE and LONGITUDE. 1-sigma uncertainties in these quantities are SIGMA_RADIAL_DISTANCE, SIGMA_LATITUDE and SIGMA_LONGITUDE. The speed of the spacecraft relative to the atmosphere, which is assumed to rotate with the same angular velocity as the solid body of Mars, is important for the atmospheric structure reconstruction. These values are archived as VREL and their 1-sigma uncertainties are archived as SIGMA_VREL. Uncertainties were found using a Monte Carlo uncertainty analysis.  Atmospheric density is related to aerodynamic deceleration by the drag equation.  m az : rho A vrel^2 CA / 2 (4)  where m is spacecraft mass az is AXIAL_ACCELERATION rho is atmospheric density A is the reference area of the spacecraft vrel is VREL, the speed of the spacecraft with respect to the atmosphere CA is the axial force coefficent, usually on the order of 2  A for both Spirit and Opportunity was that of a disk of diameter 2.648 m [SCHOENENBERETAL2005B]. m was 827.0 kg for Spirit and 832.3 kg for Opportunity [DESAI&KNOCKE2004].  A similar equation relates NORMAL_ACCELERATION and the normal force coefficient CN, so CN/CA : NORMAL_ACCELERATION / AXIAL_ACCELERATION  Atmospheric density can be derived from measured aerodynamic decelerations using the drag equation. However, this will not work at very high altitudes when densities and aerodynamic decelerations are very small. Acceleration measurements are dominated by errors and offsets at very high altitudes, implying unphysical negative densities. Also, this will not work at very low altitudes, after parachute deployment. The aerodynamic behavior of the parachute-backshell-lander system is very complex and it is not possible to determine CA accurately. Without an accurate CA, the drag equation cannot be used to determine atmospheric density from aerodynamic decelerations. So the atmospheric structure reconstruction cannot be performed over the entire trajectory.  Atmospheric pressure is related to atmospheric density by the equation of hydrostatic equilibrium.  dp/dr : rho x (gr + cr) (5)  where p is pressure r is radial distance gr, which is negative and a function of position, is the radial component of equation 1 cr is the radial component of - Omegavec x (Omegavec x rvec) Omegavec is the angular vector for the rotation of Mars rvec is the spacecraft's position vector  Atmospheric temperature is related to atmospheric pressure and density by the ideal gas law.  mu p : rho R T / NA (6)  where mu is the mean molecular mass of the martian atmosphere (43.49 g/mol, [MAGALHAESETAL1999]) R is the universal gas constant T is temperature NA is Avogadro's number  If the appropriate value of CA can be determined at each timestep, then the atmospheric density can be found at each timestep using known quantities and equation 1. Next, atmospheric pressure can be found at each timestep using equation 5 if a boundary condition is specified. The commonly-accepted boundary condition is that p0 : rho0 g0 H0, where H is the measured density scale height at the top of the atmosphere and the subscript 0 refers to values at the top of the atmosphere. This can also be expressed as a temperature, T0 : mu g0 H0 / R. Once atmospheric pressure and atmospheric density are known, atmospheric temperature can then be found at each timestep using equation 6.  Two atmospheric structure reconstructions were performed for each spacecraft. One considered uncertainties, began at about 100 km altitude and ended at parachute deployment. The other did not consider uncertainties, began at about 120 km altitude and ended at parachute deployment. The reasons for this are related to the Monte Carlo uncertainty analysis and will be described shortly. The atmospheric structure reconstruction that did not consider uncertainties was performed first and is described next.  The MER-1 atmospheric structure reconstruction that did not consider uncertainties used data between 128278199.625 and 128278444.625 seconds, inclusive. The MER-2 atmospheric structure reconstruction that did not consider uncertainties used data between 126462095.625 and 126462336.375 seconds, inclusive.  CA is a function of attitude, spacecraft shape, atmospheric density, and speed of the spacecraft relative to the atmosphere. Since the MER entry capsule is axisymmetric, the dependence on attitude can be expressed in terms of the angle of attack, alpha, which is the angle between the symmetry axis and the velocity of the spacecraft with respect to the atmosphere. Alpha is small, usually less than 5 degrees. MER project engineers estimated the likely entry trajectory and atmospheric structure before entry occurred. They used numerical simulations to determine CA as a function of alpha, or CA(alpha), and CN as a function of alpha, or CN(alpha), at a finite number of points along this trajectory. This set of simulated coefficients is called the aerodynamic database. Values of CA(alpha) and CN(alpha) between the simulated points can be found by interpolation. Interpolation required the velocity of the spacecraft relative to the atmosphere, the Mach number and the Knudsen number.  Ma : vrel / sqrt( gamma p / rho ) (7)  Kn : 1 / ( sqrt(2) pi d^2 n D ) (8)  where Ma is Mach number Kn is Knudsen number vrel is VREL, the speed of the spacecraft with respect to the atmosphere gamma is the ratio of the heat capacity at constant pressure to the heat capacity at constant volume of the martian atmosphere pi is 3.14... d is the diameter of an atmospheric molecule n, which satisfies n : NA rho / mu, is the atmospheric number density D, which satisfies pi D^2/4 : A, is the reference diameter of the spacecraft  Values corresponding to pure CO2 were used for gamma and d, so gamma : 1.4 [ATKINS2002] and d : 4.46E-10 m [SCHOENENBERETAL2005B]. The aerodynamic database has been published in graphical form in [SCHOENENBERETAL2005B]. Several different flight regimes are encompassed by the aerodynamic database: free molecular flow, transitional flow, hypersonic continuum flow, and supersonic continuum flow.  Although the aerodynamic database has been published in graphical format in [SCHOENENBERETAL2005B], it is not yet possible for it to be published in tabular format (personal communication, Desai, 2008).  First, preliminary profiles of atmospheric density, pressure, and temperature were obtained using CA:2. As outlined in [WITHERSETAL2003B, WITHERS&SMITH2006A, WITHERS&SMITH2006B], an iterative procedure was used to determine rho, alpha, CA and CN at each timestep. Results for alpha are unreliable when the ratio NORMAL_ACCELERATION / AXIAL_ACCELERATION is inaccurate, which occurs at high altitudes. Alpha and CN were set equal to zero for times before 128278242.125 s (MER-1) or 126462133.125 s (MER-2) (about 80 km). Densities from the upper ten km of the profile were used to find H0 and to provide the upper boundary condition for the equation of hydrostatic equilibrium. Pressures and temperatures were then obtained.  The results of this atmospheric structure reconstruction are archived as CA, CN, ALPHA, MA, KN, RHO, PRESS and TEMP. Uncertainties were obtained using a Monte Carlo technique. The Monte Carlo technique generated an ensemble of values of AXIAL_ACCELERATION for each timestep using the quoted nominal values and their uncertainties. If values from near 120 km were used, then some of these accelerations were negative, which correspond to unphysical negative densities. In order to minimize the occurrence of negative densities, the atmospheric structure reconstruction that considered uncertainties began at about 100 km, not 120 km. The MER-1 atmospheric structure reconstruction that considered uncertainties used data between 128278219.625 and 128278444.625 seconds, inclusive. The MER-2 atmospheric structure reconstruction that considered uncertainties used data between 126462110.625 and 126462336.375 seconds, inclusive. An ensemble of values for T0 was used as the upper boundary condition in the equation of hydrostatic equilibrium. This ensemble was generated using the value of T0 from the atmospheric structure reconstruction without uncertainties (160 K for both MER-1 and MER-2) and an assumed uncertainty of 50 K. Atmospheric structure reconstructions were performed as above using these ensemble inputs. Their standard deviations were used to generate uncertainties for the archived data products. This is an abbreviated description of the error analysis, see [WITHERS&SMITH2006B] for further information.  Derived values of atmospheric density, atmospheric pressure, and atmospheric temperature are archived as RHO, PRESS and TEMP, respectively. 1-sigma uncertainties in these quantities are archived as SIGMA_RHO, SIGMA_PRESS and SIGMA_TEMP. Derived values of CA, CN, alpha, Ma, and Kn are archived as CA, CN, ALPHA, MA and KN, respectively. 1-sigma uncertainties in these quantities are archived as SIGMA_CA, SIGMA_CN, SIGMA_ALPHA, SIGMA_MA and SIGMA_KN.  Each run of a Monte Carlo error analysis leads to slightly different uncertainties. Different runs were used to generate the data products archived here and the data products published as supplemental information to [WITHERS&SMITH2006A]. The Monte Carlo techniques were only used to generate those data products whose names begin with 'SIGMA_'. Data products such as LATITUDE and RHO were produced without consideration of uncertainties, so they do not change from one run of the data processing software to the next.  Not all data products are defined at all timesteps. Data products derived in the atmospheric structure reconstruction are not defined above about 120 km and uncertainties in these data products are not defined above about 100 km. Uncertainties in alpha and CN are also not defined above about 80 km, where alpha and CN are assumed to be zero. Such data products are given the null value -1 in the data files.  Acronyms :  AAT Atmospheric Advisory Team AIAA American Institute of Aeronautics and Astronautics B-IMU Backshell IMU DIMES Descent Image Motion Estimation System EDL Entry, Descent and Landing ET Ephemeris Time HASI Huygens Atmospheric Structure Instrument IDL Interactive Data Language, a computer programming language IMU Inertial Measurement Unit ITAR International Traffic in Arms Regulations JPL Jet Propulsion Laboratory Ls The angle between the Mars-Sun line and the Mars-Sun line at the northern hemisphere vernal equinox, known as the areocentric longitude of the Sun LST Local Solar Time MER Mars Exploration Rover MGS Mars Global Surveyor NASA National Aeronautics and Space Administration OPP Opportunity PAET Planetary Atmosphere Experiments Test PDS Planetary Data System R-IMU Rover IMU SCLK Spacecraft Clock SPI Spirit SPICAM Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars SPICE An information system provided by the Navigation and Ancillary Information Facility TES Thermal Emission Spectrometer TIRS Transverse Impulse Rockets System UTC Coordinated Universal Time

Data Coverage and Quality :  Data coverage is excellent. The only minor gaps in the data series are discussed above. None of the values of SPACECRAFT_CLOCK_START_COUNT, BACKSHELL_IMU_ACCEL_X, BACKSHELL_IMU_ACCEL_Y and BACKSHELL_IMU_ACCEL_Z were identified as erroneous. Uncertainties in the values of BACKSHELL_IMU_ACCEL_X, BACKSHELL_IMU_ACCEL_Y, BACKSHELL_IMU_ACCEL_Z and other IMU measurements from PDS volume MERIMU_1001 [KASSETAL2004] are discussed in [WITHERS&SMITH2006B].  Limitations and Caveats :  The entry states used to generate this archived data volume are not as accurate as they could be. They were determined as described in [WITHERS&SMITH2006A, WITHERS&SMITH2006B], rather than being taken from SPICE kernels. Uncertainties in the entry state were assumed to be the same as for Mars Pathfinder [MAGALHAESETAL1999]. There is no evidence to support this assumption. The PDS volume MERIMU_1001 [KASSETAL2004] contains information on spacecraft attitude during entry. This reconstruction does not use that information and instead made several assumptions concerning spacecraft attitude. Use of the archived attitude quaternions led to a realistic reconstructed trajectory for Spirit, but not Opportunity [WITHERS&SMITH2006A, WITHERS&SMITH2006B]. These attitude quaternions were derived from angular rate data recorded by gyroscopes and an initial attitude. It is possible that the initial attitude for Opportunity, but not Spirit, was unreliable, thereby contaminating the entire time series of attitude quaternions. The archived angular rate data from the gyroscopes are useless, because the orientation of the gyroscope axes with respect to the spacecraft coordinate frame has not been published [WITHERS&SMITH2006A, WITHERS&SMITH2006B].  If the trajectory and atmospheric structure are reconstructed using the alternative (SPICE) entry states, rather than the [WITHERS&SMITH2006A] entry states, changes are small. In this discussion, the 'difference' in a reconstructed quantity is the value of that quantity for the reconstruction that used the alternative (SPICE) entry state minus the value of that quantity for the reconstruction that used the [WITHERS&SMITH2006A] entry state. The 'relative difference' is the ratio of the 'difference' to the value of that quantity for the reconstruction that used the [WITHERS&SMITH2006A] entry state. For MER-1, the difference in altitude, which is about +1 m at entry and +450 m at parachute deployment, varies approximately linearly with altitude. The difference in latitude is -0.01 degrees. The difference in longitude is +0.02 degrees at entry and -0.04 degrees at parachute deployment. It varies approximately linearly with time. The difference in speed of the spacecraft relative to the atmosphere is -1.38 m/s at entry and -1.74 m/s at parachute deployment. The relative difference in density is less than 0.1% above 30 km and less than 1% at all altitudes. The relative difference in pressure is less than 0.5% at all altitudes. The difference in temperature varies smoothly between -0.5 K at 80 km and -1.5 K at parachute deployment. For MER-2, the difference in altitude, which is about +2 m at entry and +100 m at parachute deployment, varies approximately linearly with altitude. The difference in latitude is -0.05 degrees. The difference in longitude is -0.02 degrees at entry and -0.03 degrees at parachute deployment. The difference in speed of the spacecraft relative to the atmosphere is between -2.25 m/s and -2.45 m/s. The relative difference in density is less than 0.1% above 50 km and less than 1.2% at all altitudes. The relative difference in pressure is less than 0.5% at all altitudes below 110 km. The difference in temperature is between 0 K and -0.2 K at all altitudes between 30 km and 110 km. The difference in temperature is between -0.2 K and -1.5 K at all altitudes between 30 km and parachute deployment. The alternative (SPICE) entry states occur about 0.5 s before the [WITHERS&SMITH2006A] entry states. This was neglected in the comparison.  There is an inconsistency between the trajectory reconstruction and the atmospheric structure reconstruction. The trajectory reconstruction assumes an angle of attack of zero and neglects normal accelerations. The atmospheric structure reconstruction determines the value of the angle of attack, which was typically on the order of a few degrees. If the angle of attack is fixed at 0 degrees or at 2 degrees in both the trajectory reconstruction and the atmospheric structure reconstruction, then changes in the inferred trajectory and atmospheric structure are generally small. The changes in density and temperature at parachute deployment are <2% and <3K. However, if the angle of attack is fixed at 5 degrees, then the density and temperature at 30 km change by <1% and <3K, whereas the density, temperature, altitude, and atmosphere-relative speed at parachute deployment change by 7%, about 20 K, 0.2 km, and 20 m/s. These changes are similar for both Spirit and Opportunity.  Wind speeds were assumed to be zero in the trajectory reconstruction and the atmospheric structure reconstruction. Since neither the mean nor the standard deviation of the zonal and meridional wind speeds for the entries of Spirit and Opportunity were known as functions of altitude, non-zero wind speeds were not considered in the trajectory and atmospheric structure Monte Carlo error analysis. If the zonal wind is fixed at +30 m/s eastward throughout the atmosphere, then the density and temperature at 30 km change by +2% and -2K. The density, temperature, altitude, and atmosphere-relative speed at parachute deployment change by +15%, -20K, +0.4 km, and -30 m/s. If the wind direction is reversed, then the sign of these changes is also reversed. These changes are similar for both Spirit and Opportunity.  Three parts of the entry state were determined indirectly in [WITHERS&SMITH2006A, WITHERS&SMITH2006B]: flight path azimuth, latitude, and longitude. Changes in latitude and longitude at entry by approximately 1 degree merely translate the entire trajectory horizontally, with negligible change in the inferred atmospheric structure. Changes in the flight path angle at entry by approximately 5 degrees change the latitude at 30 km and parachute deployment by approximately 1 degree; changes in longitude are much smaller. Changes in the inferred atmospheric structure at 30 km are negligible, but changes in density and temperature at parachute deployment are 2% and 3K. These changes are similar for both Spirit and Opportunity.  The IMU data archived in PDS volume MERIMU_1001 [KASSETAL2004] are stated in that volume's DATASET.CAT file to be those transmitted from the spacecraft. That is, they have not undergone additional calibration. Recalibration of the MER IMU data began in 2004 and so these data should be considered preliminary (Desai, personal communication, 2004). At the time of writing, January 2008, that work is ongoing (Desai, personal communication, 2008). These recalibrated data will be delivered to the PDS in 2008 (Desai, personal communication, 2008). The reconstructed trajectory and atmospheric structure archived here and based on preliminary IMU data may in the future be superceded by reconstructions based on the more accurate recalibrated IMU data.