Abstract
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[1] Dualfrequency bistatic radar experiments were conducted with Mars Express at a rate of one to two per month during 2005. Each observation provided power measurements of orthogonally polarized surface echoes at one specular point; the ratio of these components yielded values of the dielectric constant in the range 2.0 < ɛ < 4.0. Doppler sorting of Xband (wavelength λ = 3.6 cm) echoes was used to achieve onedimensional surface resolutions of about 20 km. Although much weaker, simultaneous Sband (13cm) echoes yielded dielectric constants that are 10–50% higher than 3.6 cm echoes, consistent with deeper surface penetration.
1. Introduction
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[2] Bistatic radar is an effective method for remotely probing planetary surfaces at scales of interest to landers and rovers. The centimeter wavelengths used in today's spacecraft telecommunications systems interact most strongly with surface structure having dimensions of 1–100 cm. When a spacecraft highgain antenna (HGA) is aimed toward the specular point on a planetary surface (Figure 1), the power reflected toward an Earthbased receiver is maximized. Frequency dispersion in the quasispecular echo is proportional to the rootmeansquare tilt of reflecting facets having dimensions of a few centimeters to a few meters. Amplitude of the echo is proportional to the Fresnel reflectivity of the surface material, which is a function of the dielectric constant and can be related, through modeling, to the density of the top few centimeters of regolith material [e.g., Gold et al., 1970]. Bistatic radar was first described in detail by Fjeldbo [1964]; principles and results from its first 30 years were reviewed by Simpson [1993].
[3] Four of the earliest Mars Express (MEX) experiments will be discussed here (Table 1). In Table 1, t_{r} is the time at which an observer on Earth would have “seen” the spacecraft antenna illuminate the specular point, given by its latitude and longitude (β, Λ). The corresponding transmit time is t_{t} = t_{r} − ∣r_{R} − r_{T}∣/c, where r_{T} is the position of the spacecraft at transmission with respect to the center of Mars, r_{R} is the position of the receiver at reception, c is the speed of light, and propagation directly from spacecraft to Earth is assumed. The flight time for a photon traveling the carom path is about 5 ms longer, a difference that is not important here. These experiments were conducted at moderate to high spacecraft altitudes, incidence angles 50–70°, latitudes 50–70° from the equator (putting them largely beyond the reach of conventional Earthbased planetary radar systems), and EarthMars distances beyond about 1 AU.
Table 1. Summary of Mars Express Bistatic Radar ObservationsDate  Earth Receive Time t_{r}, UTC  DSS  Orbit  EarthMars Distance R_{R}, AU  Target 

Latitude β, °N  Longitude Λ, °E  Incidence Angle _{i}, deg  Slant Range, km 

12 May 2004  0158  14  423  2.31  63.8  59.9  59.6  11,400 
27 Feb 2005  0200  43  1430  1.84  −56.0  −36.0  61.6  10,400 
3 Apr 2005  0304  43  1555  1.58  −51.0  −70.7  51.1  7,400 
6 Jul 2005  2318  43  1894  0.97  −60.8  74.0  64.7  11,600 
[4] Mars Express adopted a fixed inertial attitude for these experiments rather than dynamically tracking the moving specular point, as has been customary for other bistatic radar operations [Simpson, 1993]. Hence conventional interpretations based on the methods of Fjeldbo [1964] apply only to a small area around (β, Λ).
2. Method
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[5] In each experiment, the spacecraft generated an unmodulated spacecraft carrier signal at two frequencies (Table 2), which was reflected from the surface. The incident RCP signal was converted to both RCP and LCP during the reflection process. The power reflection coefficients for RCP and LCP received are
respectively, where the “horizontal” (R_{H}) and “vertical” (R_{V}) terms are
ϕ_{i} is the angle of incidence (also the angle of reflection), and ɛ is the dielectric constant of the surface material.
Table 2. Nominal Performance of Key System Elements  SBand  XBand 


Frequency, MHz  2296  8420 
Wavelength λ, m  0.131  0.036 
Transmitted power P_{T}, W  5  60 
Transmit antenna bore sight gain G_{T}, dB  28  41.43 
Transmit antenna half power half beam width θ_{T},^{a} deg  2.6  0.6 
Transmitted polarization  RCP  RCP 
Ground antenna bore sight gain G_{R}, dB  63.3  74.3 
Ground antenna effective aperture A_{R}, m^{2}  2900  2660 
Received polarization  RCP/LCP  RCP/LCP 
Nominal receiving system noise temperature T_{SYS}, K  20–25  20–25 
[6] The radar equation gives the surface echo power received on Earth as
where P_{T} and A_{R} are given in Table 2, and R_{T} and R_{R} are the magnitudes of r_{T} and r_{R}, respectively. G_{T} is the gain of the transmitting antenna, which will have the boresight peak value given in Table 2; but the gain decreases away from the boresight in a pattern which is most simply characterized by the half power half beam width θ_{T}. The quantity σ is the surface radar cross section, which is often expressed as the product of a specific radar cross section σ_{0} (per unit surface area) and the area S mutually visible to both transmitter and receiver. In incremental terms
where σ_{0} can be a function of many variables including incidence angles, reflection angles, dielectric constant, and polarization. For simple surfaces (e.g., homogeneous, isotropic, and having gaussian height statistics) we can follow Hagfors [1964] and model the specific radar cross section as
where ρ_{i} is the appropriate reflection coefficient (1), C is a parameter often interpreted as the inverse mean square surface slope, and γ is the tilt angle needed for a surface element to reflect specularly. Angle γ is measured from the local mean surface normal; at the specular point itself γ = 0, but nonzero values are required elsewhere for local surface elements to reflect specularly. For most real surfaces the probability density function for tilt decreases with angle, meaning that the source of surface echoes is concentrated about the specular point for the mean spherical surface. For rougher surfaces the distribution of properly oriented facets will be broader around the specular point than for smooth surfaces. To first order, frequency dispersion in the echo signal is proportional to the breadth of this spatial distribution, so measurements of echo bandwidth can be used to infer C. Except for the smoothest surfaces, however, the Mars Express HGA beam pattern limits the echo frequency dispersion, so estimates of surface roughness must be made less directly.
[7] The experiment conducted on 6 July 2005 combined echoes having good signaltonoise with an observing geometry that is convenient for interpretation. To illustrate how the radar equation (3) and Doppler dispersion interact in generating the surface echo, we have simulated the Xband scattering processes for two instants separated by 60 s near the time of maximum echo strength. The calculations were carried out for a 401 × 401 grid with 5 km grid point spacing and a specific radar cross section σ_{0} = 1 (no dependence on angle, polarization, or dielectric constant). Mars was assumed to be a sphere with radius 3382.65 km. Increasing the radius by 1 km changed the location of the specular point by less than 0.01° in latitude and longitude, changed its incidence angle less than 0.01°, and changed its Doppler frequency by about 2 Hz compared to our frequency resolution of 24.4 Hz in processing; so the precise radius assumed, and the local topography around (β, Λ), are not important.
[8] The results in Figures 2 and 3 show clearly the importance of the spacecraft HGA pattern, which dominates the echo shape; the variations due to changes in R_{T} are inconsequential. The instantaneous specular point was chosen as the grid origin in each case; but the specular point moved southward by only 10 km during the time separating the two simulations, a minor adjustment compared with the 1000 km covered by the grid. The largest differences between the two simulations result from the eastward sweep of the inertially fixed HGA illumination and a modest migration of Doppler contours toward the northeast.
[9] For actual surfaces the assumed σ_{0} = 1 in the simulations must be replaced by a more realistic function, such as (5). As we demonstrate below, however, the shape of σ_{0} cannot be determined directly from the data collected on 6 July 2005; but the relative amplitudes of σ_{0} in the two orthogonal polarizations provide a straightforward way to obtain the surface dielectric constant. All of the factors in (3), (4), and (5) except ρ_{i} are common to both RCP and LCP, so measuring the ratio of RCP to LCP echo power yields the ratio ρ_{R}/ρ_{L}, which can be solved for ɛ (we do this by linearly interpolating between values of ρ_{R}/ρ_{L}, precomputed for known ϕ_{i} and dielectric constant steps of Δɛ = 0.01). We calculate each P_{R} by comparing it to the background noise power density N_{0} = kT_{SYS}, where k is Boltzmann's constant. N_{0} is measured simultaneously with P_{R} and is quantified through a series of straightforward, albeit time consuming, calibration steps for T_{SYS} on each receiver channel. Success in determining ɛ therefore depends on the precision with which we can calibrate T_{SYS} for each receiving channel.
3. Calibration
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[10] Figure 4 illustrates the key elements of each receiver. Before and after each experiment the background sky noise power was calibrated against a resistive load at ambient temperature while the antenna was pointed to zenith. The noise diode was calibrated at the same time, then used to provide an additive noise reference at 60–90 min intervals while the antenna was tracking Mars and the spacecraft. Neglecting small changes in the gain of the microwave front end, we can monitor T_{SYS} during the bistatic radar surface measurements with an accuracy of about 5% over time intervals of a few minutes. For comparison, the statistical uncertainty in our highest time resolution bistatic radar power spectra is about 5% (25,000 samples/s, taken 1024 points at a time, and averaged over 15 s). Other contributions to measurement uncertainty include nonlinearities at various points in the system, and occasional radiofrequency interference (particularly at Sband).
[11] During processing of the bistatic radar data, spectral regions B within the 25 kHz bandwidth were identified as being free of direct signal, echo, and interference. These were used for dynamic calculation of T_{SYS}; P_{N} = kT_{SYS}B could then be used to scale power spectra to correct amplitudes. P_{N} was then subtracted from the total spectrum leaving the echo signal and sometimes a residual, directly propagating carrier signal from the spacecraft. We elaborate on our signal model, calibration issues, and calculation of uncertainties in Appendix A.
4. Data
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[12] Figures 5 and 6 show echo spectra as the HGA beam swept over the specular point on 6 July 2005; these correspond to Figures 2 and 3, respectively. The XRCP echo power, integrated over all contributing frequency bins, peaked at 1163 × 10^{−21} W at 2319:35 Earth receive time (ERT), about 75 s after the specular point was centered in the HGA beam. The rise from and fall to the half power points took about 7 min. The integrated XLCP echo power peaked at 535 × 10^{−21} W. Both Sband channels showed a broad enhancement in power, coincident with the Xband increases, although the enhancement showed no clear rise, peak, or fall. Only two of the integrated Sband echoes exceeded 35 × 10^{−21} W, largely a consequence of lower P_{T} and G_{T} at the longer wavelength.
[13] Ratios of the measured RCP to LCP powers are shown in Figure 7, plotted as a function of incidence angle for the associated specular point. In each case the echo powers at all frequencies in a single spectrum were summed; then the results from 365 spectra, representing 15 s in time, were averaged before the ratio was calculated. The Sband points are more widely scattered; five exceeded the plotting limit (ρ_{R}/ρ_{L} < 4) and are not shown.
[14] If we average the 6 July XRCP and XLCP echo powers over the 20 min centered on the optimum specular point condition and then take their ratio, we find ρ_{R}/ρ_{L} = 2.066. Although this procedure confuses the specular point interpretation by folding data into the calculation from higher and lower values of ϕ_{i} (and possibly resulting from nonspecular processes), this was the only practical way to obtain results for the May 2004 data. For the February and April 2005 data, we generally obtained satisfactory ratios using 5 min averages. Results for both 5 and 20 min averages are shown in Table 3, where the specular point latitude, longitude, and incidence angle are listed as though the HGA radiated isotropically. We note that the specular point theory supports this procedure, provided that the specular angle at the illuminated point is used in the calculation; we discuss this and associated issues later. For the optimum specular point geometry on 6 July, ρ_{R}/ρ_{L} = 2.066 corresponds to a dielectric constant of ɛ = 2.58. The solid line in Figure 7 shows the expected ratio for a surface with ɛ = 2.58, as computed from (1).
Table 3. Summary of Long Integration ResultsDate  Center Time t_{r}, UTC  Integration, s  Specular Point  Band  RCP Power^{a}  LCP Power^{a}  Dielectric Constant 

Latitude β, °N  Longitude Λ, °E  Incidence Angle _{i}, deg  Noise Density N_{0}, zW/Hz  Echo P_{R}, zW  Noise Density, N_{0} zW/Hz  Echo P_{R}, zW 


21 May 2004  0200:00  1200  64.1  60.09  59.9  X  0.278  ^{b}  0.267  ^{b}  ^{b} 
      S  0.275  1.87 ± 0.20  0.343  1.50 ± 0.25  2.54 ± 0.26 
27 Feb 2005  0200:00  1200  −56.0  −36.0  61.6  X  0.283  47.67 ± 0.44  0.284  23.70 ± 0.44  2.10 ± 0.02 
      S  0.309  3.49 ± 0.17  0.346  3.17 ± 0.19  3.19 ± 0.14 
27 Feb 2005  0157:30  300  −56.3  −36.5  61.3  X  0.282  77.21 ± 0.87  0.283  39.22 ± 0.87  2.07 ± 0.03 
      S  0.308  6.33 ± 0.33  0.345  2.14 ± 0.37  1.63 ± 0.12 
27 Feb 2005  0202:30  300  −56.7  −35.5  62.0  X  0.283  70.00 ± 0.87  0.286  26.21 ± 0.88  1.82 ± 0.03 
      S  0.310  4.89 ± 0.33  0.346  4.21 ± 0.37  3.17 ± 0.20 
3 Apr 2005  0304:00  1200  −51.0  −70.7  51.1  X  0.359  60.35 ± 1.69  0.339  158.13 ± 1.59  3.05 ± 0.05 
      S  0.349  3.12 ± 0.31  0.397  11.18 ± 0.35  3.96 ± 0.26 
 0301:30  300  −51.6  −71.4  50.6  X  0.358  89.25 ± 1.24  0.337  232.53 ± 1.17  2.90 ± 0.03 
      S  0.345  1.68 ± 0.61  0.394  15.39 ± 0.69  ^{c} 
 0306:30  300  −50.3  −70.0  51.6  X  0.363  92.02 ± 1.59  0.343  248.07 ± 1.50  3.26 ± 0.04 
      S  0.351  4.62 ± 0.62  0.400  14.26 ± 0.70  3.68 ± 0.33 
6 Jul 2005  2317:58  1196  −60.8  74.0  64.7  X  0.781  437.50 ± 0.57  0.854  211.72 ± 0.62  2.58 ± 0.01 
      S  0.338  22.53 ± 0.12  0.394  13.13 ± 0.14  2.94 ± 0.02 
 2315:28  299  −60.4  74.0  65.0  X  0.773  522.25 ± 0.67  0.846  256.91 ± 1.03  2.68 ± 0.01 
      S  0.344  25.23 ± 0.19  0.401  13.39 ± 0.22  2.82 ± 0.03 
 2320:27  299  −61.2  74.1  64.3  X  0.775  949.44 ± 0.62  0.852  436.74 ± 0.83  2.43 ± 0.01 
      S  0.339  29.22 ± 0.19  0.395  17.70 ± 0.22  2.94 ± 0.22 
[15] Summing the power in a single echo means the associated surface footprint covers a significant area, for example, the area enclosed by the outer elliptical contour in Figure 2. Averaging these sums over time superimposes many such footprints, making the effective surface resolution still larger. When the signaltonoise ratio is high, we can evaluate power ratios in individual frequency bins, the Doppler stripe associated with the specular point pluses in Figure 2, for example, where its eastwest limits are the outer elliptical contour. Although a fixed location on the surface will drift from one frequency bin to another over time, we can calculate and remove the drift and obtain estimates at spatial resolutions commensurate with the frequency resolution in the spectra.
[16] Figure 8 shows power ratios plotted against latitude for six consecutive 15s spectra; we have corrected for frequency migration and the spectral bins have been associated with fixed latitudes on the surface. In each 15s time interval the instantaneous specular point moved 2.5 km to the south and 13.3 Hz higher in frequency, relative to the direct signal frequency f_{D}. Since a single bin corresponds to about 17 km northsouth on the surface and 24.4 Hz in frequency, the net drift of a fixed point on the surface is +0.4 bins in 15 s or 2.4 bins over the time represented by these six spectra. Although only six spectra at specular point latitudes between −60.86 and −61.07 were used in this exercise, the scatter in the data points (typically less than ±15%) suggests that valid results can be obtained as much as ±1.5° from the central latitude. In Table 4 we have listed the results for ten adjacent latitudes, centered approximately on the optimum specular point. Means and standard deviations were calculated at each latitude from the six measured power ratios; dielectric constants were derived assuming ϕ_{i} = 64.65°. The constant incidence angle is justified because δϕ_{i}/δt = −0.13°/min and the northsouth variation in the simulation grid is ∣δϕ_{i}/δy∣ < 0.0001°/km at 2318:00 ERT. There is also a westtoeast variation in the grid δϕ_{i}/δx ∼ 0.001°/km, which we expect to have little effect since each latitude bin contains echo from the same longitudes.
Table 4. Doppler Sorted Measurements (6 July 2005)Latitude β, °N  Incidence Angle _{i}, deg  Measured Power Ratio ρ_{R}/ρ_{L} (Average of Six Values)  Dielectric Constant 

−59.75  64.65  2.081 ± 0.151  2.570 ± 0.095 
−60.02  64.65  2.220 ± 0.197  2.463 ± 0.109 
−60.30  64.65  2.200 ± 0.249  2.482 ± 0.141 
−60.57  64.65  2.090 ± 0.118  2.561 ± 0.073 
−60.85  64.65  2.106 ± 0.043  2.545 ± 0.026 
−61.13  64.65  2.227 ± 0.134  2.454 ± 0.073 
−61.42  64.65  2.200 ± 0.280  2.486 ± 0.159 
−61.70  64.65  2.328 ± 0.256  2.390 ± 0.129 
−61.99  64.65  2.410 ± 0.384  2.348 ± 0.184 
−62.27  64.65  2.581 ± 0.326  2.239 ± 0.134 
5. Discussion
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[17] MEX bistatic radar experiments provide new data about the surface of Mars. Each of the targets listed in Table 1 is well outside the view of conventional rangeDoppler radar systems on Earth. Only Hellas has been probed using bistatic techniques before; those measurements were made by Viking and were near the center of the basin rather than on its southern margin as here. The MEX measurements also represent the first simultaneous use of dual frequency probing and the first use of the polarization ratio method for determining dielectric constant on a regular basis.
[18] The numerical simulations of the 6 July 2005 experiment show good agreement with the results in terms of echo offset frequency and shape. Our use of σ_{0} = 1 for the surface scattering function may have seemed restrictive; but comparison of the Xband computed spectrum with the data suggests that the echo shape is largely determined by the HGA radiation pattern and not the surface roughness. The echo shape is beamlimited if the surface has RMS roughness greater than 4–5° on scales of centimeters to meters.
[19] If we assume that radio wave scattering is described by the Hagfors function (5), we can make a simple estimate of roughness based on the observed reflectivity. The inferred MEX dielectric constant ɛ = 2.58 was derived from a polarization ratio ρ_{R}/ρ_{L} = 2.066 at an incidence angle ϕ_{i} = 64.65°. The individual reflection coefficients are found from (1) and (2) to be
From Figures 5 and 6 we see that σ_{0} = 2 provides a reasonable match to the XRCP echo and that a number slightly less than σ_{0} = 1 would match the XLCP echo. Evaluating (5) for γ = 0° gives C ∼ 22 and an RMS roughness on the order of C^{−1/2} = 12°. This is large for Mars, where typical values are more like 3–5°; Simpson et al. [1979] reported roughness values of 2.5–4.5° and a dielectric constant ɛ = 3.1 along a Viking Orbiter specular point track in Hellas between (47S, 296W) and (40S, 300W) at Sband. However, RMS values larger than 10° have been observed on very rough surfaces around the Tharsis volcanoes. The new Hellas roughness estimate may be reduced if it is found from further calibration that the P_{T}G_{T}A_{R} product based on the values in Table 2 is too large; there is some evidence in this direction from more recent experiments not covered by this paper.
[20] We have presented three methods for deriving dielectric constant from polarization ratios. The simplest is to sum all of the echo in each polarization and to take the ratio of the results. This is the only method that works for very weak signals, such as when EarthMars separation is more than 2 AU or in many of the Sband observations to date. Because it can be applied to all of the data which have been processed, we have listed these numbers in Table 3 for consistency. However, this method has an illdefined footprint; the HGA beam can sweep across a lot of terrain in 20 min and, although we have associated the incidence angle at the targeted specular point with these measurements, there is a wide range of facet geometries that contributes to these echoes. As noted earlier, however, there is virtually no ϕ_{i} variation in the northsouth direction and only a linear drift eastwest; so “corruption” introduced solely by variation of ϕ_{i} over the illuminated areas may be very modest, at least for geometries similar to those seen on 6 July 2005.
[21] Simulations with realistic functions for σ_{0} are one solution which we will be incorporating in future analyses. We note, however, that even with the simple geometry of the 6 July 2005 experiment we may have to face heterogeneous surfaces. The asymmetry and shoulder(s) on the lowfrequency side of the data spectra in Figures 5 and 6 imply that the surface north of these specular points is rougher or less reflective (or both) than the surface to the south. So a specific radar cross section that varies with position may be required for more detailed analysis within the 6 July 2005 target area.
[22] The second method for deriving dielectric constant is to take the ratio of total powers in individual spectrum pairs. The Xband results in Figure 7 are satisfactory, although the number of points shown far exceeds the number of independent measurements because adjacent footprints overlap by more than 90%. The individual footprints are smaller than the effective footprint in the 20 min case, however. Visual inspection of the Xband points in Figure 7 suggests that the individual ratios lie more above the solid curve than below, meaning that the inferred dielectric constants would be slightly lower than ɛ = 2.58. The model curve shows an increase in ρ_{R}/ρ_{L} with increasing incidence angle, whereas the data points suggest a very slight decrease. Again, more sophisticated use of the simulation, including a spatially varying σ_{0} function, may provide additional insight.
[23] The third method is to use the power measurements in individual frequency bins to compute ratios, to associate bins with fixed locations on the surface, and to track the migration of these fixed points through the drifting spectra as a function of time. Our limited exercise shows that reproducible ratios and dielectric constant estimates can be obtained with about 10% standard deviation in each. Using Doppler sorted data from 6 July, we confirmed that dielectric constant decreases as the specular point moves south, a conclusion first drawn from the whole spectrum ratios plotted against angle of incidence (Figure 7).
[24] We believe that the third method gives the best estimates of surface dielectric constant when there is sufficient signaltonoise to allow calculations at the individual frequency bin level and the frequency bin and time resolution are both meaningful. We note common details in the 15 s spectra such as notches 2–4 bins below the peak values in both XRCP and XLCP and approximately linear changes in echo powers over −1500 to −1200 Hz, particularly in Figure 5. Such features are diagnostic of surface heterogeneity and have been used in favorable circumstances to map surface morphology [Simpson et al., 1977]. Shoulders on the highfrequency side of XLCP echoes are not visible in XRCP, however.
[25] We have said very little about the Sband echoes. Weaker by a factor of 20 in P_{T} and another factor of 20 in G_{T}, as compared with Xband, the Sband signals are more difficult to detect. However, there are mitigating factors: the HGA beam is nearly 4 times broader at Sband and natural surfaces tend to have higher C values at the longer wavelengths making them easier to detect in quasispecular scattering. Although we computed a 20min power ratio from the Sband measurements on 6 July 2005, we were unable to define a consistent echo shape. For other targets, particularly where the surface is smoother, we expect to find the Sband echo to be more diagnostic of surface properties.
6. Conclusions
 Top of page
 Abstract
 1. Introduction
 2. Method
 3. Calibration
 4. Data
 5. Discussion
 6. Conclusions
 Appendix A:: Error Sources and Attributions
 Acknowledgments
 References
 Supporting Information
[26] Scatter among dielectric constant estimates made under similar conditions is less than 10%; statistical variations at the finest time resolution are about 5%, and calibrations appear to be reproducible at about the 5% level. Assuming an overall uncertainty on the order of 20%, we believe that the target to target ɛ variations listed in Table 3 are valid.
[27] We believe that at least part of the observed variation is a real response to heterogeneous surface properties. Although limited in scale, we believe our Doppler sorting demonstrates the viability of this highresolution processing for future MEX bistatic data.
[28] Although the Sband measurements are more uncertain, we have found, in all experiments conducted to date, that the dielectric constant is smaller at Xband than at Sband, in agreement with models of surface penetration. This is the first time simultaneous measurements have been used on Mars to confirm the wavelength variation in ɛ.
[29] As this paper is written, we have just passed Mars opposition when bistatic echo strengths were strongest. Five recent experiments have been conducted over Mars' south pole in hopes of learning more about the radarbright residual south polar ice cap (RSPIC) discovered by Muhleman et al. [1991]. The quasispecular bistatic mode is not likely to trigger the anomalous scattering reported earlier; but the new data may still be helpful in understanding the phenomenon. We are also seeking opportunities to schedule bistatic radar over the equatorial “Stealth” region, which is unusual in its weak scattering response [Muhleman et al., 1991].
[30] Operationally, the Mars Express Project has agreed to implement specular point tracking so that we can follow the path of maximum echo strengths with the HGA rather than sweeping once across. Potentially, this will allow us to collect hundreds of data points per observing session whereas we are limited to one (plus whatever we can extract using Doppler sorting) with inertial HGA pointing.