We present new model predictions of the earth-directed microwave brightness temperatures (TB) of the moon with dependence on phase angle over the wavelength range of 1 mm – 10 cm. Both disk average and global maps are included. The models are based on detailed thermophysical property retrievals provided by Apollo in situ measurements, Diviner and Chang E’2 (CE2) global measurements, and high quality pre-Apollo earth-based observations. Of particular importance are the advances in electrical property (loss tangent) determinations provided by the CE2 lunation amplitude data. A detailed error analyses is included to provide estimates of the TB model error bounds due to realistic assessments of the uncertainties of the contributing thermophysical properties. Disk average TB uncertainty levels of ~ +/- 4 K are found for wavelengths in the 1-10 cm range and ~ +/-8 K for ~ 1-5 mm wavelengths. Analyses are presented indicating that neither surface roughness nor potential correlation of the thermal parameter uncertainties significantly impact the model total error budget. The new models are proposed as stable absolute calibration references for earth-based microwave radiometers, in particular the vast array of earth-orbiting instruments now monitoring weather and climate properties globally.

Basic radiative transfer theory reveals that the physical temperature gradients related to heat flow (HF) within the lunar regolith below the diurnal varying layer can be constrained by measurements of brightness temperature (TB) variations over a selected wavelength range, coupled with knowledge of the regolith electrical loss properties. We present a methodology illustrating the processing using recalibrated data from the Chang’E-2 (CE2) 10 and 3.85 cm wavelength microwave radiometer (MRM) data (channels 1 and 2). The utilized data are contained in latitude bins of 1 degree width centered at latitudes of 0, 5,..., 60, north and south. Results, presented as retrieved HF gradients (dT/dz in K/m) vs longitude for each of the 25 latitude bins, provide useful constraints on the overall variability of the heat flow gradient within the 60S-60N latitude range. In particular, no evidence is found that HF variations exceeding twice the Apollo value occur widely in the extensive highland regions of low electrical loss. Limitations related primarily to signal-to-noise (SNR) issues in regions of elevated electrical loss severely reduce thermal gradient retrieval accuracy needed for correlation with enhanced thorium concentrations observed in the frontside mare. Analyses of the CE2 data reveals a noise uncertainty of ~ 0.5 K in the TB10-TB3.85 measurements, equivalent to ~ 0.7 - 1.4 (loss tangent dependent) times the Apollo measured mean HF value of 1.8e-6w/cm2. The limitations preclude meaningful identification of regional variations within a precision less than one Apollo heat flow unit. An analyses is presented demonstrating that a threefold increase in SNR would be attained by addition of a 30 cm wavelength channel to the CE2 MRM’s design, providing sufficient resolution to map regional HF variations to ~ 0.5e-6w/cm2precision, ~ 30% of the averaged Apollo sites’ measured HF value.

The Chang’E-1 and E-2 (CE-1 and CE-2) orbital sounders provided high-resolution global maps of lunar microwave brightness temperatures (TBs) at wavelengths of 0.8, 1.55, 3.85, and 10 cm covering approximately two years of data over the 2007–2010 time frame. The four-channel microwave radiometers (MRMs) effectively sampled the upper ~2 m of the lunar regolith and are diagnostic of thermal and electrical properties, including mineralogy, rock abundance, and, potentially, interior heat flow. Early comparisons of colocated data between the two instruments revealed ~10–20 K offsets between the CE-1 and CE-2 measurements that required in- flight recalibration efforts. These have included comparisons with model predictions at Apollo sites as well as adjustments of the cold sky horn reference temperatures to include contamination from lunar surface emissions. This article proposes an in- flight recalibration methodology that focuses on correction of the preflight transfer coefficients that determine the relative importance of component losses along the radiometer hardware paths connecting the antennas to the detectors. It is shown that for each channel, a single parameter, representing the ratio of the cold sky and main antenna transfer coefficients, can be constrained by knowledge of the stable regolith physical temperatures below the diurnal-varying layer and is sufficient to establish values for the complete set of hardware transfer coefficients. The in- flight comparisons proposed for recalibration are based on backside highland models of regolith thermal properties. Potential remaining offsets of the recalibrated MRM data are evaluated in terms of parameter uncertainties of our chosen nominal thermal model.

To determine the likelihood that the surface contamination effects can explain the offsets between Chang’E-1 (CE-1) and Chang’E-2 (CE-2) microwave radiometers’ (MRMs) calibrated antenna temperatures (TAs), we quantitatively estimate the effects of the lunar contamination of the CE MRMs’ cold sky horn antennas on TA. We calculated the lunar radiation contributions to the cold sky horn (${ {\Delta T}}_{\text {cosmic}}$) values from the Apollo model brightness temperatures (TBs) convolved with the CE-1 and CE-2 cold sky horn antenna patterns for the circular orbit geometries of CE-1 and CE-2. The calculations were done for all four channels and estimated 20°–40° range of the full width half maximum (FWHM) Gaussian beam sizes over a latitude range of 0°–64°. Small differences between the predawn and noon results illustrate that the local time effects are negligible. Most noteworthy is the result that the largest plausible “${ {\Delta T}}_{\text {cosmic}}$” are less than 10.8 K for CE-1 and less than 23.6 K for CE-2 at the largest assumed beam size. The latitude effect is small, although significant (~5–6 K difference), from the equator to 64°. Most importantly, the cold sky horn contamination effects are shown to produce target TA errors of only 2–5 K for CE-1 and 4–10 K for CE-2 with the largest errors occurring at the lowest TAs. These errors are inadequate to resolve much larger relative offsets reported in the CE-1 and CE-2 TA data. We thus propose an alternative recalibration scheme that focuses on the role of uncertainties in the preflight derivation of hardware loss coefficients.

An end-of-mission recalibration effort was recently completed for Topex Microwave Radiometer to generate climate data records of precipitable water vapor and cloud liquid water for 1992–2005. The TMR climate data is analysed for trends. The global trend in precipitable water vapor is found to be 0.9 ± 0.06 mm/decade. Regional precipitable water vapor trends are found to be highly correlated with regional sea surface temperature trends. The cloud liquid water trends are observed to be generally negative outside the tropics and positive in the tropics.

Two tropospheric calibration systems have been developed at the Jet Propulsion Laboratory (JPL) using different technologies to achieve different levels of accuracy, timeliness, and range of coverage for support of interplanetary NASA flight operations. The first part of this paper describes an automated GPS-based system that calibrates the zenith tropospheric delays. These calibrations cover all times and can be mapped to any line of sight using elevation mapping functions. Thus they can serve any spacecraft with no prior scheduling or special equipment deployment. Centimeter-level accuracy is provided with 1-h latency and better than 1-cm accuracy after 12 h, limited primarily by rapid fluctuations of the atmospheric water vapor. The second part describes a more accurate line-of-sight media calibration system that is primarily based on a narrow beam, gain-stabilized advanced water vapor radiometer developed at JPL. We discuss experiments that show that the wet troposphere in short baseline interferometry can be calibrated such that the Allan standard deviation of phase residuals, a unitless measure of the average fractional frequency deviation, is better than 2times10-15on time scales of 2000 to approximately 10 000 s.

The Jason microwave radiometer is calibrated using hot and cold on-Earth theoretical brightness temperature references. The retrieved path delay values are validated using collocated Topex microwave radiometers and radiosonde values. The calibrated path delay values are demonstrated to have no significant bias or scale errors. The absolute accuracy of the individual path delay values is demonstrated to have no significant bias or scale errors. The absolute accuracy of the individual path delay measurement exceeds the mission goal of 1.2 cm RMS.

We discuss the performances of a set of four microwave water vapor radiometers operating in the 20-30-GHz band during a field experiment, with an emphasis on calibration and achievable accuracy. The field experiment was conducted at the Department of Energy's Atmospheric Radiation Measurement Program's field site in north central Oklahoma, and was focused on clear-sky water vapor measurements by both radiometers and radiosondes. A comparison between two published radiometric tip curve calibration procedures is presented, and these procedures are applied to measurements from two nearly identical instruments placed a few meters apart. Using the instantaneous tip cal method of the Environmental Technology Laboratory, the brightness temperature measurements for the two identical instruments differed by less than 0.2 K over a 24-h period. Results from reference load cryogenic tests and brightness temperature cross comparisons have shown differences within 0.7 K. In addition, we compare radiometric measurements with calculations of brightness temperature based on the Rosenkranz absorption model and radiosonde observations. During the experiment, both Vaisala-type RS80 and RS90 humidity sensors were used. Our comparisons demonstrate the improvements achieved by the new Vaisala RS90 sensors in atmospheric humidity profiling, which reduce or eliminate the "dry bias" problem.

We discuss the performances of a set of four microwave radiometers during a field experiment, with an emphasis on calibration and achievable accuracy. A comparison between two alternative tip curve calibration procedures has been carried out. Additional results from reference load cryogenic tests and from brightness temperature cross-comparison are shown. Also, we compare radiometric measurements with radiosonde observations and we demonstrate the improvements achieved by the new Vaisala sensors to reduce or eliminate the "dry bias" problem.

Collocated measurements of opacity (from water vapor radiometer brightness temperatures) and wet path delay (from ground-based tracking of global positioning satellites) are used to constrain the model of atmospheric water vapor absorption in the 20-32 GHz band. A differential approach is presented in which the slope of opacity-versus-wet delay data is used as the absorption model constraint. This technique minimizes the effects of radiometric calibration errors and oxygen model uncertainties in the derivation of a best-fit vapor absorption model. A total of approximately five months of data was obtained from two experiment sites. At the Cloud and Radiation Testbed (CART) site near Lamont, Oklahoma, three independent water vapor radiometers (WVRs) provided near-continuous opacity measurements over the interval July-September 1998. At the NASA/Goldstone tracking station in the California desert two WVRs; obtained opacity data over the September-October 1997 interval. At both sites a Global Positioning Satellite (GPS) receiver and surface barometer obtained the data required for deriving the zenith wet delays over the same time frames. Measured values of the opacity-versus-wet delay slope parameter were obtained at four WVR frequencies (20.7, 22.2, 23.8, and 31.4 GHz) and compared with predictions of four candidate absorption models referenced in the literature. With one exception, all three models provide agreement within 5% of the opacity-versus-wet delay slope measurements at all WVR frequencies at both sites. One model provides agreement for all channels at both sites to the 2-3% level. This absorption model accuracy level represents a significant improvement over that attainable using radiosondes.

The stability and accuracy of the TOPEX Microwave Radiometer (TMR) measurement of the atmospheric path delay due to water vapor is assessed over the interval from launch (August 1992) through June 1998. Detailed global comparisons are made with path delays derived from the special sensor microwave imager (SSM/I) instruments and a network of 15 island radiosondes. The results provide consistent evidence that the TMR path delay measurements included an instrument-related downward drift of 1.0-1.5 mm/yr between October 1992 and December of 1996. The four-year drift correlates with an upward drift seen in the coldest TMR 18-GHz brightness temperature time series and is further supported by independent comparisons of TMR with ERS-1 and 2, GPS, and the Harvest Platform water vapor radiometer measurements. From January 1997 through June 1998, no significant relative path delay drift between TMR and SSM/I is seen in the comparison data, although anomalies do appear in early 1998. In terms of accuracy, both the SSM/I and radiosonde comparisons indicate no significant (>2%) scale error in the TMR path delay. An overall bias of <10 mm mag be present, but the comparisons are not consistent in this determination.

For pt.I see ibid., vol.33, no.1, p.125-37 (1995). The calibrated antenna temperatures measured by the TOPEX Microwave Radiometer are used to derive radiometric brightness temperatures in the vicinity of the altimeter footprint. The basis for the procedure devised to do this-the antenna pattern correction and brightness temperature algorithm-is described in the paper, along with its associated uncertainties. The algorithm is based on knowledge of the antenna pattern, the ground-based measurements of which are presented along with their analyses. Using the results of these measurements, the authors perform an error analysis that yields the net uncertainties in the derived TMR footprint brightness temperatures. The net brightness temperature uncertainties range from 0.79 to 0.88 K for the three TMR frequencies, and include the radiometer calibration uncertainties which range from 0.54 to 0.57 K. the authors also derive an estimate of the uncertainty incurred by using brightness temperatures measured in the /spl sim/40 km TMR footprint to estimate path delay in the /spl sim/3 km altimeter footprint. The RMS difference in path delay averaged over the largest TMR footprint relative to that in the altimeter footprint is estimated to be about 0.3 cm. Finally, the authors discuss the error associated with using unequal beams at the three TMR frequencies to derive path delays, and describe an approach using along-track averaging of the algorithm brightness temperatures to reduce this error.>

For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted.>

The TOPEX/Poseidon microwave radiometer (TMR) is a three-frequency radiometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit. It operates at 18, 21, and 37 GHz in a nadir-only viewing direction which is co-aligned with the T/P radar altimeters. The TMR monitors and corrects for the propagation path delay of the altimeter radar signal due to water vapor and nonprecipitating liquid water in the atmosphere. The paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (T/sub A/) from the raw digital data. T/sub A/ precision of 0.4 K is predicted on orbit in all expected thermal environments, T/sub A/ accuracy of 0.5-0.6 K is expected following a post-launch field calibration campaign. These performance figures represent a significant improvement over those of the Seasat and Nimbus-G Scanning Multichannel Microwave Radiometer on which TMR is based. The improvements are the result of specific hardware design and calibration changes. Hardware changes include a redesigned feed horn, to reduce impedance mismatches, and the addition of radomes over the feed and sky horns, to reduce thermal variations. Calibration changes involve more extensive temperature cycling and data analysis during thermal/vacuum testing.>