Researchers at Johnson Space Center (JSC) recently demonstrated that the resonant microstrip patch antenna (RMPA), which has been proven in commercial applications, can be applied with equal success to measuring prelaunch ice buildup on the insulated low-temperature external tanks of the space shuttle orbiter. To do this, they explored the following questions: Why is prelaunch ice buildup on the shuttle external tanks measured, how is the ice measured now, how does the RMPA sensor work, and will the performance of this sensor satisfy the Space Shuttle Program launch-commit criteria.
Accurately measuring prelaunch ice buildup on the orbiter external tanks is a programmatic concern. After the tanks are filled, launch countdown continues unless the "acreage ice" builds to more than 1/16 in. (1.6 mm), at which point the applicable launch-commit criterion requires that the launch be delayed. The delay is necessary because, during initial ascent through the atmosphere, an ice layer thicker than 1/16 in. (1.6 mm) could become fragmented, causing damage to orbiter heat-shield tiles and windows. Therefore, the ability to measure the thickness of ice is vital to vehicle performance and safety. At present, ice measurements are performed on the launch pad, by teams of technicians who manually scratch away the ice layer to determine its thickness. However, the large size of external tanks limits accessibility and the number of measurements that can be taken in this way. A sensor that could automatically perform these measurements would increase accessibility and reduce risk.
JSC researchers needed a sensor that could be applied to the Shuttle's external tanks; the RMPA sensor and associated microprocessor-controlled electronic circuitry satisfied their need. But there was a drawback: Inasmuch as active circuits are essential parts of an RMPA sensor, operation of the circuits could, potentially, cause localized heating that could cause inaccurate measurements. Therefore, a multiple-element RMPA that could be integrated into the tank insulation was designed.
An RMPA sensor is driven by a microprocessor-controlled microwave signal generator. It is characterized by, among other things, an electric field (E field) distributed throughout a dielectric material between a circular copper patch and an underlying copper back plane (ground plane) through the dielectric material. The RMPA sensor can be modeled as a high-Q cavity (where "Q" denotes the resonance quality factor) that capitalizes on its resonant sensitivity; this means that it offers a distinct advantage over a nonresonant electromagnetic (EM)-wave sensor.
The high-Q cavity is bounded by the circular copper patch and the ground plane. The E field within the cavity is both excited and sensed by use of a probe perpendicular to the ground plane at the feed point. An example of the E field within the cavity and the associated fringing E fields is illustrated in the figure. The magnetic (H) field, which is not shown in the figure, is orthogonal to the E field. The magnetic field in the vicinity of the edge of the circular patch is associated with currents that flow along the patch and ground-plane surfaces, and can be regarded as a diffuse magnetic boundary of the resonant cavity. The fringing E and H fields play an important role in the operation of the RMPA because they constitute the means for coupling between the internal cavity fields and the external fields.
The high-Q RMPA transmits primary electromagnetic fields and senses reflected and scattered electromagnetic fields through alteration of its resonant condition. The RMPA emits a continuous-wave signal that is partly reflected and partly transmitted at interfaces between layers of materials in and near the antenna, including layers of nearby materials (e.g., coal, rock, and/or ice) that one seeks to characterize.
The return signal, which is coupled through the fringing fields, alters the E field at the feed point. The microprocessor-controlled electronic circuitry of the RMPA changes the signal frequency until the measured impedance or admittance is real. The resonant resistance or conductance measured at the feed point can change by a significant amount when the layers of materials change.
The RMPA sensor has been proven useful in several commercial applications; for example, to measure the thickness of ice on a roadway, to measure the thickness of uncut coal in a mine, and as an ore-pass monitor. It should be possible to put the RMPA to good use in measuring ice buildup on the orbiter low-temperature external tanks. The replacement of measurements by technicians with measurements by RMPA sensors would make it possible to measure ice buildup on a greater portion of the tank surface in a timely manner; this would enable launch personnel to satisfy the applicable launch-commit criterion and to reduce the potential danger to the shuttle and its crew.
This work was done by Larry G. Stolarczyk of Raton Technology Research, Inc., for Johnson Space Center. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Larry StolarczykRaton Technology Research848 Clayton HighwayRaton, NM 87740
Refer to MSC-22766