Contemporary radio frequency (RF) communication systems and instruments require precise alignment and phase-stability to operate as designed. For example, monopulse antenna tracking systems use the RF phase of a mono pulse signal to correct the pointing of an RF antenna. As another example, systems used for ocean surface vector wind measurements rely on RF data from both horizontal and vertical polarizations in order to derive Stokes parameters. The sensitivity of such systems to changes in phase is particularly evident at high frequencies. For example, for a system operating in the Ka-band (30 GHz), a 1.1 mil (0.028 mm) change in the length of the signal path results in a 1° shift in phase. However, flex waveguides used in practical antenna systems to decouple structural loads and to absorb the mechanical tolerance buildup routinely experience mobility resulting in variations in electrical length of greater than acceptable amounts (e.g. greater than 2.4 mil (0.06 mm)).
Contemporary methods for pointing monopulse antenna tracking systems include receiving a signal at two antennas of an antenna system and measuring differences between the signal as received at the two antennas. The differences that are measured can include a difference in signal strength and/or phase. Such techniques require well characterized signal paths for received signals. However, a problem often encountered in a frontend RF system is a high phase error due to unpredictable RF phase changes or a high amplitude error due to path loss changes. Such changes can be caused by various environmental and operating conditions, which cannot be easily mitigated by design or simulated in a mathematical model. These conditions can include changes due to temperature shifts, vibrations, or mechanical displacement. The resulting errors can degrade the coherency of the signals. This degradation can in turn lead to the measured signal becoming unreliable and difficult to interpret. Degradation in coherency can also lead to an issue where control systems cannot function properly due to erroneous inputted data.
These problems can be at least partially addressed by engineering systems with high mechanical and thermal stability; however, such systems often result in structures that are relatively heavy, making them impractical for use in space or other applications in which relatively low weight is a requirement. In addition, even the most carefully engineered and produced system may not be capable of maintaining path length differences within the tightest tolerances over long periods of time or in the presence of environmental extremes. Alternatively, or additionally, the performance of a system over a wide range of operating conditions can be characterized, and adjustment factors can be applied to tune the system based on information regarding the environmental conditions while the antenna is in operation. This correction method involves only compensating for the phase error in data analysis instead of physically or electrically changing the phase of the RF pathway. However, such systems are limited in that they are incapable of sensing changes in signal path length due to unanticipated or unmeasured environmental or mechanical conditions.