Introduction

     My name is See-Chen Lee and most people call me Gary. I am currently a 2nd year MS Student in the School of Aeronautics and Astronautics at Purdue University and will continue pursuing the PhD program. Although the name of my school does not sound like it relates to meteorology, the satellites do serve as an important remote sensing tool in weather/climate study. The research I have been working on since 2003 is using a dual-frequency GPS ground based array to detect and investigate the transient ionospheric perturbations in terms of Integral Electron Content (IEC), particularly for those disturbances with no obvious source. My research advisors are Eric Calais, Jennifer Haase and Dr. James Garrison, who are also working on GPS occultation. It appears that there were lots of interesting topics in the COSMIC 2005 summer camp that were related to my research - particularly with regard to GPS ionospheric observation. Therefore, my adviser Dr. Garrison suggested to Brian Ventre and me that we participate in this event. Brian is another MS student working for professor Garrison; his research is to develop open-loop tracking software for airborne-based GPS occultation receivers.

     I studied aerospace engineering (specifically in dynamics and control) as an undergraduate and switched to remote sensing and digital data processing (DSP) in graduate school. Although the materials are interdisciplinary, the fundamental mathematics and physics principles are basically the same. Those presentations at the COSMIC colloquium showed a lot of good examples, such as the data assimilation used in climate study, which could be viewed as a statistical-based ODE/PDE solving problem; the ionosphere model is a combination of electromagnetic and fluid dynamics.

Scientific Experience

     At the beginning of the summer camp there were several presentations that introduced the theories behind GPS occultation, including signal processing, space physics, optical bending and Abel-inverting. This was in fact a good review for me since I had read papers about radio occultation a long time ago. I believe this should also be a good start for the others who were not familiar with DSP. Unfortunately there were several subjects that appeared to be somewhat difficult for me to understand, such as topics about weather forecasting models and hurricanes. This was probably due to my limited background knowledge about those subjects. After all, Brian and I were the only two engineering students, while others all came from science departments, especially meteorology.

     There were several applications of radio occultation (RO) mentioned during the colloquium. Serious issues such as bulk global climate change could be indicated by thermal expansion of the troposphere using GPS occultation with 3 standard deviations (99% confidence) in 10~20 years. This technology was also applied for planetary observation back to the 1960s. The COSMIC array that is being launched within the next two years has a vast range of applications; while the primary objective is focused on GPS radio occultation. Several functions that can be studied from COSMIC satellites were presented, such as the measurements of a photometer. The precise positioning of the satellites' orbits can also improve the understanding of the time-variant irregular gravity field of the Earth. Surprisingly, as an aerospace engineer, the new orbit insertion technique that NASA developed really amazed me. This ingenious method saves a huge amount of delta-V to separate the different satellites into their own orbits by using the irregular gravity field. Although gravity-pull technique has long been proven to be an efficient method in orbit mechanics, no one has really done it in low-earth-orbit. This colloquium was an important experience for me to learn how different knowledge and different aspects were put together; to see how a professional engineering and science program is organized, conducted and carried out.

     There were several topics in this colloquium that are directly related to my research. My thesis is focused on transient ionospheric perturbations observed from a GPS ground-based array. Typically, the electron content in the ionosphere is computed from the differences of pseudo ranges that correspond to L1 and L2 GPS frequencies. In the case of radio occultation, the electron content is measured in vertical profile (versus altitude), but in the case of the ground-based receiver this electron content profile is represented on a (nearly horizontal) curve of sub-ionospheric-point (SIP) trajectory. Since the electron content represents the integrated electron density from transmitter to receiver, these measurements in fact contain information in one dimension. Two-dimensional mapping of electron content can be calculated (i.e. ionospheric tomography) by assuming the ionosphere is time-invariant and all of the line-of-sight trajectories are approximately on the same two-dimensional plane. Unfortunately this technique cannot be applied to my research since my main concern is the ionospheric perturbation in the time series. However, one important issue - brought up by Dr. Mannucci during the colloquium - might have strong impact upon my research. He talked about the electron content measurements bias existing in both transmitters and receivers. This IEC bias is usually a constant and will not be a significant factor after filtering. However there might be an abrupt change (close to a step function) that could change the filtered waveforms. If this step function shows up in the IEC map for a single satellite, we could observe instantaneous changes across the GPS receiver array. This phenomenon requires further investigation. There was also some other useful and interesting information related to my research, such as data quality control and earthquake precursors in the ionosphere. I have to thank Tony Mannucci, Shu-Sua Chen and Scott Budzien for their valuable suggestions for my research.

Cultural Experience

     It is somewhat difficult for me to write about the cultural experience in Taiwan since I lived in Taipei for about ten years (back to the ancient time). Precisely, I came to the United States after finishing elementary and junior high schools in Taiwan. Naturally I became a translator and provided tourist assistance for the rest of the U.S. students with the Taiwanese students. This gracious opportunity offered by UCAR and NSF also allowed me to go around and visit some of my old friends and relatives in Taiwan two weeks before the summer camp started. I met the other participants at Friends Hotel in Taipei and we visited several famous tourist spots (to which I had been a million times) such as the National Palace Museum, Chiang Kai-Shek's mansion, Taroko National Park and jade markets in Taipei and Hua-Lien. Of course, the delicious Chinese foods, convenient subway system, spectacular Taipei 101 and fashion shopping malls greatly impressed everybody. But my personal favorite is the crowded, hot, loud and dirty traditional night market. At the beginning I did not expect that any American student would dare to try or even have fun in such an "awful" (but awesome!) place. But apparently I underestimated those adventurous students. Many of them were willing to try different cultures, and they enjoyed those peculiar (but excellent) foods and desserts. They even tried the stinky tofu! This tofu smells funny but tastes really good. I have to admit that I did not have the guts to try it when I was a kit because it does smell... really interesting.

     The tour of NSPO (National SPace Program Office) in Taiwan is the first time I actually got to see real satellites and their manufacturing plant with my own eyes. Even though that I am an aerospace engineer and I did build a mini remote control aircraft and mini rocket at Purdue, I have never built a satellite - not even a model or a prototype! The design of those COSMIC satellites is indeed a masterpiece of advanced technology. The propulsion system, attitude control system, power system, and multifunctional remote sensing equipment are all well integrated into those tiny octagons.

     I believe the friendships that were built between students and speakers from different locations of America and Europe are really important, because it is very possible for us to work together toward global climate studies in the future. It was also really nice to meet professors and graduate students from NTU, NUC and PCCU - one day they may do further study in the U.S. After living in America for about eight years, it is such a shame for me to realize that many local students in Taiwan are capable of speaking fluent English as a second language and some perform even better than I do...

Concluding Remarks

     This COSMIC summer camp was a wonderful and exciting experience. I am so glad that I had the chance to participate in the COSMIC colloquium and the field trip. It was a great opportunity to learn the most advanced technology and actually to meet and converse with the top researchers in the world. I have to thank the people at UCAR, especially Bill Kuo, Kim Prinzi-Kimbro and Jonathan Spangler (my roommate!) for arranging this event. I also thank students and professors from Taiwan for accommodating us. I will definitely see them again, some day, in America, or in Taiwan.