| The accurate determination of gravity field is essential to a variety of geophysical applications, including geodesy, geophysics, geodynamics and oceanographic. Among many methods to measure the Earth's gravity field, the use of satellite date has become critically important. At present, there are 3 models in gravity field measurement: high-low satellite-to-satellite tracking (SST-hl, such as CHAMP satellite), low-low satellite-to-satellite tracking (SST-ll, such as GRACE satellite) and satellite gravity gradiometry (SGG, such as GOCE satellite). And among them, the SST-11 is one of the most promising and valuable methods in measurement of gravity field.The SST-11 of gravity field measurement consists of 3 payloads: High Accuracy Accelerometer, Dual-frequency GPS Receiver, High Accuracy KBR System (K Band Ranging System). For High Accuracy Accelerometer and Dual-frequency GPS Receiver, China has put a lot of financial and human resources in the preliminary research, and achieved certain results. Due to various reasons, the research of KBR System is still in the initial stage. KBR System requires micron level measurement accuracy, which pushes it to the commanding height of ranging technology. The United States spent 20 years before the successful development of KBR System, which is now classified as high secret. KBR System is very complex and technically difficult, and there isn't mature product in our country yet. Accordingly, the development of high accuracy KBR System is the bottleneck of our country's gravity field measurement. In the KBR System, as the frequency base, the USO (Ultra-stable oscillators) 's technical performance is essential to the system's measurement accuracy, and the technical level lags greatly behind the application requirements in our country in this area.Based on domestic and international studies on SST, the paper did research on our country's KBR System concept on gravity field measurement, and conducted in-depth research on one of the key technologies-USO. The main research contents and related conclusions are as follows:1. Introducing the theory and major methods of satellite gravity field measurement, and makes analysis of the 3 existing methods in the world, and also the reasons and benefits for choosing the SST-11 are analyzed.2. Making in-depth study on inter-satellite dual-one-way microwave-ranging theory, and put forwards the KBR System main structure used in the gravity field measurement in China, and makes detailed analysis of 7 key technologies which are needed to get breakthrough, and also provides initial methods to do that.3. Making in-depth analysis on various sources of errors affecting the KBR System, and establishes the error simulation model, and makes preliminary estimation on single or general system errors.4. Introducing the development of satellite-loaded dual-frequency GPS receiver, of which the key technologies and system's designing methods are analyzed, and also analyzes the integrated platform of GPS receiver and KBR System.5. Making optimized design of USO's critical components, including quartz crystal, oscillator circuit and constant temperature control system, on the basis of analysis on the factors affecting USO frequency stability. The successfully developed short-term frequency USO stability indicators have reached 5×10-13 / s (Allan deviations).6. Making study on the KBR system's frequency multiplier circuit, and having developed 8 multiplier circuit used for KBR, and the results of test indicate that the requirements of the performance are met.The key innovative points of this paper are as follow:1. On the basis of Microwave-ranging theory analysis, the paper put forward the KBR System designing proposal for China's gravity field measurement, and this proposal has been approved by related experts.2. Making in-depth research on one of the key technologies-USO, and successfully developed the USO and the 8 frequency multiplier, and USO short-term stability indicators reached 5×10-13 / s(Allan deviations), exceeding China's requirement of 2×10-12 /s(Allan deviations).The key KBS System breakthroughs achieved in this paper are not only conducive to gravity field measurement satellite development in our country, but also have far-reaching influence on the satellite formation flying, satellite autonomous navigation, complex system design, high-stability frequency reference source, as well as high-precision measurement and related studies.
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