Core Viewpoint - The article highlights the significance of Serbia's 250th Air Defense Brigade, which gained fame for shooting down a NATO F-117A stealth fighter during the 1999 conflict, and its current collaboration with China in acquiring advanced air defense systems, specifically the FK-3 and HQ-17AE missile systems [1][3][4]. Group 1: Historical Context - The 250th Air Defense Brigade is renowned for using the SAM-3 missile to down the F-117A, marking the first instance of a stealth aircraft being shot down by a surface-to-air missile [1][4]. - The F-117A was considered invulnerable due to its stealth technology until it was successfully targeted by the older SAM-3 system, showcasing the effectiveness of training and tactics over technological superiority [6][7]. Group 2: Current Developments - Serbia has publicly showcased its new Chinese-made FK-3 and HQ-17AE air defense systems, with officials expressing pride in these acquisitions [7][9]. - The FK-3 system has a maximum range of 100 kilometers and can intercept various aerial threats, significantly enhancing Serbia's air defense capabilities [12]. - The HQ-17AE system is designed for low-altitude defense and can engage multiple targets simultaneously, further strengthening Serbia's military capabilities [15]. Group 3: User Experience and Collaboration - The 250th Air Defense Brigade has reported high satisfaction with the performance and reliability of the Chinese missile systems, emphasizing effective collaboration with Chinese manufacturers for system adaptations [9][12]. - The introduction of these systems has improved Serbia's airspace control and defense capabilities, allowing for better protection of critical infrastructure [9][12]. Group 4: Future Outlook - The brigade's commander expressed interest in continuing to monitor advancements in Chinese military technology, indicating a commitment to integrating new systems into Serbia's defense strategy [16].

Quantitative Models and Construction Methods - **Model Name**: Guozheng Aerospace and Aviation Industry Index (CN5082.CNI) **Model Construction Idea**: The index is designed to reflect the market performance of aerospace and aviation industry companies listed on the Shanghai, Shenzhen, and Beijing Stock Exchanges [41][42] **Model Construction Process**: 1. **Sample Space**: Select A-shares and red-chip enterprises' depositary receipts that meet the following conditions: - Non-ST/*ST securities - Listed for over 1 year (for STAR Market and Beijing Stock Exchange securities) or over 6 months (for other securities) - No major violations or financial reporting issues in the past year - No abnormal price fluctuations during the observation period - Belong to the aerospace and aviation industry under Guozheng's tertiary industry classification [43] 2. **Candidate Pool**: - Calculate the average daily free-float market capitalization and average daily trading volume over the past six months for eligible securities - Exclude the bottom 10% of securities ranked by trading volume if the pool exceeds 10 securities [43] 3. **Sample Selection**: - If the candidate pool contains ≤30 securities, all are included - If the pool contains 30<N≤50 securities, select securities covering 85% of free-float market capitalization, rounded to the nearest multiple of 10 [43] - If the pool contains >50 securities, select securities covering 85% of free-float market capitalization, rounded to the nearest multiple of 10, capped at 50 securities [43] 4. **Weighting**: Free-float market capitalization weighting [43] 5. **Adjustment**: Regular adjustments occur semi-annually, with temporary adjustments for special cases like delisting or corporate actions [44] **Model Evaluation**: The index is highly focused on the aerospace and aviation sector, with a strong representation of small-cap stocks and high exposure to the defense industry [44][47] Model Backtesting Results - **Guozheng Aerospace and Aviation Industry Index**: - **Annualized Return**: 6.26% (past five years) [61][62] - **Sharpe Ratio**: 0.33 (past five years) [61][62] - **Maximum Drawdown**: -55.93% (past five years) [61][62] - **Annualized Volatility**: 34.13% (past five years) [61][62] - **Recent Performance**: 33.78% return in the past year, outperforming other broad-based indices and military-themed indices [64][66] - **Bull Market Elasticity**: Demonstrated strong performance during bull market periods, with gains of 40.56%, 50.90%, and 38.36% in specific intervals [65] Quantitative Factors and Construction Methods - **Factor Name**: "Military Exposure" **Factor Construction Idea**: Focus on stocks with high exposure to the defense industry, particularly aerospace and aviation [47][55] **Factor Construction Process**: - Select stocks with significant involvement in defense-related activities, such as aircraft manufacturing, satellite technology, and unmanned systems [47][55] - Weight stocks based on their free-float market capitalization [43][55] **Factor Evaluation**: The factor achieves high representation of military-related stocks, with 97% of the index's components belonging to the defense industry [47][55] - **Factor Name**: "Aerospace Exposure" **Factor Construction Idea**: Emphasize stocks within the aerospace and aviation sub-sector [47][60] **Factor Construction Process**: - Identify stocks classified under the aerospace and aviation sub-sector [47][60] - Weight stocks based on their free-float market capitalization [43][60] **Factor Evaluation**: The factor has a high concentration in aerospace stocks, with 51% of the index's weight allocated to this sub-sector [47][60] Factor Backtesting Results - **Military Exposure Factor**: - **Representation**: 97% of index components belong to the defense industry [47][55] - **Aerospace Exposure Factor**: - **Representation**: 51% of index weight allocated to aerospace stocks [47][60] - **Unmanned Systems Factor**: - **Representation**: Over 12% of index weight allocated to stocks involved in unmanned systems, such as drones [60]