本文為荷蘭代爾夫特理工大學（作者：Joris Derksen）的碩士論文，共251頁。

無線電探測和測距系統（雷達）是海軍用於探測、跟蹤和識別敵我目標的主要感測器，雷達系統對於感知周圍戰場環境資訊是必不可少的。雷達的效能可以顯著地受到系統部署環境的影響。在某些大氣條件下，折射效應會導致電磁波傳導（electromagnetic ducts）、雷達探測漏洞、跳躍盲區和/或陰影區增加。這些現象會導致戰術上的優勢和劣勢。例如，其中一個優點是，電磁傳導導致目標可檢測範圍擴大，從而提供更多的響應時間來對進入的敵對目標採取行動。其中一個缺點是，敵方目標在雷達漏洞和跳躍盲區仍然不能被探測到，這些區域通常與電磁傳導效應共存。

由於自然環境可以顯著地影響雷達效能，因此非常希望能夠在真實的環境條件下評估雷達的效能。正確評估雷達作戰效能能夠使海軍從戰術優勢中充分受益，並最小化伴隨某些大氣條件的不利因素的影響，也有助於避免錯誤的戰場態勢感知。

多年來，用於模擬雷達傳播和效能的模型已經成熟，只要環境資料輸入也足夠精確，就能產生足夠準確的結果。隨著這些模型的成功開發，研究注意力轉移到獲取真實環境資料的方法。在遠離海岸的快速水流上方的大氣中，單垂直折射率分佈足以進行準確的雷達效能評估，這樣的分佈可以很容易地通過單個無線電探空儀氣球測量獲得。然而，近海岸線在許多情況下需要三維（3D）折射率資料。在這些情況下使用單折射率分佈可能導致錯誤的評估。

為評估雷達效能獲取3D大氣資料並不是沒有挑戰的。因此，根據大氣條件獲得輸入資料所需的精度和解析度，以充分準確地評估雷達效能是非常重要的。在通過數值天氣預報(NWP)系統獲得3D資料的情況下，通過對3D資料最小解析度要求的評估，可以允許減小資料檔案大小。目前，NWP資料檔案太大，無法通過衛星通訊（SATCOM）定期傳送給海軍艦船。這使得NWP資料的使用在一定程度上不適合於實際情況下的操作使用。

本文針對北海天氣條件的不同情況，研究了水平和時間解析度對雷達效能評估精度的敏感性，通過比較“地面真實”效能評估和“破壞”評估完成以上工作（這裡的“破壞”是指人為降低精度的資料）。在本文中，地面真實性定義為基於高解析度NWP HARMONIE4資料的採用雷達傳播模型AREPS3（3.6版）進行的評估。然後將這些評估的真相與基於水平或時間解析度降低的相同HARMONIE資料集的評估進行比較，使用多種測量精度進行比較。地面真實性的結果也與基於單垂直輪廓和標準大氣廓線的評估進行了比較。本文分析了41種不同的場景，每種場景中的艦船位置、雷達部署方位和天氣條件均不同。從KNMI可獲取的資料中，具有不同的天氣變化，如寒冷天氣鋒面（8x）、溫暖天氣鋒面（8x）、暖區（1x）、相對暖空氣在水中的平流、高氣壓系統和平靜海面條件，這些資料都是北海地區全年中非常典型的氣象資料。

真實的雷達效能評估是基於AREPS模型的，AREPS是一種先進的傳播模型，用於計算諸如傳播損耗與距離、高度的關係以及針對任何特定雷達、環境和目標的檢測概率(覆蓋率)。所有場景中都使用3300MHz中程監視雷達，目標為雷達截面2m2的小型戰鬥機，在AREPS中使用雷達和目標整合模型對雷達系統和目標進行建模。

為了獲得以下的結論，原始HARMONIENWP資料需要被輸入到AREPS模型中。因此，開發了一種新的環境模型，該模型適應HARMONIE NWP資料，從而適用於雷達效能評估，並且可以被AREPS使用。使用NWP輸入資料模擬如下：沿傳播路徑整合垂直NWP輪廓到垂直輪廓上；用Monin-Obukhov相似理論（MOST）計算表面層的折射率分佈；將上下輪廓的資料混合在一起。該模型提供了沿傳播路徑的真實折射率分佈。雖然它適合於本論文的研究目的，但它尚未被驗證用於實際操作使用。

將所需的大氣資料轉換為適合雷達效能評估的資料，獲得地面真實值評估和不同實驗場景的損壞評估，並進行比較，從而研究評估精度對資料輸入解析度的敏感性。結果表明，在寒冷或溫暖天氣鋒面附近或溫暖扇區條件下，基於單輪廓的評估與地面真實值相比具有良好的效能。因此，在這些情況下，由無線電探空儀測量的單輪廓資料足以滿足雷達效能評估。

暖幹空氣在寒冷水面平流的情況下，效能評估的準確性受到粗解析度的影響而顯著降低。在實驗中的九個場景裡，只有一個場景下使用單輪廓資料能夠滿足評估要求；在其它八個場景中使用單輪廓資料都會導致錯誤的評估，如果依賴該類檔案，可能會產生戲劇性的後果。例如，對150km以外的探測距離預測產生誤差。顯然，在這些情況下，必須獲取3D大氣資料。

高氣壓附近的評估結果與暖幹空氣流過寒冷水面的情況相似。在這些情況下，只有八個場景中的兩個允許使用單輪廓來進行精確的雷達評估。

對七種場景進行了分析，預計這些場景不會發生電磁異常傳播；這一預期是基於天氣圖的評估。在沒有傳播異常的情況下，單輪廓資料就足夠了。在這七種場景中只有五個是正確的，因此，不能僅僅通過檢視天氣圖來可靠地預測單輪廓資料是否足夠。

將基於單輪廓的評估與基於標準輪廓的評估進行比較表明，一般來說，單輪廓比標準輪廓提供更準確的雷達效能評估。這意味著，當必須採用3D資料時，這兩種評估都將出現顯著的錯誤。

通過比較相同空間解析度、不同時間條件下的地面真實資料與基於大氣資訊的評估結果，研究了評估精度對時間解析度的敏感性。結果表明，在許多情況下，每24小時測量一次是不夠的。然而，目前RNLN每24小時只進行一次大氣測量。在許多情況下的時間解析度要求不小於1小時。也就是說，在單輪廓資料滿足要求的情況下，可以降低時間解析度，並且在一些情況下可以降低到24小時。

在本文中，沒有指出實際最低要求的解析度，因為需要評估更多的真實場景；這些場景應該在雷達引數、目標引數和/或天氣條件上有所不同。此外，所需的最小解析度取決於所需的評估精度，這可能根據具體應用而變化。

除上述研究外，本文的其它目的是提供清晰易懂的雷達在非均勻大氣中傳播的文獻，並就雷達效能評估結果向荷蘭皇家海軍（RNLN）提出建議。這篇論文可作為雷達傳播特性的一般概述，也可作為教育中進一步研究的引言。對於RNLN，最重要的建議是開始使用NWP資料進行雷達效能評估。使用單一測量的折射率分佈，這是目前的測試方法，可能導致錯誤的評估和潛在的戲劇性的操作後果。由於還沒有定義最低要求的解析度，因此建議使用當前可用大氣(NWP)資料的最大解析度，並且每小時更新一次資料，儘管這在邏輯上具有挑戰性。

Within navies, radio detection and rangingsystems (radars) are the primary sensors for the detection, tracking andsometimes classification of friendly and hostile targets. They are essentialfor creating an operational picture of the surroundings and situationalawareness. The performance of radars can be significantly influenced by theenvironment in which the systems are deployed. Under certain atmosphericcondition refractive effects result in electromagnetic ducts, radar holes,skipping zones and/or increased shadow zones. These phenomena can lead totactical advantages and disadvantages. For example, an advantage is that ductslead to extended detection ranges thereby providing more response time to actagainst incoming hostile targets. A disadvantage is that hostile targets canremain undetected in radar holes and skipping zones that generally coexist withthe developed duct. As the environment can significantly affect the performanceof radars, it is highly desirable to be able to assess radar performance underprevailing conditions. The ability to assess radar performance allows navies tofully benefit from the tactical advantages and to minimise the effects of thedisadvantages that accompany certain atmospheric conditions. It also helps avoidfalse situational awareness. Over the years models for modelling radar propagation1and performance have matured to the extent that they produce sufficientlyaccurate results provided that the environmental data input is alsosufficiently accurate. With the successful development of these models attentionshifted to methods for obtaining this environmental data. In large air masses overfast bodies of water away from the coast a single vertical refractivity profilesuffices for accurate radar performance assessment. Such a profile can easilybe obtained by means of a single radiosonde balloon measurement. Nearcoastlines however three dimensional (3D) refractivity data is required in manycases. Using a single profile in these cases may result in erroneous assessments. Obtaining 3D atmospheric data for radar assessmentpurposes does not come without challenges. Because of this, it is important toget a sense of the required accuracy and resolution of input data, depending onatmospheric conditions, to assess radar performance sufficiently accurate. Incases where 3D data is obtained by numerical weather prediction (NWP) systems,assessment of the minimal 3D data resolution requirement allows the reductionof data file size. Currently NWP data file sizes are too big to send to navyships by satellite communication (SATCOM) on a regular basis. This makes theuse of NWP data, to a certain extent, unsuitable for operational use. Thisthesis studies the performance assessment accuracy sensitivity to horizontaland temporal data resolution for different cases of North Sea weather conditions. This is done by comparing “groundtruth” radar performance assessments with “spoiled” assessment2.For this thesis, the ground truths are defined as the assessments computed bythe radar propagation model AREPS3 (version 3.6) on the basis of high resolutionNWP HARMONIE4 data. These truths are then compared to assessments based on setsof the same HARMONIE data which are reduced in horizontal or temporalresolution. Comparisons are made using several measures of accuracy. The groundtruths are also compared to the assessments based on a single vertical profileand a standard atmosphere profile. In total 41 different cases are analysed. Casescenarios differed in ship position, radar deployment azimuths and weatherconditions. In the available data from the KNMI, weather conditions variedbetween cold weather fronts (8x), warm weather fronts (8x), a warm sector (1x),advection of relatively warm air over water, high pressure systems and calmconditions, which is a fairly representative data set for the conditions overthe North Sea throughout the year. The actual radar performance assessments areobtained using AREPS. AREPS is an advanced propagation model that computes, forexample, propagation losses versus range and height and detection probabilities(coverage) versus range and height for any specific radar, environment andtarget combination. The radar used was for all scenarios a 3300 MHz medium rangesurveillance radar and the target was a small fighter with a radar crosssection of 2 m2. Theradar system and target were modelled using the integrated radar and targetmodel in AREPS. In order to acquiring the results that are summarised below,raw HARMONIE NWP data needed to be fed into AREPS. For this a new environmentalmodel was developed that adapts HARMONIE NWP data so that it is suitable forradar performance assessment and can be used by AREPS5. Using NWP input datathe model subsequently: integrates vertical NWP profiles to vertical profilesalong the propagation path; computes the refractivity profile of the surfacelayer using Monin-Obukhov similarity theory (MOST); and blends the lower andupper profiles together. The model provides realistic refractivity profilesalong the propagation path. Whilst it is suitable for the purpose of thisthesis, it is has not yet been validated for operational use. Converting therequired atmospheric data into data suitable for radar performance assessment, obtainingthe ground truth assessments and the spoiled assessments of the differentexamined scenarios and comparing these, allowed the assessment accuracysensitivity to data input resolution to be studied. Results show that in thereviewed cases near a cold or warm weather front or in a warm sector,assessments based on a single profile compared well with the ground truth.Hence, in these cases a single profile measured by, for example, a radiosondewill suffice for radar performance assessment. In scenarios where warm dry airadvected over cooler water the accuracy of the performance assessment decreasessignificantly at coarser resolutions. In only one of the nine scenarios a singleprofile sufficed. Using a single profile in the other eight scenarios resultedin erroneous assessment, which may have dramatic consequences if relied upon.For example, detection range prediction errors of over 150 km occurred. It is clear that in these casesacquiring 3D atmospheric data is a must. Results for assessments in thevicinity of high pressure systems were similar to that of the scenarios wherewarm dry advected over cooler water. In these cases only two out of the eight scenariosallowed the use of a single profile for accurate radar assessment. Sevenscenarios were analysed in which it was expected that no anomalous propagationwould occur. This expectation was based on the evaluation of the weathercharts. In the absence of propagation anomalies, it was also expected that inthese cases a single profile would suffice. This was true for only five out ofthe seven cases. Hence, one cannot dependably predict if a single profile willsuffice merely by looking at weather charts. Comparing assessments based on asingle profile to the assessments based on a standard profile showed that, ingeneral, a single profile provides more accurate radar performance assessmentsthan the standard profile. This said, when 3D data is a must, both assessments willbe significantly in error. Investigating the sensitivity of assessment accuracyto temporal resolution was done by comparing the ground truth to assessments basedon atmospheric data with the same spatial resolution but of a different time.The results show that a single measurement every 24 hours is not sufficient inmany cases. Current practice in the RNLN, however, is to carry out atmosphericmeasurements only once every 24 hours. In many of the viewed cases a temporal resolutionof no less than 1 hour is required. This said, in cases where a single profilesuffices the required temporal resolution can be reduced, and in some cases to24 hours. In this thesis no actual minimum required resolutions areestablished. To do so many more scenarios should be evaluated. These scenariosshould differ in radar parameters, target parameters and/or weather conditions.Also the minimum required resolution depends on the required accuracy which mayvary with application. Besides the research described above, other goals ofthis thesis were to provide clear and understandable documentation on radarpropagation through an inhomogeneous atmosphere and to give recommendations tothe Royal Netherlands Navy (RNLN) concerning radar performance assessment. Thisthesis can be used as a general overview on radar propagation, for educationalpurposes and as an introduction to further research. For the RNLN, the most importantrecommendation is to start using NWP data for radar performance assessment. Theuse of a single measured refractivity profile, which is current practice, canlead to erroneous assessments and potentially to dramatic operationalconsequences. As no minimum required resolution is yet defined, it isrecommended, although logistically challenging, that the maximum resolution ofavailable atmospheric (NWP) data is used and that the data is updated everyhour.

1 引言

2 雷達傳播機制

3 對流層傳播

4 雷達傳播建模方法

5 環境建模

6 評估資料解析度對靈敏度影響的方法

7 精度靈敏度與資料解析度：結果、分析與討論

8 結論與建議

9 對荷蘭皇家海軍操作使用的建議

附錄A 雷達效能評估

附錄B 相似性函式

附錄C 實現MOST的MATLAB原始碼

附錄D 實驗場景設定

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