Laurent Warlouzet
email: laurent.warlouzet@sorbonne-universite.fr

The Centralization of EU Competition Policy: Historical Institutionalist Dynamics from Cartel Monitoring to Merger Control (1956-91)

Keywords: competition Policy, historical institutionalism, commission, history

The contemporary strength of EU competition policy does not stem naturally and mechanically from the Treaty of Rome, nor is it only a consequence of the spread of 'neoliberal' ideas or the single market programme. It is also the product of decades of dynamics underlined by historical institutionalism, which allowed the Commission to secure decisive powers, despite the unwilling-ness of some of the most powerful Member States. In this regard, the two most important corner-stones were Regulations 17/62 on cartels and 4064/89 on mergers. The Commission benefited from the unintended consequences of decisions taken in the Council and from the path dependencies created by Regulation 17/62. It progressively developed a centralized institutional framework with itself at the centre.

Introduction

Competition policy is probably the domain in which the European Commission has enjoyed the largest influence over economic actors, be they companies, consumers or nation-states. The supranational institution has considerable leeway to make decisions alone, and to co-ordinate the decisions of national regulators. Nowadays, the 'modernization' process of EU competition policy involves a 'decentralization' of competences. This evolution cannot be understood without considering the previous dynamic of the centralization of power. The centralization process in competition policy refers to a process by which the Commission has acquired a monopoly, both in terms of information and in terms of decisions on anti-competitive practices. By contrast, competition policy could have been carried out with a stronger role in the decision-making process for Member States and/or for ad hoc independent agencies (at the national and/or at the European level).

First, this article claims that this centralization on the Commission was a surprising outcome, considering the preferences of the larger Member States. Many alternative institutional schemes had been seriously considered, which usually relied on a non-centralized framework. Second, an important explanation of this surprising outcome is the linkage between two milestones pieces of legislation, Regulation 17/62 on cartels and Regulation 4064/89 on mergers. As a result, an exploration of archival material, some of it just recently released, brings new insights for understanding the motivations of the main decision-makers. This article relies on the records of the four main actors: the European Commission, France, Germany and the UK. 1 Third, the concepts of path dependencies and of unintended consequences underlined by historical institutionalism are especially fruitful for understanding this long-term dynamic. Two path dependencies established in 1962 played a role in 1989: the template of centralization and the negative path dependency of the backlog. At many crucial points, decision-makers both in national government and at the Commission took decisions without foreseeing many undesirable unintended consequences.

As a result, this article is a useful complement to a literature which generally puts an emphasis on other dynamics, such as the spread of neoliberal ideas, the single market programme or the pressure on nonstate actors such as companies, either directly or via case law. Section I will review the existing literature to demonstrate the relevance of historical institutionalism for the study of competition policy. Section II will delve into the origins of Regulation 17/62, before Section III examines the negotiations leading to the 'Merger Regulation' of 1989.

I. Historical Institutionalism and Competition Policy

The history of European competition policy

There is now a large body of literature on the history of European competition policy. However, most of it is divided between historical literature on the origins of Regulation 17/62 and political science literature on the dynamics leading towards the Merger Regulation. It reflects the corporatist divide between the two disciplines, even if some academics on both sides try to bridge the gap (Kaiser, 2010). A recent volume on the history of competition policy stops in the early 1980s and does not deal with mergers (Patel and Schweitzer, 2013). Several studies having a wide scope exist, but they do not deal with the decision-making process of both Regulations 17/62 and 4064/89, and consequently do not try explicitly to weave a link between both pieces of legislation. The most convincing studies deal with the role of ideas (Akman and Kassim, 2010) or with the relationships between non-state and supranational actors (Büthe, 2007; McGowan, 2007). The seminal and much-discussed book by David Gerber, for instance, devoted only a few pages to both negotiations (Gerber, 1998, pp. 349-50 and 380-1). Lastly, the reference study on competition policy, the textbook written by Michelle Cini and Lee McGowan, has an excellent historical chapter, but it is only an introductory survey (Cini and McGowan, 2009, pp. 11-40). Conversely, studies de-voted to detailed examination of the decision-making process in European institutions over a long timeframe generally treat competition policy as a minor field (Moravcsik, 1998, pp. 218-9).

The historical institutionalist approach has sometimes been applied to the history of European competition policy (Armstrong and Bulmer, 1998; Bulmer, 1994; Büthe and Swank, 2007). This article will complement their perspectives by shedding new light on the alternative institutional designs envisaged in the 1960s and on the path dependencies remaining in the 1980s, thus reinforcing the overall historical institutionalist argument. The role of unintended consequences in the development of European competition policy has already been underlined by Stephen Wilks, but only with regard to the modernization process (Wilks, 2005) or as a stimulating but undocumented intuition (Wilks and Bartle, 2002). He called Regulation 17/62 a 'slowly ticking bomb for over 20 years until it exploded in the faces of the national governments during the 1980s'.

As a result, there is room for a detailed study of the decision-making process of the crucial milestones in the development of European competition policy. A global and com-prehensive study of the 12 Member States, the five EEC institutions and non-state actors being impossible to undertake in a single article, three choices have been made. In terms of actors, only the Commission and the leading Member States have been studied. This choice is justified by the limited role of the European Parliament in the decision-making process in the period considered. The roles of the European Court of Justice (in particular through the Philip Morris case; Büthe and Swank, 2007) and of business actors (Buch-Hansen and Wigger, 2011), have already been studied with regard to the Merger Regulation.

In terms of case studies, the focus is put on two negotiations. The first is cartel control, since it was the first area of competence developed by the Commission. The second is merger control; its development is relatively easy to study because it was largely created by a single piece of legislation, Regulation 4064/89, whereas other areas have generally been developed progressively by successive landmark decisions and pieces of legislation. Moreover, merger control is a peculiar example of centralization, since there was no legal basis for it in the Treaty of Rome. Last, it became one of DG Competition's most high-profile policies once it had been created.

In terms of actors, the emphasis will be put on Germany, France and the United Kingdom, because they are the most powerful Member States and because they have among the oldest competition policy laws in Europe, set up respectively in 1957 (without taking into account the allied laws), 1953 and 1948. Most of the other Member States, including Belgium and Italy, did not have proper national legislation for a long time. The Netherlands had legislation before the signing of the Rome Treaty in 1957 but it was weakly enforced, as cartelization was widespread until the 1990s (Bouwens and Dankers, 2010). In the Commission, only the activity of the Commissioner for competition and its Directorate General (DG Comp, previously known as DG IV) will be considered.

Historical Institutionalism and EU Policies

In order to overcome the historian's tendency to indulge in a detailed chronological narrative, recourse to historical institutionalist tools is useful. Historical institutionalism (HI) posits that time matters in explaining the outcome of the decision-making process (Steinmo and Thelen, 1992). It is not similar to narrative history: 'historical institutionalism is less about drawing lessons from or documenting the past than it is about identifying the conditions under which and mechanisms by which the past affects the present and the future' (Fioretos, 2011, p. 383). It posits that temporality has an influence through three crucial mechanisms: 'path dependency', 'critical juncture' and 'unintended consequences' (Hall and Taylor, 1996). The first dynamic means that decision-makers are constrained by decisions taken earlier on, sometimes decades earlier. They do not negotiate on a blank page. 'Path dependencies' make alternative designs more difficult to promote, even those which would, in theory, be more in tune with the actors' preferences. They trigger dynamics of 'positive feedback' and 'increasing return' that reinforce institutions (Streeck and Thelen, 2005, p. 6). This concept is useful to understand the discrepancy between the aims of the main actors -in this case, the reluctance of many decision-makers of the three biggest member states against the Commission's initial proposals -and the outcome of centralization. Path dependencies can also work against the Commission, as in the case of the backlog (see below) which seriously undermined its credibility as an efficient institution. However, this 'negative' path dependency (seen from the European Commission's perspective) paradoxically helped to reinforce the centralization process.

Path dependencies originate during 'critical junctures', which are short periods of times when 'wideranging change is possible' (Capoccia and Kelemen, 2007, p. 352). The aim of this article is to demonstrate that 1962 was such a 'critical juncture', much more than the period of the Rome Treaty, by demonstrating that many 'alternative choices were available to the decision makers' at this point (Capoccia and Kelemen, 2007, p. 369), but not later on.

'Unintended consequences' mean that the outcome of negotiations can appear disappointing after a first apparent victory. This is, for example, the case when a regulation is not implemented in the way anticipated during its negotiation. As a result, HI aims to use history to understand why actors took decisions which appear, with hindsight, illogical or detrimental to their interests. As Paul Pierson underlined, it is especially suited to the study of European institutions (Pierson, 1996). Within the European institutional framework, national decision-makers are especially constrained by the partial autonomy of supranational institutions, the multiplicity of technical issues to master, the limited timehorizons of national decision-makers and their shift in policy preferences.

Against historical institutionalism, it has been argued that Member States usually have well coordinated European policies which enable them to master the EEC/EU decision-making process and to impose radical reforms, in particular through intergovernmental conferences (Kassim and Menon, 2003, pp. 130-1). Despite these criticisms, HI is especially suited to the study of competition policy, since this field has barely been affected by Treaty revision and had largely been considered as secondary by national officials for most of the period considered. Competition policy developed as a significant public pol-icy in West Germany in the 1960s, but in most other countries only in the 1980s. In the United Kingdom and in France, where provisions had already existed for decades, com-petition policy was often subdued under the influence of more powerful policies such as price policy or industrial policy. As a result, even when the Merger Regulation was negotiated in 1989, competition policy was a new area of expertise, and not a priority for decision-makers. National ministers had changed frequently and did not consider this issue as being at the core of their agenda (except in Germany). Last, the Commission has been especially important in this field since it is one of the first supranational policies, along-side the CAP and the commercial policy.

It is necessary, however, to use HI in a neutral way regarding the balance of power be-tween Member States and the Commission. Indeed, the Commission's officials can also be affected by the dynamics of path dependency and of unintended consequences. The Commission is internally divided along national origins but also with regard to political and economic conceptions.

Recourse to a classical historical methodology, that is to say, the examination of archival records, allows the researcher to go beyond the press releases, speeches and memoirs. With regard to HI, archival records are particularly useful to assess whether decision-makers were really constrained by past agreements, or whether they were free to discuss alternative designs. Taking into consideration the alternatives seriously considered by the decision-makers, that is to say, discussed within the Council either on the proposal of the Commission or of Member States, is important to show that the outcomes -in this case, centralization -are not inevitable, but are the product of a selection process among different alternatives, often forgotten in the subsequent accounts.

II. The Path Dependencies of 1962

The foundational years of EEC competition policy, namely between the negotiations of the Rome Treaty in 1956 and the passing of Regulation 17/62 in 1962, have now been thoroughly examined (Bussière, 2007; Hambloch, 2009; Montalban et al., 2011; Pace and Seidel, 2013; Pitzer, 2009; Warlouzet, 2011, pp. 269-338). Drawing on this vast body of literature -which does not use HI concepts explicitly -and on further research in archives, this contribution will focus on the question of centralization between 1956 and 1962. It will underline two points. The first is the contrast between the vague and general provisions of the Treaty of Rome regarding competition policy and the institutional outcome of Regulation 17/62. This point has already been underlined in a few studies (Gerber, 1998, p. 349), but most of them failed to underscore the fact that many alternatives were seriously discussed. Second, the importance of path dependency and of unintended consequences will be underlined by pinpointing the discrepancy between the anticipation of actors during the negotiation of the regulation and its early implementation.

The Vagueness of the Rome Treaty

The Treaty of Rome was largely influenced, with regard to the competition policy provisions, by a confrontation between the French and the Germans, whose national experiences contrasted sharply.

The French law of 1953 was embedded in price policy, as its main aim was to fight against 'restrictive practices' that had an inflationary effect. A severe stance was maintained against some types of distribution agreements as they were supposed to hamper the modernization of the distribution sector. Conversely, cartels as such were not particularly discouraged. The 1953 law had established a consultative committee linked to the Ministry of Economics. The cartels were generally cleared, and 'no drastic sanctions' were taken (Riesenfeld, 1962, p. 469).

Germany had no law on its own when the Treaty of Rome was negotiated, except the allied law of 'decartelization'. The law against the restriction of competition was eventually adopted in July 1957 but some of its features were already visible in 1956, when the Treaty of Rome was negotiated. Two of these features contrasted vigorously with the French example. First, competition policy was considered in Germany as crucial in the building of the new democratic and liberal Germany which would break from the past, as the National Socialist era was associated with cartelization (Gerber, 1998, pp. 232-65). For ordo-liberalism, a school of thought which influenced many German officials, eco-nomic liberalism was strongly linked to political liberalism and competition policy played a central role in this process. It was part of an 'economic constitution' designed to ensure that individual freedom was guaranteed. Second, the principle of prohibition had already been largely accepted in 1956. It meant that all cartels were banned, unless they were explicitly authorized by an authority. The prohibition principle of the afore-mentioned German law contrasted with the abuse principle adopted in France (all cartels are authorized unless they are explicitly banned). There was no merger control. The Minister of Economics, Ludwig Ehrard, did not want to weaken the industry with stringent provisions against concentrations.

During the Treaty of Rome negotiations, these positions translated into a clash of models. As documented in a PhD thesis based on French and EU archives, the French proposed a competition policy based on the abuse principle and on the same treatment of all restrictions to competition: cartels, concentration and individual practices (Warlouzet, 2011, pp. 274-5 and 294-6). In 1956, the French officials had in mind the failure of the ECSC's High Authority to tackle the large German companies, and more generally to implement an efficient competition policy (a failure documented in Witschke, 2009), so they considered this field relatively minor within the negotiations.2 While they did not want to establish a powerful supranational competition policy, they were interested in controlling concentration as they feared the competition of the larger German companies, even if they did not say so openly. The Germans took the reverse position: they put an emphasis on the fight against cartels, and they were more lenient against concentrations. Above all, Alfred Müller-Armack, the German negotiator and a close collab-orator of Ludwig Erhard, insisted on securing the prohibition principle. In internal documents, he explained that it was compulsory for domestic reasons: if the prohibition principle was not upheld at the EEC level, it could be threatened at the national level. [START_REF] Mcgowan | The Antitrust Revolution in Europe[END_REF] The two sides agreed on one point: they did not want to give large power to the European authorities in this remit. For the French, competition policy was only a minor field, especially considering the failure of the ECSC competition policy. For the Germans, what was most important was to preserve their future national law, whose long-standing negotiation was not yet completed. That is why both countries accepted the compromise presented by Hans von der Groeben, the president of the group negotiating the articles on competition policy, with the support of the Dutch and Belgians (Büthe and Swank, 2007, 23). 4 It left the main questions largely unanswered: article 85 EEC (article 101 TFEU) contained the prohibition principle in the first paragraph, but also the exceptions in the third paragraph. Cartels could be banned according to article 85-1, but also authorized if they fulfilled the criteria of article 85-3 (contribution to technical progress, etc.). This association of a prohibition provision with an important exception looked like the French abuse law. Article 86 EEC (article 102 TFEU) on dominant position was vague and article 87 EEC left the implementation of the first two articles to a further regulation. It was stated that this future regulation should take into account the national laws (article 87-2e) and 'the need, on the one hand, of ensuring effective supervision and, on the other hand, of simplifying administrative control to the greatest possible extent' (article 87-2b). In other words, no clear institutional framework was defined by the Treaty.

The Decisive Regulation 17/62

Regulation 17 in 1962 clarified the uncertainties of the Treaty of Rome in three ways. First, it interpreted article 85 as a ban on cartels. It followed the German interpretation rather than the French theory of 'abuse', despite the fact that the wording of article 85 was somewhat close to article 59 of the French law of 1953. Regulation 17/62 banned all anticompetitive agreements (if capable of affecting intra-EEC trade) unless the Commission was notified. It meant that a procedure dominated by the competition criterion was imposed, whereas the French negotiators pleaded for a system called the 'global economic assessment' of cartels (Warlouzet, 2011, pp. 305-6). It was broader, as it took into account all the economic consequences of a cartel (on the modernization of economic structures, for example). It was close to the criterion of the 'general economic interest'. In the French perspective, competition policy was not erected as an independent public policy relying on its own criteria.

Second, it gave clear priority to the fight against cartels (article 85 EEC) on the monitoring of dominant position (article 86 EEC). As a result, competition policy progressively asserted itself as a genuine public policy, with its own logic and independent from other fields like industrial, social or regional policy. This stance had been constantly affirmed by the Commissioner for competition, Hans von der Groeben. Third, Regulation 17/62 gave extensive power to the Commission, which received a monopoly on information, via the notification procedure, and on decisions, as the committee of Member State experts was only consultative. As article 85-1 EEC was interpreted as a full prohibition on cartels, an institution was required to grant the exemption of article 85-3 EEC. The Commission fulfilled this role. Again, the German model was probably the closest to the Regulation 17/62 system, even if it was not a pure adaptation, since the Commission was a political institution. Its function was, among others, to propose legislation and not simply to implement it. By contrast, the German competition authority created in 1957 was a purely ad-ministrative authority, albeit with a high degree of independence from the government. This outcome does not mean that the German national government's conception prevailed, but rather that a reinterpretation of the German experience by the European Commission did. Bonn had different priorities in 1960, when the negotiations over the future regulation began. It wanted to preserve the competences of its national competition authority, the Bundeskartellamt, and thus refused to envisage the centralization of decision-making with the Commission at first. The Bundeskartellamt proposed to follow the German experience more closely, by setting up an independent cartel authority, with less risk of political interference than the European Commission. 5 Eventually, Bonn altered its stance and accepted the revised proposal of Commissioner von der Groeben.

Alternative schemes were also envisaged by the French negotiators. From the start, they had defended an interpretation of the Rome Treaty based on the abuse principle and a decision-making process associating the Commission and the Member States (Warlouzet, 2011, pp. 304-7). France was not isolated. During the first extensive debate among the six governments on the Commission's proposal in November 1960, France, Italy and Belgium opposed this choice to centralize both the assessment of the agreement and the power to take final decisions under the Commission. 6 For many actors, it appeared logical to separate both tasks, as in the French experience, or to create an independent authority, as in the German and British cases. The idea of a European cartel authority was aired by the President of the Commission, the German Walter Hallstein, in 1960 in a letter to the Council of Ministers. [START_REF]Hallstein to the Council[END_REF] The German trade unions (DGB) supported an institutional framework designed to associate societal actors to the decision-making process. [START_REF] Warlouzet | [END_REF] This balance of power evolved in favour of the Commission because of von der Groeben's skills but also for various contingent reasons, including a change of government in Belgium which brought in a socialist government more hostile to cartels. More-over, the French government clearly overestimated its leeway. In a letter to de Gaulle of 9 June 1961, the Prime Minister Michel Debré still thought that it was possible to 'isolate' Germany on cartels. [START_REF] Debré Collection | DE30[END_REF] French officials did not anticipate the outcome of the negotiations, which occurred only six months later.

The outcome of the negotiations has sometimes been considered mainly as a side-effect of an intergovernmental bargain between France (which secured the CAP on 14 January 1962) and Germany (which got the agreement on competition policy; Moravcsik, 1998, p. 218; Pitzer, 2009, p. 407). This inter-governmental bargain certainly played a role, but it does not demonstrate that the Member States got what they wanted, as the gap between the initial aim of the German government and the final regulation shows. Moreover, this bargain was unnecessary to conclude the agreement. According to article 87 EEC, the regulation had to be adopted unanimously only during the first period of three years (1958-60). As a result, it could have been adopted by a qualified majority voting in 1961, so France, isolated along with Luxembourg in its opposition, could have been outvoted. In the aforementioned letter of 9 June 1961, Michel Debré, an outspoken adversary of supranationalism, worried about the growing use of qualified majority voting within EEC institutions. [START_REF] Debré Collection | DE30[END_REF] In December, the French Department in charge of co-ordinating European policy explicitly envisaged the scenario of France being outvoted in the Council. [START_REF][END_REF] This demonstrates that the threat of qualified majority voting was real.

Commissioner von der Groeben also made concessions. He agreed to alter his initial proposal of 1960, in particular by taking into account the European Parliament report of Deringer and specific French requests. For example, the French policy against vertical agreements was more severe than that of its neighbours, including Germany (Riesenfeld, 1962, p. 473), as the German law targeted horizontal rather than vertical restraints (Gerber, 1998, p. 295). Thus France secured article 22, which stipulated that, within a year of the regulation's entry into force, the Council (on a proposal of the Commission) was to examine the possibility of making the notification procedure mandatory for certain types of agreements. This clearly meant distribution agreements according to the French interpretation, which was accepted by DG IV. [START_REF]Verloren van Themaat to Fontanet[END_REF] These additions did not contradict, and even reinforced, the main features of Regulation 17/62, namely a genuine pro-competition approach and full centralization under the Commission.

Path Dependency during Implementation

The implementation of Regulation 17/62 quickly became a nightmare for the Commission. In April 1963, the Commission received 36,000 notifications, while DG IV had only 78 officials in 1964 (Pitzer, 2009, p. 416; McGowan, 2010, pp. 128-9). As competition policy was still a new field in Europe, setting criteria to gauge the cartels proved to be a near-impossible task. This created a strong and negative path dependency for the Com-mission for decades. The DG IV policy was largely reactive rather than proactive, since it had to cope with these notifications (Cini and McGowan, 2009, p. 215).

The origins of this backlog are usually attributed to the notification procedure of Regulation 17/62 but, strictly speaking, this is not exactly the case. In fact, the Commission received only 800 notifications in November 1962. As a result, Commissioner von der Groeben apparently sought to encourage further notifications. 13 He secured the adoption of Regulation 153/62, which introduced a simplified notification procedure for certain exclusive agreements, and the Commission published a communication on the subject of notification (EEC Commission, 1962). Nevertheless, European business organizations remained wary, as they did not know whether or not they had to notify their agreements. In a letter to UNICE (Union des industries de la Communauté européenne), the main EEC-wide business organization, von der Groeben expressly asked companies to notify their distribution agreements, including those for which the need to notify was uncertain. 14 He said that the Commission would be 'moderate' and 'understanding' for the companies which 'trusted' it. This decision greatly contributed to the flow of notification. The vast majority of these (almost 90 per cent) were exclusive dealing agreements.

Von der Groeben probably underestimated the extent of this flood of declarations, and thought that he could count on the support of the Council to get block exemptions (the exemption of a category of agreements from the obligation to notify, in order to clear a large number of cases in one go). However, the Member States did not want to devolve more powers to an authority which had not proved its ability to efficiently implement Regulation 17/62. 15 Von der Groeben made his first proposal in November 1962, but the first group exemption regulation was adopted only in 1967.

The study of the adoption of Regulation 17/62 and of its early implementation demonstrates that these events of 1956-62 produced a three-fold path dependency: first, the prohibition principle and the focus on cartels meant that competition policy could progressively emerge as an independent public policy, as in the German case; second, the centralization of information and of decision-making under the Commission was a surprising outcome, considering the wording of the Treaty and the alternatives considered by major actors. Critics of this centralization were widespread at the beginning of the negotiations, so the choice of a more decentralized institutional framework could have been perfectly possible. Third, the mismanagement of the notification process led to a backlog problem which affected DG IV for decades. These three dynamics all had a direct influence on the Merger Regulation 20 years later.

III. The 1989 Merger Regulation

The Merger Regulation of 1989 empowered the Commission to review all mergers of 'European dimension', i.e. beyond a certain threshold. As it was implemented effectively from the start, by contrast with the ECSC Treaty's merger provisions or Regulation 17/62, it is a clear watershed in the history of European competition policy. As a result, it brought about a large number of studies. They generally underline the role of market dynamics, in particular the momentum of the Single European Act, or the role played by transnational networks of private actors fostering a neoliberal agenda (Buch-Hansen and Wigger, 2011, pp. 78-87). A decisive role is generally attributed to the pressure created by the Philip Morris judgment of 17 November 1987 (Joint cases 142 and 156/84). When the ECJ recognized the right of the Commission to exert a merger control on the basis of article 85 EEC, business companies feared for the legal security of their operations. Thus, they put pressure on their national governments to agree on the long-awaiting Merger Regulation, first proposed by the Commission in 1973 but adopted only in December 1989. This momentum created by the Philip Morris judgment is considered as the most important factor leading to the adoption of the merger control by the most convincing account of the negotiation -either almost exclusively (Büthe and Swank, 2007; Pollack, 2003, pp. 284-91) or in conjunction with other factors, such as the leader-ship of the commissioners for competition (Cini and McGowan, 2009, p. 31; Montalban et al., 2011, pp. 28-9) or Member States' bargaining (Armstrong and Bulmer, 1998, pp. 100-6; Bulmer, 1994; Schwartz, 1993). This article does not dismiss the importance of these factors. Rather, it aims to show the limits of the explanations based on the market dynamic, the decisive role of the path dependencies of Regulation 17/62 and the importance of unintended consequences for French officials.

The Limits of the Market-based Explanations

The Philip Morris judgment was certainly a major breakthrough, but it was insufficient to bring about a successful conclusion of the negotiation, for two reasons. First, the deterrence argument (the fact that the Commission could use article 85 and 86 directly, thus putting pressure on the Member States to agree on a regulation) had existed before Philip Morris. The French archives reveal that Commissioner Sutherland had already used this threat before this ruling, at the end of 1985, and that the French officials took this seriously. 16 Second, even the legal service of the Commission considered, in its internal opinion on the Philip Morris judgment, that it was insufficient to provide a sound basis for a sound European merger policy. 17

The more general explanation about the role of the single market dynamic is also important but insufficient. There was no article reinforcing competition policy in the Single European Act of 1986, which by contrast insisted on regional policy, industrial policy and research and technology policy. It was the implementation of the SEA and the single market dynamic that were important (Jabko, 2006, p. 19). Qualified majority voting was not extended to merger control. Indeed, a completion of the single market without a merger regulation would have been perfectly possible, as many important actors were contemplating a new failure of this 16-year-long negotiation in 1989.

In April 1989, the new Commissioner for competition, Leon Brittan, asked DG IV about the merger case under review and for possible new cases. The note from Brittan's cabinet elaborated: 'The reason for this initiative is Sir Leon's concern that we may be losing momentum on the Merger Regulation. If no progress is made on 3 May, we should also consider the preparations of guidelines on the application of Article 85 and 86. We could also make public the fact that we are preparing them'. 18 Brittan clearly envisaged the failure of the negotiations. He reflected on an alternative to the regulation, even though this move was also a tool to put pressure on the Member States.

The same uncertainty over the conclusion of the negotiations also loomed large in the national officials' reflections. Some French officials cast doubts on the Commission's ambitions for the same reasons as in the 1960s: the proposed regulation was too supranational and too market-friendly. In October 1989, senior officials from the Ministry of Economics and Finance (including Jean-Claude Trichet as head of the Treasury) warned that a failure of the negotiations was better than poor legislation. 19 In London, the official position was still negative in early November 1989: 'The UK continues to maintain its general reservation on the principle of a Regulation'. 20 The staunchest opponent was Germany, which feared that the European merger regime could undermine its own national merger regime, in particular if it was more lenient. 21 In July 1989, in a meeting between Brittan and national 16 NA-FR, 19900452/25, notes SGCI, 13 November 1985 and25 November 1985. 17 officials, the German representative put forward the idea of a 'European antitrust office'. 22 The old idea of an independent com-petition agency remained present at the highest level, threatening the Commission's ambitions of centralization. Until the last moment, Germany's agreement was not guaranteed. In the end, purely political factors played an important role in Bonn's final acquiescence. It is well known that the quick pace of reunification after the fall of the Berlin Wall on 9 November 1989 worried some EEC member states. In late November and early December, the priority of Kohl and Genscher was to dissipate the impression that Germany 'emerged as a brake on the European unification process' (Dyson and Featherstone, 1999, p. 364). They felt compelled to reaffirm the West German European credentials during the Strasbourg summit of 8-9 December 1989, in particular by agreeing to set a date for an intergovernmental conference on the EMU. What is less well known is that, according to the declaration of Minister of Economics Helmut Haussmann and his Secretary of State Otto Schlecht to the FDP Bundestag group, the question of the merger regulation 'played a role' during the European Council. [START_REF] Adl | A49/273[END_REF] Bonn probably considered it impossible for Germany to be the only country to block the merger regulation negotiations in these circumstances.

The Role of Negative Path Dependencies

Paradoxically, the Commission was also able to benefit from negative path dependencies, that is to say institutional legacies triggered by the critical juncture of 1962 that put into question its administrative efficiency. The most important was the backlog of Regulation 17/62. Even though it was milder in the 1980s than in the late 1960s, it was still weighing heavily on DG IV resources. There were still more than 3,500 cases pending on 31 December 1986 (Commission of the European Communities, 1987, p. 55). When he became Director General for Competition in 1990, Claus-Dieter Ehlermann still considered his main task with regard to cartels to be alleviating the burden of the remaining 3,000 cases awaiting a decision. [START_REF]Interview of the author with Claus-Dieter Ehlermann[END_REF] This led important Member States to believe that the merger regulation would probably not lead to an assertive policy. When the Director General of DG IV, Caspari, summarized the negotiation for the new commissioner in January 1989, he underlined: 'Particularly from countries in this group [FRG, UK, France], the argument has been put forward that the Commission services are not capable of treating in depth the great number of notifications expected'. 25 In Germany, almost everybody anticipated a lenient merger policy. For example, the debate within the FDP group of the Bundestag on 7 November 1989 revolved around the consequences of this possible weak policy. Some feared that it would be a threat to the German merger policy. They supported the position of the Bundeskartellamt, which was very critical. By contrast, Haussmann and Schlecht, who negotiated the regulation in Brussels, appeared far less hostile to mergers. Schlecht explicitly criticized the Bundeskartellamt's position and underlined the necessity for companies to grow. 26 At the same time, the Minister of Economics had authorized a controversial German merger between Daimler-Benz and MBB, despite the Bundeskartellamt's opposition to it. In any case, none of them anticipated a smooth and bold merger policy.

The backlog also helped to overcome fears on the part of some French officials of an overactive European merger policy. Overall, bar notable exceptions like the aforementioned Trichet note, a sizeable number of French officials were relatively confident in the last stage of the negotiations. They anticipated a lenient policy, more favourable to mergers than both the direct application of article 85-86 and of certain foreign legislations. A recurring theme was the possibility of using the merger regulation to 'avoid the protectionism of the Bundeskartellamt'. 27 This comment dates back to 1988 and applies to the Bundeskartellamt's prohibition of a takeover of a German company (Grunding) by a French one (Thomson) in 1983. The confidence of those French officials can appear strange retrospectively, as it occurred in the context of a growing assertion of the EEC state aid policy which was targeting France in particular. The memory of the implementation of 17/62 certainly played a role, both with the backlog and with the subsequent focus on exclusive dealing agreements, which concurred with French priorities much more than with German ones. For example, the German representative at the advisory commit-tee voted against Grunding-Consten, the first decision to ban a cartel, made in 1964. 28 Germans supported exclusive dealing agreements as they were useful to penetrate foreign markets. As a result, the Commission's cartel policy resulting from the decisions of 1962 helped the French to accept centralization in the 1980s.

Another consequence of the backlog was the concentration of the debate on the means of ensuring effective implementation. Caspari worried about the 'administrative work-load' and the need to find ways to alleviate it within the procedure set out by the future regulation. 29 In the negotiation with Member States, Commissioner Brittan insisted on the necessity to create simple and quick procedures and to provide more staff to DG IV to implement the regulation. 30 Likewise, in Paris, French officials worried about the risk of a 'bureaucratic procedure' and about the necessity to ensure a quick decision by the Commission. 31 This is why the debate concentrated on the question of the dual implementation of national and European authorities, and on the delay of the procedure (in order to ensure a quick assessment of mergers). Sutherland had already progressed on these issues, but it was Brittan who secured the final agreement, with further concessions regarding the exclusive implementation of the regulation to mergers and the so-called 'German' clause concerning national authorities. 32 This focus meant that two other features of the regulation, the centralization under the Commission and the focus on the competition criterion, were less discussed. Many countries, including France and Spain, wanted to include non-competition criteria but their strategies were uncoordinated. 33 As a result, the final outcome was disappointing in this regard, with a competition criterion largely bolstered (Armstrong and Bulmer, 1998, 105). To conclude, the backlog inherited from Regulation 17/62 helped to allay the fears of an overactive merger policy from France, which was a potential opponent of a competition-only regulation, but also of certain German officials, who were not so hostile to mergers.

Unintended Consequences for France

The conclusion of the Merger Regulation was a success for France. The government had stressed since June 1989 that it was of the utmost importance to reach an agreement on the regulation before the end of December in order to achieve success during the French Pres-idency (which lasted during the second semester of 1989). 34 The British archival records show clearly that the last round of the negotiations (November-December 1989) was played between the Germans and them on the issue of national authorities' competences. [START_REF]DTI to Redwood[END_REF] France appeared as an honest broker determined to reach a compromise in good time.

However, most French decision-makers certainly had not anticipated that the first merger to be banned would target a French company. On 2 October 1991 the Commission banned the merger between the Franco-Italian company ATR and the Canadian De Havilland. This interdiction was seen as particularly problematic in France, as it threatened the emergence of a 'European champion' in high technology. For perhaps the first time, European competition policy created a political crisis in Paris, with members of parliament shouting 'Delors démission!' ('Delors, resignation!') in the French National Assembly. [START_REF][END_REF] Ironically, according to George Ross, during the internal debate within the Commission, President of the Commission Jacques Delors tried to oppose his colleague Leon Brittan's proposed decision to ban this merger. However, facing the prospect of being outvoted within the college of commissioners, he chose to abstain (Ross, 1995, pp. 178-80). This outcome is a spectacular illustration of the notion of 'unintended consequences', as most of the French officials negotiating the regulation did not anticipate such an outcome. As a result, on 15 November 1991, a note from the European Department insisted on the necessity to 'reexamine the functioning' of Regulation 4064/89 within the Council. 37 To be fair, some French officials did voice concerns. In October 1989, Head of Treasury Jean-Claude Trichet warned that a merger regulation based on a com-petition-only criterion could 'hamper the formation of a competitive European industry'. 38 Political priorities, but also the path dependencies of Regulation 17/62, limited the impact of this warning.

Conclusion

Regulation 17/62 was a critical juncture, as it created a path dependency for European competition policy by solving the uncertainties of the Treaty of Rome. Many alternatives to the institutional design finally chosen were seriously considered, such as associating the Member States to the decision-making process, setting up an independent authority or adopting the abuse principle. The balance of power at the end of 1960, at the beginning of the negotiation on the future Regulation, was not in favour of the solution of a double centralization (in terms of information and of decisions).

Regulation 17/62 created three long-term path dependencies that were difficult to over-come: the development of competition policy as an independent public policy regulated by its own criteria, a double centralization of the decision-making process under the Com-mission and the backlog. These three path dependencies influenced the negotiations of the Merger Regulation, and were interrelated with unintended consequences. The experience of the backlog was useful to alleviate the fear of a proactive European merger control pol-icy from France, but also for several German officials. This was a dramatic error in the French case, as the indignant reaction to the ATR/De Havilland merger illustrated. The notion of 'unintended consequences' also concerned the Commission, which did not anticipate the extent of the backlog problem in the 1960s. It was triggered partly by an un-anticipated consequence of the French insistence on targeting distribution agreements and of the Council's reluctance to grant block exemptions. Last, market-based dynamics were important but not sufficient. Overall, no single actor was able to dominate the process. French influence on both the implementation of Regulation 17/62 and the adoption of Regulation 4064/89 has often been underestimated, as has been German reluctance to grant powers to the Commission in the 1960s.

Using primary sources is useful to test the notions of the long-term impact of path de-pendency and of 'unintended consequences'. It is a useful complement to previous studies which put an emphasis on market dynamics, such as the rise of neoliberal ideas or the pressure of business. It is helpful to understand the constraints of actors in their response to this changing context. Decision-makers took decisions taking into account economic, institutional, but also purely political factors, as the examples of France and Germany in 1989 demonstrate. This changing context interacted with long-term path dependencies to limit the alternatives available, and in the end to support the centralization process. However, this was not a mechanical process, as the failure of the negotiations remained an option. While it can appear logical today to have a centralized merger policy as a complement to the single market, this was not a preordained outcome.

37 NA-FR, 5AG4/PHB7/1, note SGCI, 15 November 1991. 38 NA-FR, 19900452/29, Trichet for the Minister, note of 4 October 1989.

  EUA, BAC104/1993/159, note Dewost, 25 January 1989. 18 EUA, BAC104/1993/160, note Jonathan Faull, 26 April 1989. 19 NA-FR, 19900452/29, Trichet for the Minister, note of 4 October 1989. 20 NA-UK (FOIA), note by Redwood (DTI), 3 November 1989. 21 AdL, A49/272, fax BMWi, meeting of the economic Commission of the Bundestag on 25 October 1989.

These official archives are opened after 30 years. For1988-89,FOIA procedures have been used to gain access to archives from the European Commission (thereafter : EUA) and national archives in the UK (thereafter : NA-UK) and in France (thereafter : NA-FR). In Germany, national archives (thereafter : NA-FRG) were used for the 1960s and private archives ('Archiv des Liberalismus' in Gummersbach, thereafter: AdL) for 1988-89. French archives also include the Foreign Affairs archives (thereafter : FAA-FR) and the Michel Debré's paper deposited at Sciences-po.

FAA-FR, DECE613, note FG, 14 April 1956; DECE614, note FV, 5 December 1956.

EUA, CM3/236, note Secretariat, debates on 7 September 1956; note of 5 November 1956 by Meyer-Cording; note of8 November 1956, published in Schulze and Hoeren, 2000, pp. 194-5 and 204-5.

EUA, CM3, document of 20 November 1956 ('groupe du Marché commun').

NA-FRG,B102/134644, note of 3 October 1960; note on a meeting on

October 1960; B102/134647, note Epphardt, 15 June 1961.

NA-FR, 1979.0791/262, note on the conference of experts, 22-23 November 1962. FAA-FR, DECE1259, note MC, 15 January 1963.

EUA, BAC89/1983/9/213, von der Groeben to de Koster,23 January 1963.

FAA-FR, RPUE615, note Cuvillier, 11 February 1964.


Mehrez Zribi

Nicolas Baghdadi

Christine Guérin

come

Analysis of Surface Roughness Heterogeneity and

Scattering Behavior for Radar Measurements Mehrez Zribi, Nicolas Baghdadi, and Christine Guérin Abstract-The use of a theoretical backscatter model to analyze medium to low spatial resolution microwave data is still very complicated, particularly because of the difficulty in defining a unique roughness parameter, capable of adequately representing heterogeneous terrain. In this paper, an approach is proposed for roughness analysis and the modeling of backscattering, under conditions of surface heterogeneity. This paper is based on the use of a semiempirical backscattering model, defined with a single roughness parameter Zs = s 2 /l (s being the root mean square surface height and l the correlation length). The proposed backscattering model has been validated with integral equation model simulations, for high radar incidence angles, and within its domain of roughness validity. A range of experimental measurements was used to validate the model expressions. The effective low spatial resolution roughness, inferred from signals backscattered from a surface of heterogeneous roughness, is defined for different roughness classes. Index Terms-Heterogeneity, modeling, roughness, surface scattering.

I. INTRODUCTION

I N RECENT years, various efforts have been devoted to the analysis of the backscattering characteristics of bare soils. Initially, different backscattering models (theoretical, semiempirical, and empirical) were developed [START_REF] Ulaby | Microwave Remote Sensing Active and Passive[END_REF]- [START_REF] Baghdadi | Soil moisture estimation using multi-incidence and multi-polarization ASAR SAR data[END_REF]. More recently, several studies have proposed various approaches to improve roughness descriptions [START_REF] Mattia | Backscattering properties of multi-scale rough surfaces[END_REF]- [START_REF] Baghdadi | Semi-empirical calibration of the IEM backscattering model using radar images and moisture and roughness field measurements[END_REF]. Such descriptions are essential to an accurate analysis and interpretation of backscattering behavior. Zribi et al. [START_REF] Zribi | Validation of a rough surface model based on fractional Brownian geometry with SIRC and ERASME radar data over Orgeval site[END_REF] introduced fractal and Brownian approaches, whereas Li et al. [START_REF] Li | A generalized power law spectrum and its applications to the backscattering of soil surfaces based on the integral equation model[END_REF] proposed a general powerlaw description of roughness spectra. Zribi and Dechambre [START_REF] Zribi | An new empirical model to retrieve soil moisture and roughness from radar data[END_REF] later introduced a description based on the Zs parameter [Zs = s 2 /l, where s is the root mean square (rms) surface height and l is the correlation length]. All of these descriptions have been applied to practical situations, at the scale of an agricultural field of homogenous roughness.

Simultaneously, the effectiveness of low-resolution spaceborne scatterometers for land-surface characterization has been demonstrated by a large number of studies related to vegetation, moisture, and roughness [START_REF] Frison | Use of ERS-1 wind scatterometer data over land surfaces[END_REF]- [START_REF] Prigent | Estimation of the aerodynamic roughness length in arid and semi-arid regions over the globe with the ERS scatterometer[END_REF]. The questions addressed in M. Zribi this paper concern the applicability of a backscattering model, and the meaning of surface roughness in the case of large (scatterometer) footprints containing zones of different and heterogeneous roughness. Various authors have introduced fixed empirical parameters in order to fit backscattering models to their data [START_REF] Jarlan | Mapping of Sahelian vegetation parameters from ERS scatterometer data with an evolution strategies algorithm[END_REF], [START_REF] Zine | Land surface parameter monitoring with ERS scatterometer data over the Sahel: A comparison between agro-pastoral and pastoral areas[END_REF]. However, in some cases, the derived roughness values appear unrealistic when compared with in situ roughness measurements made at a local scale. Grippa and Woodhouse [START_REF] Grippa | Validation of surface scattering models across large footprints for global scatterometer applications[END_REF] analyzed the behavior of different backscattering models (geometrical optics (GEOs), physical optics (PHYs), small perturbation model (SPM), integral equation model (IEM), and the semiempirical model by Oh et al. (OH) [START_REF] Oh | An empirical model and an inversion technique for radar scattering from bare soil surfaces[END_REF]) when used in the presence of heterogeneous soils. Only OH model expressions appear to incorporate heterogeneity. It is however important to note that although the rms height is factored into this model, variations in correlation length are not taken into account. Grippa and Woodhouse [START_REF] Grippa | Validation of surface scattering models across large footprints for global scatterometer applications[END_REF] proposed modified models that incorporate heterogeneity by means of a probability distribution. However, this approach requires other hypothesis, particularly for the roughness probability distribution.

For these reasons, the principal objective of this paper is to reveal the relationship between effective roughness at a low spatial resolution and radar backscattering, in the case of bare soils. Analytical backscattering models such as IEM, which require several roughness parameters as input (rms height, correlation length, surface spectrum) are very difficult to use for the analysis of such relationships. Furthermore, Grippa and Woodhouse [START_REF] Grippa | Validation of surface scattering models across large footprints for global scatterometer applications[END_REF] have shown that these models are not suitable for retrieving surface characteristics from a heterogeneous area. For example, as discussed in the same paper, backscattering depends quadratically on the rms height for SPM. Consequently, if the correlation length is constant, the rms height corresponding to the sum of backscattering from two surfaces with two different rms heights (s A , s B ) could be computed as s = (s 2 A + s 2 B )/2. In order to overcome the difficulties encountered when using analytical models with different roughness input parameters, it is proposed in this paper to develop a "semiempirical" model that is characterized by a single surface roughness parameter, and which remains in agreement with the IEM model. There is a strong need for improved clarity in the discussion of the relationship between low spatial resolution roughness (including heterogeneous roughness) and backscattering.

This paper is organized into five sections, of which Section II presents the semiempirical model developed by the authors, and discusses its validation using the IEM model. Section III describes the comparison between the developed model and experimental radar data obtained with different types of roughness, and Section IV analyzes the concept of an effective low spatial resolution roughness computed from heterogeneous roughness values. Finally, the authors' conclusions are presented in Section V.

II. SEMIEMPIRICAL MODEL DEVELOPMENT

A. Analysis of the Relationship Between Backscattering Signal and the Parameter Zs

The characterization of surface roughness is one of the key requirements in achieving correct analysis of backscattering behavior. It is noteworthy that an increase in the number of surface roughness parameters in a model also increases the difficulty of data inversion. In fact, in the case of some specific radar configurations, the use of more than two roughness parameters, in addition to a surface dielectric constant, renders the inversion inaccurate. Zribi and Dechambre [START_REF] Zribi | An new empirical model to retrieve soil moisture and roughness from radar data[END_REF] proposed the introduction of the parameter Zs = s 2 /l, combining the effects of rms height and correlation length. This is in effect the product of the rms height s, related to the power of the surface height variations, with the ratio s/l, which represents a slope factor. The underlying idea is to introduce the influence of a slope, which is an important soil feature in estimating σ 0 . Small values of Zs correspond to small values of s and/or large values of l, whereas large values of Zs correspond to large values of s or small values of l. A smooth soil surface (without clods) is generally characterized by a small value of s and a medium to large value of l, thus to a small value of Zs. Ploughed soil often corresponds to a large value of s and to a medium to large value of l, thus to a medium to large value of Zs. Very cloddy soil, even with a small s, is characterized by a very small value of l and thus leads to large values of Zs. Fig. 1 provides values of Zs computed for a large range of rms soil heights and correlation lengths. In view of the very strong correlation demonstrated by Zribi and Dechambre [START_REF] Zribi | An new empirical model to retrieve soil moisture and roughness from radar data[END_REF] between backscattered radar signals and Zs, the latter has been chosen as the unique roughness parameter to be used in the model proposed in this paper.

B. Proposed Semiempirical Model

In order to analyze the effect of surface roughness on the backscattering behavior for different spatial resolutions, it is very important, as noted in the Introduction, to use a backscattering model with just one roughness parameter. For this reason, our objective in this section is to propose a new semiempirical model dependent on a single roughness parameter Zs. The proposed model is based on a complete separation between roughness and moisture effects, a hypothesis that has been found to be robust in various theoretical and experimental studies [START_REF] Dubois | Measuring soil moisture with imaging radars[END_REF], [START_REF] Zribi | New methodology for soil surface moisture estimation and its application to ENVISAT-ASAR multi-incidence data inversion[END_REF]. The semiempirical model is adjusted to fit the IEM single backscattering model [START_REF] Fung | Backscattering from a randomly rough dielectric surface[END_REF], [START_REF] Wu | A transition model for the reflection coefficient in surface scattering[END_REF] written as

σ 0 pp = k 2 2 exp -2k 2 z s 2 ∞ n=1 s 2n I n pp 2 W (n) (-2k x , 0) n! p = h, v (1)
where k z = k cos θ, k x = k sin θ, θ is the radar incidence angle, k is the wavenumber, and s is the rms surface height. I n pp is a function of the radar incidence angle, the dielectric constant of the soil s, and the Fresnel reflection coefficient.

W (n) (-2k x , 0)
is the Fourier transform of the nth power of the surface correlation function.

Although the theoretical domain of validity of the above expression is generally limited to ks < 3 [START_REF] Fung | Microwave Scattering and Emission Models and Their Applications[END_REF], various experimental studies [START_REF] Rakotoarivony | Radar backscattering over agricultural bare soils[END_REF], [START_REF] Zribi | Backscattering behavior and simulation comparison over bare soils using SIRC/XSAR and ERASME 1994 data over Orgeval[END_REF] have shown that the single-scattering model is limited to approximately ks < 1.2. Therefore, in the model presented here, surface roughness will be limited to smooth and medium values (ks < 1.2). It is important to note that this is the domain in which a roughness-induced backscattering exhibits the strongest dynamic range, since at higher roughness values, the radar signal approaches saturation. In this paper, the developed model is limited to high incidence angles only (θ > 35 • ); at lower angles the single roughness parameter representation would be inadequate, since new effects, resulting from low roughness frequencies, would not be accounted for.

The proposed model is written as

σ vv 0 = 0.5(kZs) 0.84 cos θ 4 sin θ 3 α 2 vv ( 2
)
σ hh 0 = 3.2110 p(θ) (kZs) q(θ) cos θ 4 sin θ 3 α 2 hh (3)
with

α hh = R ⊥ = cos θ -ε r -sin 2 θ cos θ + ε r -sin 2 θ α vv = (ε r -1) sin 2 θ -ε r (1 + sin 2 θ) [ε r cos θ + ε r -sin 2 θ] 2
R ⊥ Fresnel coefficient for horizontal (HH) polarization; ε r relative dielectric constant; k wavenumber; θ incidence angle.

q(θ)= 2.6289θ 2 -3.2561θ + 1.969, with θ in radians p(θ)= 2.303θ 2 -2.3217θ,
with θ in radians.

The expressions given in ( 2) and (3) were derived as follows. The SPM has been used, allowing roughness and moisture effects to be separated.

The backscattering coefficient is written as

σ 0 = g(R, θ) • f (ε, θ)
and the roughness component is given by

g = k 4 s 2 W (2k sin θ, 0). (4)
In general, exponential correlation function is used to describe natural soil surfaces, thus leading to

g = k 4 s 2 l 2 (1 + (2kl sin θ) 2 ) 1.5 .
(

) 5
By taking (2kl sin θ) 1, the above expression for g can be simplified to

g ≈ k 8 s 2 l sin 3 θ = kZs 8 sin 3 θ . ( 6
)
This hypothesis is particularly useful for high incidence angles. With a view to adjusting the proposed model to the IEM, the roughness component is expressed here as a function of Zs. As simulations show that the SPM and IEM models have approximately the same behavior as a function of surface moisture, it is proposed to adjust the model presented here to the IEM by considering two variables: a parameter (a) used to define the power law of (kZs), and a proportionality constant (b)

g = b (kZs) a sin 3 θ . ( 7
)
The proposed semiempirical model depends only on the Zs parameter to describe surface roughness. This limitation to one parameter is very interesting for radar measurement inversion. However, it is important to note that our hypotheses concerning the correlation function shape or the expression of the model limit necessarily the validity domain of the model, particularly for high radar frequencies.

C. Comparison With IEM Simulations

In order to validate the proposed model, use of the IEM has been restricted to its domain of validity (ks < 1.2). Simulations were made for a large range of surface roughness parameters. Heterogeneous roughness parameters were generated using a Monte Carlo approach, based on uniform rms height (0.3-1.2 cm) and correlation length (3-20 cm) distributions. Three hundred IEM simulations were made using an exponential correlation function with constant soil moisture (30%). For HH polarization, the rms error does not exceed 0.82 dB (0.6 dB for 35 • , 0.62 dB for 40 • , 0.69 dB for 45 • , and 0.82 dB for 50 • ). This result convincingly supports the assumption that Zs can replace all the roughness parameters used in the IEM model, and justifies the use of the proposed model to analyze roughness behavior for the proposed domain of validity.

In order to characterize moisture effects, the same function can be used as that derived for the SPM model. Table I illustrates the good degree of coherence between IEM and proposed model simulations, where an rms error less than 1 dB is found (except for one case) over a large range of soil moistures.

III. ANALYSIS BASED ON EXPERIMENTAL RADAR DATA

In the previous section, the coherence between the IEM and proposed model has been demonstrated. The purpose of this section is to compare the roughness component behavior predicted by the present model with that found in experimental radar data.

A. Database Description

For this paper, data acquired over agricultural watersheds in France were used [START_REF] Baghdadi | Soil moisture estimation using multi-incidence and multi-polarization ASAR SAR data[END_REF], [START_REF] Zribi | An new empirical model to retrieve soil moisture and roughness from radar data[END_REF]. Radar soundings were carried out at high incidence angles, all in HH polarization, by various active orbiting sensors (ASAR/ENVISAT, RADARSAT-, SIR-C). The following three experimental campaigns are considered in this paper. Simultaneously to the above-described radar acquisitions, ground measurements were carried out in a large number of test fields. The soil moisture was measured within the upper 5 cm using a gravimetric method and/or a time-domain reflectometry (TDR) probe. All measurements used in this paper correspond to high moisture values (higher than 30% for volumetric moisture). Therefore, in the following discussions, the moisture levels are considered to be approximately constant. Roughness measurements were made using a pin profiler (total length of 1 and 2 m and a resolution of 5 and 10 mm, respectively). In order to guarantee sufficient statistical accuracy, ten profiles were measured for each test field. For the full set of measurements, the rms height ranges between 0.38 and 2.5 cm, the correlation length varies between 2.5 and 12 cm and the corresponding parameter Zs ranges between 0.016 and 2.64 cm. Roughness values are illustrated in Fig. 3.

B. Validation of the Model Behavior Within Its Validity Domain

In this section, it is shown that the semiempirical model provides simulated backscattering results in good agreement with experimental radar data.

In the case of the HH polarization, the backscattering signal could be written as where g = (kZs) q(θ) .

σ hh 0 [dB] = 10 log (g(R, θ)) + 10 log (f (ε, θ)) (8)
For the purposes of the present discussion, soil moisture is assumed to be almost constant (higher than 30%), and analysis of the results is centered principally around the influence of soil roughness on backscattering.

Fig. 4 provides a comparison between experimental radar data and model simulations for 21 different test fields, for which measured roughness respects the domain of validity (ks < 1.2) of the semiempirical model, as illustrated in Fig. 3. The data correspond to various incidence angles (37.4 • , 39 • , 40 • , 44 • , 47 • ) in HH polarization. The rms error between experimental and predicted signals is equal to 1.33 dB, thus demonstrating that the use of a single roughness parameter can provide reliable results of good quality. Concerning the soil moisture variation effect on the proposed model, it is in good agreement with IEM model behavior as it is illustrated in Table I.

C. Analysis of the Model Behavior Outside the Bounds of Its Validity Domain

In this section, experimental radar data are compared with results derived from the semiempirical model, for roughness values outside its limits of validity (ks > 1.2). Fig. 5 compares, as in Fig. 4, radar data with results predicted by the model, for a range of incidence angles. In this case, a discrepancy is observed between data and simulations, with an rms error of 2.7 dB. The model is seen to overestimate the backscattered signal, a result generally observed with the IEM model for high surface roughness [START_REF] Zribi | Backscattering behavior and simulation comparison over bare soils using SIRC/XSAR and ERASME 1994 data over Orgeval[END_REF]. In practice, the radar data saturates more quickly than the signal levels predicted by the backscattering model.

IV. CHARACTERIZATION OF LOW SPATIAL RESOLUTION ROUGHNESS

A. Analysis Within the Model's Domain of Validity

In this section, the semiempirical model is used to analyze the impact of heterogeneous roughness on low spatial resolution data.

The incoherent sum of signals backscattered by n fields is written as

σ low 0 = n c i σ i 0 (s i , l i , ε i ) (9)
where c i signal fraction corresponding to the ith field; σ i 0 backscattered signal from the ith field; s i rms height of the ith field; l i correlation length of the ith field; ε i dielectric constant of the ith field. Assuming constant soil moisture content for all fields and combining (2), (3), and ( 9), a low-roughness parameter Zs can be written as

Zs low = n c i Zs 0.84 i 1/0.84 for VV polarization (10)
Zs low = n c i Zs q(θ) i 1/q(θ)
for HH polarization. [START_REF] Davidson | On the characterization of agricultural soil roughness for radar remote sensing studies[END_REF] These equations provide relationships between the Zs parameter for n different local fields, and the effective low spatial resolution parameter Zs low of the combined fields. Equation [START_REF] Davidson | On the characterization of agricultural soil roughness for radar remote sensing studies[END_REF] shows that for HH polarization, the effective Zs for heterogeneous surfaces is dependent on incidence angle only. The principal objective of this paper is to provide an analysis of radar backscattering behavior as a function of heterogeneous roughness at low spatial resolutions. Having defined effective low spatial resolution roughness, the authors undertook to compare IEM simulations with the backscattering behavior predicted by the proposed model, using various surfaces of heterogeneous roughness.

For each of 300 different cases, the incoherent sum of IEM model simulations for n fields with different roughness characteristics was compared with the developed model using the effective Zs parameter computed from [START_REF] Mattia | Backscattering properties of multi-scale rough surfaces[END_REF] or [START_REF] Davidson | On the characterization of agricultural soil roughness for radar remote sensing studies[END_REF]. In order to correctly validate the proposed model at low spatial resolution, it was chosen to consider different types of roughness distribution to represent surface heterogeneity: For each different case, various types of roughness distribution were considered (uniform, Gaussian, Weibull, etc.). Fig. 6 shows the outcome of these comparisons for four incidence angles (35 • , 40 • , 45 • , and 50 • ), thus demonstrating that a good degree of coherence exists between the two methods.

This result confirms the potential of an effective Zs parameter in describing low spatial resolution roughness, when used with the proposed semiempirical model to simulate low spatial resolution backscattering.

After the analysis made with IEM model, a comparison is made between the proposed semiempirical model and experimental radar data in HH polarization. Each datum corresponds to the radar signal computed from three or four fields with a different surface roughness. Table II illustrates the values of roughness (rms height, correlation length) for each datum. Zs low (representing the heterogeneous roughness for each datum) is computed using [START_REF] Davidson | On the characterization of agricultural soil roughness for radar remote sensing studies[END_REF] with roughness measurements for each field. Fig. 7 illustrates this comparison with six different data (corresponding to 21 different fields). It shows a good agreement between data and model with an rms error equal to 0.47 dB.

B. Discussion of the Model Outside Its Range of Validity

The study with the semiempirical model is made for fields with a surface roughness ranging between smooth and medium. When a greater range of surface types is present (smooth,

c i Zs B i (θ) i 1/B t (θ) (12)
where B i (θ) is the power corresponding to each type of roughness.

For low spatial resolution data with topographic (local angle) variations, the effective Zs parameter must also take into account variations in incidence angle. With the mean local angle taken to be zero, the effective Zs parameter for VV polarization can then be written as

Zs low = n c i Zs 0.84 i cos θ 4 i sin θ 3 i α 2 vvi / cos θ 4 sin θ 3 α 2 vv 1/0.84 (13)
where θ mean incidence angle; θ i θ -θ local ; θ local local angle of the ith field. The preceding discussions show that even for low-resolution radar data, the effective roughness parameter Zs has a distinct physical meaning, which is closely linked to the local roughness characteristics of the terrain under study.

C. Inversion Model

After the validation of our model to simulate radar signal over heterogeneous soil surfaces, we propose to apply the inversion model to retrieve surface roughness from radar measurements over heterogeneous surfaces. For our case, radar data exist only in HH polarization with one incidence angle for each experimental campaign. Therefore, radar inversion is made only to calculate roughness parameter Zs and not moisture, which is considered as known (measured). We consider 17 data with for each case a heterogeneous soil with three or four different agricultural fields. Fig. 8 illustrates the comparison between retrieved roughness from radar data (with incidence angle raging between 37.4 • and 47 • ) using our model and roughness computed from the three or four measured soil roughness of each field, using [START_REF] Davidson | On the characterization of agricultural soil roughness for radar remote sensing studies[END_REF]. We observe a good agreement with an rms error equal to 0.023 cm. This result confirms the potential of this model to retrieve heterogeneous surface roughness in spite of its validity domain limitation for high radar frequencies.

V. CONCLUSION

The objective of this paper is to analyze the role of roughness, for the case of heterogeneous terrains, in a surface scattering model. A semiempirical model is proposed, which clearly distinguishes between roughness and moisture effects. Roughness is described with just one parameter Zs = s 2 /l. The proposed model has been validated by comparing its predictions with those obtained with the IEM single-scattering model. Good agreement is found between these models for high incidence angles (θ > 35 • ) and for small or medium degrees of roughness (ks < 1.2). A comparison made with experimental radar data shows that model predictions and observed data are in good agreement, for homogenous surfaces with ks < 1.2. For a terrain characterized by a high degree of roughness, the model is found to overestimate the backscattering coefficient. In the case of heterogeneous terrains observed with low spatial resolution, the proposed model allows a single value of effective surface roughness to be used. For a case study of n fields with heterogeneous roughness characteristics, the backscattering predictions obtained with the semiempirical model are shown to be well correlated with the result obtained by incoherently summing n IEM simulations and experimental radar data. The effective roughness of a heterogeneous area is defined for different types of surface roughness, for both HH and VV polarizations. The comparison between computed heterogeneous roughness over different test sites and retrieved roughness using inversion model and radar data shows a good agreement.

The proposed model with a limited roughness validity domain is certainly more adapted to low frequencies up to C-band.

Further studies will propose a global mapping of the Zs parameter, based on data obtained from existing low resolution microwave sensors, in particular ASAR (ENVISAT) and the European Remote Sensing Satellite 2 or the Advanced Scatterometer wind scatterometer.

Fig. 1 .

 1 Fig. 1. Illustration of the parameter Zs as a function of rms height and correlation length.

Fig. 2

 2 compares the results predicted by the IEM and proposed models, for HH and vertical (VV) polarizations in C-band, using four different incidence angles 35 • , 40 • , 45 • , and 50 • . Good agreement is found between the two models; in particular, for the VV polarization, the rms error is less than 0.26 dB (0.25 dB for 35 • , 0.23 dB for 40 • , 0.24 dB for 45 • , and 0.26 dB for 50 • ).

Fig. 2 .

 2 Fig. 2. Comparison between backscattering predictions from the IEM and the proposed semiempirical model for HH and VV polarizations, for different incidence angles: (a) 35 • , (b) 40 • , (c) 45 • , and (d) 50 • .

1 )

 1 Orgeval (1994): The Orgeval watershed is located in the east of Paris, France. The soil texture is approximately constant over the whole basin: 17% clay, 78% silt, and 5% sand. The Orgeval (1994) radar data were acquired with the SIR-C radar. 2) Pays de Caux (1998): The test site is the Blosseville watershed located in the Pays de Caux in northern France. The soil is characterized by a loamy texture (13% clay, 60% silt, and 22.4% sand). The radar data were acquired by RADARSAT-I. 3) Villamblain (2003): The Villamblain site is located in the center of France with a very flat topography. The site is characterized by soils composed of about 60% loam, 30% clay, and 10% sand. The data were acquired by the ASAR radar instrument on ENVISAT.

Fig. 3 .

 3 Fig. 3. Illustration of roughness measurements of different test fields used in the proposed model validation.

Fig. 4 .

 4 Fig. 4. Comparison between experimental radar data and simulations using the proposed model, within its validity domain (ks < 1.2) for various incidence angles and HH polarization.

Fig. 5 .

 5 Fig. 5. Comparison between experimental radar data and simulations using the proposed model for high roughness (ks > 1.2) and various incidence angles and HH polarization.

Fig. 6 .

 6 Fig. 6. Comparison between an incoherent sum of IEM simulations computed for heterogeneous roughness cases, and simulations from the semiempirical model using effective roughness values for the parameter Zs for four incidence angles (35 • , 40 • , 45 • , and 50 • ).

Fig. 7 .

 7 Fig. 7. Comparison between experimental radar data and simulations using the proposed model for low-resolution data with heterogeneous roughness and HH polarization.

Fig. 8 .

 8 Fig. 8. Comparison between retrieved roughness from radar measurements and coarse-resolution roughness computed from real measurements over different homogenous soil surfaces.

TABLE I RMS

 I ERRORS BETWEEN IEM AND PROPOSED MODEL SIMULATIONS,

FOR DIFFERENT CONFIGURATIONS OF MOISTURE (Mv) AND INCIDENCE ANGLE (θ)

TABLE II ILLUSTRATION

 II OF ROUGHNESS PARAMETERS (RMS HEIGHT, CORRELATION LENGTHS) MEASURED IN THE SIX DIFFERENT HETEROGENEOUS SOIL SURFACES USED FOR COMPARISON BETWEEN MODEL SIMULATION AND RADAR DATA

ACKNOWLEDGMENT

The authors would like to thank the European Space Agency for having providing them with ASAR images free of charge, through project number 351 ENVISAT/ASAR. The authors would like to thank also all the team for their logistic support in terrain campaigns.